CBSE Class 11 Biology Chapter 16: Digestion and Absorption NCERT Solutions

Solution: Here is a differentiation between the given biological processes:

1. Respiration vs. Combustion

   Respiration	Combustion
   It is a biochemical process occurring within living cells.	It is a physiochemical process that occurs outside living cells.
   It is a biologically controlled process involving enzymes.	It is an uncontrolled process that does not require enzymes.
   Energy is released gradually and captured in the form of ATP molecules.	Energy is released rapidly as heat and light, with no ATP generation.
   The process is slow and occurs in multiple steps.	The process is fast and occurs in a single step.
   It involves specific respiratory substrates like glucose.	It involves the burning of various combustible materials.

2. Glycolysis vs. Krebs' cycle

   Glycolysis	Krebs' cycle
   It is a linear pathway where glucose is broken down.	It is a cyclic pathway where acetyl-CoA is oxidized.
   It occurs in the cytoplasm of the cell.	It occurs in the mitochondrial matrix.
   It occurs in both aerobic and anaerobic respiration.	It occurs only during aerobic respiration.
   The net yield from one glucose molecule is 2 ATP and 2 NADH2.	The net yield from two acetyl-CoA molecules is 2 ATP (or GTP), 6 NADH2, and 2 FADH2.
   The end product is pyruvic acid.	The end products are CO2, ATP, NADH2, and FADH2.

3. Aerobic Respiration vs. Fermentation

   Aerobic Respiration	Fermentation
   Requires oxygen for the complete breakdown of the respiratory substrate.	Occurs in the absence of oxygen.
   Takes place primarily in the mitochondria.	Occurs in the cytoplasm.
   The end products are carbon dioxide and water.	The end products are typically ethyl alcohol and carbon dioxide (in alcoholic fermentation) or lactic acid (in lactic acid fermentation).
   Complete oxidation of the respiratory substrate occurs.	Incomplete oxidation of the respiratory substrate occurs.
   A large amount of energy is released, producing approximately 36 ATP molecules per glucose molecule.	A small amount of energy is released, producing only 2 ATP molecules per glucose molecule.

Solution: Respiratory substrates are organic molecules within living organisms that are oxidized during cellular respiration to release energy. This energy is typically captured in the form of ATP. While carbohydrates, particularly glucose, are the most common and preferred respiratory substrates, other molecules like fats, proteins, and organic acids can also serve this role depending on the organism and physiological conditions.

Solution: Glycolysis is the initial stage of cellular respiration, occurring in the cytoplasm, where one molecule of glucose (a 6-carbon sugar) is broken down into two molecules of pyruvic acid (a 3-carbon compound). This process involves a series of ten enzymatic reactions and yields a net gain of ATP and NADH. Here is a schematic representation:

Glucose (6C) + 2 ATP → Glucose-6-phosphate (6C) + 2 ADP

Glucose-6-phosphate (6C) ⇌ Fructose-6-phosphate (6C)

Fructose-6-phosphate (6C) + ATP → Fructose-1,6-bisphosphate (6C) + ADP

Fructose-1,6-bisphosphate (6C) → Dihydroxyacetone phosphate (3C) + Glyceraldehyde-3-phosphate (3C)

Dihydroxyacetone phosphate (3C) ⇌ Glyceraldehyde-3-phosphate (3C)

Each Glyceraldehyde-3-phosphate (3C) molecule undergoes the following steps:

Glyceraldehyde-3-phosphate (3C) + NAD⁺ + Pi → 1,3-Bisphosphoglycerate (3C) + NADH + H⁺

1,3-Bisphosphoglycerate (3C) + ADP → 3-Phosphoglycerate (3C) + ATP

3-Phosphoglycerate (3C) ⇌ 2-Phosphoglycerate (3C)

2-Phosphoglycerate (3C) → Phosphoenolpyruvate (PEP) (3C) + H₂O

Phosphoenolpyruvate (PEP) (3C) + ADP → Pyruvic acid (3C) + ATP

Overall Net Yield per Glucose molecule: 2 Pyruvic acid, 2 ATP, and 2 NADH + H⁺.