Kingdom Protoctista (Protista) - Theory & Question Answers

 Biology XI Notes - Kingdom Protoctista (Protista) - Theory & Question Answers

KINGDOM PROTOCTISTA (PROTISTA)

This kingdom includes all the unicellular eukaryotic organisms, which are no longer classified as animals, plants, or fungi, e.g., Euglena, Paramecium, etc. Multi-cellular algae and primitive fungi have also been included.

DIVERSITY AMONG PROTISTA (PROTOCTISTA): Due to diversification, this kingdom is regarded as a polyphyletic group of organisms. The kingdom is divided into three groups:

• Plant-like prototists: Algae
• Fungi-like prototists: Primitive fungi
• Animal-like prototists: Protozoa

PLANT LIKE PROTOCTISTS: ALGAE (CHLORELLA AND ULVA):

Algae are responsible for more than half of the amount of photosynthesis carried out in the world. Algae ranges from unicellular to filamentous and to huge multicellular structure.

Chlorella:

Introduction: It is plant-like prototist and grows in fresh water and usually floats in stagnant water of ponds, pools, ditches, etc. It is easily cultured.

Structure: Chlorella is single-celled, spherical, and solitary. The cell is surrounded by a cell wall. The body comprises a single nucleus and cup-shaped chloroplast. Pyrenoid is usually absent.

Reproduction: Chlorella reproduces asexually by autospores. The protoplast divides into 8 to 16 daughter protoplasts and develops a wall to produce non-motile autospores. After maturation, the autospores are released from the parent body. Each autospore develops into a new Chlorella.

Economic Importance:

• Chlorella cells have very high food value. They contain about 50% proteins, 20% fats, and 20% carbohydrates, amino acids, and vitamins; therefore, they are considered an alternative source of food.
• An antibiotic, chlorellin, is produced from Chlorella, which is useful for controlling bacterial diseases.
• Chlorella is an experimental organism in research of photosynthesis as it can be grown easily.

Ulva:

Introduction: Ulva is commonly known as Sea-lettuce. It is marine algae and grows between the low and high tides in Pakistan. It is found in Kemari and Manora coast. Sometimes, Ulva attaches with the rocky edges of the coast.

Structure: The plant body of Ulva is called the thallus (root, stem, and leaves are not distinguishable), which consists of erect, broad, sheet-like branches or blades. At the base of the plant, long thread-like cells are present, called holdfast, which helps in attachment with rocks bordering the tide pools. The thickness of the thallus is only two cells. Only the outer surface of Ulva is exposed to the aquatic environment. The thallus of Ulva has two types:

• Sporophyte having 26 chromosomes.
• Gametophyte having 13 chromosomes.

Morphologically, both are exactly similar, so termed as isomorphic.

Life Cycle Of Ulva: Ulva completes its life cycle in two phases. Ulva shows isomorphic alternation of generation.

Asexual Phase: Asexual reproduction takes place in diploid sporophyte plants. The spores are formed in all cells of the thallus except basal cells by meiosis. In each parent cell, 8-16 haploid zoospores are produced, which are quadriflagellated. The formation of spores is continuous until all the cells are used and nothing remains of the thallus. The liberation of spores takes place due to the incoming tides. The zoospores swim in water and lose their flagella and grow into new haploid Ulva plants.

Sexual Phase: In Ulva, sexual reproduction takes place in gametophyte plants. Sexual phase is isogamous. The haploid Ulva thallus produces the haploid gametes by mitosis. Gamete is smaller than the zoospores and biflagellate. The fusion of two haploid gametes results in the formation of a quadriflagellated diploid zygote. It loses its flagella after a period of swimming and secretes a wall, which develops into a new diploid sporophyte plant.

Alternation Of Generation Of Ulva: Ulva has distinct isomorphic alternation of generation. In the life cycle, Ulva has two morphologically identical generations. One produces gametes and is called the gametophyte, the second produces zoo spores and is called the sporophyte. They alternate to each other. The gametophyte is haploid (n) with 13 chromosomes. Zygote formed by the fusion of gametes has 26 chromosomes. This is called diploid or ‘2n’ zygote. Zygote develops into diploid (2n) gametophyte.

Euglena:

Introduction: Euglena is unicellular algae that is advanced compared to blue-green algae. It is typical algae and has both plant and animal characteristics.

• Plant-Like Characters:

  • Euglena has chloroplast.
  • It performs photosynthesis.

• Animal-Like Characters:

  • Euglena has a mouth, gullet, and flagella.
  • The body is surrounded by a pellicle.

Structure: Euglena is microscopic and spindle-shaped. The cell of Euglena is surrounded by a pellicle, and the cell cytoplasm is divided into two parts. The outer part is called ectoplasm, and the inner part is called endoplasm. The cell cytoplasm contains a plate-like structure called chloroplast, which has chlorophyll. It performs photosynthesis. The center part of the cell has a nucleus. The mouth is present at an interior part of the cell, which is open into a tube called cytostome. At the base of the cytostome, a large reservoir is present, which contains a contractile vacuole. A red color eye spot is present near the reservoir.

• Movement:
  Euglena has a long flagellum that arises from the mouth. It helps in the movement of Euglena.

• Reproduction:
  Euglena has asexual reproduction. During reproduction, the cell of Euglena simply divides by fission, and two Euglena are produced.

FUNGI LIKE PROTOCTISTA (SLIME MOLD, WATER MOLD):

Slime Mold: Slime molds are fungi-like prototists. They form a special group of organisms, which are animal-like in their body structure and plant-like in their reproduction. They are creeping multinucleated masses of cytoplasm, which look like egg white.

• Habitat:
  They grow rapidly up to one foot in damp and shady places, crawl over the grass, decaying leaves, old and rotten logs of wood. They have saprophytic nutrition because they lack chlorophyll.

• Structure:
  The body of the slime mold consists of an irregularly shaped mass of protoplasm, which is naked because a proper body wall is absent. The naked protoplasm is bounded by a non-cellular, thin, and flexible slimy layer. Due to the presence of the slimy layer, it is termed as slime mold. The protoplasm in slime layer contains plasma membrane, and cytoplasm is divided into outer ectoplasm and inner endoplasm. Slime mold has no proper shape and size.

Life Cycle Of Slime Mold: The life cycle of slime mold consists of the following stages:

1. Plasmodium Stage:
   The plasmodium consists of cytoplasm with numerous nuclei, food vacuoles, and undigested food particles. It occupies several square centimeters and looks like a creeping mass resembling a large amoeba, engulfing and digesting bacteria. The life cycle of the slime mold is completed in two phases:

   • Asexual phase.
   • Sexual phase.

2. Asexual Phase:
   In dry and warm weather, the plasmodium changes into clusters of fruiting bodies, which vary in shape and color, e.g., small golf balls, feathers, or worms. Part of each fruiting body produces a large number of spores, which are microscopic, asexual reproductive cells having a single nucleus and thick protective wall. The spores may remain inactive for a long time or may germinate soon, producing one or more tiny cells having a pair of flagella. Plenty of water and suitable temperature are required for spore germination. Slime mold resembles fungi in that they show fruiting and thick-walled spores.

Sexual Reproduction: The flagellated cells function as sex cells (gametes) and fuse together. Although the gametes are identical in structure, slime mold shows true sexual reproduction. The fusion of gametes results in the formation of cells which become amoeboid and form a new plasmodium which is multinucleated.

Water Molds (Oomycotes) Phytophthora: Phytophthora infestans is an example of a water mold. It is a pathogenic organism causing late blight in potatoes.

• Structure:
  The body of Phytophthora is known as mycelium. It is branched in nature and composed of many thread-like structures called hyphae. The mycelium is unseptate or coenocytic. The mycelium is either intercellular or intracellular and absorbs food material from host cells by haustoria.

• Reproduction:
  Phytophthora has two types of reproduction.

  • Asexual Reproduction:
    In asexual reproduction, zoospores are produced in reproductive organs called sporangia. At the time of reproduction, many erect branches arise from the mycelium called sporangiophores. They come out from the stomata of lower epidermis of host leaves. They produce more branches at the tips of which oval or lemon-shaped sporangia are formed. Many sporangia are produced due to continuous growth. A small papilla is developed at the anterior end of each sporangium. After maturation, the sporangia are scattered by wind or rain splash to different places. Further development of sporangia occurs in two ways depending upon the conditions.

    • In Dry Condition:
      In dry seasons at high temperatures, the sporangia germinate directly into a germ tube, which forms new mycelium.

    • In Moist Conditions:
      In moist conditions at low temperatures, each sporangium produces eight biflagellate zoospores. After maturation, it ruptures at the point of the papilla, and the spores are liberated. The zoospores swim for some time with the help of flagella in rain or dew drops on the leaves of the host plant. Afterwards, they lose their flagella and develop a thick wall around themselves. During favorable conditions, each zoospore forms a germ tube. It enters the host leaf through stomata or cuticle and produces a new mycelium of the Phytophthora.

  • Sexual Reproduction:
    In Phytophthora, the sexual reproduction is oogamous type. In this method, two entirely different types of male and female reproductive organs are produced. The male reproductive organ is called antheridium, and the female reproductive organ is known as oogonium. Both of them may develop on the same hypha or on two adjacent hyphae. The oogonial hypha penetrates the antheridium, where the male and female nuclei fuse. After fertilization, the thick-walled oospore develops. Sometimes parthenospores are developed without fertilization. The oospore germinates with a germ tube which develops into mycelium. Reduction division occurs during the germination of the oospore.

Animal-Like Protists: Phylum Protozoa:

General Characteristics:

• This group contains about 50,000 species.
• Protozoans are small, usually microscopic unicellular organisms that were previously regarded as animals.
• The cell mostly contains one nucleus, but in some, two nuclei are also present, which may be similar (monomorphic) or dissimilar (dimorphic).
• Some of the protozoans are multinucleated.
• Some of the species occur independently (solitary), whereas some form loose colonies in which individuals are all alike.
• Asexual reproduction takes place by fission and budding.
• Sexual reproduction occurs by the conjugation of adults or by the fusion of gametes.
• They are aquatic and are found in both fresh and marine water.
• Parasitic or symbiotic protozoans are found over or inside animals and plants.
• Respiration mostly occurs through the general body surface.
• Some of them have constant body shapes, whereas some change their shape constantly.

Classification Of Protozoa:
According to their mode of locomotion, protozoans are divided into five classes.

1. Class Flagellata:
   Characteristics:

   • The class is so named because its members contain one or more whip-like structures - flagella.
   • Some of them contain plastids, containing chlorophyll; hence these flagellates are called phytoflagellates.
   • The phytoflagellates feed autotrophically.
   • The flagellates which do not contain chloroplasts and chromatophores feed heterotrophically and are called zooflagellates.
   • Body (cell) contains only one nucleus. Example: Euglena.

2. Class Sarcodina:
   Characteristics:

   • Members of this class have no specific organs for locomotion.
   • They move through cytoplasmic projections called pseudopodia.
   • They have no constant body shape.
   • Body (cell) contains only one nucleus.
   • Some parasitic species e.g., Entamoeba histolytica cause disease of dysentery in humans.
   • Certain shelled sarcodines have deposited millions of skeletons under sea which give clues about possible petroleum deposits. Example: Amoeba.

3. Class Ciliata:
   Characteristics:

   • Members of this class contain cilia all over the body for locomotion.
   • Nearly all the ciliates are free-living.
   • Ciliates contain two types of nuclei: A macronucleus and from one to as many as 80 micronuclei.
   • The large macronucleus controls everyday activities.
   • The small micro-nuclei function in sexual reproduction.
   • Many of them contain a groove called gullet through which engulfing of food takes place. Example: Paramecium.

4. Class Suctoria:
   Characteristics:

   • Suctorians also contain two types of nuclei. The meganucles and micronucles.
   • Young individuals have cilia and are motile whereas adults have no cilia and are sedentary.
   • Body bears tentacles.
   • In some have tentacles are pointed to pierce the prey. Some have adhesive knobs to catch and hold the prey.
   • Reproduction is asexual by budding. Example: Acineta.

5. Class Sporozoa:
   Characteristics:

• This class contains exclusively parasitic forms.
• They occur both as intra and inter-cellular parasites.
• Some of them cause serious diseases Coccidiasis and malaria in poultry and man respectively.
• Sporozoans do not have any locomotory organelles. Example: Plasmodium.

Life Cycle Of Malaria Parasite: Plasmodium

Discovery Of Plasmodium:
Plasmodium is a unicellular protozoan which is the causative agent of malaria. It is first discovered by Laveran in 1878 in the blood of a malarial patient. Later on, Ronald Ross discovered the presence of plasmodium inside the stomach of the female Anopheles mosquito in 1897. After that, in 1898, Grassi discovered the life cycle of plasmodium inside the female Anopheles mosquito.

The parasite of malaria, plasmodium, completes its life cycle in two hosts: man being the secondary host involving asexual cycle (Schizogony) and Anopheles mosquito being the primary host involving sexual cycle (Gametogony).

Asexual Cycle In Man - Schizogony:
While biting a healthy person, an Anopheles mosquito inoculates its saliva to check blood clotting. Along with its saliva, sporozoites of plasmodium also enter the body. Schizogony comprises the following stages:

• Pre-erythrocytic Cycle
• Erythrocytic Cycle
• Post-erythrocytic Cycle

Pre-erythrocytic Cycle:
It is completed in the following stages:

• Sporozoites:
  When the infected female Anopheles mosquito bites a healthy person, it transfers the plasmodium in man. This stage of plasmodium is known as sporozoite. Once within the human blood, the sporozoites circulate for about half an hour and then get into the liver to invade the hepatic cells.

• Schizont:
  After penetrating the hepatic cells, each sporozoite grows for a number of days by absorbing food material and becomes a large, rounded shape structure called schizont.

Cryptozoites:
After that, the nucleus of schizont divided and produces uninucleated cryptozoites which are liberated and invade other healthy hepatocytes.

Meta Cryptozoites:
The cryptozoites invade other fresh liver cells and multiply, producing an enormous number of meta-cryptozoites after repetition of the pre-erythrocytic phase.

Erythrocytic Cycle:
This phase is completed into the following stages:

• Trophozoite:
  Meta cryptozoites leave the hepatocytes and invade the R.B.C (red blood cells) after escaping into the bloodstream. Each meta cryptozoite absorbs the food material from the R.B.C and becomes a rounded-shaped structure called trophozoite.

• Signet Ring Stage:
  When the trophozoite grows in size, the nucleus is pushed to one side into the peripheral cytoplasm. It resembles a signet ring and is referred to as the signet ring stage.

• Amoeboid Trophozoite:
  After that, trophozoite becomes irregular in shape, and pseudopodia-like structures are produced. It ingests the hemoglobin of R.B.C and converts it into a poisonous substance called haemozoin. After active feeding, it becomes rounded, grows in size, and becomes a schizont.

• Merozoites:
  The nucleus of trophozoite divides and produces 10-12 nuclei which convert into merozoites. With the rupture of RBC, the merozoites are liberated into the blood plasma. The poisonous substance haemozoin is also released with the merozoites, causing the symptoms of malaria. The merozoites again invade the fresh RBC to repeat the cycle. The time taken to complete one erythrocytic cycle depends upon the species of plasmodium.

Post Erythrocytic Cycle:
Some merozoites repeated the erythrocytic phase and reached the liver cells, undergoing schizoid development. This is known as the post-erythrocytic cycle.

Sexual Phase: (Gametogony)
When successful asexual multiplication is achieved, the merozoites do not proceed further with the erythrocytic phase but after entering the RBC increase in size to become gametocytes.

Types Of Gametocytes:
Merozoites convert into two types of gametocytes:

• Male or microgametocyte.
• Female or macrogametocyte.

The gametocytes do not divide, but remain within their host blood until they are ingested by the anopheles mosquito in which they continue their development.

Sexual Cycle in Mosquito:
The sexual cycle of plasmodium is completed in the gut of the female anopheles mosquito. This cycle is completed in the following stages.

Gametogony:
The gametocytes are taken up along with the blood into the stomach of the mosquito. The female gametocytes soon become macrogametes by absorbing food and getting ready to be fertilized. Each male gametocyte forms 6 to 8 male sperm-like microgametes by a process of exflagellation.

Conjugation or Syngamy:
The two gametes of opposite sexes fuse together to form a zygote. The process is called syngamy. The zygote becomes worm-like ookinete. It penetrates into the stomach wall to settle down just under the mid-gut. Here, after absorbing nutrients, it becomes rounded and encysts to form the oocyst.

Sporogony:
After 6 to 7 days, the sporogony takes place, which is completed in the following stages.

Formation of Sporozoites:
The oocyst absorbs the food from the wall of the stomach and enlarges in size. After that, the nucleus is divided into 10,000 nuclei, which change into spindle-like sporozoites very soon.

Liberation of Sporozoites:
When the blister of the sporoblast is ruptured, all the sporozoites come into the haemolymph (blood) of the mosquito and reach the salivary gland through the blood. Due to the bite of the mosquito, sporozoites enter the blood system of a healthy person, causing malaria, and this cycle is repeated again.

Symptoms of Malaria:
The symptoms of malaria first appear when infected R.B.C.s are broken. The time taken by the parasite from its entry into the man up to the breaking down of R.B.C.s is called the incubation period. The symptoms that appear in this period of infection include fever, nausea, loss of appetite, constipation, and insomnia. Soon, headache, muscular pains, and aches in the joints develop, followed by chills.

At the onset of malarial fever, the patient suffers from shaking chills and sweating. The body temperature may rise as high as 106°F. After a few hours of fever, there is profuse sweating, and finally, the fever disappears. The recurrence of symptoms usually occurs after 48 hours or sometimes earlier.

Kingdom Prokaryota (Monera) - Theory & Question Answers Chapter # 06

 Biology XI Notes - Kingdom Prokaryota (Monera) - Theory & Question Answers

Chapter # 06
Theory & Question Answers
Section III - Biodiversity

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KINGDOM PROKARYOTA (MONERA)

Prokaryotae is a group of living organisms which are unicellular, having prokaryotic or a primitive nucleus, i.e., lack nuclear membrane, nucleolus, and nucleoplasm. Prokaryotic cells lack all membrane-bound organelles, e.g., mitochondria, plastids, endoplasmic reticulum, etc. Bacteria and cyanobacteria (blue-green algae) are included in this group.

BACTERIA:

• Discovery:

  • Antony Van Leeuwenhoek (1673) was the first to observe bacteria. In 16 he wrote an article on bacteria, the very first description of bacteria.

• Habitat:

  • Bacteria are found everywhere in the world. They are found in all possible environmental conditions of earth.

• Structure:

  • Bacteria are considered the smallest and simplest living organisms. Bacteria are the pioneers of cellular organization and strictly unicellular size.

• Size:

  • Bacterial cell measures from 0.2 microns (μ) to 2 μ in breadth and 2 to 10 μ in length.

Parts Of Bacteria: A bacterial cell may consist of the following parts.

• Flagella:

  • They are long thread-like structures. They originate from the basal body in the cytoplasm. They are made up of flagellin protein. Flagella help in motility.

• Pilli:

• They are hollow, filamentous appendages smaller than flagella. They help in conjugation and not in locomotion.

• Capsule:

  • It is a protective shield composed of polysaccharides and proteins. Some bacteria have a slime capsule that provides greater pathogenicity (disease-causing ability) and protects them against phagocytosis.

• Cell Wall:

  • The cell wall is found surrounding the cell membrane. It is rigid, determines the shape, and protects from osmotic lysis. It is composed of amino acids, sugar, and chitin, but cellulose is totally absent. Most bacteria have a unique macromolecule peptidoglycan. Teichoic acid, lipoprotein, and lipopolysaccharides are also present in the cell wall. Archaebacteria do not contain peptidoglycan.

• Cell Membrane:

  • It lies inside the cell wall. It is attached to the cell wall at a few places. It has many pores. Chemically, it is made up of lipids and proteins. The cell membrane performs the function of respiration as mitochondria are absent in them. It also acts as a selective membrane.

• Cytoplasm:

  • It is granular, present between the cell membrane and nuclear region. It has many but small vacuoles, free ribosomes, and glycogen particles. It has no membranous organelles like Golgi compounds, endoplasmic reticulum, etc.

• Mesosomes:

  • These are invaginations of the cell membrane into the cytoplasm. Their function is to help in DNA replication, cell division, respiration, and in the export of enzymes.

• Nucleoid:

  • There is a distinct nuclear region in the bacterial cell containing genetic material DNA in the form of a single circular molecule called the chromatin body or bacterial chromosome.

• Plasmids:

  • It is a small fragment of extra genetic material. Plasmids serve as vectors in genetic engineering.

Form (Shapes) Of Bacteria: There are four shapes of bacteria.

• Cocci: (Singular Coccus)
• They are spherical, non-flagellated, and according to cell arrangement, they are:
• Monococcus: Bacterium found in solitary form.
• Diplococcus: Found in pairs.

• Streptococcus:
  It is a long chain of cocci.

• Tetrad:
  Having four cocci.

• Sarcina:
  Cube of eight cocci.

• Staphylococcus:
  Has grape-like arrangement.

• Bacilli: (Singular Bacillus)
  They are rod-shaped. They may be flagellated. Bacilli are of the following types:

  • Bacillus:
    A single rod-shaped bacterium.
  • Diplobacillus:
    A pair of bacilli.
  • Streptobacillus:
    A chain of bacilli.

• Spirilla: (Singular Spirillum)
  They are spiral or corkscrew-shaped. E.g., spirochaeta.

• Vibrio Or Commas:
  They are slightly curved, e.g., vibrio cholera. They may be flagellated.

Diversity: Bacteria are difficult to classify. Taxonomists classify bacteria using a variety of criteria.

On The Basis Of Flagella: On the basis of presence, pattern of attachment, and the number of flagella present, bacteria are classified into the following taxonomic groups.

• Atrichous:
  Bacteria are without any flagella.

• Monotrichous:
  When a single polar flagellum is present.

• Lophotrichous:
  If a tuft of flagella is present only at one pole of bacteria.

• Peritrichous:
  Flagella surround the whole cell.

On The Basis Of Staining: Christian Gram developed the technique of gram staining, dividing bacteria into two groups.

• Gram positive: Bacteria stained purple
• Gram negative: Bacteria stained pink.

On The Basis Of Origin: Some microbiologists place bacteria into two major categories.

• Eubacteria: True bacteria
• Archaebacteria: Ancient bacteria

Occurrence: Bacteria are omnipresent, which can be found distributed on the earth from air to soil and water and from dead to living organisms.

Nutrition: There are two types of bacteria on the basis of nutrition.

• Autotrophic bacteria.
• Heterotrophic bacteria.

Autotrophic Bacteria: The bacteria which utilize CO₂ and get energy from sunlight or from some chemical reactions are called autotrophic bacteria. They can synthesize organic compounds. They may be further divided into two groups.

• Photosynthetic Autotrophs:
  These bacteria have chlorophyll called bacteria chlorophyll or chlorobium chlorophyll dispersed in the cytoplasm. They utilize atmospheric CO₂ and get energy from sunlight and perform the process of photosynthesis, but they use hydrogen sulfide (H₂S) instead of water as a source of hydrogen and liberate sulfur instead of oxygen.

  $$CO_2 + 2H_2S \xrightarrow{\text{Light}} (CH_2O)_n + H_2O + 2S$$

• Chemosynthetic Bacteria Or Chemoautotrophs:
  These bacteria use inorganic compounds as a source of carbon and get energy by oxidation and reduction processes. They oxidize compounds like nitrate, sulfur, ammonia, and ferrous iron.

Heterotrophic Bacteria: Most bacteria are heterotrophic, which cannot synthesize their organic compounds from simple inorganic substances. According to their mode of feeding, heterotrophic bacteria may be saprotroph, symbiotic, or parasite. These bacteria cannot synthesize their organic compounds; they get energy by decomposition of organic compounds and live either as parasites or saprophytes.

• Parasite:

                  These bacteria obtain their food from the body of living organisms. They live in or on the                       body surface of other organisms and are fully dependent on the host. These bacteria cause                       diseases in hosts and sometimes cause death.

• Saprophytes:

  • They obtain their food from dead organic substances. The soil humus is formed from the decay of plants and animals, and it is rich in organic compounds. The bacteria produce enzymes that break down the complex substances of humus into simpler compounds. These compounds are easily absorbed by bacteria, and they use them as a source of energy.

• Symbiotic Bacteria:

• Some bacteria form an association with other organisms, and both get benefits from each other; such bacteria are called symbiotic bacteria. For example, Rhizobium radicicola bacteria live in the roots of leguminous plants (e.g., pea plants). These bacteria convert nitrogen into its compounds, nitrites, and nitrates. This process is called nitrogen fixation. As a result of this process, bacteria get food material from plants. Such bacteria are called symbiotic bacteria.

Nitrogen Fixation By Bacteria: It is a process in which nitrogen is changed into ammonium and ammonia compounds. Because nitrogen cannot be used in a free state by plants, it is only used when it is converted into simple nitrogenous compounds. This process is mainly performed by bacteria.

In the first step, dead plants, animals, and excretory nitrogenous products are converted into simpler compounds like water, CO₂, and amino acids by the activity of fungi and bacteria. This process is called ammonification. Then the nitrifying bacteria act upon ammonia and ammonium ions and change them into nitrites. The conversion of ammonia and ammonium ions into nitrites takes place by Nitrosomonas bacteria, and nitrites are changed into nitrates by Nitrobacter bacteria. Some bacteria live in the roots of leguminous plants. In the roots of these plants, small rounded bodies are produced, called nodules, in which bacteria are present. These bacteria convert nitrogen into nitrites and nitrates, which are used by plants. As a result of this process, the bacteria get their food from the plant. This process is called symbiosis. In this nitrogen fixation, Rhizobium bacteria take part.

Another way is non-symbiotic nitrogen fixation. Two types of bacteria, Azotobacter and Clostridium, help in this process. They convert nitrogen into chemical compounds like nitrites and nitrates. They do not form any association with plants. Some bacteria are called denitrifying bacteria (Bacillus denitrificans). They change the ammonia and nitrates into free nitrogen. In this way, the nitrogen remains in constant amount in the atmosphere by a cycle called the nitrogen cycle.

Respiration: Bacteria are also classified according to their need for oxygen in respiration.

• Aerobes:
  Require oxygen for respiration.

• ingdom Prokaryota (Monera) - Theory & Question Answers

• Anaerobes:
  Respire without oxygen.

• Facultative Bacteria:
  Respire with or without oxygen.

• Microaerophilic:
  Bacteria require a low concentration of oxygen for growth.

• Obligate Anaerobes:
  Some bacteria are killed in the presence of oxygen; they are called obligate anaerobes.

• Facultative Anaerobes:
  Others use oxygen but can respire without it; they are called facultative anaerobes.

• Obligate Aerobes:
  Bacteria which can only survive in the presence of oxygen are obligate aerobes.

Locomotion In Bacteria:
Some bacteria use flagella for locomotion. Flagella are attached with a unique wheel-like structure. Bacterial flagella can rotate rapidly.

• Taxis Behavior:
  Flagellated bacteria show orientation toward various stimuli, a behavior called taxis.

• Chemotactic Bacteria:
  Bacteria are moving toward chemicals or away from toxic chemicals.

• Phototactic Bacteria:
  These are moving toward or away from light.

• Magnetotactic Bacteria:
  These detect earth’s magnetic field using magnets formed from iron crystals within their cytoplasm.

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Growth:
During favorable conditions, bacteria can grow very rapidly. Important environmental factors affecting growth are temperature, nutrient availability, pH, ionic concentration, and oxygen (absence or presence).

Four distinct phases are recognized in the bacterial growth curve.

• Lag Phase:
  Inactive phase or phase during which bacteria prepare themselves for division.

• Log Phase:
  Bacteria grow and multiply very rapidly.

• Stationary Phase:
  Bacterial multiplication is equal to the death rate.

• Death/Decline Phase:
  Death rate is more rapid than multiplication.

Reproduction:
Bacteria reproduce by two methods.

• Asexual reproduction
• Genetic recombination

Asexual Reproduction:

• Fission:
  Bacteria generally reproduce asexually by a process called binary fission. Fission is the fastest mode of asexual reproduction found in living organisms, particularly unicellular organisms.

  Fission takes place when there is an ample supply of food and moisture with favorable conditions. During fission, first, the hereditary material (DNA) in the form of chromatin-body is replicated. Chromatin bodies formed then move apart. A construction appears around the middle of the cell and it splits into two parts. These parts grow in size and form mature bacterial cells. The single fission takes about 20-30 minutes to complete.

• Endospore Formation:
  During endospore formation, when conditions are not favorable, the whole protoplasmic content shrinks into a small mass. A cyst is formed inside the parental wall around this mass to form an endospore. When the parental wall ruptures due to decay, the endospore is set free. On the return of favorable conditions, this endospore enlarges to form a mature bacterial cell.

Genetic Recombination: The re-union of genetic material from two different sources is called genetic recombination. Three methods by which genetic recombination takes place in bacteria are conjugation, transduction, and transformation.

• Conjugation:
  Conjugation is a simple process of genetic recombination in which genetic material is transferred from one bacterium to another through a tube called the conjugating tube or cytoplasmic bridge. This process was carried out experimentally by Laderberg and Tatum in 1946.

• Transduction:
  It is a mode of genetic recombination in which genetic material is transferred from one bacterium to another by a third party, which is a bacteriophage. This process was carried out experimentally by Laderberg and Zinder in 1952.

• Transformation (Transforming Principle):
  It is a process of transmitting genetic information from one bacterium to another bacterium through the environment, causing it to transform (undergo change). This principle was first notified by Fred Griffith in 1928.

Importance And Control:

Importance of Bacteria:
Useful Bacteria:

• Decomposers:
  Bacteria are an important biotic component of each and every ecosystem. They act on dead plant and animal bodies, decomposing various organic compounds into simple forms such as nitrates, sulfates, phosphates, etc., for utilization by green plants again. Nitrifying bacteria convert the proteins of these dead bodies into nitrates. Soil bacteria increase the fertility of the soil by bringing about physical and chemical changes in the soil.

• Alimentary Canal Bacteria:
  They help herbivores in the digestion of cellulose by an enzyme called cellulase. They are present in the appendix or in the cecum of cows, goats, etc.

• Industrial Bacteria:
  They are symbionts that help in curing and ripening tobacco leaves, fermentation of sugar into alcohol, ripening of cheese, retting of fibers, curdling of milk, conversion of hides into leather, etc.

• Medicinal Bacteria:
  Valuable antibiotic drugs have been obtained from bacteria, e.g., thyrothricin, subtilin. Riboflavin is a vitamin produced by clostridium.

• Genetically Engineered Bacteria (Biotechnology):
  E. coli has already been programmed to make human growth hormones for the treatment of growth deficiencies and insulin for diabetics.

Harmful Bacteria:

• Pathogenic Bacteria:
  They are responsible for many diseases in humans, animals, and plants. They may be called the invisible enemies of man. Some diseases found in humans due to bacteria are typhoid, tetanus, food poisoning, diphtheria, tuberculosis, etc. Plant diseases caused by bacteria are black rot of cabbage, citrus canker, fire blight of pear and apple, and ring rot of potato, etc.

• Food Spoilage:
  Bacteria spoil food by fermentation, putrefaction, or decomposition.

Control Of Bacteria:
The term control of pathogenic organisms refers to bringing infections in a population to a tolerable limit. Control of bacteria is essential to prevent diseases and avoid spoilage of food and other industrial products. Several measures are taken to control infectious microorganisms. Such measures involve the following:

• Treatment of the infected individuals.
• Prophylactic treatment of the population at risk through immunization or vaccination.
• Disruption of the life cycle of the pathogen at all possible stages.
• In case of epidemics, prevention of the spread of infection to non-infected individuals through quarantine.
• Identification and control or treatment of the reservoir hosts if any.
• Health awareness in masses primarily to reduce the risk factors related to some diseases.
• Establishing a surveillance system.
• Killing of bacteria is brought about by several sterilization methods, like exposing bacteria to ultraviolet rays or high temperatures. Certain antiseptics, antibodies, and chemotherapy agents are used to kill the bacteria present in a living tissue.

Use And Misuse Of Antibiotics:

• Antibiotics:
  Antibiotics are chemical substances produced by certain microorganisms that inhibit or kill other microorganisms.

• Uses:
  Antibiotic drugs are of two types:

  • Narrow spectrum: Effective against only certain types of bacteria.
  • Broad spectrum: Effective against a wide range of bacteria.

• Misuse:
  Antibiotics also have many side effects, and their unnecessary and prolonged use disturbs the metabolic activities of the user.

Cyanobacteria: (Blue Green Algae)

Salient features:

• They are Prokaryotic.
• They are unicellular or may occur in colony form.
• Cell wall is double layered.
• Protoplasm is differentiated into an outer colored region (chromoplasm) and an inner colorless region (centroplasm).
• Chromoplasm contains various pigments in which chlorophyll-x and phycocyanin are in abundance, imparting bluish-green color.
• They are aquatic (freshwater, with a few marine forms).
• Total absence of sexual reproduction.
• Asexual reproduction takes place by means of hormogonia, zoospores, akinetes, and fragmentation.
• Nostoc may be taken as a typical example of Blue-green algae.

NOSTOC:

Nostoc: It is a blue-green algae.

Habit And Habitat: Nostoc is found in freshwater as well as in a terrestrial habitat. It always occurs in colonies. The colony of Nostoc floats on water like a ball. The terrestrial forms are found in wet soil.

Structure: Nostoc is a unicellular alga found in a colony as filamentous algae. Many filaments of Nostoc are intermixed in a gelatinous mass, forming a ball-like structure called coenobium. Each filament is enveloped in a mucilaginous sheath. It is unbranched, with a narrow curved structure. The filaments are made up of round or oval cells, which are arranged in the form of beaded strings.

Heterocysts: Here and there in the filaments, transparent, enlarged, thick-walled, round or barrel-shaped cells are found, which are known as heterocysts. They are terminal or intercalary in position. Heterocysts are specialized cells that differ from vegetative cells in lacking protoplasm.

Akinetes: Some of the cells in filaments accumulate reserve food materials and swell. These are known as akinetes.

Cell Wall Structure: The cell wall is made up of two layers. The outer thick layer is made up of cellulose mixed with pectic compounds, and the inner thin layer is made up of only cellulose.

Protoplasm: The protoplasm is differentiated into:

• Chromoplasm:
  Outer peripheral zone. It is colored and has pigments like chlorophyll, xanthophyll, carotene, phycocyanin, and phycoerythrin.

• Centroplasm:
  Inner central zone. It is an irregular structure that consists of an incipient nucleus. It looks like a mass of chromatin granules.

Reproduction In Nostoc: Nostoc reproduces asexually, and there is no sexual reproduction. The following are the methods of reproduction:

• Fragmentation:
  Sometimes the filament divides into pieces by some means. These pieces develop into new filaments.

• Hormogonia:
  The cells of the filament break at the point of heterocyst. The fragment between two heterocysts is known as hormogonium. Each hormogonium develops into a new filament.

• Akinetes:
  These are the resting spores formed under unfavorable conditions. During the formation of an akinete, a vegetative cell enlarges, accumulates reserve food within it, and becomes thick-walled. The akinetes are resistant to unfavorable conditions. On the approach of favorable conditions, they germinate into new filaments.

Importance Of Cyanobacteria:

• Capable of making organic compounds.
• Able to fix atmospheric nitrogen.
• Nostoc anabaena is used as nitrogen fertilizer in agriculture.

Variety of Life - Theory & Question Answers Chapter # 05

 Biology XI Notes - Variety of Life - Theory & Question Answers

Chapter # 05
Theory & Question Answers
Section III - Biodiversity

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VARIETY OF LIFE

Classification / Taxonomy: Classification or Taxonomy is defined as techniques of describing, naming, and classifying living organisms on the basis of the similarities and dissimilarities of characters.

Character: A character can be defined as any attribute as a descriptive phrase, referring to form, structure, or behavior of a specific organism for a particular purpose; thus, a character is anything or any feature whose expression can be measured or assessed.

Bases Of Classification: Following are the bases of classification.

• Homology:

  • The living organisms of a particular group have many fundamental similarities in their structure. It is believed that they originated from the same structure in a common ancestor and were thus once controlled by the same gene. Structures that are similar because of the common origin but may differ functionally are said to be homologous, e.g., the flipper of a whale, limbs of a cat and man, and the wings of a bat are homologous. All these organisms are placed in class Mammalia. The study or phenomenon of homologous origin is said to be homology.

• Biochemistry:

  • The bacteria-like organisms, which look alike, can also be classified by comparing the chemical substances which they contain. By using techniques such as chromatography, electrophoresis, comparison of sequence of amino acids in proteins, or order of bases in DNA, help to classify organisms and to determine their evolutionary relationship.

• Cytology:

  • Microscopic study of cells can be useful in the classification of living organisms at the kingdom, generic, and species levels. Prokaryotic and eukaryotic organisms have been identified on cytological bases. Electron microscopic studies proved that bacteria and cyanobacteria have incomplete nuclei, so they are placed in kingdom Monera. The number of chromosomes can enable entomologists to classify locusts and grasshoppers. Seeds and pollen grains surface studies can be used in classifying flowering plants.

• Genetics:

• The main and final tool helping in classifying organisms is genetics. All morphological, cytological, and biochemical characters are based upon genetic constitution.

Concept Of Species And Hierarchy Of Biological Classification:

• Species:

  • It is the basic unit of classification. A species is a group of organisms that have numerous physical features in common and are normally capable of interbreeding and producing viable fertile offspring.

• Genera: (Singular Genus)

  • Closely related species are grouped together into genera.

• Family:

  • Closely related genera are grouped into a family.

• Order:

  • Many families with certain similar characters are grouped in order.

• Class:

  • It is a group of similar orders.

• Phyla / Division:

  • Many closely related classes constitute a division (in plants) and phyla (in animals).

• Kingdom:

  • It is the highest level of classification. All phyla or divisions are included in the kingdom.

Taxonomic Hierarchy: The ascending series of successively larger, more inclusive, groups of organisms make up the Taxonomic hierarchy. Each grouping of organisms within the hierarchy is called a Taxon, and each taxon has a rank and a name.

Nomenclature: The system used to give scientific names to living organisms is called nomenclature.

Binomial Nomenclature: It is the modern system of naming species, introduced by a Swedish biologist Linnaeus. According to him, every species shall have a particular name consisting of two words—a substantive or generic and another adjective or specific.

The first substantive or generic part is the name of the genus to which the species belongs, while the second adjective or specific part is a designation for that particular species based on certain definite and specific characters which differentiate the species from other species.

Rules Of Binomial Nomenclature:

• The first word (generic part) of the name is capitalized.
• Both words of the name are printed in italics.
• If the name is handwritten or typed, it is underlined.

Two To Five System Of Classification:

• Two Kingdoms System:

• Until quite recently, living organisms were divided into two kingdoms.

• The animal kingdom:
• This kingdom contained mainly motile organisms which fed heterotrophically. Unicellular heterotrophs (Protozoa) were put in this kingdom.

• The Plant Kingdom:
  • This kingdom contained mainly static organisms which fed autotrophically by photosynthesis. Unicellular autotrophs (Protophyta) were put in the plant kingdom with the algae. Fungi and bacteria were attached to the plant kingdom because they possessed a rigid cell wall.

Problems With Two Kingdom System: There are three main problems with having only two kingdoms.

• The first problem concerns the unicellular flagellates, such as the protozoa in the animal kingdom. However, some euglenoids, including euglena itself, contain chlorophyll and feed autotrophically by photosynthesis.
• The second problem concerns fungi, which were put in the plant kingdom. Fungi are really very different from green plants. Not only do they lack chlorophyll and feed heterotrophically, but their cellular structure differs from that of plants in several ways.
• The third problem concerns bacteria. The electron microscope has shown bacteria to have a simple prokaryotic cell structure and markedly different from all eukaryotes.

The Five Kingdom System: To cope with these problems, a number of alternative classificatory schemes have been suggested, all involving more than two kingdoms.

Robert. H. Whittaker’s Scheme (1969): The most supportive scheme of classification was proposed by Robert. H. Whittaker in 1969. He based his classification on two main criteria:

• The Level Of Organization:

  • Prokaryotes
  • Unicellular eukaryotes
  • Multicellular eukaryotes

• The Modes Of Nutrition:

  • Ingestive heterotrophs
  • Absorptive heterotrophs
  • Photosynthetic autotrophs

On this basis, Whittaker proposed the following five kingdoms.

• Kingdom Prokaryotae (Monera):

  • Level of organization - prokaryotes
  • Mode of Nutrition - Feed by a variety of different methods

• Kingdom Protista:

  • Level of organization - Unicellular eukaryotes
  • Mode of Nutrition - Feed by a variety of different methods
  • The protozoa and unicellular algae are brought together in this kingdom.

• Kingdom Fungi:

• Level of organization - Multicellular eukaryotes
• Mode of Nutrition - Absorptive heterotrophs.
• The fungi are put in this kingdom.

• Kingdom Plantae:

  • Level of organization - Multicellular eukaryotes
  • Mode of Nutrition - Photosynthetic autotrophs.

• Kingdom Animalia:

• Level of organization - Multicellular eukaryotes
• Mode of Nutrition - Ingestive heterotrophs.
• These are motile organisms.

Objections (Difficulties): The five-kingdom system solves many difficulties, but it also creates some major snags related to the protist kingdom. Separating the unicellular algae from the simple multicellular algae is not entirely satisfactory as they have certain features in common. Modification of Whittaker’s scheme was put forward by L. Margulis and K. Schwartz. The two American biologists L. Margulis and K. Schwartz suggested that all algae, unicellular and multicellular, should be included in the Protist kingdom, and this kingdom should be called the Protoctista. According to them, there are five kingdoms of living organisms. According to the modification of Whittaker’s scheme put forward by L. Margulis and K. Schwartz, there are five kingdoms of living organisms as listed below.

• Kingdom Prokaryotae: (Monera)

  • It includes almost all the prokaryotes, e.g., bacteria and cyanobacteria, etc.

• Kingdom Protoctista: (Protista)

  • It includes all the unicellular eukaryotic organisms, which are no longer classified as animals, plants, or fungi, e.g., Euglena, Paramecium, Chlamydomonas, Plasmodium, etc. Multicellular algae and primitive fungi have also been included.

• Kingdom Fungi:

  • It includes non-chlorophyllous, multicellular (except yeast) organisms having a chitinous cell wall and a coenocytic body called mycelium, e.g., Agaricus (mushroom), yeast, etc. They are absorptive heterotrophs.

• Kingdom Plantae:

  • It includes all the eukaryotic multicellular chlorophyllous photosynthetic autotrophs having cell walls made up primarily of cellulose, zygote retained to become an embryo, and exhibiting heterotrophic alternation of generation, e.g., Moss, Fern, Pine, Apple, etc.

• Kingdom Animalia:

  • It includes all eukaryotic, non-chlorophyllous, multicellular, ingestive heterotrophs with no cell wall, e.g., Hydra, earthworm, human, etc.

Viruses: The term virus is derived from a Latin word Venome, which means poison. They were first reported by a Russian biologist Iwanowsky in 1892. In 1935, Wendy Stanley succeeded in isolating tobacco Mosaic Virus (TMV) from infected tobacco leaves and prepared their pure crystal.

Characteristics Of Viruses:

• Viruses are non-cellular obligate parasites.
• They have a protein coat and a nucleic acid core.
• They range in size from 20 nm to 250 nm.
• Viruses may be virulent, i.e., destroying the host cell, or they may also be temperate, i.e., becoming integrated into their host genomes and remaining stable for long periods of time.
• They exhibit the process of replication.

Structure: They appear like little spheres or golf balls, rod-shaped, like tadpoles, and may be polyhedral. A virus consists of the following parts:

Viral Genome:

• It is a nucleic acid core. It may consist of a single or several molecules of DNA or RNA. The smallest viruses have only four genes, while the largest have several hundreds.

Capsid:

• An outer protein coat that encloses the viral genomes is called a capsid. The capsid is made up of protein subunits called capsomeres. The capsid is a thick protective sheath.

Viral Envelopes:

• Some viruses have accessory structures called viral envelopes. They are membranes cloaking their capsids. The presence and absence of envelopes classify viruses as enveloped and unenveloped viruses. Viral envelopes help viruses infect their hosts.

Tail Fibres:

• Tail fibers are additional protein fibers arising from the base plate. They help in attachment with the host. Bacteriophages contain several tail fibers.

Classification Of Viruses: The diversity of viruses is great, mostly related to the modes of origin. Viruses are classified into the following eight groups.

Unenveloped Plus-Strand RNA Viruses:(Polio Viruses, Rhino Viruses)They are called plus strand because they act directly as mRNA after infecting the host cell, attaching to the host’s ribosomes and being translated. As indicated by their name, these viruses lack envelopes and consist only of a nucleic acid core surrounded by a protein capsid. They infect plants and bacteria, causing polio and cold in human beings.

Unenveloped Plus-Strand RNA Viruses: (Polio Viruses, Rhino Viruses)
They are called plus strand because they act directly as mRNA after infecting the host cell, attaching to the host’s ribosomes and being translated. As indicated by their name, these viruses lack envelopes and consist only of a nucleic acid core surrounded by a protein capsid. They infect plants and bacteria, causing polio and cold in human beings.

• Enveloped Plus-Strand RNA Viruses: (Hepatitis A and C Viruses)
  The enveloped plus strand RNA viruses, all of which parasitize animals, are distinguished from the members of the preceding group by their lipid-rich envelopes. They infect arthropods and vertebrates, causing leukemia and yellow fever in human beings.

• Minus-Strand RNA Viruses: (Rhabdo Viruses And Pox Viruses)
  Minus-strand RNA viruses are distinguished from plus-strand RNA viruses because they carry the RNA strand complementary to the mRNA that carries the genetic information of the appropriate mRNA, which then functions in the cell. They infect plants and animals, causing flu, mumps, and rabies in human beings.

• Retrovirus:
  A virus that is replicated in a host cell via the enzyme reverse transcriptase to produce DNA from its RNA genome. They are enveloped viruses. Retroviruses are either single-stranded RNA (e.g., HIV) or double-stranded DNA (e.g., Hepatitis B) viruses.

• Double-Strand RNA Viruses: (Reo Viruses)
  These are double-stranded, icosahedral RNA viruses that infect plants and animals, causing Colorado tick fever in human beings.

• Small-Genome DNA Viruses: (Parvo Viruses)
  Many DNA viruses have small genomes; some of these viruses have single-stranded DNA, while others have double-stranded DNA. Among them are the parvoviruses, which infect animals. They are icosahedral and about 20 nanometers in diameter. They infect animals causing viral hepatitis and warts in human beings.

• Medium-Genome And Large-Genome DNA Viruses: (Herpes Viruses)

         The herpes viruses, one of the major groups of large-genome, double-stranded DNA viruses.                 They cause herpes, shingles, cancer, and poxes in human beings.

• Bacteriophage:
  A long DNA molecule is coiled within the head. They infect bacteria only.

Life Cycle Of Bacteriophage: Bacteriophage can reproduce by two alternative mechanisms:

• Lytic cycle
• Lysogenic cycle

Lytic Cycle: A viral reproductive cycle that culminates in the death of the host cell is known as the lytic cycle. This type of life cycle is referred as lytic because it always leads to the lysis (break / open) of the host bacterial cell. Virus produces only by lytic cycle is a virulent virus. Life cycle of lytic phage can be explained by considering T₄ phage and it infects the bacterium Escherichia coli.

Steps Of Lytic Cycle: Lytic cycle completed into following steps.

• Attachment:

  • The T₄ phage uses its tail fibers to stick to specific receptor sites on the outer surface of an E. coli.

• Injection:

  • From tail fibers, an enzyme lysozyme is released which dissolves the bacterium surface. Sheath contracts and a pore is made into the bacterial cell wall; through this pore, viral DNA is injected into the host bacterial cell. The empty capsid of the phage is left as a ‘ghost’ outside the host.

• Replication:

  • Viral DNA controls all the metabolic activities of the bacterial cell and directs the production of DNA and phage proteins.

• Assemblage:

  • The phage parts come together. Three separate sets of proteins assemble to form phage heads, tails, and tail fibers forming daughter phages.

• Lysis:

• These phages then direct production of lysozyme, an enzyme that digests the bacterial cell wall. With a damaged wall, osmosis causes the cell to swell and finally to burst, releasing 100 to 200 phage particles.

The Lysogenic Cycle: The lysogenic cycle replicates the viral genome without destroying the host. Viruses that are capable of using both modes of reproduction within a bacterium are called temperate viruses (Lambda). The lambda or temperate phage resembles T₄, but its tail has only one short tail fiber.

Steps Of Lysogenic Cycle:

• Adsorption:

  • Infection of an E. coli cell by X begins when the phage binds to the surface of the cell.

• Injection:

  • The Lambda injects its DNA into the host. The DNA molecule forms a circle.

• Prophage Formation:

  • The circular DNA molecule is incorporated by genetic recombination into a specific site on the host cell’s.

Biology XI Notes - Variety of Life - Theory & Question Answers

The Lysogenic Cycle: The lysogenic cycle replicates the viral genome without destroying the host. Viruses that are capable of using both modes of reproduction within a bacterium are called temperate viruses (Lambda). The lambda or temperate phage resembles T₄, but its tail has only one short tail fiber.

Steps Of Lysogenic Cycle:

• Adsorption:

  • Infection of an E. coli cell by X begins when the phage binds to the surface of the cell.

• Injection:

  • The Lambda injects its DNA into the host. The DNA molecule forms a circle.

• Prophage Formation:

  • The circular DNA molecule is incorporated by genetic recombination into a specific site on the host cell’s chromosome (chromatin body). It is known as Prophage.

• Replication:

  • The phage genome is mostly silent within the bacterium; every time the E. coli cell prepares to divide, it replicates the phage DNA along with its own and passes on the copies to the daughter cells. It is usually an environmental trigger, such as radiation or the presence of certain chemicals, which switches the virus from the lysogenic to the lytic mode.

Viroids And Prions:

• Viroids:

  • Viroids are the class of pathogens as small and simple as viruses. These are tiny molecules of naked circular RNA that infect plants, can replicate, and these RNA molecules can disrupt the metabolism of a plant cell and stunt the growth of the whole plant.

• Prions:

• Prions are infectious proteins that cause a number of degenerative brain diseases, including scrapie in sheep and the mad-cow disease. According to one hypothesis, a prion is a misfolded form of protein that converts the normal protein to the prion version.

Viral Diseases: (Transmission / Spread And Control)

Animal Diseases:

• Poliomyelitis:

  • Caused by: Polio viruses (unenveloped plus-strand RNA viruses)

  • Pathology: Virus affects the central nervous system. The destruction of motor neurons in the spinal cord results in paralysis.

  • Symptoms: Nausea, fever, headache are initial symptoms.

  • Treatment and control; Oral vaccination: Colds, Immunization methods are very difficult if not impossible.

  • Caused by: Rhinoviruses (unenveloped plus-strand RNA virus)

  • Pathology: Virus infects the upper respiratory tract.

  • Symptoms: Sneezing, cough, headache, nasal discharge, and sore throat.

• Dengue, Yellow Fever And Encephalitis:

  • Caused by: Arboviruses (enveloped plus-strand RNA viruses) are arthropod-borne viruses. Encephalitis is a sudden onset of severe headache, chills, fever, nausea, and vomiting. Some types of encephalitis may result in death, coma, and paralysis.

  Yellow fever is an acute, mosquito-borne illness; severe cases are characterized by jaundice and hemorrhage. Dengue is a mosquito-borne infection characterized by fever, muscle, and joint pain.

• Rabies:

  • Caused by: Rhabdovirus (minus-strand RNA virus) spread by small mammals such as dogs, raccoons, foxes.
  • Pathogenesis: It is an acute infection of the central nervous system.
  • Symptoms: Nervous dysfunction, increased salivation, and perspiration hydrophobia.
  • Treatment And Control: Control by vaccinating pets and humans.

• Measles And Mumps:

  • Caused by: Paramyxoviruses (minus-strand RNA virus)
  • Symptoms of Measles: Fever, redness of eyes, sneezing, rashes on the body.
  • Mumps: Characterized by the enlargement of one or more parotid glands.
  • Treatment And Control: The development of MMR vaccines has reduced the incidence of these diseases in humans.

• Flu:

  • Caused by: Flu viruses (minus-strand RNA viruses)
  • Symptoms: Chills, headache, dry cough, high fever, and muscular pain.

• AIDS:

• Acquired immune deficiency syndrome.
• Caused by: HIV (retroviruses)

Plant Diseases: Plant viruses can stunt any plant growth and diminish crop yields. One of the best-known plant diseases is called tobacco mosaic virus.

Transmission: There are two major routes by which a plant viral disease can spread.

• Horizontal Transmission:
  • A plant is infected from an external source of virus through injured parts or through insects.
• Vertical Transmission:
  • A plant inherits a viral infection from a parent.

Control: Agriculturists have not yet devised a cure for most viral diseases of plants. They have focused largely on reducing the incidence and transmission of diseases and producing virus-resistant varieties of crop plants.

Human Immune Deficiency Virus (HIV) - A Retrovirus:

AIDS - Acquired Immune Deficiency Syndrome:

Pathology: It is a disorder that impairs the body’s lymphocytic cell T4 immune system in humans. The body’s immune system breaks down, leaving the patient exposed to a variety of diseases.

The Infectious Agent: The infectious agent that causes AIDS is the glycoprotein of the viral envelope.

Transmission: The HIV can only survive in body fluids and is transmitted by blood or semen. 90% transmission occurs by sexual contact. People can contract the disease through:

• Intimate Sexual Contact:

  • It passes from the infected partner to an unaffected partner through sexual contact.

• Infected Blood Entering The Blood Stream:

  • By intravenous drug users using unsterilized needles and syringes.
  • By infected blood transfusion to uninfected individuals.
  • Close contact between infected and non-infected people through cuts and open wounds.
  • Through infected mother to the baby by placenta or by feeding milk.

Sign And Symptoms:

• Short flu.
• Skin cancer - Kaposi’s sarcoma.
• Weight loss.
• Fever.
• Diarrhea.
• Dementia.
• Septicaemia (blood poisoning)
• Severity of immune deficiency varies
• Irreversible dementia. Mental agility and behavioral changes.

Control - Treatment And Prevention:

• Best known drug - Azidothymidine or Zidovudine (formerly known as AZT)
• Ribavarin - antiviral drug
• Sumarin - antiparasitic drug

Prevention:

• Reduction of transmission by the use of clean needles and sterilized syringes.
• Education about the disease plays an important role in the prevention of disease.

Hepatitis: Hepatitis is an inflammation of the liver.

Causative Agent Of Hepatitis: Viral infection, toxic agents, or drugs.

Symptoms: It is characterized by jaundice, abdominal pain, liver enlargement, fatigue, fever.

Types Of Hepatitis:

• Hepatitis A:

  • It is caused by an enveloped RNA virus. Transmitted by contact with faces from infected individuals.

• Hepatitis B (Serum Hepatitis):

  • It is caused by Retrovirus. Enveloped double-stranded DNA virus. The hepatitis B virus poses a serious public health problem. It is transmitted through sexual contacts, skin contacts, blood transfusion.

• Hepatitis C:

  • Hepatitis C is caused by enveloped plus-strand RNA viruses. Viruses pass through blood transfusion, from mother to child during pregnancy, and by sexual contact.

THE CELL - THEORY & QUESTION ANSWERS Chapter # 04 Theory & Question Answers

 BIOLOGY XI NOTES

THE CELL - THEORY & QUESTION ANSWERS
Chapter # 04
Theory & Question Answers
Section II - Unity of Life
THE CELL

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DISCOVERY OF CELL:
The discovery of cells and their structure became possible with the development of optical lenses and with the construction of compound microscope, which was invented by David Jansen in 1590 and in 1610 Galileo designed it properly. Cells were first described in 1665 by Robert Hooke. Later, Robert Brown in 1831 discovered a spherical body, the nucleus, in the cells of orchids. In 1839 Theodore Schwann, observed that the nucleus was surrounded by a fluid in the cell.

DEFINITION OF CELL:
According to T. Schwann "cell as a structure which consists of a nucleus surrounded by semi fluid substance enclosed by a membrane". Later it was pointed out that plant cell has a cell wall in addition to the cell membrane. After proposed of cell theory a comprehensive definition of cell established. Now, a cell is defined as the structural and functional unit of living organisms, made up of protoplasm containing nucleus surrounded by cytoplasm and bounded by cell membrane.

CELL THEORY:
In 1938 a German botanist M. J. Schleiden concluded that plants were made of cells. In 1939 a German zoologist T. Schwann published a comprehensive report on the cellular basis of animals. The observations of Schleiden, Schwann and others led to the formation of cell theory. The cell theory has a far reaching effect in all fields of biological researches. The salient feature of cell theory are as follows.

• All organisms are composed of one or more cells.
• The cell is the structural and functional unit of life. According to them cell is a 'de novo' structure could arise from noncellular materials, this idea was not convincing. In 1855 Rudolf Virchow established the fact and added third point to the cell theory.
• Cells can arise only by division of pre-existing cell, it is not a 'de novo' structure.

MICROSCOPE:
It is an instrument which is used to study cells and microorganism.

Kinds Of Microscope:
According to the source of light microscope is of three types.

• Light Microscope:
  It utilizes visible light as the source of illumination. It can magnify an object very high but it resolving power is limited.
• X-Ray Microscope:
  In this type short wave length X — rays are used as source of illumination.

Electron Microscope:
In which electron beam is used as source of illumination. Most of the sub cellular structure are studied by electron microscope. It uses electromagnets instead of glass lenses.

RESOLUTION Vs MAGNIFICATION:
The three attributes of microscopes are magnification, resolution and contrast.

Resolution:
It is minimum resolved distance. The resolving power is the minimum distance by which two adjacent form or object must be separated. Light microscope has limited resolving power which is about 500 times better than unaided human eye, while electron microscope are capable of resolving objects about 10,000 times better than unaided human eye.

Contrast:
Contrast is important in distinguishing one part of cell from another. In light microscope contrast is often obtained by fixing and staining the material.

Magnification:
It is a means of increasing the apparent size of the object. Magnification of microscope = Power of eye piece X magnifying power of objectives

Techniques To Isolate Components Of Cell:
Isolation of cellular components is necessary to study and observe various aspects of cell, isolation of cellular components to determine their chemical composition is called fractionation. It is stepwise process.

• First step is to break or open a large number of similar type of cells in ice cold environment.
• ii. The cells are usually placed in a homogenizer and motor are broken. The freed content of the cells are subjected to spinning action known as centrifugation.
• Initially at a low speed large particles like cell nuclei, settle down are in the sediment smaller particles are still in the supernatant.
• Later on at higher speed smaller particles are separated out.
• The various cell fraction can then be biochemically examined.

EUKARYOTE AND PROKARYOTE:

Prokaryote	Eukaryote
Organisms which do not contain membrane nucleus in their cells are called Prokaryote.	Eukaryote are those organisms having a true nucleus in their cells.
These organisms do not contain other membrane bounded structure like mitochondria, plastids, endoplasmic reticulum etc. Their nuclear material is usually coiled and concentrated in a region of  the cell called the nucleoid.	They contain a variety of membrane bound organelles like mitochondria plastids, endoplasmic reticulum etc. Their nuclear material is bounded by nuclear membrane and form true nucleus.
Ribosomes are of small size and freely scattered in cytoplasm. Ribosomes are of large size and present either on endoplasmic reticulum or free in cytoplasm. Nucleoplasm is absent. Nucleoplasm is present. Mesosomes are simple infoldings of the plasma membrane responsible for respiration, photosynthesis, nitrogen fixation. Respiratory enzymes are present in mitochondria, photosynthesis takes place by plastids. These are unicellular organisms, may live in colony form. They are unicellular as well as multi-cellular organisms. Examples are bacteria and cyan bacteria. Examples are plants, animals, fungi, etc. STRUCTURE OF EUKARYOTIC CELL: A typical eukaryotic cell consists of following parts. Plasma membrane. Cell wall (absent in animals) Nucleus Cytoplasm and cytoplasmic organelles. (The image contains a diagram labeled "The structure of the plasma membrane," showing various components such as extrinsic protein, intrinsic protein, lipids, glycoproteid, phospholipid, cholesterol, integral protein, hydrophobic α-helix, etc.) PLASMA MEMBRANE OR CELL MEMBRANE: The nucleus and cytoplasm in all types of cells is enclosed in a membrane called plasma membrane. In plants and bacterial cell membrane is surrounded by cell wall. It is thin, delicate, and elastic and has capability of limited self-repair. Molecular Structure Of Plasma, Membrane: All biological membranes have the same molecular organization. They consist of a double layer (bilayer) of phospholipids interspersed with proteins. Phospholipid molecules are arranged in two parallel layers. Their nonpolar hydrophobic ends face each other, whereas their polar hydrophilic ends are associated with carbohydrates, protein, etc. Fluid Mosaic Model Of Plasma Membrane: In 1972 Singer and Nicolson proposed a working model of the Plasma membrane known as the fluid mosaic model. According to it, the cell membrane consists of: Lipid Bilayer: It is retained as a core of the cell membrane. These lipid molecules are present in a fluid state. Protein Molecules: The proteins are present in the lipid layer like floating icebergs. The proteins occur as a mosaic of discontinuous particles that penetrate deeply into and even completely through the lipid sheet. The proteins associated with the lipid bilayer can be divided into two groups. Integral Proteins: (Intrinsic Proteins) A class of proteins that are directly incorporated within the lipid bilayer. Peripheral Proteins: (Extrinsic Proteins) A class of proteins located entirely outside of the lipid bilayer. Functions Of Plasma Membrane: The main and most important functions of the plasma membrane: Protection of Cell. To regulate the flow of solution and material in and out of the cell with certain limitations called differential or selective permeability. This transport is necessary to maintain suitable pH, ionic concentration for enzyme activity and excrete toxic substances. Types Of Transport: Passive Transport Active Transport The passive transport includes processes of diffusion and osmosis, which do not require energy. The active transport includes endocytosis and exocytosis. The active transport requires energy. Endocytosis: Endocytosis is the process of taking in material in a cell. It may be phagocytosis (cell eating process) in which solid particles are ingested by the cell, e.g., destroying of harmful bacteria by W. B. C. or pinocytosis (cell drinking process) when liquid material is taken in. Exocytosis: The process by which material moves outside of a cell. CELL WALL: Plant cell possesses a cell wall which lies outside the membrane. It is a non-living component of the cell, secreted by the protoplasm. In the plant cell, it is mainly composed of cellulose, a polysaccharide. Pectin and a few other compounds may also be found in the cell wall. Structure: Ultra microscopic structure of the cell wall shows that cellulose makes the fibers which lie at different angles. The cell wall is composed of three layers. Middle Lamella Primary wall Secondary wall Middle lamella: The first formed cell plate works as a cementing layer between two daughter cells and is called the middle lamella. It is composed of calcium and magnesium pectate. Primary Wall: The primary wall is the first product of the cell, synthesized by protoplast. It is 1.3 m thick and elastic. The primary wall contains up to 50% hemicellulose, cellulose up to 25%, and a small amount of pectic substances. Hemicellulose forms a matrix of the wall in which cellulose microfibrils are embedded. Secondary Wall: It is produced inside the primary wall by the deposition of cellulose. It is 5 - 10 m thick and rigid. It mainly consists of cellulose or varying mixtures of cellulose. Plasmodesmata: At places in the cell wall, the deposition of wall material does not take place. These places are known as plasmodesmata, through which cellular contents of neighboring cells remain in communication with each other. Functions Of Cell Wall: The framework around the plasma membrane, affords protection, gives a definite shape, and provides rigidity to the plant body as a whole. Being hydrophilic, cellulose is capable of imbibing water. The cell wall acts as a permeable structure that helps in the transportation of material. NUCLEUS: The nucleus is the most significant, important, and prominent part of the cell, which controls all its activities. Discovery: The nucleus was discovered by Robert Brown in 1831. Shape: It is commonly spherical or oval in shape but may be lobed or elongated. Size: Its size varies between 5 to 25 μm. Numbers: Most cells have one, some have two or more nuclei. Some small organisms have several small nuclei per cell (coenocytic). Position: In animal cells, the nucleus is present in the center of the cell. In plant cells, it is located in the center in a young cell, while in a mature cell, it comes to lie on one side due to a large vacuole. Structure Of Nucleus: The nucleus consists of the following parts: Nuclear membrane Nucleoplasm Nucleolus Image: Structure of nucleus and its components. Nuclear membrane: The nucleus is enveloped by a double membrane called the nuclear membrane. It is perforated by nuclear pores through which certain substances pass freely. Nucleoplasm Or Karyolymph: The nucleus is filled with a gel-like protein-rich substance called nucleoplasm or karyolymph. It is granular in nature and denser than the cytoplasm. In the nucleoplasm, a network of fine loosely connected threads is present, called the chromatin network or nuclear reticulum, which is composed of DNA and protein. From this network, chromosomes are produced during cell division.  Chromosomes: Chromosomes are thread-like structures present inside the nucleus, bearer of hereditary characters in the form of genes, present in pairs in an individual, and their number remains constant from generation to generation in a given species. Number Of Chromosomes: Fruit fly: 8 Human: 46 Corn: 20 Structure Of Chromosomes: A typical chromosome is composed of two parts. Chromatid In mature cells, each chromosome consists of two very thin threads called chromatids. Chromatid consists of one or more threads called chromonema, which have bead-like areas called chromomeres. Centromere: Both chromatids are joined at a point called the centromere. The centromere has a disc-shaped protein called kinetochore. Spindle fibers attach to the kinetochore. Types Of Chromosomes: Types of chromosomes depend on the position of the centromere. Metacentric: In these chromosomes, the centromere is present almost in the center. These chromosomes have two equal arms resembling the letter V. Submetacentric: The position of the centromere is just away from the center. Chromosomes have two unequal arms. These are J or L-shaped chromosomes. Acrocentric Or Subtelocentric: The centromere is subterminal. These rod-shaped chromosomes have one arm very small and the other very long. Telocentric: The location of the centromere is at the end of the chromosome. Nucleolus: Inside the nucleus, a spherical body called the nucleolus is present. The nucleolus synthesizes RNA and ribosomes in eukaryotic cells. Functions Of Nucleus: Controlling Centre: The nucleus controls all the activities of the cell. Inheritance Of Characters: Inside the nucleus, chromosomes are formed during cell division, containing DNA as hereditary material. Therefore, it plays a significant role in the inheritance of characters from parents to offspring. CYTOPLASM & CYTOPLASMIC ORGANELLES: Cytoplasm: The protoplasm outside the nucleus is called cytoplasm. It is a semifluid colloid. The cytoplasm exhibits active streaming movement called cyclosis. Cytoplasm divides into two parts: Cytosol Cytoplasmic organelles Cytosol: It is a fluid matrix part of the cytoplasm. It is a cell solution in which organelles reside. Cytosol is a watery solution of salts, sugar, amino acids, proteins, fatty acids, nucleotides, and other materials. The cytosol is organized into a three-dimensional network of fibrous proteins called the cytoskeleton. Cytoplasmic Organelles: There are many minute bodies present in the cytoplasm called organelles. Organelles reside in the cytoplasm, occupying as much as half of the volume of the cell. A variety of cytoplasmic organelles are present in cells. ENDOPLASMIC RETICULUM: It is a complex network of channels, which extends from the plasma membrane to the nuclear membrane. Composition: They are composed of lipoprotein. Types Of Endoplasmic Reticulum: There are two types of endoplasmic reticulum. A Granulated Or Smooth Endoplasmic Reticulum (SER): It is not associated with ribosomes. It is present in steroid-producing cells like adipose cells (fat cells), interstitial cells, glycogen-storing cells (liver), and muscle cells. Granulated Or Rough Endoplasmic Reticulum (RER): It is heavily coated with ribosomes on its outer surface. It is mostly present in protein-synthesizing cells, e.g., mammalian salivary glands and pancreas. Functions: It serves as a supporting platform for the ribosomes. It forms a structural framework of the cell. It contains enzymes and takes an active part in various metabolic reactions in the cell. It provides a conducting pathway for the circulation of various substances within the cell. It provides passage for RNA to pass from the nucleus to the cytoplasm. It involves protein and lipid synthesis. It helps in the detoxification of harmful drugs. It involves the storage and release of Ca²⁺ ions. MITOCHONDRIA: Mitochondria are one of the cell’s most important organelles. They are the center of aerobic respiration. Position: They are universally present in the cytoplasm of plant and animal cells. Shape: They appear as minute granules, vesicles, rodlets, threads, or strings. Size: Each mitochondrion is 0.2 to 1.0 nm in diameter and about 10 µm long. Structure: Each mitochondrion is bounded by two thin membranes, made of lipids and proteins. The outer membrane is smooth, while the inner membrane forms irregular, incomplete partitions called cristae. On these cristae, enzymes and co-enzymes are present, which metabolize carbohydrates (starch), fatty acids (lipids), and amino acids (protein). Energy is released in the form of ATP and stored within mitochondria. Inside the mitochondria, granular fluid like organic matrix is present, consisting of numerous chemical compounds. Functions: Cellular respiration takes place in the mitochondria. They form and release ATP (an energy-rich compound). Mitochondria are known as the “Powerhouse” of the cell, where energy is stored and released whenever and wherever required by a living body. GOLGI APPARATUS: (DICTYOSOME) It was discovered by Camello Golgi in 1898. It is a canalicular system with sacs. It has parallel arranged, flattened, membrane-bound vesicles which lack ribosomes. Structure: Each of them is disc-shaped and consists of central, flattened, plate-like compartments called cisternae. A peripheral network of interconnecting tubules and vesicles and Golgian vacuoles. Functions: They perform the function of collection, packaging, and distribution. Manufacture certain macromolecules. Cell wall and cell plate material is Golgi product. LYSOSOMES: These are spherical bodies, a few micrometers in diameter. These were isolated by De Duve in 1949. They are most abundant in those animal cells which exhibit phagocytic activity. Each lysosome is bounded by a single membrane. Lysosomes originate from the Golgi apparatus. Lysosomes contain acid phosphatase enzymes and hydrolytic enzymes (digestive enzymes). These enzymes can digest the phagocytosed food particles. They are also involved in autophagy (self-eating). During this process, some parts of the cell are digested, resulting in the degeneration of the cell, so lysosomes have been referred to as “suicide sacs”. Lysosomal Storage Diseases: Disturbance in lysosome function influences human health. Diseases characterized by the deficiency of a lysosomal enzyme and the corresponding accumulation of undegraded substrate are called lysosomal storage disorders. In 1965, W. G. Hers explained that the absence of a lysosomal enzyme, α-glucosidase, leads to storage of undigested glycogen, causing swelling of organelles and damage of the cell and tissues. More than 30 disorders have been diagnosed. Some are described as under: Disease Consequences Tay-Sachs disease Mental retardation, blindness, death by age 3. Gaucher’s disease Liver and spleen enlargement, erosion of bones. Krabbe’s disease Long bones, mental retardation in infantile form only. PLASTIDS: Plastids are special protoplasmic membrane-bound organelles. These are chemical synthesizers and food storage bodies. Position: Plastids are present in plant cells and in some protists. Types Of Plastids: Plastids are of three types. Chloroplast: The chloroplasts are green plastids containing chlorophyll. They are found in all green parts of the plant, particularly in leaves. Functions: They have the ability to convert solar energy into chemical or food energy, therefore manufacturing carbohydrates by the process of photosynthesis. Chromoplast: These plastids are present in all colored parts of plants except green. These are mostly present in petals of flowers and fruits; they have pigments like xanthophylls and carotene and anthocyanin. Functions: Due to the chromoplast, insects are attracted towards flowers to take part in pollination. Leucoplast: The leucoplasts are usually colorless and contain starch. These plastids develop in the absence of sunlight and are commonly found in all underground parts of plants. Functions: They convert sugar into starch and store food material as carbohydrates, lipids, and proteins. Chloroplast As Energy Converting Organelles: Chloroplasts are the most common type of plastids in plants. They have the ability to convert solar energy (light energy) into chemical or food energy by the process of photosynthesis. Structure Of Chloroplast: Each chloroplast consists of three parts. An outer double membrane. Inside the chloroplast, a set of flattened sacs called thylakoids form a membranous system. In some regions, thylakoids are stacked, forming structures called grana. The fluid outside the thylakoids is called stroma. During photosynthesis, chlorophyll captures solar energy and transfers it to other molecules in the thylakoid membranes. These molecules transfer the energy to ATP, and other energy carriers diffuse into the stroma, where energy is used to derive the synthesis of sugar. Chloroplast converts solar energy into food or chemical energy through the process of photosynthesis. So, chloroplast is an energy-converting organelle. Proplastids: Proplastids are immature, colorless plastids occurring in cells of meristematic tissues. In mature cells, proplastids develop into chloroplasts, chromoplasts, or leucoplasts. Peroxisome: These are microbodies surrounded by a single membrane. These are present in animal and plant cells. They contain enzymes that transfer hydrogen atoms to oxygen to form hydrogen peroxide (H₂O₂). It is a toxic molecule that is immediately broken down to water by the enzyme catalase. Peroxisomes are found in cells that metabolize alcohol, for example, in liver and kidney cells, where they break down and detoxify fully, half of the alcohol a person drinks. Glyoxysome: They are specialized glyoxysomes, present only in plant cells. Structurally, each glyoxysome is a single-layered membrane-bounded microbody, enclosing a fine granular stroma. Functions: Glyoxysome contains enzymes that initiate the conversion of fatty acids into sugar. Cytoskeleton: The cytosol is organized into a three-dimensional network of fibrous proteins called the cytoskeleton. The cytoskeleton plays vital roles in cell division, mitosis, meiosis, cytokinesis, and cell wall formation. It also maintains the cell shape and differentiation of cells to form different parts. There are three types of cytoskeleton elements found in cells. Microfilaments: Microfilaments are solid, made of protein (two actin chains and some actin with myosin). They can be readily assembled and disassembled. Microfilaments are about 8 nm in diameter and several cm in length. They occur in bundles in the cytoplasm, where they are associated with cell motility, such as cytoplasmic streaming and muscle contraction. Intermediate Filaments: They are solid strands of 8 to 11 nm in diameter and 10 to 100 µm in length. Their shape is rope-like in different tissues. They are made up of at least five different types of proteins. In hair and skin, they consist of keratin protein. They are important in maintaining the cell shape, attachment of muscle cells, and support of nerve cell processes (axon). Microtubules: Hollow tube-like structures with a diameter of 25 nm and more than 50 nm in length. They are composed of protein tubulin. The single microtubule consists of numerous subunits of tubulin arranged in 13 columns, called proto-filaments. They are responsible for the movement of chromosomes during cell division, movement of organelles within the cytoplasm, and movement of cilia and flagella. Ribosome: The ribosomes are small, dense, rounded, and granular particles of ribonucleoprotein. It has 65% ribosomal RNA and 35% proteins. Due to the high concentration of RNA, they are called ribosomes. They were first discovered by G. E. Palade in 1955. They are found freely in the cytoplasm as well as attached to the endoplasmic reticulum. They are formed in the nucleolus inside the nucleus. Structure: Each ribosome is composed of two unequal subunits. The larger subunit is dome-shaped, and the smaller one forms a cap on the flat surface of the larger subunit. The attachment of the two subunits is controlled by Mg. Functions: They serve as sites where proteins are synthesized; hence, they are called protein factories of the cell. Centriole: An animal cell has a centrosome near the nucleus. Each centrosome consists of two rod-like centrioles. Centrioles are short, barrel-shaped structures of microtubules situated at right angles to each other. Each centriole is composed of nine sets of triplet microtubules arranged in a ring. Function: During cell division, the centrioles replicate and move opposite to each other and form a nuclear spindle. Vacuoles: Generally, vacuoles (except food vacuoles) are non-protoplasmic fluid-filled cavities in the cytoplasm. They are surrounded by a membrane called the tonoplast. In animal cells, they are many and small in size. They are temporarily formed at the time of their need, while in plant cells, permanent and large-sized vacuoles are present. Function: In plant cells, vacuoles are filled with cell sap and act as a storage house; they also play a role in plant defense and are necessary for plant cell enlargement. In animal cells, vacuoles are rich in hydrolytic enzymes, including proteases, ribonucleases, and glycosidases.

The ENZYMES

 iology XI Notes

Enzymes – Theory & Question Answers
Chapter # 03
Theory & Question Answers
ENZYMES

Introduction:
The term “enzyme” was coined by Friedrich Wilhelm Kuhne (1878). It was coined for the active ingredient in the yeast extract that promotes fermentation. Enzyme literally means in yeast, but it is now used as the collective name for the many hundreds of compounds that have since been extracted from cells and shown to have a catalytic action on specific chemical reactions.

Definition:
Enzymes may be defined as organic substances capable of catalyzing specific chemical reactions in the living system.
OR
Enzymes are organic catalysts which speed up chemical reactions in organisms.

Ribozymes:
Generally, enzymes are proteinaceous in nature, but Thomas Cech and Sidney Altman discovered that certain molecules of ribonucleic acid also function as enzymes. These molecules are called ribozymes.

Energy Of Activation:
Chemical transformation requires that certain covalent bonds be broken within the reactants. To do so the reactant, must contain sufficient kinetic energy (energy of motion) to overcome a barrier called energy of activation. Enzymes lower down the energy of activation. They can do this because they form a complex with their substrate (S) at the active site.

Characteristics Of Enzymes:
Nature:
Most of the enzymes are proteinaceous in nature. They may be entirely consist of protein e.g. amylase or pepsin or may contain, along with protein, a non-protein part e.g. holoenzyme.

React With Acidic And Alkaline Substances:
They react with both acidic and alkaline substances due to the presence of protein as their major part.

Endoenzyme And Exoenzyme:
Enzymes are produced in the protoplasm. They may act within the cell in which they are produced. These enzymes are called intra cellular enzymes or endoenzymes. Enzymes may diffuse out of the cell and act upon some outside medium called extracellular enzymes or exoenzymes.

Specificity Of Enzymes Action:
With a few exceptions, enzymes are specific in their action. A given enzyme can act only upon a particular substrate or a particular group of substrates, and each kind of metabolic reaction is catalyzed by a specific kind of enzyme e.g. sucrase acts upon sucrose (cane - sugar), lactase on lactose (milk sugar), lipase on fats.

Size Of Molecules:
Their molecules are much greater in size than the substrate.

Active Site:
The specificity of enzymes is due to their primary amino acid sequence; only a small part of which reacts with the substrates called the active site.

Catalytic Properties:
Enzymes influence the speed of chemical reactions. They are not utilized or consumed in the chemical reaction nor do they appear in the end products of the reaction. A very small quantity of the enzymes can catalyze the transformation of a very large quantity of the substrate and at a much greater speed than that of other chemically catalyzed organic reactions.

Enzyme Activators:
Enzymes activities can be accelerated by certain ions or salts called activators e.g. Mn, Ni, Mg, Cl.

Enzyme Inhibitors:
Enzyme activities can be inhibited by certain factors called inhibitors e.g. substrate concentration, enzyme concentration, pH.

Heat Sensitive – Enzymes Are Thermolabile:
Enzymes are inactivated by excessive heat. This property of enzymes relates to the fact that they are proteins. Enzymes are denatured at high temperatures.

PH Sensitive:
Enzymes are sensitive to pH. Every enzyme has its own range of pH in which it functions most efficiently e.g. pepsin only functions in an acidic medium. Trypsin functions in an alkaline medium.

Unchange:
They remain chemically unchanged during and after the chemical reactions.

Mode Of Action:
When an enzyme-controlled reaction takes place, the enzyme and substrate molecules combine with each other and form an enzyme — substrate complex. Action of enzyme is related to its structure which is complex and three-dimensional. Each enzyme has a dimple or groove of a specific shape called the active site, into which substrate can fit.

Lock & Key Theory:
An explanation of the specificity of enzymes, enzyme and substrate fitting together like a lock and key. Lock and key theory was proposed by Fischer in 1898, which was later improved by Paul Filder and D.D Woods. They proposed that a particular enzyme acts on a particular substrate like a particular lock can be unlocked by a particular key. Each enzyme has an active site of particular shape where the particular substrate is attached and produces the product after the catalytic action of enzyme.

Induce Fit Model:
According to the Kosh - Land in 1959 recent research on enzymes has suggested that the active site may not necessarily be exactly the right shape to begin with. It is believed that when the substrate combines with the enzyme it causes a small change to occur in the shape of the enzyme molecule, thereby enabling the substrate to fit more snugly into the active site.

Types Of Enzymes:
Enzymes are generally proteinaceous in nature. According to their composition they are divided into two types.

• Simple enzymes (Proteozyme)
• Conjugated enzymes (Holoenzyme)

Simple Enzymes (Proteozyme):
If an enzyme consists only of protein it is called simple enzyme or proteozyme e.g. amylase, peptidase, lipase.

Conjugated Enzyme (Holoenzyme):
If an enzyme contains another group with protein it is called conjugated enzyme. The protein part of a conjugated enzyme is called apoenzyme and non-protein part is called prosthetic group. Enler (1932) proposed that conjugated enzyme showing complete activity be called holoenzyme.

On the basis of the nature of prosthetic group conjugated enzymes or holoenzyme are of two types.

• Cofactor:
  The term cofactor is usually used for inorganic ions which are necessary to the functioning of an enzyme. Thus Cl⁻ is a cofactor for α - amylase. Role of Mg, Mn, Ca, K on enzymes like phosphatases, phosphorylase, amidase, peptidase, carboxylase are well known coenzyme.

• Coenzyme:
  The enzymes for which prosthetic group is organic substance called coenzyme. e.g. NAD Nicotinamide Adenine Dinucleotide ATPase Adenosine Triphosphate.

Factors Affecting Enzyme Activity:
Following are the factors which increase or decrease the activity of enzyme.

• Concentration Of Substrate:

At constant enzyme concentration, increase in substrate concentration causes an initial increase in the velocity of reaction till it reach a maximum and then remains constant. With further increase of substrate concentration the velocity of reaction decreases, indicating that at high substrate concentration enzyme activity is inhibited.

Effect Of Temperature:
Enzymes are sensitive to temperature. Each enzyme has optimum temperature for maximum activity, above and below this temperature its rate of reaction decrease. Most of the enzymes are highly active at about 37°C and all are completely destroyed at 100°C; whereas at minimum i.e. 0°C activity is reduced to minimum but enzymes are not destroyed.

Effect Of pH:
Enzymes are also sensitive to acids, alkalis and certain salts. Each enzyme acts best in a certain hydrogen - ion concentration pH e.g. pepsin of stomach has an optimum of 1.4, catalase shows optimum activity in a neutral medium, sucrose and lipase in acid medium and trypsin in alkaline medium.

Co – Enzymes, Activators & Inhibitors:
Substances inhibited or accelerated the enzyme activity are called co - factor. Co - factors have been divided into three categories

• Co - Enzyme:
  The cofactor is a complex non protein organic molecule known as a coenzyme e.g. CoA, NAD, FAD etc. most enzymes are co - enzyme.

• Activators:
  Inorganic substances which increase the activity of an enzyme are called activators. Magnesium (Mg) is an inorganic activator for the enzyme phosphatase and Zinc ion (Zn) is an activator for enzyme carbonic anhydrase.

• Inhibitors:
  Inhibitors are substances which slow down the catalytic function of enzymes. They may be introduced from outside or they may be normally present in the cells. Their molecules react with groups at or near the active site of the enzyme protein, thus making them inactive. Inhibitors are of two types.

• Competitive Inhibitors:
  These inhibit only one enzyme or a group of closely related enzymes and are also called specific inhibitors. They have a molecular structure which is similar to that of the substrate. By virtue of this, the inhibitor molecule combines with active site of the enzyme forming a reversible complex which does not react to form end products. Thus the inhibitor competes with the substrate for the active site of enzyme and decreases the net amount of the enzyme available for reaction with the substrate. Examples of competitive inhibitors are malonic acid inhibits the activity of enzyme. Succinic dehydrogenase and Fluorocitric acid inhibits the activity of aconitic hydratase.

• Non-Competitive Inhibitors:
  These are general poisons like carbon monoxides, cyanides, and ions of heavy metals like Cu++, Zn++ and Hg++. They inhibit many enzymes and are also called nonspecific inhibitors. They obstruct enzymatic reactions by binding to a part of the enzyme away from the active site. This interaction causes the enzyme molecule to change its shape, rendering the active site unreceptive to the substrate or leaving the enzyme less effective at catalyzing the conversion of substrate to product. In noncompetitive inhibition, a molecule binds to an enzyme other than its active site, which is called allosteric site and inhibitor is called allosteric inhibitor.

• Feedback Inhibition:
  The activity of almost every enzyme in a cell is regulated by feedback inhibition, which is an example of negative feedback. When the product is in abundance, it binds competitively with its enzyme’s active site; once the product is used up, inhibition is reduced and more product can be produced.

• Effect Of Water:
  Water influences the rate of enzymatic activity. Most of enzymes perform their activity by hydrolytic mechanism. In seed germination, water activates enzymes, and germination proceeds.

• Radiation:
  Enzymes actively decrease rapidly by exposure to ultraviolet light and also to β, γ and X-rays.