12.9 Exercise Problems

Q1. Differentiate between
(a) Respiration and Combustion
(b) Glycolysis and Krebs’ cycle
(c) Aerobic respiration and Fermentation

Answer: (a) Respiration and combustion

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Respiration } & & \text { Combustion } \\
\hline \text { 1. } & \text { It is a biochemical process. } & \text { 1. } & \text { It is a physiochemical process. } \\
\hline \text { 2. } & \text { It occurs in the living cells. } & \text { 2. } & \begin{array}{l}
\text { It does not occur in the living } \\
\text { cells. }
\end{array} \\
\hline \text { 3. } & \text { ATP is generated } & \text { 3. } & \text { ATP is not generated } \\
\hline \text { 4. } & \text { Enzymes are required } & \text { 4. } & \text { Enzymes are not required } \\
\hline \text { 5. } & \begin{array}{l}
\text { It is a biologically-controlled } \\
\text { process. }
\end{array} & \text { 5. } & \text { It is an uncontrolled process. } \\
\hline
\end{array}
\)

(b) Glycolysis and Krebs cycle

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Glycolysis } & & \text { Krebs cycle } \\
\hline \text { 1. } & \text { It is a linear pathway. } & \text { 1. } & \text { It is a cyclic pathway. } \\
\hline \text { 2. } & \text { It occurs in the cell cytoplasm. } & \text { 2. } & \text { It occurs in the mitochondrial matrix. } \\
\hline \text { 3. } & \text { It occurs in both aerobic and anaerobic respiration. } & \text { 3. } & \text { It occurs in aerobic respiration. } \\
\hline \text { 4. } & \begin{array}{l}
\text { It generates } 2 \mathrm{NADH}_2 \text { and } 2 \\
\text { ATP molecules on the } \\
\text { breakdown of one glucose } \\
\text { molecule. }
\end{array} & \text { 4. } & \begin{array}{l}
\text { It produces } 6 \mathrm{NADH}_2, 2 \mathrm{FADH}_2 \text {, and } \\
\text { 2 ATP molecules on the breakdown } \\
\text { of two acetyl-CoA molecules. }
\end{array} \\
\hline
\end{array}
\)

(c) Aerobic respiration and fermentation

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Aerobic respiration } & & \text { Fermentation } \\
\hline \text { 1. } & \text { Oxygen is used for deriving energy } & \text { 1. } & \text { Occurs in the absence of oxygen } \\
\hline \text { 2. } & \text { Occurs in the cytoplasm and mitochondria } & \text { 2. } & \text { Occurs in the cytoplasm } \\
\hline \text { 3. } & \text { End products are carbon dioxide and water } & \text { 3. } & \text { End products are ethyl alcohol and carbon dioxide } \\
\hline \text { 4. } & \begin{array}{l}
\text { Complete oxidation of the } \\
\text { respiratory substrate takes place }
\end{array} & \text { 4. } & \begin{array}{l}
\text { Incomplete oxidation of the } \\
\text { respiratory substrate takes place } \\
\end{array} \\
\hline \text { 5. } & \text { About 36 ATP molecules are produced } & \text { 5. } & \text { Only 2 ATP molecules are produced } \\
\hline
\end{array}
\)

Q2. What are respiratory substrates? Name the most common respiratory substrate.

Answer: The compounds oxidised during the process of respiration are called respiratory substrates. Carbohydrates, especially glucose, act as respiratory substrates. Fats, proteins, and organic acids also act as respiratory substrates.

Q3. Give the schematic representation of glycolysis? 

Answer: 

Glycolysis:
1. The process by which the conversion of glucose into pyruvic acid takes place to release energy.
2. This process takes place in the cytoplasm of the cell.
3. This process might be aerobic or anaerobic.
4. It is a process in which 10 reactions take place, which are catalyzed by enzymes.

The steps involved in glycolysis are:
1. In the preparatory phase of glycolysis, under the effect of the hexokinase and other enzymes, D-glucose is converted to fructose 6-phosphate with some rearrangements.
2. Under the effect of the aldolase enzyme, fructose 1,6-bisphosphate is broken down to dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).
3. The last five steps of glycolysis are called the pay-off phase. G3P is converted to 1,3-bisphosphoglycerate. After which, ATP and 3phosphoenolglycerate are formed.
4. One molecule of water is removed from 2-phosphoglycerate, and phosphoenol pyruvic acid (PEP) is formed.
5. Finally, from PEP, pyruvate and ATP are formed.

Q4. What are the main steps in aerobic respiration? Where does it take place?

Answer: Aerobic respiration is an enzymatically controlled release of energy in a stepwise catabolic process of complete oxidation of organic food into carbon dioxide and water with oxygen acting as terminal oxidant. It occurs by two methods, common pathway and pentose phosphate pathway. Common pathway is known so because its first step, called glycolysis, is common to both aerobic and anaerobic modes of respiration. The common pathway of aerobic respiration consists of three steps – glycolysis, Krebs’ cycle and terminal oxidation. Aerobic respiration takes place within mitochondria. The final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria.

Q5. Give the schematic representation of an overall view of Krebs’ cycle.

Answer: 

Q6. Explain ETS.

Answer: An electron transport chain or system (ETS) is a series of coenzymes and cytochromes that take part in the passage of electrons from a chemical to its ultimate acceptor. Reduced coenzymes participate in electron transport chain. Electron transport takes place on cristae of mitochondria [oxysomes ( \(F_0-F_1\), particles) found on the inner surface of the membrane of mitochondria]. NADH formed in glycolysis and citric acid cycle are oxidised by NADH dehydrogenase (complex I) and the electrons are transferred to ubiquinone. Ubiquinone also receives reducing equivalents via FADH2 through the activity of succinate dehydrogenase (complex II). The reduced ubiquinone is then oxidised by transfer of electrons of cytochrome c via cytochrome Fc, complex (complex III). Cytochrome c acts as a mobile carrier between complex III and complex IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and \(a_3\) and two copper centres. When the electrons are shunted over the carriers via complex I to IV in the electron transport chain, they are coupled to ATP synthetase (complex V) for the formation of ATP from ADP and Pi. Oxygen functions as the terminal acceptor of electrons and is reduced to water along with the hydrogen atoms. Reduced coenzymes (coenzyme I, II and FAD) do not combine directly with the molecular \(\mathrm{O}_2\). Only their hydrogen or electrons are transferred through various substances and finally reach \(\mathrm{O}_2\). The substances useful for the transfer of electron are called electron carriers. Only electrons are transferred through cytochromes (Cyt \(\mathrm{F}_1\) Cyt c, \(\mathrm{C}_2\), a , \(\mathrm{a}_3\) ) and finally reach molecular \(\mathrm{O}_2\). Both cytochrome a and \(\mathrm{a_3}\) form a system called cytochrome oxidase. Copper is also present in \(\mathrm{Cyt}~ \mathrm{a}_3\) in addition to iron. The molecular oxygen that has accepted electrons now receives the protons that were liberated into the surrounding medium to give rise to a molecule of water. The liberated energy is utilised for the synthesis of ATP from ADP and Pi.

Q7. Distinguish between the following:
(a) Aerobic respiration and Anaerobic respiration
(b) Glycolysis and Fermentation
(c) Glycolysis and Citric acid Cycle

Answer: (a) Differences between aerobic and anaerobic respiration are as follows:

\(
\begin{array}{|l|l|}
\hline \begin{array}{l}
\text { Aerobic respiration is a type of respiration } \\
\text { in which foodstuffs(usually carbohydrates) } \\
\text { are completely oxidised to carbon dioxide } \\
\text { and water, with the release of chemical } \\
\text { energy, in a process requiring atmospheric } \\
\text { oxygen. The reaction can be summarized } \\
\text { by the equation: } \\
\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \rightarrow 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}+\text { energy } \\
\end{array} & \begin{array}{l}
\text { Anaerobic respiration is a type of respiration } \\
\text { in which foodstuffs (usually carbohydrates) are } \\
\text { partially oxidised, with the release of chemical energy, } \\
\text { in a process not involving atmospheric oxygen. } \\
\text { Since the substrate is never completely oxidised the } \\
\text { energy yield of this type of respiration is lower than } \\
\text { that of aerobic respiration. It occurs in some yeasts } \\
\text { and bacteria and in muscle tissue when oxygen is } \\
\text { absent. }
\end{array} \\
\hline
\end{array}
\)

(b) Differences between glycolysis and fermentation are as follows:
\(
\begin{array}{|l|l|l|}
\hline & \text { Glycolysis } & \text { Fermentation } \\
\hline \text { (i) } & \begin{array}{l}
\text { It is the first step of respiration which } \\
\text { occurs without requirement of oxygen and } \\
\text { is common to both aerobic and anaerobic } \\
\text { modes of respiration. }
\end{array} & \begin{array}{l}
\text { It is anaerobic respiration or respiration which } \\
\text { does not require oxygen. }
\end{array} \\
\hline \text { (ii) } & \text { Glycolysis produces pyruvic acid. } & \begin{array}{l}
\text { Fermentation produces different products. The } \\
\text { common ones are ethanol (and } \mathrm{CO}_2 \text { ) and lactic } \\
\text { acid. }
\end{array} \\
\hline \text { (iii) } & \begin{array}{l}
\text { It produces two molecules of NADH per } \\
\text { glucose molecule. }
\end{array} & \begin{array}{l}
\text { It generally utilises NADH produced during } \\
\text { glycolysis. }
\end{array} \\
\hline \text { (iv) } & \begin{array}{l}
\text { It forms 2 ATP molecules per glucose } \\
\text { molecule. }
\end{array} & \text { It does not produce ATP. } \\
\hline
\end{array}
\)

(c) Glycolysis is defined as the chain of the reaction for the conversion of glucose into pyruvate lactate and thus producing ATP.

\(
\begin{array}{|l|l|}
\hline \text { Glycolysis } & \text { Citric acid } \\
\hline \text { First step of cellular respiration. } & \text { Second step of cellular respiration. } \\
\hline \text { A linear process. } & \text { A cyclic process. } \\
\hline \text { End product is organic carbon substance. } & \text { End product is an inorganic substance. } \\
\hline \text { Consumes two ATP. } & \text { Consumes no ATP. } \\
\hline \text { CO2 is not released. } & \text { CO2 is released. } \\
\hline
\end{array}
\)

Q8. What are the assumptions made during the calculation of net gain of ATP?

Answer: For the theoretical calculation of ATP molecules, various assumptions are made, which are as follows.
(a) It is assumed that various parts of aerobic respiration such as glycolysis, TCA cycle, and ETS occur in a sequential and orderly pathway.
(b) NADH produced during the process of glycolysis enters into mitochondria to undergo oxidative phosphorylation.
(c) Glucose molecule is assumed to be the only substrate while it is assumed that no other molecule enters the pathway at intermediate stages.
(d) The intermediates produced during respiration are not utilized in any other process.

Q9. Discuss “The respiratory pathway is an amphibolic pathway.”

Answer: Respiration is generally assumed to be a catabolic process because, during respiration, various substrates are broken down for deriving energy. Carbohydrates are broken down to glucose before entering respiratory pathways. Fats get converted into fatty acids and glycerol whereas fatty acids get converted into acetyl CoA before entering respiration. In a similar manner, proteins are converted into amino acids, which enter respiration after deamination.
During the synthesis of fatty acids, acetyl CoA is withdrawn from the respiratory pathway. Also, in the synthesis of proteins, respiratory substrates get withdrawn. Thus, respiration is also involved in anabolism. Therefore, respiration can be termed as amphibolic pathway as it involves both anabolism and catabolism.

Q10. Define RQ. What is its value for fats?

Answer: Respiratory quotient (RQ) or respiratory ratio can be defined as the ratio of the volume of \(\mathrm{CO}_2\) evolved to the volume of \(\mathrm{O}_2\) consumed during respiration. The value of respiratory quotient depends on the type of respiratory substrate. Its value is one for carbohydrates. However, it is always less than one for fats as fats consume more oxygen for respiration than carbohydrates.
It can be illustrated through the example of tripalmitin fatty acid, which consumes 145 molecules of \(\mathrm{O}_2\) for respiration while 102 molecules of \(\mathrm{CO}_2\) are evolved. The RQ value for tripalmitin is 0.7.

Q11. What is oxidative phosphorylation?

Answer: “Oxidative phosphorylation is the process of ATP formation when electrons are transferred by electron carriers from NADH or FADH2 to oxygen” What is Oxidative Phosphorylation? Oxidative phosphorylation is the final step in cellular respiration. It occurs in the mitochondria. It is linked to a process known as the electron transport chain.

Q12. What is the significance of step-wise release of energy in respiration?

Answer: During oxidation within a cell, all the energy contained in respiratory substrates is not released freely in a single step. It is released in a series of slow stepwise reactions controlled by enzymes and it is trapped as chemical energy in the form of ATP. The significance of this stepwise release of energy is that some energy is used to synthesise ATP which is stored for the later utilisation wherever, required. Hence, ATP acts as the energy currency of the cell. The energy stored in ATP can be utilised following (i) In various energy-requiring processes of organisms. (ii) The carbon skeleton produced during respiration is used as precursors for the synthesis of other molecules.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Energy is released during the oxidation of compounds in respiration. How is this energy stored and released as and when it is needed?

Answer: Energy released during the oxidation of compounds in respiration is immediately stored in ATP in the form of chemical bonds.
\(
\text { ADP + iP + energy -> ATP }
\)
As and when needed, this bond energy is broken and utilised
\(
\text { ATP -> ADP + iP + energy }
\)

Q2. Explain the term “Energy Currency”. Which substance acts as energy currency in plants and animals?

Answer: Every function of the cell requires energy. Energy currency stores and releases the energy as and when needed in the cell. ATP is called energy currency in both plants and animals.

Q3. Different substrates get oxidized during respiration. How does Respiratory Quotient (RQ) indicate which type of substrate, i.e., carbohydrate, fat or protein is getting oxidized?
\(
R \cdot Q .=\frac{A}{B}
\)
What do A and B stand for?
What type of substrates have R.Q. of \(1,<1\) or \(>1\) ?

Answer: \(\quad \mathrm{RQ}=\frac{\text { Volume of } \mathrm{CO}_2 \text { evolved }}{\text { Volume of } \mathrm{O}_2 \text { consumed }}\)
For fat and protein RQ is less than 1.
For carbohydrate RQ is 1 .
RQ of organic acid is more than 1.

Q4. \(\mathrm{F}_1\) particles participate in the synthesis of _____.

Answer: \(F_0-F_1\) particles participate in the synthesis of ATP.

Q5. When does anaerobic respiration occur in man and yeast?

Answer: In alcoholic fermentation (by yeast), incomplete oxidation of glucose occurs under anaerobic conditions by sets of reactions where pyruvic acid is converted to ethanol and \(\mathrm{CO_2}\).
PA \(\rightarrow\) Ethanol \(+\mathrm{CO_2}\)
In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid.

Q6. Which of the following will release more energy on oxidation? Arrange them in ascending order.
a. \(1~ \mathrm{gm}\) of fat
b. \(1~ \mathrm{gm}\) of protein
c. \(1~ \mathrm{gm}\) of glucose
d. 0.5 g of protein +0.5 g glucose

Answer: The asscending order of substrate that will release more energy on oxidetiom will be as follows
1 gm protein < 0.5 gm in protein \(<1~ \mathrm{gm}\) glucose \(<1~ \mathrm{gm}\) fat +0.5 gm glucose

Q7. The product of glycolysis (under hypoxia) in skeletal muscle and anaerobic fermentation in yeast are respectively _____________ and ________________.

Answer: The product of aerobic glycolysis in skeletal muscle and anaerobic fermentation in yeast are respectively pyruvic acid and ethanol \(+\mathrm{CO}_2\).

SHORT ANSWER TYPE QUESTIONS

Q1. If a person is feeling dizzy, glucose or fruit juice is given immediately but not a cheese sandwich Explain.

Answer: Glucose or fruit juice is absorbed easily through the alimentary canal. In the cells glucose is oxidised and energy is released immediately. A cheese sandwich provides energy after digestion and absorption which takes long time.

Q2. What is meant by the statement “aerobic respiration is more efficient.”?

Answer: Aerobic respiration is more efficient because fermentation accounts for only a partial breakdown of glucose, whereas in aerobic respiration it is completely degraded to \(CO_2\) and \(H_2O\). Also, in fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid, whereas many more molecules of ATP are generated under aerobic conditions.

Q3. Pyruvic acid is the end product of glycolysis. What are the three metabolic products of pyruvic acid produced under aerobic and anaerobic conditions? Write their name in the space provided in the diagram.

Answer: i. Under aerobic condition \(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\) is formed
ii. Under anaerobic condition in our skeletal muscles lactic acid is formed
iii. Under anaerobic condition in yeast ethyl alcohol \(+\mathrm{CO}_2\) is produced

Q4. The energy yield in terms of ATP is higher in aerobic respiration than anaerobic respiration. Why anaerobic respiration occurs even in
organisms that live in aerobic condition like human beings and angiosperms?

Answer: The energy yield in terms of ATP is higher in aerobic respiration than anaerobic respiration. Why anaerobic respiration occurs even in organisms that live in aerobic condition like human beings and angiosperms?

Q5. Oxygen is an essential requirement for aerobic respiration but it enters the respiratory process at the end? Discuss.

Answer: Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.

Q6. Respiration is an energy releasing and enzymatically controlled catabolic process which involves a step-wise oxidative breakdown of organic substances inside living cells. In this statement about respiration explain the meaning of 1) Step-wise oxidative breakdown, and 2) Organic substances (used as substrates).

Answer: 1. Step-wise oxidative breakdown: During oxidation within a cell, all the energy contained in respiratory substrates is not released free into the cell, or in a single step. It is released in a series of slow step-wise reactions controlled by enzymes, and it is trapped as chemical energy in the form of ATP. Hence, it is important to understand that the energy released by oxidation in respiration is not (or rather cannot be) used directly but is used to synthesise ATP, which is broken down whenever (and wherever) energy needs to be utilised.
2. Organic substances (used as substrates): The compounds that are oxidised during this process are known as respiratory substrates. Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substances in some plants, under certain conditions.

Q7. Comment on the statement – Respiration is an energy producing process but ATP is being used in some steps of the process.

Answer: In the respiration pathway, there are some steps where energy is utilised for phosphorylation. For example, conversion of glucose to glucose-6-phosphate consume one ATP. But at the end of the respiratory process many more ATP are produced.

Q8. The figure given below shows the steps in glycolysis. Fill in the missing steps A, B, C, D and also indicate whether ATP is being used up or released at step E?

Answer: 

Step A: fructose 6 – phosphate
Step B: fructose 1, 6 biphosphate
Step C: triose phosphate
Step D: triose biphosphate
And finally, at Step E, we can conclude that the energy is being used up.

Q9. Why is respiratory pathway referred to as an amphibolic pathway? Explain.

Answer:  During breakdown and synthesis of protein, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.

Q10. We commonly call ATP as the energy currency of the cell. Can you think of some other energy carriers present in a cell? Name any two.

Answer: Yes, some other energy carriers are also present in a cell like GTP (guanosine triphosphate), ADP (adenosine diphosphate) and creatine phosphate.

Q11. ATP produced during glycolysis is a result of substrate level phosphorylation. Explain.

Answer: ATP produced during glycolysis is a result of substrate level phosphorylation because these ATP are produced without the electron transport system (ETS) and chemiosmosis.
During substrate level phosphorylation ATP is directly synthesised from ADP and inorganic phosphate (iP).

Q12. Do you know any step in the TCA cycle where there is substrate level phosphorylation. Which one? 

Answer: During TCA cycle there is one step where substrate level phosphorylation takes place. This occurs during conversion of succinyl-CoA to succinic acid.

Q13. A process is occurring throughout the day, in ‘ X ‘ organism. Cells are participating in this process. During this process ATP, \(\mathrm{CO}_2\) and water are evolved. It is not a light dependent process.
a. Name the process.
b. Is it a catabolic or an anabolic process?
c. What could be the raw material of this process?

Answer: a. The name of the process is cellular respiration.
b. It is a catabolic process because it involves the breakdown of the glucose molecule.
c. Raw materials involved in the cellular respiration process are Glucose molecule and oxygen.

Q14. When a substrate is being metabolized, why does not all the energy that released in one step. It is released in multiple steps. What is the advantage of step-wise release?

Answer: The complete combustion of glucose, which produces \(\mathrm{CO}_2\) and \(\mathrm{H}_2 \mathrm{O}\) as end products, yields energy most of which is given out as heat.
\(
\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \rightarrow 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}+\text { Energy }
\)
If this energy is to be useful to the cell, it should be able to utilise it to synthesise other molecules that the cell requires. The strategy that the plant cell uses is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat. The key is to oxidise glucose not in one step but. in several small steps enabling some steps to be just large enough such that the energy released can be coupled to ATP synthesis.

Q15. Respiration requires \(\mathrm{O}_2\). How did the first cells on the earth manage to survive in an atmosphere that lacked \(\mathrm{O}_2\) ?

Answer: During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products. The combustion reaction requires oxygen. But some cells live where oxygen may or may not be available. There are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen. Even among present-day living organisms, we know of several that are adapted to anaerobic conditions.

Q16. It is known that red muscle fibres in animals can work for longer periods of time continuously. How is this possible?

Answer: Muscle contains a red coloured oxygen storing pigment called myoglobin. Myoglobin content is high in some of the muscles which gives a reddish appearance. Such muscles are called the Red fibres. These muscles also contain plenty of mitochondria which can utilise the large amount of oxygen stored in them for ATP production. These muscles, therefore, can also be called aerobic muscles.

Q17. The energy yield in terms of ATP is higher in aerobic respiration than during anaerobic respiration. Explain.

Answer: The energy yield in terms of ATP is higher in aerobic respiration than during anaerobic respiration because fermentation (anaerobic respiration) accounts for only a partial breakdown erf glucose, whereas in aerobic respiration it is completely degraded to \(CO_2\) and \(H_2 O\) . So, in fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid, whereas many more molecules of ATP are generated under aerobic conditions.

Q18. RuBP carboxylase, PEP carboxylase, Pyruvate dehydrogenase, ATPase, cytochrome oxidase, Hexokinase, Lactate dehydrogenase. Select/choose enzymes from the list above which are involved in
a. Photosynthesis
b. Respiration
c. Both in photosynthesis and respiration

Answer: a. Photosynthesis: RuBP carboxylase, PEPcase, ATPase
b. Respiration: Hexokinase, ATPase, Pyruvate dehydrogenase, Cytochrome oxidase
c. Both in photosynthesis and respiration: ATPase

Q19. How does a tree trunk exchange gases with the environment although it lacks stomata?

Answer: In stems, the ‘living’ cells are organised in thin layers inside and beneath the bark. They also have openings called lenticels. The cells in the interior are dead and provide only mechanical support. Thus, most cells of a plant have at least a part of their surface in contact with air. This is also facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces.

Q20. Write any two energy yielding reactions of glycolysis.

Answer: The conversion of BPGA to 3-phosphoglyceric acid (PGA), is an energy yielding process; this energy is trapped by the formation of ATP. Another ATP is synthesised during the conversion of PEP to pyruvic acid.

Q21. Name the site (s) of pyruvate synthesis. Also, write the chemical reaction wherein pyruvic acid dehydrogenase acts as a catalyst.

Answer: Pyruvate is synthesized in the cytoplasm of the cell by the process of glycolysis. 1 molecule of glucose forms 2 molecules of pyruvate through a series of reactions.
Pyruvic acid dehydrogenase catayses the reaction in which pyruvate forms acetyl Co-A. It requires \(\mathrm{NAD}^{+}\), Co-enzyme A and \(\mathrm{Mg}^{2+}\) ions for its activity. The reaction as follows

\(
\text { Pyruvic acid }+\mathrm{CoA}+\mathrm{NAD}^{+} \xrightarrow[\text { Pyruvate dehydrogenase }]{\mathrm{Mg}^{2+}} \text { Acetyl } \mathrm{CoA}+\mathrm{CO}_2+\mathrm{NADH}+\mathrm{H}^{+}
\)

Q22. Mention the important series of events of aerobic respiration that occur in the matrix of the mitochondrion and in the inner membrane of the mitochondrion.

Answer: For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate is transported from the cytoplasm into the mitochondria. The crucial events in aerobic respiration are: The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of \(\mathrm{CO}_2\).

The passing on of the electrons removed as part of the hydrogen atoms to molecular \(\mathrm{O}_2\) with simultaneous synthesis of ATP. What is interesting to note is that the first process takes place in the matrix of the mitochondria while the second process is located on the inner membrane of the mitochondria.

Q23. Respiratory pathway is believed to be a catabolic pathway. However, nature of TCA cycle is amphibolic. Explain.

Answer: During breakdown and synthesis of protein, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.

LONG ANSWER TYPE QUESTIONS

Q1. In the following flow chart, replace the symbols a,b,c and d with appropriate terms. Briefly explain the process and give any two application of it.

Answer: 

Q2. Given below is a diagram showing ATP synthesis during aerobic respiration, replace the symbols A, B, C, D and E by appropriate terms
given in the box.

Answer: 

 

Diagrammatic presentation of ATP synthesis in mitochondria

Q3. Oxygen is critical for aerobic respiration. Explain its role with respect to ETS.

Answer: Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor. Unlike photophosphorylation where it is the light energy that is utilised for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilised for the same process. It is for this reason that the process is called oxidative phosphorylation.

Q4. Enumerate the assumptions that we undertake in making the respiratory balance sheet. Are these assumptions valid for a living system? Compare fermentation and aerobic respiration in this context.

Answer: The Respiratory Balance Sheet:
It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised; but in reality this can remain only a theoretical exercise. These calculations can be made only on certain assumptions that:
There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another.
The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation. None of the intermediates in the pathway are utilised to synthesise any other compound.
Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages. But this kind of assumptions are not really valid in a living system; all pathways work simultaneously and do not take place one after another; substrates enter the pathways and are withdrawn from it as and when necessary; ATP is utilised as and when needed; enzymatic rates are controlled by multiple means. Yet, it is useful to do this exercise to appreciate the beauty and efficiency of the living system in extraction and storing energy. Hence, there can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose.

Q5. Give an account of Glycolysis. Where does it occur? What is the end product? Trace the fate of these products in both aerobic and anaerobic respiration.

Answer: Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms. In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid. In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway. Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme hexokinase. This phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. Subsequent steps of metabolism of glucose and fructose are same. In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose. Pyruvic acid is then the key product of glycolysis. The metabolic fate of pyruvate depends on the cellular need. There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to \(\mathrm{CO_2}\) and \(\mathrm{H_2 O}\) , however, organisms adopt Krebs’ cycle which is also called as aerobic respiration. This requires \(\mathrm{O}_2\) supply.