CBSE Class 11 Biology Chapter 14: Respiration in Plants NCERT Solutions

In aerobic respiration, the process relies heavily on the presence of oxygen. Oxygen serves as the final electron acceptor in the electron transport chain (ETC), which is the last stage of aerobic respiration. At the end of the ETC, electrons are passed to oxygen, which then combines with protons (H+) to form water ($H\_2O$). This step is crucial for regenerating $NAD^+$ and $FAD$, allowing glycolysis and the Krebs cycle to continue, and it is the primary mechanism for generating a large amount of ATP.

Therefore, oxygen is the ultimate hydrogen acceptor and electron acceptor in aerobic respiration.

Answer: (b) oxygen

Glycolysis begins with the phosphorylation of glucose. This reaction involves the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate. This step is catalyzed by the enzyme hexokinase. This phosphorylation traps glucose within the cell and makes it more reactive for subsequent steps.

Let's look at why the other options are incorrect:

• Phosphoglucomutase: This enzyme isomerizes glucose-6-phosphate to glucose-1-phosphate.
• Phosphoglucoisomerase: This enzyme catalyzes the conversion of glucose-6-phosphate to fructose-6-phosphate, an isomer.
• Phosphorylase: This enzyme typically catalyzes the addition of inorganic phosphate to a substrate, often breaking a bond, and is not directly involved in the initial phosphorylation of glucose in glycolysis.

Answer: (c) hexokinase

Glycolysis, the breakdown of glucose, occurs in the cytoplasm and produces two molecules of pyruvic acid. Under aerobic conditions, pyruvic acid enters the mitochondria and undergoes oxidative decarboxylation. This process, catalyzed by the pyruvate dehydrogenase complex, converts pyruvic acid into acetyl Co-A, releasing one molecule of carbon dioxide ($CO\_2$). The acetyl Co-A then enters the Krebs cycle for further oxidation.

The other options describe fates of pyruvate under different conditions:

• Lactic acid is formed during anaerobic respiration in muscle cells and some bacteria.
• Ethanol and $CO\_2$ are products of anaerobic respiration in yeast and some plant tissues.
• While $CO\_2$ and $H\_2O$ are the ultimate end products of complete aerobic respiration, the direct fate of pyruvate under aerobic conditions is its conversion to acetyl Co-A.

Answer: (c) acetyl Co - A + $CO\_2$

The Electron Transport System (ETS) is a series of protein complexes and electron carriers embedded within the inner mitochondrial membrane. These complexes are responsible for accepting electrons from $NADH$ and $FADH\_2$ (produced during glycolysis and the Krebs cycle) and passing them along in a stepwise manner. As electrons move through the chain, energy is released and used to pump protons ($H^+$) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient that drives ATP synthesis.

Answer: (c) inner membrane

The rate of respiration is generally proportional to the metabolic activity of the tissue. Actively growing and metabolically demanding tissues respire at a higher rate to meet their energy requirements. Among the given options, a germinating seed exhibits the highest rate of respiration. This is because upon imbibition of water, dormant seeds become metabolically active. Hydrolytic enzymes are activated to break down stored food reserves (like starch, fats, and proteins) into simpler molecules, providing energy and building blocks for rapid growth and development into a seedling. Growing shoot apices and root tips are also metabolically active, but the initial surge of activity during seed germination is typically the highest.

Answer: (b) Germinating seed

The statement that mitochondria are the 'powerhouses of the cell' refers to their primary role in generating the majority of the cell's supply of adenosine triphosphate (ATP), which is used as a source of chemical energy. The most direct evidence supporting this is that mitochondria synthesise ATP through the process of oxidative phosphorylation, which occurs during the electron transport chain and chemiosmosis. While the other statements are correct observations about mitochondria (they have a double membrane, contain Krebs' cycle enzymes and cytochromes, and are widespread), they do not directly explain *why* they are called powerhouses. The synthesis of ATP is the defining characteristic that earns them this title.

Answer: (a) Mitochondria synthesise ATP