Ch9RespPhotoSyn

Life and Energy

• Cells require energy from external sources to perform work.

  • Work includes assembling polymers, membrane transport, movement, and reproduction.

  • Animals obtain energy by feeding on organisms (either animals or photosynthetic organisms).

Ecosystem Energy Flow

• Energy enters ecosystems as sunlight and exits as heat.

• Essential chemical elements are constantly recycled in ecosystems.

• Photosynthesis produces O₂ and organic molecules, which are utilized in cellular respiration.

• Cellular respiration generates ATP from the chemical energy stored in organic molecules.

Overview of Cellular Respiration

• Cellular respiration encompasses both aerobic and anaerobic respiration, although it typically refers to aerobic respiration.

• Main components:

  • Aerobic Respiration: Consumes O₂ and organic molecules, yielding ATP.

  • Anaerobic Respiration: Similar to aerobic, but uses other compounds instead of O₂.

• Key equation for cellular respiration:

  • Equation: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat).

Catabolic Pathways and Energy Release

• Catabolic pathways yield energy by oxidizing organic substrates (e.g., glucose).

• These pathways are central to cellular respiration and are characterized by the release of stored energy through the breakdown of complex molecules.

Redox Reactions in Cellular Respiration

• Redox Reactions: Involve the transfer of electrons, which releases energy stored in organic molecules.

• Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons.

  • The electron donor is termed the reducing agent, while the electron acceptor is called the oxidizing agent.

The Role of NAD+ in Energy Harvesting

• During cellular respiration, glucose is oxidized while O₂ is reduced.

• Electrons from organic compounds are usually first transferred to NAD+, acting as an oxidizing agent.

  • Each NADH generated represents stored energy to produce ATP.

Stages of Cellular Respiration

• 1. Glycolysis: Breaks down glucose into pyruvate.

  • Operates in the cytoplasm and can occur with or without O₂.

• 2. Citric Acid Cycle (Krebs Cycle): Completes glucose breakdown.

• 3. Oxidative Phosphorylation: Main process for ATP synthesis, utilizing the electron transport chain and chemiosmosis.

Oxidative Phosphorylation

• Major ATP generation mechanism derives from redox reactions.

• Glycolysis and the citric acid cycle produce NADH and FADH2 that donate electrons to the electron transport chain.

• The flow of electrons through the chain facilitates ATP production via oxidative phosphorylation.

Fermentation and Anaerobic Respiration

• Fermentation enables ATP production without O₂ via glycolysis, producing NAD+ from organic molecules such as pyruvate or acetaldehyde.

• Two primary types of fermentation include:

  • Alcohol Fermentation: Converts pyruvate to ethanol (used in brewing).

  • Lactic Acid Fermentation: Converts pyruvate to lactate (used in muscle cells during exertion).

Energy Yield of Cellular Respiration

• Typical yield is about 32 ATP molecules per glucose molecule oxidized.

• Energy from glucose is released gradually through multiple steps to increase efficiency and minimize energy lost as heat.

Regulation of Cellular Respiration

• Feedback inhibition is the main control mechanism.

• ATP concentration influences respiration rate: low ATP increases respiration; high ATP decreases it.

• Enzyme activity in the catabolic pathway is regulated to optimize energy production.