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Question-1

Give two types of cellular respiration.

Solution:
The two types of cellular respiration are (i) Aerobic respiration and (ii) Anaerobic respiration.

Question-2

What is respiratory quotient (RQ)? Give the RQ for carbohydrates.

Solution:
The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time is called respiratory quotient (RQ).



The value of RQ for carbohydrates is



Question-3

Name the energy currency of the cells.

Solution:
ATP is the energy currency of the cells.

Question-4

What is compensation point?

Solution:
The light intensity, at which the rate of photosynthesis is just equal to the rate of respiration, is called compensation point.

Question-5

What is significance of F0 – F1 combination in mitochondria?

Solution:
F0 – F1 combination in mitochondria maintains the proton gradient on the two sides of the membrane.

Question-6

How does respiration differ from combustion?

Solution:

Respiration

Combustion

(i) Respiration is a biochemical process and occurs in living cells. (i) Combustion is a physiochemical and non cellular process.
(ii) It is under biological control. (ii) It is an uncontrolled process.
(iii) Energy is released in stages as chemical bonds are broken in steps. (iii) Energy is released in a single step as most of the chemical bonds break simultaneously.
(iv) Only a part of energy is lost as heat. (iv) Most of the energy is liberated as heat.
(v) Expect in a few cases, light is not emitted during respiration. (v) Light is often emitted during combustion.
(vi) Temperature is not allowed to rise. (vi) Temperature becomes very high.
(vii) Most of the energy is trapped in ATP molecules. (vii) No ATP is formed during combustion.
(viii) Each step in respiration is catalysed by an enzyme. (viii) Enzymes are not involved in combustion.
(ix) A number of intermediates are formed for the synthesis of different organic compounds. (ix) No intermediates are produced in combustion.
(x) The oxidation of substrate occurs by the loss of hydrogen or electrons. Molecular oxygen is involved only in the last step. (x) The substrate is directly oxidised in combustion.

 

Question-7

Write the importance of anaerobic respiration.

Solution:
(i) Anaerobic respiration is important during periods of oxygen deficiency.

(ii) Various types of wines, beers and whiskeys are prepared through alcoholic fermentation of sugary solutions with yeast.

(iii) Carbon dioxide, released by yeast cells in alcoholic fermentation is used in baking industry for making bread light and spongy.

(iv) Dairy industry depends upon the action of lactic acid bacteria, which converts milk sugar, lactose to lactic acid. Lactic acid coagulates the milk protein casein and fuses droplets of milk fat.

(v) Vinegar is obtained by the fermentation activity of acetic acid bacteria.

(vi) Tea and tobacco leaves are cured by fermentation with certain bacteria.

(vii) Fermentation is also employed in the preparation of ensilage, cleaning of hides, retting of stem fibres and production of industrial alcohols and organic acids.

(viii) Decomposition of dead bodies of organisms is carried out by fermentation.

Question-8

A total of 38 ATP molecules are produced per glucose molecule oxidized but net gain is 36 ATP molecules. What happens to 2 ATP molecules?

Solution:
Two molecules of ATP are lost as heat.

Question-9

Discuss the external factors influencing respiration.

Solution:
The external factors, which influence respiration are,

(i) Temperature: Most of the plants respire normally between 5oC and 25o C. Further rise in temperature causes decrease and finally inhibition of respiration due to denaturation of enzymes. The actual range of temperature at which respiration occurs varies considerably in different plants. Some temperate and arctic plants can respire well near or below 0oC. Similar hardiness is observed in dormant seeds, which do not lose viability even at -40o C.

(ii) Light: The effect of light on the rate of respiration is indirect. Under suitable light, the rate of photosynthesis is optimum, which supplies respiratory substrates at a moderate rate. Active input of respiratory substrates ultimately increases the respiration rate.

(iii) Oxygen: Oxygen is the most important factor, affecting the rate of aerobic respiration. In absence of O2, plants continue to respire anaerobically. Absence of O2 does not affect anaerobic organisms. However, higher plants fail to survive for long under anaerobic conditions, due to accumulation of toxic alcohol and carbon dioxide.

(iv) Carbon dioxide: The concentration of carbon dioxide in the atmosphere is almost constant. Therefore, it does not affect the rate of respiration. Under controlled conditions, a higher concentration of carbon dioxide decreases the rate of respiration.

(v) Water: The rate of respiration is decreased, when the amount of available water becomes low, because the respiratory enzymes become inactive in the absence of this medium.

(vi) Inorganic salts: The rate of respiration increases when a plant or tissue is transferred from water to a salt solution. The amount by which respiration is increased over normal is called ‘ salt respiration’.

(vii) Organic substances: Within limits, plant hormones, auxins and gibberellins and many herbicides enhance the rate of respiration. Certain organic substances such as cyanides, azides, carbon monoxide, etc. inhibit respiration, because they act as enzyme inhibitors. Small quantities of anesthetics such as chloroform, ether, etc. increase the rate of respiration. But in higher doses they function as inhibitors of respiration.

(viii) Injuries: Injuries initiate meristematic activity in the area of wound, which enhances the rate of respiration to supply more energy for the healing of the injury.

Question-10

Name the phenomenon by which carbohydrates are oxidized to release CO2, H2O and energy.

Solution:
The phenomenon by which carbohydrates are oxidized to release CO2, H2O and energy is called aerobic respiration.

Question-11

What is the significance of pentose phosphate pathway?

Solution:
Pentose phosphate pathway is significant in the following ways:

(i) Pentose phosphate pathway constitutes an alternate pathway for the breakdown of carbohydrates in respiration.

(ii) It produces NADPH + H+ for some synthetic processes.

(iii) It produces ribose 5-phosphate, which is used in the synthesis of nucleic acid.

(iv) Erythrose 4-phosphate produced in pentose phosphate pathway is required for the synthesis of lignin, anthocyanin, IAA and a number of other compounds.

Question-12

In which organelle does Kreb’s cycle occur in a living cell?

Solution:
Mitochondria is the organelle where Kreb’s cycle occurs in a living cell.

Question-13

What are the internal factors influencing respiration?

Solution:
The internal factors influencing respiration have been listed below:

(i) Protoplasmic factors: The rate of respiration is influenced by the amount and state of protoplasm. Young growing cells show higher rate of respiration than mature cells. Dormant tissues have very low rate of respiration.

(ii) Respiratory substrate: Within limit, the rate of respiration shows a linear relation with the concentration of available respiratory substrate, particularly sugars.

Question-14

Where does glycolysis take place in a cell?

Solution:
Glycolysis takes place in the cytoplasm of the cell.

Question-15

What is the unit of oxidative phosphorylation?

Solution:
Oxysomes is the unit of oxidative phosphorylation.

Question-16

What is electron transport chain?

Solution:
The inner mitochondrial membrane contains some proteins, which act as H+ ions and electron transporting enzymes. The enzymes are arranged in an ordered manner in a specific series called electron transport chain. An electron transport chain is a series of enzymes and cytochromes in the inner mitochondrial membrane that take part in the passage of electrons from a substance to its ultimate acceptor. The electron carriers include flavins, iron-sulphur complexes, quinines and cytochromes. Most of them are prosthetic groups of proteins.

Specific enzymes of the electron transport chain receive electrons from reduced prosthetic groups. The electrons are then transported successively from one enzyme or cytochrome to the next in a down hill journey with a loss of energy at each step. At the end of the chain, the electrons and the accompanying protons combine with oxygen to form water. Oxygen is thus, the terminal electron acceptor of the mitochondrial respiratory chain.

At each step of electron transport, the electron acceptor has a higher electron affinity than the electron donor. The energy from such electron transport is utilized in transporting protons from the matrix, across the inner membrane to its outer end. There are three such sites in the electron transport chain. This creates a higher proton concentration outside the inner membrane than in the matrix.

Question-17

What is the full form of EMP pathway?

Solution:
EMP - Embden – Mayerhof – Parnas pathway.

Question-18

Why does strenuous exercise cause muscular fatigue?

Solution:
Skeletal muscles usually derive their energy by anaerobic respiration. After vigorous exercise, lactic acid accumulates, leading to muscular fatigue. During rest, however, the lactic acid is reconverted to pyruvic acid and this is channeled back into the aerobic respiration pathway.

Question-19

What is oxidative phosphorylation and substrate level phosphorylation?

Solution:
Oxidative phosphorylation is the synthesis of ATP with the help of energy released by the oxidation of reduced co-enzymes during terminal oxidation.

Substrate level phosphorylation is the phosphorylation of ATP or some other nucleoside diphosphate directly from a metabolite.

Question-20

What is the final acceptor of electrons in electron transport chain?

Solution:
Oxygen is the final acceptor of electrons in electron transport chain.

Question-21

Why is less energy produced during anaerobic respiration?

Solution:
Less energy is produced during anaerobic respiration because

(i) Incomplete breakdown of respiratory substrate takes place.

(ii) Some of the products of anaerobic respiration can be oxidised further to release energy, which shows that anaerobic respiration does not liberate the whole of energy contained on the respiratory substrate.

(iii) ATP as electron transport is absent hence, NADH2 is not produced.

(iv) Oxygen is not utilized for securing electrons and protons.

Question-22

What is the common step in aerobic and anaerobic pathway?

Solution:
Glycolysis is the common step in aerobic and anaerobic pathway.

Question-23

Out of the following substances, carbohydrates, fats and proteins, which gets easily oxidised by oxygen to produce energy during respiration?

Solution:
The carbohydrates – glucose and fructose are broken down during respiration very easily to produce energy. The carbohydrate gets hydrolysed to glucose and fructose for respiration. When fats are used as respiratory substrate they are first hydrolysed to fatty acids and glycerol by the action of enzymes and are then oxidised in respiration. Proteins are rarely used as respiratory substrates. But when they are used, they are first broken down to amino acids.

Question-24

Name the enzyme, which is represented by oxysomes.

Solution:
ATPase is the enzyme, which representes oxysomes.

Question-25

What role is played by ATPase?

Solution:
ATPase helps in the formation of ATP from ADP, Pi and energy by downward flow of protons.

Question-26

Write the mechanism of respiration.

Solution:
During both aerobic and anaerobic respiration the glucose molecule gets broken down into an intermediate pivotal compound the pyruvic acid by a process known as glycolysis. But further breakdown of this pivotal compound differs in two types of respiration.

(a) Breakdown of pyruvic acid in anaerobic respiration - In this process, in the absence of oxygen the pyruvic acid is incompletely reduced to ethyl alcohol.

(b) Breakdown of pyruvic acid in aerobic respiration – In this process, the pyruvic acid is completely oxidised into carbon dioxide and water in the presence of oxygen. This process occurs in the mitochondria of the cell and is known as Kreb’s cycle.

Question-27

In which part of the mitochondria is the electron transport chain located?

Solution:
The electron transport chain is located on the cristae of inner mitochondrial membrane.

Question-28

Where do TCA cycle enzymes occur?

Solution:
TCA cycle enzymes occur in the matrix of mitochondria.

Question-29

Point out the differences between cellular respiration and alcoholic fermentation.

Solution:

Question-30

What is the common substrate of cellular respiration?

Solution:
Glucose is the common substrate of cellular respiration.

Question-31

Where is ETS found?

Solution:
Electron Transmitter System (ETS) is found in the inner membrane of mitochondria.

Question-32

Explain the role of NAD in glycolysis.

Solution:
NAD plays an important role during glycolysis, where 3-phosphoglyceraldehyde is converted into 1,3 disphosphoglycerate in the presence of inorganic phosphate and the enzyme – glyceraldehydes phosphate dehydrogenase. In this reaction NAD is reduced to NADH2.

Question-33

Give the site of glycolysis in cells.

Solution:
Cytoplasm is the site of glycolysis in cells.

Question-34

Name the final acceptor of electrons in electron transmitter system.

Solution:
Oxygen is the final acceptor of electrons in electron transmitter system.

Question-35

What is the unit of oxidation photophorylation?

Solution:
Oxysome is the unit of oxidation photophorylation.

Question-36

What are the four parts of cellular respiration?

Solution:
The four parts of cellular respiration are,

(i) Glycolysis

(ii) Decarboxylation

(iii) TCA

(iv) Electron transport chain

Question-37

Give the chemical equation for respiration.

Solution:

Question-38

Name the three end products of glycolysis.

Solution:
The three end products of glycolysis are, 

(i) Two molecules of pyruvic acid, 

(ii) 2ATP, and

(iii) 2NADPH2.

Question-39

Write the differences between respiration and combustion.

Solution:
Respiration Combustion
Occurs in the cells of living organisms. Does not occur in living things.
Energy is liberated in small quantities during each step of a series of reactions. Unlimited energy is liberated once during combustion.
The liberated energy is stored in ATP molecules. No energy is stored during combustion.

 

Question-40

What is the main source of energy for endergonic activity in living cells?

Solution:
The main source of energy for endergonic activity in living cells is cellular respiration.

Question-41

Name one physico-chemical catabolic exergonic reaction.

Solution:
Respiration is the physico-chemical catabolic exergonic reaction.

Question-42

What is the main respiratory fuel?

Solution:
Glucose is the main respiratory fuel.

Question-43

Provide alternative name for Kreb’s cycle.

Solution:
TCA cycle or citric acid cycle is the alternative name for Kerb’s cycle.

Question-44

Give full form of ETS.

Solution:
ETS – Electron Transport System.

Question-45

Name the raw material in cellular respiration.

Solution:
Glucose is the raw-material in cellular respiration.

Question-46

What is the utility of stepwise oxidation during respiration?

Solution:
The following are the advantages of stepwise oxidation during respiration:

(i) Stepwise release of the chemical bond energy facilitates the utilization of a relatively higher proportion of that energy in ATP synthesis.

(ii) Activities of enzymes for the different steps may be enhanced or inhibited by specific compounds. This provides a means of controlling the rate of the pathway and the energy output according to the need of the cell.

(iii) The same pathway may be utilized for forming intermediates used in the synthesis of other biomolecules like amino acids.

Question-47

Explain the structure and function of mitochondria.

Solution:
Mitochondria are spherical or elongated or rod like cytoplasmic organelles. Mitochondria are actively associated with the process of aerobic respiration and release of energy for the cell activities. In this process of biological oxidation of the carbohydrates and fats large amount of energy is released. This energy is utilized by the mitochondria for the synthesis of ATP.

Mitochondria are made up of two layers of unit membrane. The outer membrane is smooth while inner membrane has a number of folds or cristae. The space between outer and inner membranes is the outer chamber. The space surrounded by the inner membrane is the inner chamber. It is filled with a dense homogeneous fluid.

Each cristae runs at right angle to the long axis of the mitochondria. They increase surface area of inner membrane. The latter has a number of oxysomes which are sites of oxidative phosphorylation.

The oxysomes are stalked tennis rocket shaped bodies present in the cristae of the mitochondria. Each oxysome has a head, stalk and base. There are elementary particles in the mitochondria.

Functions of mitochondria:

(i) The mitochondria are called "Power houses of the cell where sugar is oxidized during cellular respiration and the energy thus released is stored as ATP.

(ii) Mitochondria contain several respiratory enzymes which are related with release of energy.

(iii) Mitochondria contain DNA and hence help in cytoplasmic inheritance.

(iv) They are useful in coupling of phospholipids RNA and DNA.

(v) Mitochondria regulate the entry and exit of water and other substances through its own membrane.

Question-48

Explain in brief the phase of cellular respiration, which occurs in cytoplasm and does not require oxygen.

Solution:
Glycolysis is also called as Embeden-Mayerhoff-Parnas pathway (Glykis = Sweet, lysis= loosening).

Glycolysis means "the splitting of sugar". A glucose molecule is converted into 2 molecules of pyruvic acid during glycolysis. It takes place in the cytoplasm and is anaerobic pathway. Glycolysis occurs in ten steps. Glucose is converted into glucose 6-phosphate through phosphorylation under the influence of hexokinase and ATP molecule. Glucose-6-Phosphate is converted into Fructose-6-phosphate. This chemical reaction occurs in the presence of enzymes phospho glucoisomerase.

Fructose-6-phosphate yields Fructose-1, 6-disphosphate. Fructose-1, 6-diphosphate splits into 2 molecules of 3-carbon sugar phosphates-dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL). PGAL is converted in to 1, 3-diphosphoglyceric acid. 2 ATP molecules are released when 1, 3-diphosphoglyceric acid is converted into 3-phosphoglyceric acid, 3-phosphoglyceric acid is converted into 2-phosphoglyceric acid. It is converted into 2-phosphoenol pyruvic acid in the presence of aldolase. 2-phosphoenol pyruvic acid is converted into pyruvic acid in the presence of pyruvate kinase and 2 molecules of ATP are released.

Question-49

Explain the processes of glycolysis with the help of flow chart only, indicating various steps.

Solution:

 

Glycolysis means "the splitting of sugar". A glucose molecule is converted into 2 molecules of pyruvic acid during glycolysis. It takes place in the cytoplasm and is anaerobic pathway. Glycolysis occurs in ten steps. Glucose is converted into glucose 6-phosphate through phosphorylation under the influence of hexokinase and ATP molecule. Glucose-6-Phosphate is converted into Fructose-6-phosphate. This chemical reaction occurs in the presence of enzymes phospho glucoisomerase.

Fructose-6-phosphate yields Fructose-1, 6-disphosphate. Fructose-1, 6-diphosphate splits into 2 molecules of 3-carbon sugar phosphates-dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL). PGAL is converted in to 1, 3-diphosphoglyceric acid. 2 ATP molecules are released when 1, 3-diphosphoglyceric acid is converted into 3-phosphoglyceric acid, 3-phosphoglyceric acid is converted into 2-phosphoglyceric acid. It is converted into 2-phosphoenol pyruvic acid in the presence of aldolase. 2-phosphoenol pyruvic acid is converted into pyruvic acid in the presence of pyruvate kinase and 2 molecules of ATP are released.

Question-50

How may ATP molecules are produced by complete aerobic and anaerobic respiration of one glucose molecule?

Solution:
ATP molecules produced during aerobic and anaerobic respiration of one molecule of glucose.
Names Process involved Consumed Net Gain
1.Glycolysis Phosphorylation of glucose

Energy trapped 2NADH2 (in ETS)

-2

 

+ 4

¾

 

2 ATP

6 ATP

2. Kreb’s cycle Energy trapped NADH2 (Decarboxylation) NADH2 (CAC)

FADH2 (CAC)

¾

2 x 3

 

6 x 3

2 x 3

2 ATP

6 ATP

 

18 ATP

4 ATP

     Total 38 ATP

Complete oxidation of one molecule of glucose yields 38 molecules of ATP but net gain is 36 ATP molecules because 2 ATP molecules are lost as heat. Net yield in Kreb’s cycle (aerobic oxidation) of pyruvic acid is 30 ATP molecules.

Question-51

How do glycolysis and Kreb’s cycle differ?

Solution:
Differences between glycolysis and Kreb’s cycle:
Glycolysis Kreb’s cycle
Linear pathway of nine steps occuring in the cytoplasm of the cell. Cyclic pathway consisting of eight steps occuring in the mitochondria.
No generation of CO2, but consumption of 2 ATP under aerobic and anaerobic conditions. Generation of CO2 and no consumption of ATP under aerobic conditions.
End products are 2 molecules of pyruvic acid. End products are CO2 and H2O with release of more energy.

 

Question-52

Differentiate between Aerobic and Anaerobic respiration.

Solution:
Difference between aerobic and anaerobic respiration:
Aerobic respiration Anaerobic respiration
It takes place in the presence of oxygen. It takes place in the absence of oxygen.
It involved two steps. The first step is glycolysis, which is carried out in cytoplasm and the second step in Kreb’s cycle, which takes place in mitochondria. The complete process takes place outside the mitochondria i.e., in the cytoplasm.
Complete oxidation of glucose takes place. Incomplete oxidation of glucose takes place.
During this process 38 ATP per one-gram mole of glucose are formed. During this process 2 ATP molecules per one-gram mole of glucose are formed.

 

Question-53

When and where does anaerobic respiration occur in man and yeast?

Solution:
The anaerobic respiration occurs in man during the heavy exercise in the muscles. In yeast the anaerobic respiration occurs during fermentation.

Question-54

Why is less energy produced during anaerobic respiration than in aerobic respiration?

Solution:
The less energy produced during anaerobic respiration than in aerobic respiration is because of the following reasons,

(i) The end products of anaerobic respiration can be further oxidized to release energy,

(ii) The regeneration of NAD does not yield ATP, as the electrons are not transported to oxygen.

Question-55

Where is the respiratory electrons transport system located in a cell?

Solution:
The respiratory electrons transport system in the cell is located in the membranes of mitochondria.

Question-56

What compound is the terminal electron acceptor in aerobic respiration?

Solution:
Oxygen is the terminal electron acceptor in aerobic respiration.

Question-57

What is anaerobic respiration? Give its chemical equation.

Solution:
When respiration takes place in the absence of oxygen, it is termed as anaerobic respiration.
The process can be shown by the following equation:

Question-58

Differentiate between respiration and combustion.

Solution:
Difference between breathing and respiration:
Breathing Respiration
It is a biophysical process. It is a biochemical process.
Oxygen is taken in and carbon dioxide is given out. Water, carbon dioxide and energy is released by the oxidation of carbohydrates.

 

Question-59

Define aerobic respiration.

Solution:
When respiration takes place in the presence of oxygen, it is termed as aerobic respiration. In other words respiratory substrates are broken down in the presence of oxygen. The process can be shown by the following equation.

In this process 38 ATP molecules are produced in complete oxidation of one gm mole of glucose.

Question-60

What is compensation point?

Solution:
Compensation point – At the given low concentration of carbon dioxide and non-limiting light intensity, the photosynthetic rate of a given plant will be equal to the total amount of respiration. Atmospheric concentration of carbon dioxide in which photosynthesis just compensation for respiration is called carbon dioxide compensation point. The carbon dioxide compensation point is reached when the amount of carbon dioxide uptake is equal to that generated through respiration at a non-limiting light intensity. Net photosynthesis under these condition is zero. In C3 plant, the CO2 compensation point is usually much higher than in C4 plants.

Question-61

What are NAD and ATP? Explain their role in respiration?

Solution:
NAD – It is the short form of coenzyme Nicotinamide Adenine Dinucleotide. It is also called DPN (Diphosphophyridine nucleotide).

Role of NAD in respiration - NAD plays an important role during glycolysis, where 3-phosphoglyceraldehyde is converted into 1,3, disphosphoglycerate in the presence of inorganic phosphate (Pi) and the enzyme – glyceraldehydes phosphate dehydrogenase. In this reaction NAD is reduced to NADH2.

ATP – It is the short form of Adenosine Triphosphate. It is a high energy compound present in the living cells. It is the energy currency of cells. In this compound adenine base is attached with sugar and three phosphate molecules.

Role of ATP – ATP plays an important role in the conversion and transfer of energy from catabolic to anabolic reactions. During the formation of ATP, energy is stored and during hydrolysis, energy is released. This stored energy in the form of ATP is utilized at the time needed for growth, maintenance and synthesis of molecules. ATPs are formed in aerobic respiration. During aerobic respiration 38 molecules of ATP per-glucose molecule oxidized are produced.

Question-62

Explain the formation of NADH and ATP during glycolysis in aerobic respiration.

Solution:
Nicotiamide Adenine Dinucleotide Hydrogen is formed by the reduction of NAD. NAD plays an important role during glycolysis. Where 3-phosphoglyceraldehyde is converted into 1,3-diphosphoglycerate in the presence of inorganic phosphate and the enzyme glyceraldehde phosphate dehydrogenase.

ATP – Adenosine triphosphate: It is a high energy compound present in the living cells. During the formation of ATP, energy is stored and during hydrolysis energy is released. 2 molecules of ATP are formed from ADP when 1,3-diphosphoglycerate is converted into 3,phosphoglycerate. Two molecules of ATP are formed from ADP when phosphoenal pyruvate is converted into pyruvic acid at the end of glycolysis. In this way four molecules of ATP are formed and two molecules are used during the conversion of glucose into glucose-6-phosphate and later fructose –1,6-phosphate. So there is net gain of two ATP molecules during glycolysis.

Question-63

How would you demonstrate that yeast could respire aerobically and anaerobically? Explain the pathway of anaerobic respiration in yeast by figure.

Solution:
Experiment - Take a pinch of dry baker’s yeast suspended in water. Add 10 per cent glucose solution to it in a test tube (tube A). Cover the surface of the liquid carefully with oil to prevent contact with air. Close the test-tube tightly with a rubber stopper. Insert one end of a short bent glass through it to reach the air inside the tube. Connect the other end of the glass tube by a polyethylene or rubber tubing to another bent glass tube fitted into a stopper. Dip the open end of the glass tube into lime-water in a test-tube (tube B).

See that the stoppers of both tubes are fitted tightly to prevent leakage of gases. Keep the first test tube in warm water (370 to 380 C) in a beaker. The lime water gradually turns milky. This indicates the evolution of carbon dioxide from the yeast preparation. Observe the level of the lime water in the delivery tube. It does not rise. It shows that there is no decline in the volume of gas in test-tube A, and consequently no utilization of oxygen by yeast. Keep the preparation for a day or two. On opening stopper of tube A, a smell of alcohol would be noticed indicating the formation of ethanol. From this activity it may be inferred that yeast respires anaerobically to ferment glucose to ethanol and carbon dioxide.

Aerobic respiration by yeast:

Experiment: Aerobic respiration of yeast can be demonstrated using a similar apparatus used for anaerobic respiration. Make two differences; the container for the yeast culture should be large enough to leave sufficient space above the glucose solution containing yeast. The surface of the solution should not be covered with oil to allow easy contact with air.

Observe the lime-water. It turns milky indicating evolution of carbon dioxide. Note the level of water in the delivery tube. It also rises test-tube B indicating a fall in gas volume in tube A, in account of utilization of oxygen by yeast. Observe the smell. No smell of alcohol after the reaction in tube A is noticed.

Question-64

Write explanatory notes on: 

(a) Glycolysis, 

(b) Electron transport chain.

Solution:
(a) Glycolysis - A process in which glucose (sugar) is partially broken down by cells in enzyme reactions that do not need oxygen. Glycolysis is one method that cells use to produce energy. When glycolysis is linked with other enzyme reactions that use oxygen, more complete breakdown of glucose is possible and more energy is produced.

(b) Electron transport chain - The H ions and electrons removed from the respiratory substrate during oxidation do not directly react with oxygen. Instead they reduce acceptor molecules such as NAD+ and FAD to NADH and FADH2 respectively. Next these molecules transfer their electrons to a system of electron acceptors and transfer molecules. In this some proteins of the inner mitochondrial membrane act as electron transporting enzymes. The proteins are arranged in an ordered manner in the membrane and function in a fixed sequence. The assembly of electron transport enzymes is termed as the mitochondrial respiratory chain or the electron transport chain. Specific enzymes of this chain receive electrons from reduced prosthetic groups. NADH or FADH2 produced by glycolysis and the TCA cycle. The electrons then get transported is successively in order of energy yielding reactions. AT the end of the chain the electrons and the accompanying protons (H+) get combined with oxygen as a result of which water is formed. Oxygen acts as terminal electron acceptor of the mitochondrial respiratory chain.

At each step of electron transport chain, the electron acceptor has a higher electron affinity than the donor. The energy from the transport of electron is utilized in transporting protons (H+) from the mitochondrial matrix to the inner membrane and finally to its outside. As a result of this a higher proton concentration occurs outside the inner membrane as compared to the matrix. This difference in the proton concentration is termed as proton gradient.

Question-65

Explain the main steps in aerobic respiration.

Solution:
Aerobic respiration includes those portions of the respiratory metabolic pathway that are oxygen dependent. This includes the transition reactions, the Krebs cycle, and the electron transport system. All of these occur within the mitochondria in eukaryotic cells. In prokaryotes, the enzymes and carrier molecules involved in the aerobic pathways are embedded in the plasma membrane, and the reactions occur in the cytoplasm at the surface of the membrane.

Pyruvate is one of the substances that is allowed to cross the membrane of the mitochondria into the organelle. As pyruvate enters the transition reactions, it is oxidized and decarboxylated, becoming a 2-carbon acetate molecule. The acetate is then joined to the carrier molecule acetyl coA, forming acetyl coA, which is delivered to the Krebs cycle. In the course of the transition reactions, one carbon dioxide is produced from each pyruvate molecule, and one molecule of NAD+ is reduced to NADH.

To begin the Krebs cycle, the acetate group is delivered to a molecule of oxaloacetate [4-carbon compound found in the matrix of the mitochondrion]. The 2-carbon acetate group joins to the 4-carbon oxaloacetate to form the 6-carbon tricarboxylic compound citrate. [The Krebs cycle is also known as the tricarboxylic acid cycle or the citric acid cycle, named for this first step of the reaction.] The Coenzyme A molecule is released to be recycled, picking up the next acetyl unit that passes through the transition reactions.
In a two step process, citrate if first dehydrated, then rehydrated, to produce isocitrate, an isomer of citrate.The isomerization prepares the molecule for an oxidative decarboxylation in the next step of the cycle.

In the fourth step, a carboxyl group is removed, and two hydrogens are removed, creating a 5-carbon molecule of [[alpha]] ketoglutarate. The hydrogens [one proton and 2 electrons] are passed on to NAD+ [the second proton remains free].
In another complex reaction, a second oxidative decarboxylation occurs. Coenzyme A temporarily binds to the resulting 4-carbon compound, producing a molecule of succinyl coA. NAD+ again accepts the electrons and one of the protons removed from the [[alpha]]-ketoglutarate.

The coenzyme A is removed, and enough energy is released to allow a substrate level phosphorylation. The phosphate group is bound briefly to the succinyl group, then is transferred to a molecule of GDP [guanosine diphosphate] to produce GTP. GTP transfers the phosphate group to ADP, producing a molecule of ATP. The 4-carbon succinate continues in the Krebs cycle, while the coenzyme A and the GDP are recycled.
Succinate is oxidized, passing the 2 removed hydrogens to the carrier molecule FAD. The remaining 4-carbon compound is fumarate.

Water is added to the fumarate to produce malate, in preparation for the last step of the cycle.

In the final step, a last oxidation occurs, removing 2 hydrogens, which are passed on to NAD+, and re-creating oxaloacetate. The cycle has been completed, and is ready to begin again.

In a single cycle of the Krebs cycle, 3 molecules of NAD+ have been reduced, and 1 molecule of FAD was reduced. One ATP molecule was produced through substrate level phosphorylation during the Krebs cycle. Two carbon dioxide molecules were released. Another NAD+ was reduced during the transition reactions, and one carbon dioxide was released during this reaction. All of the carbon-carbon and carbon-hydrogen bonds of the original pyruvate molecule have been broken.

If two pyruvates [the products of a single glucose molecule from the glycolyitic pathway] pass through these aerobic pathways, the total recovery will be doubled: 8 NADH, 2 FADH2, and 2 ATP will be recovered.

If the two ATP from the glycolytic pathway are added, this makes a total of 4 ATP recovered at the end of the Krebs cycle. 4 X 7.3 = 29.2 kcal of energy recovered from the original glucose molecule. Although this is better than the recovery of glycolysis alone, it is still inefficient. [about a 4 % efficiency rating.]

Aerobic respiration, however, is not yet over. The carrier molecules FADH2 and NADH hold electrons that remain at a high energy level, and can release that energy through continued oxidation. In the final steps of aerobic respiration, these electrons are passed through a series of oxidation-reduction reactions in the electron transport system.

Question-66

How are glycolysis Kreb’s TCA cycle and the electron transport chain linked?

Solution:
The Krebs cycle is a series of reactions, which occurs in the mitochondria and results in the formation of ATP and other molecules, which undergo further reactions to form more ATP.

In order for the pyruvic acid molecules from glycolysis to enter the Krebs cycle, they must first undergo one additional chemical reaction called the oxidation of pyruvic acid. In this reaction, one carbon atom and two oxygen atoms are removed, forming CO2 (carbon dioxide). In addition, a molecule of the coenzyme NAD+ becomes NADH in the process. The remaining molecule has two carbons and is known as acetyl coenzyme A and has a molecular formula of CH3CO. Remember, this process happens to both of the two pyruvic acid molecules formed during glycolysis.

In general, the Krebs cycle consists of the bonding of the two-carbon acetyl coenzyme A with the four-carbon compound called oxaloacetic acid. The resulting compound, which has six carbon atoms, undergoes a series of chemical reactions (which are outlined below), resulting in the formation of energy (in the form of ATP, NADH, and FADH2) and oxaloacetic acid, which can then bond with another molecule of acetyl coenzyme A so that the cycle can run again. The Krebs cycle occurs two times for each glucose molecule from glycolysis, since it occurs for each of the two molecules of acetyl coenzyme A formed by the oxidation of the two molecules of pyruvic acid. For each turn of the Krebs cycle, one molecule of ATP, three molecules of NADH, and one molecule of FADH2 (a coenzyme similar to NADH) are formed. Therefore, for one glucose molecule, the Krebs cycle results in the formation of two molecules of ATP, six of NADH, and two of FADH2. The NADH and FADH2 then undergo the electron transport chain, resulting in the production of more ATP.

The electron transport chain is the final stage of aerobic respiration. It utilizes the molecules of NADH and FADH2 formed during glycolysis and the Krebs cycle to produce great amounts of ATP. During glycolysis and the Krebs cycle, the NAD+ and FAD coenzymes were passed high-energy electrons. The purpose of the electron transport chain is to pass those electrons to other carrier molecules, which hold the electrons at slightly lower energy levels. As the electrons are passed from high to low energy levels, energy is released. After many steps, the electrons are finally accepted by oxygen at the lowest energy level, producing water.

At some steps in the electron transport chain, the jumps from one energy level to the next are significantly larger than in others. The energy that is released in these steps is enough to power a process, which drives ADP and phosphate molecules together to produce ATP.

Question-67

Illustrate the mechanism of electron transport system.

Solution:
The H ions and electrons removed from the respiratory substrate during oxidation do not directly react with oxygen. Instead they reduce acceptor molecules such as NAD+ and FAD to NADH and FADH2 respectively. Next these molecules transfer their electrons to a system of electron acceptors and transfer molecules. In this some proteins of the inner mitochondrial membrane act as electron transporting enzymes. The proteins are arranged in an ordered manner in the membrane and function in a fixed sequence. The assembly of electron transport enzymes is termed as the mitochondrial respiratory chain or the electron transport chain. Specific enzymes of this chain receive electrons from reduced prosthetic groups. NADH or FADH2 produced by glycolysis and the TCA cycle. The electrons then get transported is successively in order of energy yielding reactions. AT the end of the chain the electrons and the accompanying protons (H+) get combined with oxygen as a result of which water is formed. Oxygen acts as terminal electron acceptor of the mitochondrial respiratory chain.

At each step of electron transport chain, the electron acceptor has a higher electron affinity than the donor. The energy from the transport of electron is utilized in transporting protons (H+) from the mitochondrial matrix to the inner membrane and finally to its outside. As a result of this a higher proton concentration occurs outside the inner membrane as compared to the matrix. This difference in the proton concentration is termed as proton gradient.




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