Test: Respiration in Plants - 1 - NEET MCQ

The TCA cycle begins with a condensation reaction. In this step, an acetyl group, derived from the breakdown of glucose or fatty acids, combines with oxaloacetic acid (OAA), a four-carbon molecule, in the presence of water. This condensation reaction forms a six-carbon compound called citric acid or citrate.

The complete reaction is as follows:
Acetyl-CoA + OAA + H₂O → Citric Acid (Citrate)

The condensation step is significant because it marks the entry of the acetyl group into the TCA cycle and the beginning of a series of enzymatic reactions that ultimately lead to the release of energy and the regeneration of OAA.

Topic in NCERT: Tricarboxylic acid cycle

Line in NCERT: "The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid."

During anaerobic respiration, less energy is produced compared to aerobic respiration because incomplete oxidation of glucose occurs. Anaerobic respiration occurs in the absence of oxygen, typically in situations where oxygen is not readily available or during strenuous exercise when oxygen demand exceeds supply.

Topic in NCERT: Fermentation

Line in NCERT: "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."

The two molecules of pyruvic acid (produced from one glucose molecule during glycolysis). The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle, more commonly called as Krebs’ cycle. Thus, acetyl-CoA interconnecting between two cyclic pathways.

Topic in NCERT: Tricarboxylic acid cycle

Line in NCERT: "The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle,"

Plants unlike animals have no special systems for breathing or gaseous exchange. Stomata and lenticels allow gaseous exchange by diffusion.

(a) Stomata: Stomata are tiny openings present on the surface of leaves, primarily on the underside. These openings are surrounded by specialized cells called guard cells. Stomata regulate the exchange of gases, including the intake of carbon dioxide (CO₂) for photosynthesis and the release of oxygen (O₂) and water vapor (H₂O) during transpiration. Stomata play a crucial role in the gas exchange process in plants.

(d) Lenticels: Lenticels are small openings or pores in the bark of woody plant stems. They allow for gaseous exchange between the internal tissues of the stem and the external environment. Lenticels facilitate the exchange of gases, such as oxygen (O₂) and carbon dioxide (CO₂), in stems and other woody parts of plants.

Topic in NCERT: Do plants breathe?

Line in NCERT: "Plants, unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose."

The correct statements are:

(A) The oxidation of pyruvic acid molecules formed in glycolysis occurs inside the mitochondria.
(C) Under anaerobic conditions, the pyruvic acid formed during glycolysis is reduced to either ethyl alcohol or lactic acid.

Explanation:

• (A) Correct: Pyruvic acid (a 3-carbon compound) produced in glycolysis enters the mitochondria, where it is oxidatively decarboxylated to form Acetyl CoA (a 2-carbon compound) before entering the Krebs cycle.

• (B) Incorrect: Acetyl CoA is a 2-carbon compound, not 3-carbon.

• (C) Correct: In anaerobic conditions, pyruvate is reduced to:

  • Lactic acid (in animal cells/muscles via lactate fermentation).

  • Ethanol + CO₂ (in yeast/plant cells via alcoholic fermentation).

• (D) Incorrect: Acetyl CoA enters the Krebs cycle (in mitochondria), not the Calvin cycle (which fixes CO₂ in chloroplasts using RuBisCO).

Final Answer:

(A) and (C) are correct

Phosphofructokinase is a kinase enzyme that phosphorylates fructose-6-phosphate in glycolysis. The enzyme catalysed transfer of a phosphoryl group from ATP is an important reaction in a wide variety of biological processes.

In animal cells, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase. The reducing agent is NADH+ and H+ which is reoxidised to NAD+ in both the processes.

Topic in NCERT: Glycolysis

Line in NCERT: "The initial stage of cellular respiration takes place in the cytoplasm. Each glucose molecule is broken through a series of enzyme catalysed reactions into two molecules of pyruvic acid. This process is called glycolysis."

Citrate is first isomerised to isocitrate and then followed by decarboxylation, leading to the formation of α-ketoglutaric acid.

Topic in NCERT: Tricarboxylic acid cycle

Line in NCERT: "The reaction is catalysed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate. It is followed by two successive steps of decarboxylation, leading to the formation of α-ketoglutaric acid."

• Oxysomes refer to small round structures present within the folds of the cristae of the inner mitochondrial membrane. It is also known as F₀-F₁ particles.
• F₀ and F₁ particles are found in the inner mitochondrial region and are attached to the cristae and help in ATP production and oxidation.

Cytochrome c is a water soluble protein.

Topic in NCERT: Cytochrome c

Line in NCERT: "Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV."

Acetyl CoA is the connecting link between glycolysis and Kreb's cycle. The end product of glycolysis is pyruvate. It is converted to Acetyl CoA which enters the krebs cycle by combining with oxaloacetic acid (OAA) to form a 6 - C compound citric acid.

Topic in NCERT: Tricarboxylic acid cycle

Line in NCERT: "The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid."

CoA is acetylated to acetyl CoA by the breakdown of carbohydrates through glycolysis and by the breakdown of fatty acids through β-oxidation.

Topic in NCERT: Breakdown of pyruvate

Line in NCERT: "Pyruvic acid is transported into the mitochondria where it is converted into acetyl CoA with the release of CO2."

The ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed in respiration is called the respiratory quotient(RQ) or respiratory ratio

Topic in NCERT: Respiratory quotient

Line in NCERT: "The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio."

Formation of pyruvate is not a prerparatory phase or energy spending phase of Glycolysis. In this phase one ATP molecule is released.

Topic in NCERT: Glycolysis phases

Line in NCERT: "Pyruvic acid is then the key product of glycolysis."

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.

Topic in NCERT: Glycolysis, TCA cycle and ETS pathway

Line in NCERT: "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 value of R.Q. depends upon the nature of the respiratory substrate (the organic food matter oxidised in respiration) and its oxidation.

Topic in NCERT: Respiratory quotient

Line in NCERT: "The respiratory quotient depends upon the type of respiratory substrate used during respiration."

There can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose. 

Topic in NCERT: Net gain of ATP in aerobic respiration

Line in NCERT: "Hence, there can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose."

A (Molecular oxygen): Oxygen is a hydrogen acceptor in the electron transport chain (ETC). Hence, A matches with 2 (Hydrogen acceptor).

B (Electron acceptor): Cytochrome-c acts as the electron acceptor in the ETC, so B matches with 3 (Cytochrome-c).

C (Pyruvate dehydrogenase): Pyruvate dehydrogenase converts pyruvate to acetyl Co-A, making C match with 4 (Acetyl Co-A).

D (Decarboxylation): Decarboxylation often occurs in metabolic processes, including the conversion of α-ketoglutaric acid to succinyl-CoA. Thus, D matches with 1 (α-ketoglutaric acid).

The correct matching is:

A - 2 (Hydrogen acceptor)

B - 3 (Cytochrome-c)

C - 4 (Acetyl Co-A)

D - 1 (α-ketoglutaric acid)

Therefore, the correct answer is Option C.