Respiration in Plants

Malonate closely resembles the substrate succinate in structure. It competes with succinate for the active site of succinic dehydrogenase and blocks it, so the enzyme cannot convert succinate to fumarate. This is a classical example of competitive inhibition.

Carboxypeptidase is a metalloenzyme. Zinc is its cofactor, which is essential for its catalytic activity in the hydrolysis of peptide bonds.

Citric acid cycle – mitochondrial matrix Glycolysis – cytoplasm Electron transport system – inner mitochondrial membrane Proton gradient – intermembrane space of mitochondria Correct match: A-II, B-I, C-IV, D-III.

ATP is utilised at two steps: first in the conversion of glucose into glucose 6-phosphate and second in the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate.

Pyruvate when enters into mitochondrial matrix, it undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase. Glycolysis is often referred to as the EMP pathway. Oxidative phosphorylation is a process embedded in the inner membrane of mitochondria (called electron transport chain and ATP synthase). The reaction in TCA cycle is catalysed by the enzyme citrate synthase.

In aerobic respiration the electrons removed from the substrate are finally passed to an external electron acceptor, molecular oxygen.
In fermentation there is no external electron acceptor: glucose is only partially oxidised to pyruvic acid, and the NADH produced is re-oxidised by transferring its electrons to an internal organic molecule derived from the substrate itself.
In alcoholic fermentation acetaldehyde is reduced to ethanol; in lactic acid fermentation pyruvic acid is reduced to lactic acid.
Only a small amount of energy is released (net gain of 2 ATP per glucose).
Glycolysis by itself only produces pyruvate and NADH; photorespiration consumes energy.
Hence the process is fermentation.

In aerobic respiration the electrons removed from the substrate are finally passed to oxygen, which is an external electron acceptor.
Fermentation takes place in the absence of oxygen, and no external electron acceptor is used.
The $NADH + H^+$ produced in glycolysis is reoxidised to $NAD^+$ by passing its electrons to an organic molecule formed from the substrate itself: pyruvic acid (giving lactic acid) or acetaldehyde (giving ethanol and $CO_2$).
So in fermentation the substrate is only partially oxidised, and an internal organic compound acts as the electron acceptor. Only a small amount of energy (a net gain of 2 ATP per glucose) is released.
Glycolysis is only the first step, common to both aerobic respiration and fermentation, and by itself does not include the fate of the reduced coenzymes.
Photorespiration is not an energy-releasing process; it consumes energy.
So the process is fermentation.

In glycolysis, glyceraldehyde-3-phosphate (a triose phosphate) is oxidised; it gives electrons (and $H^+$) to $NAD^+$, forming NADH. So triose phosphate is the electron donor.
In alcoholic fermentation by yeast, pyruvic acid is first decarboxylated to acetaldehyde: $CH_3COCOOH \rightarrow CH_3CHO + CO_2$
Acetaldehyde then accepts electrons from NADH and is reduced to ethanol: $CH_3CHO + NADH + H^+ \rightarrow C_2H_5OH + NAD^+$
So acetaldehyde is the final electron acceptor. (Pyruvic acid is the acceptor in lactic acid fermentation.) Oxygen is not involved.
So in alcohol fermentation, triose phosphate is the electron donor while acetaldehyde is the electron acceptor.

In one turn of the citric acid cycle, there is one substrate level phosphorylation: Succinyl-CoA $\xrightarrow{succinate\ thiokinase}$ succinate, with GDP → GTP (which then forms ATP from ADP).

The enzymes of the TCA (Krebs) cycle are present in the mitochondrial matrix, with one exception.
Succinate dehydrogenase, which oxidises succinate to fumarate and reduces FAD to $FADH_2$, is bound to the inner mitochondrial membrane.
It is a part of complex II of the electron transport system (succinate : ubiquinone oxidoreductase), and so it provides a direct link between the citric acid cycle and the membrane-bound electron transport chain.
The enzyme contains a covalently bound FAD group and iron-sulphur clusters, through which the electrons from succinate are passed on to ubiquinone.
Isocitrate dehydrogenase and malate dehydrogenase are soluble enzymes of the matrix.
Lactate dehydrogenase is not an enzyme of the TCA cycle at all; it is a cytoplasmic enzyme of lactic acid fermentation.
So the enzyme is succinate dehydrogenase.

ETS Complex I is NADH Dehydrogenase (A-I); ETS Complex II is Succinate Dehydrogenase (B-IV); ETS Complex III is Cytochrome $bc_1$ (C-II); ETS Complex IV is Cytochrome C oxidase (D-III).

Statements A, C, D and E are correct. Statement B is incorrect because glycolysis (the first step of cellular respiration) occurs in the cytoplasm, not entirely within the mitochondria.

The pyruvate dehydrogenase enzyme complex, which catalyses the conversion of pyruvate to acetyl CoA (the link reaction), requires magnesium ($Mg^{2+}$) as a cofactor.

Decarboxylation occurs twice in each turn of the TCA (Krebs) cycle: once when isocitrate is converted to alpha-ketoglutarate, and again when alpha-ketoglutarate is converted to succinyl-CoA, releasing two molecules of $CO_2$ per cycle.

Fatty acids, after beta-oxidation, are broken down into acetyl CoA units, which then enter the TCA cycle — thus fatty acids connect to the respiratory pathway via acetyl CoA.

In a single Krebs cycle decarboxylation occurs at two steps: isocitrate is oxidatively decarboxylated to 5-C $\alpha$-ketoglutarate, and $\alpha$-ketoglutarate is decarboxylated to 4-C succinyl-CoA. Isocitrate itself is decarboxylated only once.

Porins are proteins that form huge pores in the outer membranes of plastids, mitochondria and bacteria. Leg-haemoglobin is the pink pigment of root nodules that acts as an oxygen scavenger. $H^+$ accumulates in the lumen of the thylakoid during light reactions. Respiration is an amphibolic pathway as it involves both anabolism and catabolism.

The Krebs cycle starts with condensation of acetyl CoA (2C) with oxaloacetic acid (4C) to form citric acid (6C), not with pyruvic acid.

Pathway A is glycolysis, B is the Krebs cycle and C is the electron transport system. ATP is produced in all three, so arrows 4, 8 and 12 can all be ATP.

Energy released by the complete oxidation of one mole of glucose = 686 kcal.
Energy stored in the high energy phosphate bond of one mole of ATP = 12 kcal.
If all the energy released could be trapped in ATP, the maximum number of ATP molecules formed per molecule of glucose would be:
$\dfrac{686}{12} = 57.1$
So a maximum of about 57 ATP molecules could be generated.
In actual aerobic respiration only 36 to 38 ATP molecules are formed per glucose molecule, because the rest of the energy is lost as heat; but the question asks for the theoretical maximum.
So the answer is fifty-seven.