Cellular Respiration and Photosynthesis: Energy Processes
Cellular Respiration: Stages and Significance
Aerobic respiration, mediated by the aerobic cell, covers the transformation of pyruvic acid into acetyl-CoA, the Krebs cycle, the electron transport chain, and oxidative phosphorylation. Pyruvic acid from glycolysis is completely oxidized to CO2 and water, but little use is made of O2 as the final electron acceptor.
Stages of Aerobic Respiration
- Transformation of pyruvic acid into acetyl-CoA: This is an oxidative decarboxylation catalyzed by a multienzyme complex called pyruvate dehydrogenase and occurs in the mitochondrial matrix.
- Krebs Cycle: A series of reactions in which organic acids are developed. The Krebs cycle develops in the mitochondrial matrix where the necessary enzymes are found.
- Electron Transport Chain: Comprises a series of oxidoreductase enzymes. The electrons collected in the catabolic phases pass to the final acceptor, which is O2. This occurs in the mitochondrial cristae membrane.
- Oxidative Phosphorylation: Consists of ATP production in the mitochondria using the energy released in the electron transport process. ATP is synthesized by the action of the ATP synthase enzyme linked to the inner mitochondrial membrane.
Aerobic Respiration
Aerobic respiration is a process of total glucose oxidation that occurs in the mitochondria. The resulting pyruvic acid from glycolysis is oxidized by decarboxylation to form CO2. Reduced nucleotides (NADH and FADH2) are transferred to the final acceptor, O2, through the action of the electron transport chain. Additionally, oxidative phosphorylation produces ATP. The final process yields 36 or 38 ATP, depending on the cell type.
Glycolysis
Glycolysis is a set of anaerobic reactions that occur in the cytosol of the cell. Glucose (6C) is degraded, yielding two molecules of pyruvate (3C). Glycolysis is utilized by almost all cells to obtain energy from glucose. It is a primitive pathway that occurs in both prokaryotic and eukaryotic cells. The end result is two molecules of pyruvate, 2 ATP, 2 NADH, and 2 H+.
Fermentation
Fermentation is a process where the electron transport chain does not interfere. The final acceptor of protons is always an organic compound, and an organic compound is always among its products. It is an anaerobic process because O2 is not involved. Its energetic yield is very low because only 2 ATP are obtained by substrate-level phosphorylation. It is a process found in organisms such as bacteria.
Types of Fermentation
- Alcoholic Fermentation: An anaerobic pathway where the final acceptor is a simple organic molecule that transforms into another reduced organic molecule. This pathway originates ethanol from pyruvic acid produced during glycolysis, catalyzed by alcohol dehydrogenase.
- Lactic Acid Fermentation: An anaerobic pathway where the final acceptor is a simple organic molecule that transforms into another reduced organic molecule. This pathway originates lactic acid from pyruvic acid produced during glycolysis.
Fermentation processes are partial; the final products still contain high energy in their molecules, resulting in low energetic yield.
Photosynthesis
A photosystem is a structural unit of the thylakoid membrane involved in the production of solar energy uptake and high-energy electron release. A photosystem is composed of an antenna complex and a reaction center, along with a donor and acceptor of electrons. The antenna complex is formed by belts of pigment molecules such as chlorophyll, xanthophylls, and carotenoids. These are responsible for absorbing light, and the antenna traps photons. When a molecule is excited, it captures a photon and transfers the resonance energy to the reaction center.
Photophosphorylation
Photophosphorylation is ATP synthesis due to light, generating electron transport across the thylakoid membrane. There are two types:
- Cyclic Photophosphorylation: Describes a cyclical movement aimed at ATP synthesis. In this case, only photosystem I (PSI) is involved, and water photolysis does not occur, O2 is not released, and reduced coenzymes (NADPH + H+) are not generated.
- Acyclic Photophosphorylation: Characterized by the participation of both photosystems. It produces water photolysis, ATP, O2 release, and reduced coenzymes required for ATP synthase during the dark phase.
Comparison of Oxidative Phosphorylation and Photophosphorylation
During the light phase of photosynthesis and cellular respiration, energetic jumps produce sufficient energy release to make ATP. Oxidative phosphorylation refers to ATP production in the mitochondria by the energy released during the electron transport process. ATP is synthesized through the ATP synthase enzyme linked to the inner mitochondrial membrane. Photophosphorylation refers to ATP production in the chloroplast through the ATP synthase linked to the thylakoid membrane.