THE CELL

 Biology XI Notes

The Cell – Short Questions Answers
Chapter # 04
Short Questions Answers
Section II – Unity Of Life

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THE CELL

Q.1: What is a cell?

Ans:
The cell is the smallest fundamental, structural and functional unit of living organisms OR cell is the basic unit of life.

The discovery and study of cells became possible after the invention of microscope. The microscope was invented by an Italian scientist Galilio in 1610.

The word cell was first used by a scientist Robert Hooke in 1665. He observed a piece of cork under microscope and found many small chambers in it. He named each chamber as a cell. Another scientist Robert Brown discovered a rounded body in the cells of Orchids in 1831. This body was named as nucleus.

Q.2: Write a short note on Cell Theory?

Ans:
Cell Theory: This theory was proposed by two German scientists, Schleiden (1838), Schwann (1839) and Virchow. It is a fundamental theory. The main points of this theory are as follows:

• All organisms are composed of one or more cells.
• The cell is the structural and functional unit of life. Schleiden and Schwann described the cell a de novo structure i.e. it could arise from non-cellular material, but a German scientist Rudolf Virchow in 1855 added a third very important point to the cell theory.
• A new cell is always produced only by the division of pre-existing cell. It is not a de novo structure.

Importance Of Cell Theory:
It brought a great revolution in the field of biology and proved that the body of living organisms performs all activities and functions by the interaction of cell units.

Q.3: Write a note on Microscope?

Ans:
MICROSCOPE:
Microscopes are the instruments which are used to observe microorganisms or small organisms.

Kinds Of Microscopes:
There are different kinds of microscopes. These kinds are as follows:

• Light microscope
• X-ray microscope
• Electron microscope

Light Microscope:
In this microscope, visible light is used as a source of illumination.

• Simple Microscope (Dissecting Microscope):
  In this microscope, a single lens is used to study the object.

• Compound Microscope:
  In this microscope, more than one lens is used.

X-Ray Microscope:
In this microscope, short-wave length X-rays are used as a source of light. This microscope is very efficient to observe the three-dimensional structure of cell parts.

Electron Microscope:
In this microscope, an electron beam is used for illumination. In this type of microscope, an electron beam is emitted from the source (cathode or metal filament). This beam passes through the specimen, and the image is seen on a photographic film or on a screen.

Q.4: What is the technique to isolate the components of the cell?

Ans: COMPONENTS OF THE CELL

• To determine the chemical composition of various parts of a cell, its components are isolated; this process is known as fractionation.

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In fractionation:

• Many similar types of cells are placed in a cold environment in a homogenizer, and then a spinning action is applied, called Centrifugation.
• At low speed, larger particles like cell nuclei are separated, and these are settled down at the bottom, in the sediments.
• Smaller particles remain in the fluid, which are transferred into another test tube. These particles are centrifuged at high speed, which helps to separate these particles in various fractions. After their separation, they can be studied easily.

Q.5: What are the types of cell?

Ans:
According to the structure, the cells are classified into two groups:

• Prokaryotes
• Eukaryotes

Prokaryotic Cell:
These cells are found in simple living organisms like blue-green algae, bacteria. These cells do not contain a complete and true nucleus. Nuclear membrane and nucleolus are absent, so instead of a true nucleus, the genetic material is found in the cytoplasm. They contain mesosomes. Their genetic material does not contain Histone protein, so true chromosome is absent. Their nuclear material is usually coiled and present in a region, called nucleoid. These cells also do not have those organelles which are covered by membrane such as mitochondria, golgi bodies, lysosomes, and chloroplasts.

Eukaryotes:
These organisms contain a true nucleus in their cells. They contain all necessary membrane-bound organelles, such as mitochondria, golgi bodies, plastids, lysosomes, etc., which provide particular structural and functional organization to the cells.

Eukaryotes also contain chromosomes with DNA and Histone protein. Eukaryotes are either Unicellular or Multi-cellular.

Q.6: Name the different parts of a cell?

Ans:
A typical eukaryotic cell consists of the following parts:

• Plasma membrane
• Nucleus
• Cytoplasm and cytoplasmic organelles
• Cell wall: It is found in plant cell, bacterial cell, fungal cell, but absent in animal cell.

Q.7: Describe the structure and functions of Plasma Membrane?

Ans: FLUID MOSAIC BILAYERED MODEL OF CELL MEMBRANE
On the inner side of cell wall around the protoplasm, a thin, delicate and elastic membrane is found, called plasma membrane. It is also called plasma-lemma. It consists of proteins and phospholipids. Many other types of lipids are also present, such as cholesterol. It controls the process of osmosis.

The structure of plasma membrane was proposed by two scientists, Singer and Nicholson. This structure is called Fluid mosaic model. The cell membrane consists of a double layer of phospholipids; along these lipids proteins are also present. This is a bilayered structure of plasma membrane.

Each layer of phospholipids has two ends:

• Hydrophobic End:
  It is non-polar end i.e. the inner part.

• Hydrophilic End:
  It is polar end i.e. the outer part.

The inner, non-polar hydrophobic ends are present opposite to each other. In these layers, proteins are present in different manner. There are two types of proteins:

• Extrinsic proteins
• Intrinsic proteins

Extrinsic Proteins (Peripheral Proteins):
These proteins are present along the surface of lipids. These are also called peripheral proteins. They have loose attachment with membrane surface.

Intrinsic Proteins (Integral Proteins):
These proteins are found deeply in the lipid layers. They help in the movement of water-soluble ions outside or inside the cell.

Functions Of Cell Membrane:

• It protects the cytoplasm of a cell.
• It maintains the cell shape.
• It is semi-permeable membrane. It helps in permeability i.e. allows to pass different solutions of molecules in and out of the cell.

Q.8: Describe the permeability of Plasma Membrane (Cell membrane)?

Ans:
The process of flow of solutions and important materials in and out of a cell known as selected or differential permeability. This permeability is of two types:

• Passive transport
• Active transport

Passive Transport:
It is of two types:

• Diffusion Or Passive Permeability:
  When two solutions or substances of different concentration are mixed, it is called diffusion. In this process, the movement of molecules is from high to low concentration without membrane. It helps in the diffusion of many substances like gases, O₂ and CO₂.

• Osmosis:
  When a solvent moves from the region of higher concentration to the region of lower concentration in the presence of a semi-permeable membrane, it is called osmosis. It helps to maintain osmoregulation i.e., keeps the balance between the osmotic pressure inside and outside the cell.

Active Transport:
It is opposite to diffusion. In this process, the molecules move from lower to the higher concentration. In this type of transport, energy is required. Active transport is of two types:

• Endocytosis:
  When the cell membrane takes in certain materials by infolding in the form of vacuole, it is called endocytosis. It is of two types:

  • Phagocytosis
  • Pinocytosis

  Phagocytosis:
  In this process, the plasma membrane absorbs solid particles; for example, the white blood cells absorb unnecessary harmful particles from the blood. It is also called cell-eating process.

  Pinocytosis:
  It is a process of taking fluid in large amount. When the living cells take up fluid vesicles, it is called pinocytosis. It is also called cell drinking process.

• Exocytosis:
  When the cell membrane allows the movement of certain materials out of the cell, it is called exocytosis. It is the process of membrane fusion after the movement of materials.

Q.9: Write a note on Cell Wall?

Ans:
The cell wall is present in plant cells on the outer side of the plasma membrane. It is composed of cellulose, but it also contains lignin and pectin which make it stronger. In a young cell, the cell wall is thin and delicate, but in a large cell it becomes thick and strong. The cell wall consists of three layers:

• Primary wall
• Secondary wall
• Middle Lamella

Primary Wall:
It is found around a young plant cell on the outer side of the plasma membrane. It is thin and elastic. It contains hemicellulose (50%), cellulose (25%), and a small amount of pectic substance.

Secondary Wall:
It is formed on the inner side of the primary wall in an old and large cell. After its formation, the cell wall becomes thick and non-elastic. It contains cellulose with lignin and other substances.

Middle Lamella:
It is the layer between two cells. It helps to attach the cells. It is made up of calcium and magnesium pectates.

Functions Of Cell Wall:

• It gives a definite shape and structural framework to the cell.
• It protects the inner contents of the cell.
• It provides mechanical support to the cell.
• It is a permeable membrane for diffusion and helps in the absorption of minerals and solutes along with water in the cells of root hairs.

Q.10: What are Plasmodesmata?

Ans:
The cell wall separates the protoplasm of one cell from the adjacent cell, but in many cases this separation is not in a proper way. In such cases, minute pores are present in the cell wall. Through these pores, the protoplasts of the adjacent cells communicate with each other by certain protoplasmic strands, called plasmodesmata. The plasmodesmata take part in the translocation of nutritive materials and transmission of stimuli from cell to cell.

Q.11: Describe the structure and functions of Nucleus?

Ans:
The nucleus was discovered by a scientist Robert Brown in 1831. It is spherical or oval in shape and is located in the centre in a young cell, while in a mature cell, it comes to lie on one side.

STRUCTURE OF NUCLEUS:

• Nuclear-Membrane:
  The nucleus is surrounded by a thin, transparent membrane known as the nuclear membrane. It separates the cytoplasm from the nucleus. The nuclear membrane has numerous large pores which help in direct communication between cytoplasm and the nucleoplasm.

• Nucleoplasm Or Karyolymph:
  In the nucleus, there is a dense but clear mass of protoplasm, called nuclear-sap or nucleoplasm. It contains enzymes and other complex substances which take part in the formation of DNA and RNA.

• Chromatin Network:
  In the nucleoplasm, a network of fine loosely connected threads is present, called the chromatin network. The chromatin is the hereditary material. In division of the nucleus, the chromatin forms a definite number of thread-like structures, called chromosomes, which contain hereditary units on their surface, called genes. Chemically, chromatin consists of Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA).

• Nucleolus:
  In each nucleus, one or more globular bodies are present, called nucleolus. They consist of proteins and RNA. The nucleolus produces ribosomes, which take part in protein synthesis.

FUNCTIONS OF NUCLEUS:

• The nucleus controls all the vital activities of a cell, so it is considered as the brain of the cell.
• It produces chromosomes during cell division. The chromosomes transfer hereditary characters from parent cell to daughter cell.
• It directly takes part in cell division and reproduction.
• It produces DNA and RNA. DNA is a genetic material, and RNA takes part in protein synthesis.
• In the nucleolus of the nucleus, ribosomes are produced, which help in protein synthesis.

Q.12: Describe the structure and types of Chromosomes?

Ans: STRUCTURE OF CHROMOSOMES:
Structurally, the chromosomes are composed of proteins and DNA. Their number in a living body is constant, and it remains unchanged from generation to generation.

A visible and prominent chromosome can be studied very easily during cell division. Each chromosome consists of two thread-like structures, called chromatids. These chromatids are attached to each other by a small body, known as centromere. The part of chromatid from centromere to end is called arm.

Types of chromosomes according to the position of centromere:

• Telocentric:
  These are rod-shaped chromosomes. These have centromere at their proximal end.

• Acrocentric:
  These are also rod-shaped chromosomes, but the centromere is between two chromatids in such a manner that one arm is short and another is long.

• Sub-Metacentric:
  These are J or L-shaped chromosomes. In these chromosomes, the centromere is present in the centre in such a manner that two unequal arms are formed.

• Metacentric:
  These are V-shaped chromosomes. In these chromosomes, the centromere is present almost in the centre and two equal arms are formed.

Q.13: Describe the structure and functions of Endoplasmic Reticulum?

Ans:
It is a network of fine tube-like structures, which extend from cell membrane to the nuclear membrane. It is of two types. It consists of Lipoproteins.

• Agranulated or smooth endoplasmic reticulum (SER).
• Granulated or rough endoplasmic reticulum (RER).

Smooth endoplasmic reticulum does not contain ribosomes on their surface. It is found in steroid-producing cells, like fat cells, liver, and muscles.

In skin, the smooth endoplasmic reticulum converts cholesterol into vitamin D (a lipid compound). This vitamin helps to make bones strong and healthy.

Rough endoplasmic reticulum contains ribosomes on their outer surface. This reticulum is found in the cells which take part in protein synthesis, such as pancreas and salivary glands of mammals.

Functions:

• It helps in the exchange of important materials between cytoplasm and nucleus.
• It is involved in protein synthesis due to the presence of ribosomes.
• It is the passage for RNA to transfer from nucleus to the cytoplasm.
• It takes part to neutralize the harmful effect of drugs.
• It helps in detoxification of chemicals.
• In skin, smooth endoplasmic reticulum converts cholesterol into a lipid compound, called Vitamin-D in the presence of sunlight. This vitamin makes the bones healthy and strong.

Q.14: Write a note on Mitochondria (Chondriosomes)?

Ans: MITOCHONDRIA OR CHONDRIOSOMES:
They are small spherical or plate-like bodies present in the cytoplasm. Mitochondria are transferred from mother to the new generation. These are present in eggs and not in sperms, so mother transfers them into the new generation. Mitochondria have their own DNA, so they take part in the production of their component protein. (Mitochondria consist of 3 parts.)

• An Outer Membrane:
  It is smooth and consists of proteins and lipids.

• Inner Membrane:
  It forms numerous folds; these cristae enzymes and co-enzymes are present which help in the oxidation of starch, fatty acids, and amino acid. These compounds are converted into CO₂ and water. In this process, energy is released in the form of ATP. This energy is stored in the mitochondria.

• Matrix:
  It is the central granular part. It contains many organic compounds. Mitochondria are the main centers of the intercellular energy production; they are called the powerhouse of the cell. Almost all the respiratory activities take place in mitochondria, and they contain a number of enzymes.

Q.15: Write a note on Golgi apparatus (Dictyosomes)?

Ans: GOLGI APPARATUS: (DICTYOSOMES)
The Golgi bodies are found only in certain types of cells. In plant cells, they are present throughout the cell, while in animal cells, a single Golgi complex is present. Golgi bodies were discovered by a scientist Golgi.

Golgi bodies are found in plant and animal cells. In animal cells, a single Golgi apparatus is present, but in plant cells, they are more. The Golgi complex of plants and lower invertebrates is called dictyosomes.