Aerobic respiration

Aerobic respiration is a fundamental biological process that releases energy from organic molecules, primarily glucose, using oxygen. It is an enzyme-controlled process that takes place in virtually all living cells, continuously providing them with a constant supply of energy.

The primary purpose of aerobic respiration is to synthesize adenosine triphosphate (ATP), which serves as the universal energy currency for cells. This ATP then fuels a wide array of biological processes and activities, including:

• Anabolic reactions: The synthesis of complex molecules from simpler ones, such as protein synthesis and DNA replication.

• Active transport: Moving substances across cell membranes against their concentration gradient.

• Movement: Such as muscle contraction in animals, and other cellular movements like cilia and flagella.

• Maintenance of body temperature: In mammals and birds.

• Cell division, growth, and tissue repair.

Aerobic respiration involves four main stages:

1. Glycolysis:

   • Occurs in the cytoplasm of the cell.

   • Involves the phosphorylation of glucose (using 2 ATP molecules) and its subsequent splitting into two 3-carbon molecules of pyruvate.

   • During the oxidation of triose phosphate to pyruvate, reduced NAD (NADH) is produced, and there is a net gain of 2 ATP molecules per glucose molecule via substrate-level phosphorylation.

2. Link Reaction:

   • If oxygen is available, pyruvate is actively transported into the mitochondrial matrix.

   • Pyruvate is oxidized to acetate, producing reduced NAD and releasing carbon dioxide (decarboxylation).

   • Acetate then combines with coenzyme A to form acetyl coenzyme A. This reaction occurs twice per glucose molecule.

3. Krebs Cycle (Citric Acid Cycle):

   • Takes place in the mitochondrial matrix. The cycle occurs twice for every glucose molecule.

   • Acetyl coenzyme A combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate).

   • Through a series of oxidation-reduction reactions, the cycle generates reduced NAD, reduced FAD, and a small amount of ATP via substrate-level phosphorylation. Carbon dioxide is produced and released as a waste product (decarboxylation).

4. Oxidative Phosphorylation:

   • This is the final stage and occurs on the inner mitochondrial membrane (cristae).

   • The energy carried by electrons from reduced NAD and reduced FAD is used to make ATP.

   • Electrons move down an electron transport chain, releasing energy. This energy is used to pump protons (H+) across the inner mitochondrial membrane, creating a proton gradient.

   • Protons then return to the matrix through ATP synthase (chemiosmosis), driving ATP synthesis.

   • Oxygen acts as the final electron acceptor, combining with protons and electrons to form water.

The overall aerobic respiration of one glucose molecule theoretically yields around 38 molecules of ATP, although in practice, it is closer to 30-32 ATP molecules. This is a significantly greater yield compared to anaerobic respiration, which only produces 2 ATP molecules per glucose.

While glucose is the main respiratory substrate, other complex organic molecules like lipids (fatty acids and glycerol) and proteins (amino acids) can also be broken down and used in aerobic respiration, entering the pathway at different stages (e.g., Krebs cycle). Lipids, with more hydrogen atoms per molecule, provide a greater energy value per unit mass than carbohydrates or proteins.

The rate of aerobic respiration can be measured using a respirometer, by observing the uptake of oxygen or release of carbon dioxide. The Respiratory Quotient (RQ), calculated as the ratio of carbon dioxide produced to oxygen consumed, can indicate the type of substrate being respired.