Problem 102 - Objective Biology for NEET

Question

Assertion: The breakdown of glucose to pyruvic acid is called glycolysis. Reason: For complete oxidation of glucose organism adopt Kreb's cycle.

Step-by-Step Solution

Answer

The statement in the exercise is accurate. Glycolysis is indeed the process involving the breakdown of glucose to form pyruvic acid. However, for complete oxidation of glucose, it's not just glycolysis that's involved. It is necessary to undertake the process of the Kreb's cycle, consuming the pyruvate produced by glycolysis and subsequently generating energy in the form of ATP.

Step 1: Explaining Glycolysis

Glycolysis is a sequence of reactions that converts glucose into pyruvate with the associated net gain of 2 ATP. The word glycolysis comes from 'glyco-' (sugar) and '-lysis' (to break), meaning it is the metabolic pathway that transforms glucose into pyruvate. In other words, it is the breakdown of glucose to pyruvic acid.

Step 2: Connecting Glycolysis and Kreb's Cycle

Glycolysis is only the first step in glucose oxidation, resulting in the production of pyruvate. For the complete oxidation of glucose to occur, the process needs to continue. The pyruvate enters the mitochondria to be metabolized further by the Kreb's Cycle, also known as the citric acid cycle or tricarboxylic acid cycle.

Step 3: Explaining Kreb's Cycle

The Kreb's cycle is the sequence of reactions by which most living cells generate energy during the process of aerobic respiration. It takes place in the mitochondria, consuming pyruvate, and produces CO2 as a waste product, which the body discards. Energy, trapped in the form of ATP, is also produced.

Key Concepts

Metabolic Pathways

Metabolic pathways are vital routes within cells that organize chemical reactions. These pathways transform substrates through a series of steps, each controlled by a specific enzyme, to generate energy or cellular components. Think of them like an assembly line in a factory, each worker adding a piece until the final product is made.

In the context of glucose metabolism, glycolysis is the initial metabolic pathway that begins the energy extraction from glucose. It marks the entry point of carbohydrates into the energy-generating systems. Beyond just energy, metabolic pathways also create building blocks for nucleic acids, lipids, and proteins, all crucial for cell maintenance and growth.

Glucose Oxidation

Glucose oxidation refers to the complex process of breaking down glucose to extract energy. The first phase, glycolysis, splits glucose into two three-carbon molecules called pyruvate. This transformation occurs in the cytosol and does not require oxygen, making it an anaerobic process.

Glucose oxidation continues in the presence of oxygen with the conversion of pyruvate to acetyl-CoA, which enters the Kreb's cycle in the mitochondria for further breakdown. In the absence of oxygen, alternatively, pyruvate is transformed into lactate or ethanol, depending on the organism, in a process known as fermentation.

Aerobic Respiration

Aerobic respiration is like the marathon race of the cellular energy processes. It requires oxygen and entails multiple stages including glycolysis, the Kreb's cycle, and the electron transport chain. After glycolysis, pyruvate enters the mitochondrion where it's converted to acetyl-CoA, the starting point for the Kreb's cycle.

The Kreb's cycle then churns out electron carriers loaded with high-energy electrons, which are cashed in for ATP in the electron transport chain. This process is incredibly efficient, generating up to 34 ATP molecules for each molecule of glucose in contrast to the mere 2 ATP from glycolysis alone.

ATP Production

ATP, or Adenosine Triphosphate, is the energy currency of the cell, powering nearly every cellular activity. ATP production is the main goal of glucose metabolism, which includes glycolysis and the Kreb's cycle. During glycolysis, a small profit of 2 ATP is made. However, the Kreb's cycle and subsequent electron transport chain really boost the earnings, maximizing the yield from each glucose molecule.

Energy Investment Vs. Energy Payoff

Initially, glycolysis consumes 2 ATP, but it pays back by generating 4 ATP directly and 2 more indirectly through electron carriers (NADH), which are used later in the electron transport chain for more ATP. The Kreb's cycle itself directly generates 2 ATP per glucose molecule but also produces more NADH and a related electron carrier FADH2, which eventually lead to the production of the bulk of the ATP in aerobic respiration.