Cellular Metabolism: Glycolysis and Krebs Cycle Explained

Metabolism

Metabolism is the set of chemical reactions and energy processes taking place in a cell to meet its needs for material and energy. We differentiate between:

• Catabolism: The degradation of organic matter to release energy.
• Anabolism: The synthesis of complex chemicals using simpler ones, which consumes energy.

Glycolysis

Glycolysis is a complex process that enables the transformation of glucose to extract the energy contained within it.

• Location: Cytoplasm.
• Oxygen Requirement: Does not require oxygen (anaerobic).
• Organisms: Performed by both autotrophic and heterotrophic organisms.

It consists of 10 reactions, each catalyzed by a specific enzyme, transforming one glucose molecule into two 3-carbon molecules of pyruvic acid.

Stages of Glycolysis

• 1st Stage: Energy is supplied by 2 ATP to phosphorylate glucose and fructose, resulting in two molecules of glyceraldehyde-3-phosphate (3C).
• 2nd Stage: Oxidation yields energy and reducing power. Glyceraldehyde-3-phosphate is oxidized to form one molecule of NADH and one ATP.
• 3rd Stage: Pyruvic acid is formed, yielding an additional ATP.

Glycolysis Balance

• Consumption: Glucose and 2 ATP.
• Products: 2 molecules of pyruvic acid, 4 ATP, and 2 NADH.
• Net Balance: 2 ATP and 2 NADH.

Chemical Equation:
glucose + 2 P_i + 2 ADP + 2 NAD⁺ -> 2 Pyruvic Acid + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

Krebs Cycle

The Krebs cycle is part of cellular respiration occurring in aerobic conditions, where energy is extracted from pyruvic acid derived from proteins, fatty acids, or carbohydrates.

• Location: Mitochondrial matrix.
• Function: Forms ATP and reduced substances (NADH and FADH₂) through oxidation to generate energy.

Initial Stage: Oxidative Decarboxylation of Pyruvate

Pyruvic acid produced in the cytosol enters the mitochondrial matrix via specific transporters. This reaction is catalyzed by the pyruvate dehydrogenase multienzyme complex:

• Loss of a carboxyl group: Transformed into CO₂.
• Oxidation: The energy released is trapped in the formation of acetyl-CoA, releasing a molecule of NADH.

Krebs Cycle Reactions

A cyclic set of reactions, each involving a specific enzyme, producing the complete oxidation of acetyl groups to CO₂. Its primary functions include:

• Collection of reducing power: NADH and FADH₂.
• Collection of metabolic precursors.
• Obtaining energy: In the form of GTP.