Biology XI Notes - Bioenergetics - Theory & Question Answers
Chapter # 11
Theory & Question Answers
Section IV - Functional Biology
➔ Bioenergetics
The energy is used as fuel for life which is derived from light energy trapped by plant cells and converted into energy-rich compounds. Animals obtain their energy by eating plants or by eating the organisms that eat plants.
“Capturing and conversions of light energy from one form to another in living system and its utilization in metabolic activities is called bioenergetics”.
Role of ATP As Energy Currency:
Organic molecules especially carbohydrates are degraded to release energy, CO₂ and H₂O. Some of this energy is used to produce ATP. It shows that ATP is the common energy currency of cells, when cell require energy, they spent ATP for that under cellular condition is produce 7.3 K.Cal/mol on conversion into ADP.
A−P−P−P→A−P−P+P+7.3K.cal/mol
ATP acts as a mediator, capable of receiving energy from one reaction and transfers this energy to derive another reaction.
Photosynthesis:
6CO2+12H2OLight, ChlorophyllC6H12O6+6H2O+O2“Biochemical anabolic process during which simple carbohydrates are manufactured from CO₂ and water in chlorophylls cells and in presence of sunlight. O₂ is given out as by product”.
Reactants And Products Of Photosynthesis:
The reactants of photosynthesis are:
- Water: absorbed by the roots from soil.
- CO₂: enters into the plant from the atmosphere through the stomata.
- Light energy: the source of light energy is sun.
The important product of photosynthesis is glucose.
Role Of Chlorophyll And Other Pigment:
- Substances in plants that absorb visible light are called pigments. These pigments are most important in conversion of light energy to chemical energy. The most important pigments required in the process are the chlorophylls, the carotenoid and phycobilin pigments. The empirical formula of the chlorophyll-a molecule is C₅₅H₇₂O₅N₄Mg and that of chlorophyll-b molecules is C₅₅H₇₀O₆N₄Mg. Chlorophyll ‘a’ is bluish green, whereas chlorophyll - b is yellowish green.
Chlorophyll:
- Chlorophyll is organized along with other molecules into photosystem, which has light gathering “antenna complex”, consisting of a cluster of few hundred of Chlorophyll ‘a’, Chlorophyll ‘b’ and carotenoid molecules. The number and variety of pigment molecules enable a photosystem to harvest light over a large surface than single pigment molecule. When any antenna molecule absorbs a photon, the energy is transmitted from pigment molecules to pigment molecules until it reaches a particular chlorophyll - a, which is structurally same to other chlorophyll molecules but located in the region of photosystem called “reaction centre”, where the first light driven chemical reaction of photosynthesis occur.
Carotenoids:
- The chloroplast also has a family of carotenoids, which are in various shades of yellow and orange. These are present in the thylakoid membrane along with two kinds of chlorophyll. Carotenoids can absorb wavelength of light that chlorophyll cannot absorb and transfer to chlorophyll - a. On the other side excessive light can damage chlorophyll. Instead of transmitting energy to chlorophyll, some carotenoids can accept energy from chlorophyll, thus providing a function known as photoreception.
Role Of Light:
The plant is capable of using only a very small portion of incident electromagnetic radiation that falls on a leaf or the radiation that is absorbed by the pigment complex of the leaf.
Light has a dual nature. Light energy captured in the light harvesting complex which is efficiently and rapidly transferred to the chlorophyll molecules present in the photosynthetic reaction centres. When a photon of light hits these chlorophyll - a molecules the energy of these photons is absorbed and results in the elevation of an electron from the ground state to an excited state. A photon of red light has enough energy to raise an electron to excited state - 1 and this energy is sufficient to initiate useful chemical reactions and all other events of photosynthesis. The energy transferred by blue or red photons to the photosynthetic electron transport chain is exactly the same, the extra energy delivered by the absorption of a blue photon is rapidly lost by radiation less de-excitation producing an electron in excited state - 1. The movement of energy within the thylakoid membrane is very quick occurring within nanoseconds. During the transfer of electrons some energy is lost. The excitation energy can be used or lost in different ways. It can be used for photochemistry (i.e. it enters the photosynthetic electron transport chain) alternatively it can be dissipated as heat or reemitted as fluorescence.
Role Of Water:
- Photosynthesis is a redox process. It requires H₂ and electron, to fulfill this requirement H₂O is split and electrons are transferred along with Hydrogen ion (H) from H₂O to CO₂ reducing it to sugar.
Role of CO₂:
- Scientists have been studying the diffusion of CO₂ through the stomatal pores of a leaf for more than sixty years. This CO₂ provides the carbon for the basic skeleton to photosynthetic product. The opening and closing of stomata have an important effect on the regulation of photosynthetic activity; particularly in C₃ plants, which incorporate CO₂ directly into phosphorylated sugar intermediate biphosphate.
Process of Photosynthesis:
- The process of photosynthesis consists of two main types of reactions:
Light reaction
Dark reaction
Light Reaction:
- In the light-dependent reactions, chlorophyll and other molecules in the membrane of the thylakoids capture light energy and convert some of it into the chemical energy-carrier molecules i.e. ATP and NADPH + H⁺.
Dark Reaction:
- In the dark reactions or light-independent reactions, enzymes in the stroma use the chemical energy of the carrier molecules (ATP and NADPH + H⁺) to drive the synthesis of glucose or other organic molecules.
Phases of Photosynthesis:
Photosynthesis is divisible into two phases:
- Light reaction
- Dark reaction
F. Blackman describes the phases of photosynthesis in 1905.
Photochemical Reactions/Light Dependent Reactions:
Reactions initiated with the chloroplast by the excitation of the electrons of the chlorophyll upon impinging of sunlight
Or
In this series of reactions the chloroplast captures light energy and light energy is converted to mechanical energy (in the form of ATP and NADPH₂).
Mechanism Of Photochemical Reactions:
- Light reaction consists of series of reactions, it is also called Hill reaction because an American physiologist Robert Hill discovered the mechanism of light reaction in 1937. Photochemical reactions progress in the following sequence.
Photolysis of Water:
It involves the breakdown of water molecules in the presence of sunlight.
H2O→2H++2e−+21O2It takes place within the thylakoids of the chloroplasts. The pair of electrons produced is utilized in the reaction of PS-II while the pair of protons is used for the reduction of NADP to form NADPH₂.
Photosynthetic Phosphorylation:
It is a process in which ATP molecules are formed due to the addition of Pi (inorganic phosphate). The chlorophyll absorbs sunlight therefore electrons of chlorophyll jump into the higher energy orbital’s and pass through different electron acceptors for the production of ATP and NADPH₂. The ATP and NADPH₂ are most important compounds of dark reaction.
There are two types of photophosphorylation.
- Non-cyclic photophosphorylation.
- Cyclic photophosphorylation.
Non-Cyclic Photophosphorylation:
- The two pigment systems like PS-I and PS-II are involved in a non-cyclic photophosphorylation. When PS-I chlorophyll “a” absorb sunlight the electrons jumps into the higher energy orbital’s and passes through first electron acceptor called Ferrodoxin and finally reached at the last electron acceptor NADP which already reduced in the form of NADPH₂ by the addition of H⁺ ion of water molecule. The deficiency of electron in PS-I chlorophyll “a” is fill up by the help of water, electron which is transfer by the help of chlorophyll “b”. The electrons of chlorophyll “b” are first transferred in plastoquinone and different cytochromes. Therefore the electrons are transfer from higher energy to lower energy levels. So, it produces energy in the form of ATP. It is a non-cyclic reaction.
Cyclic Photophosphorylation:
- The pigment system-I mainly involved in cyclic photophosphorylation because PS-I absorbs sunlight and releases electrons which are passing through different electron acceptors like ferredoxin and plastoquinone. During this process electrons are transferred from higher energy orbitals to lower energy levels. So, it produces energy in the form of ATP.
Reduction of NADP to NADPH₂:
From cytochromes the electron pair is transferred to plastocyanin.
Plastocyanin transfers the electron pair to the activated PS-I to reduce it.
The PS-I is once again activated by impinging of sunlight to transfer its excited electrons to the sunlight to transfer its excited electrons to the electron acceptor ferredoxin reducing substance.
Frs hands over the pair of electrons to nicotinamide adenine dinucleotide phosphate where they in the presence of two protons combine to produce the reduced form of the compound.
NADP+2H++2e−→NADPH2
End Products of Photochemical Reactions:
We obtain an ATP molecule during the photochemical reactions which is utilized in the dark reactions as a phosphorylating agent.
The molecule of NADPH₂ produced during the photochemical reactions is a powerful reducing agent which is involved in the reactions of ribulose-5 phosphate.
Finally, four important events take place during light dependent reaction of photosynthesis:
- Photolysis of water.
- Electron transport chain i.e. PSII and PSI.
- Reduction of NADP to NADPH + H⁺.
- Synthesis of ATP by Photophosphorylation.
Dark Reactions / Thermo Chemical / Light Independent Reactions / Or Calvin-Benson Cycle:
The process during which CO₂ is reduced by utilizing the products of light reaction to form hexose sugars. In this reaction hydrogen is separated from water and reacts with the carbon of carbon-dioxide and form simple carbohydrate. It was discovered by Blackman in 1905. In previous years two scientists Calvin and Malvin along with their co-workers use radioactive carbon isotope (C₁₄). They studied different steps of this reaction and found that how carbon is converted into sugar compound.
Thermochemical reactions are a series of chemical reactions taking place in the following sequence:
- The Calvin cycle consists of 13 main reactions catalyzed by enzymes as shown in figure. The C₃ cycle is divided into three distinct phase for the convenience to study.
Carboxylation:
- Carboxylation or carbon fixation - during which CO₂ is fixed into organic molecules.
Formation of Ribulose-1-5-Diphosphate:
In this step two molecules of ATP activate a compound Ribulose which is converted into Ribulose 1-5-diphosphate.
Ribulose+2ATP→2ATP+Ribulose1−5Diphosphate
Formation of 6-Carbon Compound (Unstable):
Ribulose 1-5-diphosphate reacts with the environmental CO₂ and forms an unstable compound i.e., 6-carbon compound.
3CO2+3Ribulose1−5diphosphateRubisco(6G3P)6−carboncompound(Unstable)
Reduction:
- Reduction or synthesis of phosphoglyceraldehyde (PGAL) by the reduction of organic molecules.
Formation of Phosphoglyceric Acid (PGA):
6-carbon compound is cleavage into 2 molecules of 3-phosphoglyceric acid, it is 3-carbon compounds. 3-PGA is considered as very stable compound in photosynthesis. It is the key molecule for fixation of CO₂.
Formation of Di-Phosphoglyceric Acid:
One ATP molecule combines with 3-phosphoglyceric acid and converts it into 1-3 diphosphoglyceric acid.
3−phosphoglycericacid+ATP→1−3diphosphoglycericacid
Formation of Phosphoglyceraldehyde:
1-3-DIPGA combines with hydrogen coming from NADPH₂ compound and it is converted into 3-phosphoglyceraldehyde and NADP is separated.
1−3diphosphoglycericacid+NADPH2→phosphoglyceraldehyde+NADP
Formation of Fructose-6-Phosphate:
- One part of PGAL is isomerized to form dihydroxyacetone phosphate, then another part of PGAL combines with its isomer and forms fructose 1-6-diphosphate, and then it is ultimately converted into fructose-6-phosphate.
Pathway:
Phosphoglyceraldehyde → Dihydroxyacetone → Fructose-1-6-Diphosphate → Fructose-6-Phosphate
In this way hexose sugar is formed.
Formation of Carbohydrate:
- PGAL is converted to certain six-carbon carbohydrates such as Fructose-6-phosphate which is isomerized into glucose-6-phosphate which is ultimately converted into glucose.
Pathway:
Fructose-6-Phosphate → Glucose
Glucose molecules form large molecules called starch after polymerization. Starch is stored in the cells while water is discharged during starch formation.
NC6H12O6→N(C5H10O5)+NH2O
Regeneration:
- Where the reduced carbon can be utilized either to regenerate the carbon acceptor molecules or for metabolism. Many carbon rearrangements take place during this phase.
RUBP is produced at the end of dark reaction which is again utilized in the fixation of fresh CO₂ molecule. Dark reaction can be summarized up and written as a balanced equation in the following manner.
6CO2+ATP+NADPH2→[ADP+Pi]+6NADP+C6H12O6The Calvin Cycle of Dark Reaction:
- Ribulose-1-5-Diphosphate
- CO₂
- 6-Carbon Unstable Compound
- 3-Phosphoglyceric Acid
- 1-3-Diphosphoglyceric Acid
- 3-Phosphoglyceraldehyde
- Ribulose-5-Phosphate, Glucose, Fructose, Sucrose
Alternative Mechanisms of Carbon Fixation in Hot Arid Climate:
- On a hot, dry day, most plants close their stomata, a response that conserves water. This response also reduces photosynthetic yield by limiting access to CO₂ with stomata even partially closed, CO₂ concentration begins to decrease in the air spaces within the leaf, and concentration of O₂ released from photosynthesis begins to increase. These conditions within the leaf favor a wasteful process called photorespiration. In certain plant species alternate mode of carbon fixation that minimize photorespiration even in hot, arid climates have evolved. The two most important of these photosynthetic adaptations are C₄ photosynthesis and CAM.
Figure 11.6: The C₄ anatomy and pathway
The C₄ plants are so named because they go through the Calvin cycle with an alternate mode of carbon fixation that forms four carbon compound (oxaloacetate) as its first product i.e., oxaloacetate. The four carbon compounds release CO₂, which is reassimilated into organic material by Rubisco and the Calvin cycle. Among the C₄ plants important to agriculture are sugarcane and corn, members of grass family.
A second photosynthetic adaptation to arid conditions has evolved in succulent plants, many cacti, pineapples and representatives of several other plant families. These plants open their stomata during the night and close them during the day, just reverse of normal behavior. Closing stomata during the day helps desert plants conserve water, but it also prevents CO₂ from entering the leaves. During the night, when their stomata are open, these plants take up CO₂ and incorporate it into a variety of organic acids. This mode of carbon fixation is called crassulacean acid metabolism or CAM. The CAM plants store these organic acids until moving in their vacuoles. During the day, when the light reactions can supply ATP and NADPH 4-H for the Calvin cycle. These acids release CO₂ to compete with O₂. In this ratio of CO₂ maintain inside the leaves. This CO₂ is fixed through C₃ cycle.
Figure 11.7: The C₄ and CAM photosynthesis compared
Cellular Respiration:
The aerobic breakdown of glucose molecule with accompanying synthesis of ATP is called cellular respiration.
C6H12O6+6O2→6CO2+6H2O+Energy (ATP + Heat)
Oxidative Phosphorylation:
- In the process of respiration glucose loses hydrogen atoms as it is converted to carbon-dioxide. Simultaneously molecular oxygen gains hydrogen atoms and is being converted to water. Each hydrogen atom contains one electron and one proton. Thus transfer of hydrogen atoms is the transfer of electrons and protons. The movement of electrons from one molecule to another is an oxidation and reduction or redox reaction. Redox reaction is coupled reaction and requires both donor and acceptor of electrons. In the process of respiration glucose is oxidized with the loss of electrons and oxygen is reduced by the gain of electrons. During redox reaction, electrons give up energy which is used in synthesis of ATP from (ADP) Adenosine di phosphate and inorganic phosphate (Pi). This synthesis of ATP is called Oxidative Phosphorylation.
Fermentation:
- Originally defined by W. Pasteur as respiration in the absence of air, it is an alternative term used for anaerobic respiration, the production of ethyl alcohol from glucose is called Alcoholic fermentation and that of lactic acid as lactic acid fermentation.
Economic Importance of Fermentation:
Fermentation, though an inefficient method of harvesting biological energy, is an efficient source of many valuable products such as ethyl alcohol, lactic acid, propionic acid and butanol. Thus it has been of great interest to human beings brewing and dairy industries rely on fermentation. It is the source of ethyl alcohol in wines and beers. Wines are produced by fermenting fruits particularly grapes. Beers are produced fermenting malted cereals such as barley.
Yeast cells are used to make dough rise as it is baked to make bread. Cheese, yoghurt and other dairy products are produced by microbial fermentation. Lactic acid which is slightly sour, acid imparts flavor to yoghurt and cheese. Dairy products containing lactic acid are more resistant to spoilage. The characteristic flavor of pickles is due to lactic acid and acetic acid. Acetone and other industrially produced solvents are also by-products of fermentation.
Glycolysis:
In Glycolysis, Glucose a six carbon molecule is degraded through sequential enzyme dependent reactions into two molecules of Pyruvic acid, a three carbon compound.
C6H12O6Glycolysis2C3H4O3Site:
- Glycolytic enzymes are soluble in the cytoplasmic matrix where they catalyze the reactions involved in glycolysis.
Steps:
Glycolysis is achieved in 9 successive steps taking place as follows:
Dephosphorylation: (Step # 01)
- It is the first step of glycolysis in which glucose molecule breaks into glucose-6-phosphate by the help of enzyme glucokinase. The ATP molecule is converted into ADP and this process is called dephosphorylation.
Isomerization: (Step #02)
It is the 2nd step of glycolysis in which glucose-6-phosphate is converted into its isomer fructose-6-phosphate by the help of enzyme isomerase. This processing is known as isomerism.
Glucose−6−phosphate(6C)Isomerasefructose - 6 - phosphate(6C)
Dephosphorylation: (Step #03)
It is the third step of glycolysis in which fructose-6-phosphate is converted into fructose 1,6 diphosphate by the help of enzyme phospho fructokinase. ATP molecule converted into ADP.
Fructose−6−phosphate(6C)Phosphofructokinasefructose - 1,6 - diphosphate(6C) ATP→ADP+Pi
Glycolysis: (Step #04)
It is enzymatic splitting in which fructose-1, 6-diphosphate is split into 3 phosphoglyceral aldehyde and dihydroxy acetone phosphate by the help of enzyme aldolase. This is the reaction from which glycolysis derives its name.
Fructose−1,6−diphosphate(6C)Aldolase3 - phosphoglyceral aldehyde(3C)+Dihydroxy acetone phosphate(3C)
Dehydrogenation: (Step #05)
It is the 5th step of glycolysis in which 3-phospho glycerol aldehyde and dihydroxyacetone phosphate is converted into 2 molecules of 1, 3-diphosphoglyceric acid by the help of an enzyme dehydrogenase. Both these compounds are interconvertable.
3−phosphoglyceralaldehyde(3C)+Dihydroxy acetone phosphate(3C)Dehydrogenase1,3−diphosphoglyceric acid NAD→NADPH2
Phosphorylation: (Step #06)
It is the 6th step of glycolysis in which 3-phosphoglyceric acid and ATP are formed when 1,3-diphosphoglyceric acid reacts with ADP by the help of enzyme transphosphorylase.
1,3−diphosphoglyceric acid+ADP+PiTransphosphorylase3−phosphoglyceric acid+ATP
Isomerization: (Step #07)
It is the 7th step of glycolysis in which 3-phosphoglyceric acid is converted into 2-phosphoglyceric acid due to the change of position of phosphate group by the help of enzyme mutase.
3−phosphoglyceric acid(3C)Mutase2−phosphoglyceric acid(3C)
Dehydration: (Step #08)
It is the 8th step of glycolysis in which 2-phosphoglyceric acid is converted into phosphoenol pyruvic acid with the loss of one molecule of water by the help of enzyme phosphoglyceric kinase or enolase. This reaction is known as dehydration.
2−phosphoglyceric acid (3C)Phosphoglyceric kinase or EnolasePhosphoenol pyruvic acid (3C)
Dephosphorylation: (Step #09)
It is the 9th and last step of glycolysis in which phosphoenol pyruvic acid reacts with ADP and forms pyruvic acid and ATP by the help of enzyme phosphor pyruvic kinase.
Phosphoenol pyruvic acid + 2ADPPhospho pyruvic kinasepyruvic acid + 2ATP
Energy Yield in Glycolysis:
- In the 6th and 9th step, 4 ATP molecules are produced while in the 1st and 3rd step 2 ATP molecules are consumed in glycolysis. So, the total energy of glycolysis is only 2 ATP molecules.
CO2←in yeast→Pyruvic acid→in muscles→lactic acid
Figure 11.10: The pathway of glycolysis. All of these reactions take place in the cytosol. In both alcoholic and lactic acid fermentation, the electrons removed from PGAL.
Glycolysis is the universal energy-harvesting process of life. Metabolic machinery of glycolysis is found in all organisms from unicellular bacteria and yeasts to multi-cellular bodies of plants, animals, and human beings. Glycolysis occurs freely in an aerobic environment within the cytoplasm without being associated with organelle membrane structure. Net input and output of glycolysis can be summarized as under.
Breakdown of Pyruvic Acid:
The molecular remains of glycolysis are two molecules of pyruvic acid. There are three major pathways by which it is further processed. Under anaerobic conditions it either produces ethyl alcohol (Alcoholic fermentation) or lactic acid (Lactic acid fermentation) or produces carbon dioxide and water via Kerb's cycle under aerobic conditions.
Alcoholic Fermentation:
Each pyruvic acid molecule is converted to ethyl alcohol in two steps. In the first step, pyruvic acid is decarboxylated under the action of enzyme to produce acetaldehyde, a two-carbon molecule. NADH+H⁺ reduces acetaldehyde to ethyl alcohol.
CH3⋅CO⋅COOHEnzymeCH3⋅CHO+CO2 CH3⋅CHO+NADH+H+→CH3⋅CH2⋅OH+NAD+
- Ethyl alcohol is toxic. Plants never use it. Neither it can be converted to carbohydrate nor it breaks up in presence of oxygen. Accumulation of ethyl alcohol is tolerable to a certain level. Plants must revert to aerobic respiration before the concentration exceeds that tolerable limit, otherwise, they will be poisoned.
Lactic Acid Fermentation:
When NADH+H⁺ transfers its hydrogen directly to pyruvic acid, it results in the formation of lactic acid.
CH3⋅CO⋅COOH+NADH+H→CH3⋅CHOH⋅COOH
- During extensive exercise such as fast running, muscle cells of animals and human beings respire anaerobically. Due to inadequate supply of oxygen, pyruvic acid is converted into lactic acid. Blood circulation removes lactic acid from muscle cells. When lactic acid cannot be removed as fast as it is produced, it accumulates in the cells and causes muscle fatigue. This forces the person to quit or reduce exercise until normal levels are restored to depleted cells.
Kerb's Cycle / Citric Acid Cycle:
- The Kerb's cycle is also known as the citric acid cycle in which pyruvic acid further oxidized in the presence of oxygen and form a bulk amount of energy. Hans Kerb discovers the cycle and the enzymes are present in the mitochondria.
Preparatory Step of Kerb's Cycle:
2(CH3−C−COOH)−Pyruvic acid+2(Co.A.Sh)→2(CH3−C−SCOA)acetyl co-enzyme.a.+2CO2+2[2H+]Enzyme
Condensation: (Step #01)
It is the first step of Kerb’s cycle in which acetyle co-enzyme a combines with a 4 carbon compound i.e., Oxaloacetic acid with the help of enzyme citrace synthetase and forms a 6 carbon compound citric acid.
Acetyle COA + Oxaloacetic acid (4C)Citrace synthetaseCitric acid (6C) NAD→NADH+HCO.A.
Dehydration: (Step #02)
During this process citric acid is converted into cis-aconitic acid in the presence of enzyme aconitase. During this process one molecule of hydrogen is released. It is also called dehydration.
Citric acid (6C)AconitaseCis - aconitic acid H2O
Hydration: (Step #03)
During this process hydration of cis-aconitic acid occurs and it is converted into iso-citric acid by using one molecule of hydrogen. The enzyme used in this step is aconitase.
Cis - aconitic acid + H₂O→iso - citric acid
Dehydrogenation: (Step #04)
During this process iso-citric acid is converted into oxalo succinic acid by releasing one molecule of hydrogen. This hydrogen molecule adds with NAD compound and forms NADH+H compound. This process is known as dehydrogenation and the enzyme used in this process is iso-citric dehydrogenase.
iso - citric acidiso-citric dehydrogenaseoxalo succinic acid (6C) NAD→NADH+H
Decarboxylation: (Step #05)
During this step oxalo succinic acid is converted into α-ketoglutaric acid by releasing one molecule of CO₂ in the presence of an enzyme called oxalo succinic carboxylase.
Oxalo succinic acidOxalo succinic decarboxylaseα - ketoglutaric acid (5C) CO2
Decarboxylation & Dehydrogenation: (Step #06)
In this step α-ketoglutaric acid combines with Co-A and forms succinyle co-A by releasing one molecule of CO₂. In this step hydrogen molecule is also released and NAD compound converts into NADH+H compound.
α - ketoglutaric acidCO-A + CO₂Succinyle CO - A (4C) NAD→NADH+H
Phosphorylation of ADP: (Step #07)
CO-A is released form Succinyle CO - A leaving behind succinic acid and Phosphorylation of ADP occurs and converts it into energy molecule i.e., ATP molecule.
Succinyle CO - A (4C)CO-Asuccinic acid (4C) ADP+Pi→ATP
Dehydrogenation: (Step #08)
In this step dehydrogenation of succinic acid occurs and converts it into fumaric acid by releasing 2 ions of hydrogen and forming FADH₂ in the presence of an enzyme succinic dehydrogenase.
Succinic acid (4C)Succinic dehydrogenaseFumaric acid (4C) FAD→FADH2
Dehydration: (Step #09)
In the presence of enzyme fumarase, fumaric acid is converted into malic acid by releasing one molecule of water.
Fumeric acidFumeraseMalic acid H2O
Dehydrogenation: (Step #10)
It is the last step of Kerb's cycle in which malic acid is converted into oxaloacetic acid in the presence of enzyme malic dehydrogenase. During this process, one molecule of hydrogen is released, which on combining with NAD compound forms NADH+H. This process is so-called dehydrogenation.
Malic acidMalic dehydrogenaseOxalo acetic acid NAD→NADH+H
Electron Transport Chain (E.T.C) or Respiratory Chain:
The NADH₂ molecules produced during various stages of Kerb’s cycle are brought to the electron transport chain. The electron pair produced by the dissociation of hydrogen atoms is first accepted by FAD when it gets reduced. During this process, an ATP molecule is produced.
The transfer of the electron pair from FAD to Cyt B doesn’t involve the production of ATP molecules. The reduced Cyt B transfers the electron pair to Cyt C, producing another ATP molecule. Cyt C is oxidized, and the electron pair is transferred to Cyt A and then transferred to cytochrome oxidase (Cyt A₃), resulting in ATP formation. The electron pair can be retrieved from Cyt A only by the help of oxygen, which combines with them in the presence of H to form H₂O.
Energy Flow Through The Ecosystem:
Energy is the ability to do work. According to the law of thermodynamics, energy may be transformed from one form into another but can never be created or destroyed. In all processes of life, such as growth, development, and reproduction, there is a flow of energy, without this flow of energy continuity of life stops, and the ecosystem is disturbed. In an ecosystem, all the living organisms are linked together because they obtain energy from each other. In all ecosystems, the main source of energy is the sun. This energy is used by green plants in photosynthesis to prepare food material. This plant food is utilized by herbivore animals, and in the next step, carnivores eat herbivores; in this way, the flow of energy occurs from sun to plants, from plants to herbivores, and from herbivores to carnivores. From their body, the energy is released in air in the form of heat. This is the flow of energy.
Sun→Plants→Herbivores→Carnivores→Air- Food Chain (Flow of energy)
Energy is obtained from the sun in the form of light or radiant energy; when it reaches at the earth; it is transferred into heat energy. A part of this energy is used by green plants and they convert it into chemical energy. This energy is present is the form of molecules. The chemical energy is then changed into mechanical energy when it is utilized in various functions of the body. When the energy is changed from one form to another, a part of it is lost. Plants obtain sunlight for photosynthesis, some light is used in the process and some part is lost in the form of heat. The plant energy is taken by animals. In the body of animals a great amount of this energy is converted into heat and its less amount remains in the fresh protoplasm in the form of chemical energy. In this manner in every step when energy migrates from one body to another body, a part of it is liberated out.
Example:
- Let us consider that one square meter of an ecosystem receives 3000 calories of light energy.
- Half of this energy (1500 calories) is absorbed by autotrophic plants.
- About 1 to 5% of this energy is used to form food material.
- About 15 to 75 calories are transferred to the consumers of various levels.
Further loss occurs in respiration and only 10% of this energy i.e. 1.5 calories are reached to the secondary consumers. It indicates that there is ten times reduction in energy at each trophic level.
| Sunlight total energy 3000 cal | Heat loss 1500 cal | Plant Autotrophs |
|---|---|---|
| 15 cal. | ||
| Heat loss | ||
| Herbivore | ||
| 1.5 cal. | ||
| Carnivores |
Trophic Levels: Food is very important for all living organisms because it provides energy. In an ecosystem the flow of energy occurs through a chain, for example, plants are eaten by herbivores and the herbivores are eaten by carnivores, thus the food manufactured by plants travel from producers to primary consumers i.e. herbivores and then to secondary consumers i.e. carnivores. “This stepwise process through which food energy moves, with repeated stages of eating and being eaten is known as food chain”
Grass ➔ Sheep ➔ Man
Food chain represents various levels of nourishment. These levels are called trophic levels. The green plants occupy the first trophic level. It is the primary producer level. The herbivores form the second level or primary consumer level. These trophic levels are arranged in a systematic manner producer – Bacteria (decomposers). In each step the number and mass of organisms is limited by the amount of energy available. Because some energy is lost in the form of heat, thus the steps become progressively smaller near the top. These trophic levels are shown graphically by means of pyramids, called ecological pyramids. In the pyramid the producer level constitute the base of the pyramid and tertiary consumers or decomposers level make the apex.
Pyramid of Energy: This pyramid shows the rate of energy flow or productivity at successive trophic levels. This pyramid is always upright, and it gives the best picture of the overall nature of the ecosystem. It indicates the amount of energy available for successively higher trophic levels. In most cases, there is always a gradual decrease in the energy content at successive trophic levels from the producers to various consumers.
The Efficiency of Energy Flow and Its Significance: The most approximate rate of product obtained from photosynthesis during one year in a place of an ecosystem is known as productivity. The productivity cannot be estimated from the crop of the field because the obtained productivity is different. In an ecosystem, the productivity depends upon the sunlight which is used in photosynthesis. Plants convert the energy of sunlight into food energy or chemical energy by photosynthesis; it is called primary productivity, which is stored in the form of food in the body of the plant.
The whole energy of primary productivity is not used in the growth and development processes; a part of it is absorbed by chlorophyll to prepare organic matters. The energy which is present in the form of chemical energy in the plant body is termed as gross primary productivity. About 20% of this gross primary productivity is consumed in respiration and other processes, after that the rest of the energy is called net primary productivity. This is the approximate rate of total product of a particular period (one year) which is obtained as a result of photosynthesis. Herbivores again obtain their food from plants, in this way energy is transferred from plants into the body of herbivores. From herbivores, this energy is migrated into carnivores when they eat herbivores. The productivity at the level of consumers is known as secondary productivity. In the transfer of energy from producers to herbivores, 10% of it is retained, and from herbivores to carnivores, 20% of it is migrated.
Advantage of Short Food Chain: There is a great advantage of a short food chain because there is less loss of energy in a short food chain. In a large food chain, such as a food web in which many food chains are linked together, there is a great loss of energy in the form of heat.
Example: Man uses both plants and animals as food. When he eats plants, he is an herbivore, i.e., a primary consumer; when he eats animals, he is a secondary consumer, and when he eats fishes, he becomes a tertiary consumer. In a short food chain, the loss of energy is less.
Energy Flow (Productivity)
Sun
- Energy transfer: 0.2%
First energy level
- Producers (Green plants)
- Energy transfer: 5-20%
Second energy level
- Primary Consumers (Herbivores)
- Energy transfer: 5-20%
Third energy level
- Secondary Consumers (Carnivores)
- Energy transfer: 5-20%
Fourth energy level
- Tertiary Consumers (Carnivores)
This image visually represents the flow of energy through different trophic levels, starting from the sun, which transfers only 0.2% of its energy. Producers (green plants) absorb 5-20% of this energy, which then flows to herbivores (primary consumers) and progressively to carnivores (secondary and tertiary consumers) with similar rates of energy transfer.
