Cellular Respiration: How Cells Harvest Chemical Energy

Chapter 6: How Cells Harvest Chemical Energy

Introduction to Cellular Respiration

Cellular respiration is a fundamental process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the universal energy currency of the cell. Oxygen acts as a key reactant in this process, and the breakdown of food molecules also generates heat.

• Cellular respiration is the process that breaks down sugars and other food molecules to generate ATP and heat.

• Brown fat cells have a unique pathway that generates heat instead of ATP, serving a thermogenic function.

• This chapter explores the stages of cellular respiration and how cells produce ATP in the presence of oxygen.

Cellular Respiration: Aerobic Harvesting of Energy

Cellular respiration is an aerobic process, meaning it requires oxygen to efficiently harvest energy from organic molecules.

• Most organisms rely on aerobic respiration to meet their energy needs.

• Energy is harvested in a controlled manner, allowing cells to capture usable energy as ATP.

Photosynthesis and Cellular Respiration: Energy for Life

Photosynthesis and cellular respiration are interconnected processes that sustain life by cycling energy and matter through ecosystems.

• Photosynthesis uses sunlight to convert carbon dioxide and water into organic molecules and oxygen.

• Cellular respiration consumes oxygen to break down organic molecules, releasing carbon dioxide, water, and capturing energy as ATP.

• Energy flow in ecosystems begins with sunlight and is transformed by photosynthesis and cellular respiration.

Key Equations

• Photosynthesis: $6CO_2 + 6H_2O + ext{light energy} ightarrow C_6H_{12}O_6 + 6O_2$

• Cellular Respiration: $C_6H_{12}O_6 + 6O_2 ightarrow 6CO_2 + 6H_2O + ext{ATP}$

Breathing and Cellular Respiration

Breathing supplies oxygen for cellular respiration and removes carbon dioxide, linking respiratory and metabolic processes.

• Respiration refers to the exchange of gases: oxygen intake and carbon dioxide release.

• Oxygen from the environment is used in cellular respiration, and carbon dioxide is expelled as a waste product.

• Breathing and cellular respiration are closely related but distinct processes.

Energy Transfer and ATP Production

Cellular respiration is an exergonic process, meaning it releases energy. This energy is captured in ATP molecules, which power cellular activities.

• About 34% of the energy in glucose is captured as ATP; the rest is lost as heat.

• ATP is required for both body maintenance and voluntary activities.

• Maintaining energy balance is essential for health.

Electron Transfer in Cellular Respiration

Cells extract energy from fuel molecules through the transfer of electrons in redox reactions.

• Electrons are removed from fuel molecules (oxidation) and transferred to NAD+ (reduction), forming NADH.

• NADH passes electrons to the electron transport chain, where energy is released as electrons "fall" to oxygen.

• Oxygen's high electronegativity makes it an effective final electron acceptor.

Stages of Cellular Respiration

Cellular respiration occurs in three main stages, each with distinct roles in energy extraction and ATP synthesis.

• Stage 1: Glycolysis

  • Occurs in the cytosol.

  • Breaks down glucose into two molecules of pyruvate.

  • Net products: 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule.

• Stage 2: Pyruvate Oxidation and Citric Acid Cycle

  • Occurs in mitochondria.

  • Completes the breakdown of glucose to carbon dioxide and water.

  • Supplies electrons to the next stage.

• Stage 3: Oxidative Phosphorylation

  • Involves electron transport and chemiosmosis.

  • NADH and FADH2 shuttle electrons to the electron transport chain in the inner mitochondrial membrane.

  • Most ATP is produced in this stage.

  • Electrons are finally passed to oxygen, forming water.

Glycolysis: Chemical Energy Harvest

Glycolysis is the first stage of cellular respiration, where glucose is oxidized to pyruvate, yielding ATP and NADH.

• Glucose is primed with ATP and split into two three-carbon intermediates.

• These intermediates are oxidized to form pyruvate.

• ATP is produced by substrate-level phosphorylation, where a phosphate group is transferred from an organic molecule to ADP.

• Net products per glucose: 2 ATP, 2 NADH, 2 pyruvate.

Glycolysis: Energy Investment and Payoff Phases

Glycolysis consists of two phases: energy investment and energy payoff.

• Energy Investment Phase:

  • Consumes 2 ATP to add phosphate groups to glucose.

  • Glucose is split into two three-carbon sugars.

• Energy Payoff Phase:

  • Produces 4 ATP and 2 NADH.

  • Results in a net gain of 2 ATP per glucose molecule.

Summary Table: Stages of Cellular Respiration

Stage	Location	Main Products
Glycolysis	Cytosol	2 ATP, 2 NADH, 2 Pyruvate
Pyruvate Oxidation & Citric Acid Cycle	Mitochondria	CO2, NADH, FADH2, ATP
Oxidative Phosphorylation	Inner Mitochondrial Membrane	ATP, H2O

Additional info: The notes above expand on the brief points in the slides, providing definitions, context, and examples for key concepts in cellular respiration.