Respiration in Plants Test - 83

Cellobiose, the disaccharide obtained by partial hydrolysis of cellulose, consists of two D-glucopyranoses joined by a 1,4,-beta-glycoside bond. Cellobiose is reducing sugar, because the anomeric carbons on the right-hand sugar are part of a hemiacetal.
Cellobiose :  4-O--D-Glucopyranosyl-D-glucose. Thus, option A is correct.
The other options are not related to the cellobiose. Thus, other options are incorrect.

Pyruvate decarboxylation is far from equilibrium biochemical reaction, that uses pyruvate to form acetyl-CoA, releasing NADH, a reducing equivalent, and carbon dioxide via decarboxylation. This reaction is very common in most organisms as a link between glycolysis and the citric acid cycle. In organisms that perform aerobic respiration, the reaction is usually catalysed by the pyruvate dehydrogenase complex. Thus, oxidative decarboxylation is a reaction that changes pyruvate to acetyl CoA. So, option A is correct.

Succinate dehydrogenase enzyme catalyzes the oxidation of succinic acid by removal of its two hydrogen atoms and forms fumaric acid. The removed hydrogen atoms reduce the FAD into $FADH_2$ that enters mitochondrial electron transport chain. Thus, the correct answer is option A.

The respiratory quotient (RQ) is the ratio of CO$_{2}$ produced to O$_{2}$ consumed while food is being metabolized.
RQ = CO$_{2}$ eliminated/O$_{2}$ consumed.
The respiratory quotient for carbohydrate metabolism can be demonstrated by the chemical equation for oxidation of glucose.
C$_{6}$H$_{12}$O$_{6}$ + 6 O$_{2}$ 6 CO$_{2}$+ 6 H$_{2}$O
Because the gas exchange in this reaction is equal, the respiratory quotient for carbohydrates is: RQ = 6 CO2 / 6 O2 = 1. Thus, option A is wrong.
The chemical composition of fats differs from that of carbohydrates in that fats contain considerably fewer oxygen atoms in proportion to atoms of carbon and hydrogen. The substrate utilization of palmitic acid is:
C$_{16}$H$_{32}$O$_{2}$ + 23 O$_{2}$ 16 CO$_{2}$ + 16 H$_{2}$O
Thus, the RQ for palmitic acid is approximately 0.7. RQ = 16 CO$_{2}$ / 23 O$_{2}$ = 0.696. Thus, option B is wrong.
The respiratory quotient for protein metabolism can be demonstrated by the chemical equation for oxidation of albumin:
C$_{72}$H$_{11}$2N$_{18}$O$_{2}$S + 77 O$_{2}$ 63 CO$_{2}$ + 38 H$_{2}$O + SO$_{3}$ + 9 CO(NH$_{2}$)2
The RQ for protein is approximately 0.8. RQ = 63 CO$_{2}$/ 77O$_{2}$ = 0.8. Thus, option C is wrong.
In case of organic fats, volume of CO$_{2}$ given out during respiration is more than the volume of O$_{2}$ used. Thus, option D is correct.

In citric acid cycle, isocitrate gets converted into ketoglutarate by decarboxylation reaction which further undergoes second decarboxylation to form succinate.

• The rate-limiting step in glycolysis is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, which is catalyzed by the enzyme phosphofructokinase. So, phosphofructokinase is called a rate-limiting enzyme in glycolysis. 
• It is an energetically favorable step, irreversible and a different pathway must be used to do the reverse conversion during gluconeogenesis. This makes the reaction a key regulatory point. Thus, option B is correct.
• Glucokinase or hexokinase is involved in the conversion of glucose to glucose-6-phosphate. This step is not the rate-limiting step in glycolysis. 
• Phospho hexoisomerase catalyzes the conversion of glucose-6-phosphate to fructose-6-phosphate. This step is not the rate-limiting step in glycolysis.
• Aldolase catalyzes the conversion of fructose-1.6-bisphosphate to glyceraldehyde -3- phosphate and dihydroxyacetone phosphate. This step is not the rate-limiting step in glycolysis.

Glycolysis breaks down glucose to two molecules of pyruvate outside the mitochondria. Glycolysis occurs in the cytoplasm. This pyruvate then diffuses into mitochondrial matrix where enzymes are available for Krebs cycle. 

If R.Q.is $0.6$ in respiratory metabolism, it would mean that the oxidation of the respiratory substrate consumed more oxygen than the amount of $CO_2$ released.

Correct answer: option (B)
Explanation for the correct option: (B)

• RQ refers to respiratory quotient i.e., the ratio of carbon dioxide released to the oxygen consumed.
• During anaerobic respiration, carbon dioxide is released but oxygen is not consumed.
• Following is the equation for anaerobic respiration.

C6​H1​2O6​ → 2C2​H5​OH + 2CO2​ + energy

• In the equation, it can be observed that 2 molecules of CO2 are released but no O2​ is used.
• So, the ratio becomes as: 2CO2​/O2​ = 2/0 = infinity

Glucose 6-phosphate acts as an allosteric modulator for hexokinase. Glucose 6-phosphate binds to the regulatory site on the hexokinase which causes conformation changes on the active site of hexokinase and so, the substrate cannot bind to hexokinase. This declines the activity of hexokinase. The increase in glucose-6-phosphate does not cause a decline in activity of hexokinase.