Cellular Respiration and Photosynthesis

Ecosystem Energy Flow

  • Light energy is captured through photosynthesis in chloroplasts, converting CO2 and H2O into organic molecules and O2. This process is crucial for the energy flow in ecosystems.
  • Cellular respiration occurs in mitochondria, breaking down organic molecules to produce ATP, which powers most cellular work and releases heat energy.

Cellular Respiration

  • Cellular respiration typically refers to aerobic respiration, summarized by the equation:
    C6H{12}O6 + 6 O2
    ightarrow 6 CO2 + 6 H2O + ext{Energy (ATP + heat)}

Catabolic Pathways

  • Key catabolic pathways include glycolysis, fermentation, aerobic respiration, and anaerobic respiration (using electron acceptors such as NO3-, SO4^2-, and S).
  • Glycolysis occurs in the cytosol and involves the investment of energy followed by energy release.

Redox Reactions

  • Redox reactions involve the transfer of electrons, where one molecule is oxidized, losing electrons, and another is reduced, gaining electrons. For example, methane (reducing agent) interacts with oxygen (oxidizing agent).
  • The transfer of electrons from donor to acceptor releases energy, which is utilized for cellular work.

Glycolysis Overview

  • Glycolysis comprises energy-requiring and energy-releasing steps:
    • Investment Phase:
    • Glucose (6 carbons) is converted into G3P (3 carbons) with the consumption of 2 ATP.
    • Payoff Phase:
    • Generates a total of 4 ATP (net gain of 2 ATP) and produces 2 NADH.

Citric Acid Cycle

  • The citric acid cycle occurs in the mitochondrial matrix where pyruvate is oxidized. Key products per glucose include:
    • 4 ATP, 6 CO2, 10 NADH, and 2 FADH2.
  • This cycle completely oxidizes glucose, removing all possible electrons.

Electron Transport Chain and Oxidative Phosphorylation

  • Located in the cristae of mitochondria, the electron transport chain facilitates electron transfer via redox reactions, ultimately using O2 as the final electron acceptor, forming H2O.
  • Chemiosmosis couples the electron transport to ATP synthesis, using the proton gradient to power ATP synthase, resulting in the production of 30-36 ATP per glucose.

Fermentation

  • In the absence of O2, glycolysis still occurs and couples with fermentation or anaerobic respiration. Two types of fermentation are:
    1. Lactate Fermentation: Occurs in bacteria and muscle tissue, producing lactate.
    2. Alcoholic Fermentation: Occurs in yeasts, producing ethanol and CO2.

Poisoning Mechanisms

  • Cyanide poisoning: It inhibits cytochrome c oxidase in the electron transport chain, disrupting ATP production. Some plants naturally contain cyanide without succumbing to its toxic effects, often due to alternative oxidase pathways that sidestep the inhibited complex.

Conclusion

  • Cellular respiration is a complex process that efficiently transforms energy from organic substrates into usable forms (ATP) while producing byproducts that are recycled in ecosystems. Understanding these pathways is critical for biology and ecology.