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Cellular_Respiration-chapter_6

How Cells Harvest Chemical Energy

Chapter Overview

  • Introduces the processes of photosynthesis and cellular respiration as fundamental energy-harvesting mechanisms for life.

Life Requires Energy

  • Photosynthesis:

    • Formula: Sunlight + CO₂ + H₂O → Glucose + O₂

  • Cellular Respiration:

    • Consumes O₂, breaking down organic molecules into CO₂ + H₂O, releasing ATP.

    • Almost all eukaryotic cells rely on cellular respiration for energy.

  • True or False:

    • Plant cells perform photosynthesis and animal cells perform cellular respiration.

  • ATP generated from cellular respiration powers various cellular activities.

  • Ecosystem interdependence: Photosynthesis and cellular respiration are crucial for energy flow in ecosystems.

Cellular Respiration: Harvesting of Energy

Breathing and Respiration

  • Breathing involves the exchange of CO₂ and O₂.

  • In cellular respiration, the O₂ obtained through breathing is used by cells to breakdown fuels, releasing CO₂ as a waste product.

    • O₂ moves from lungs to bloodstream, then to muscles.

Types of Respiration

  • Anaerobic Respiration: Occurs in the absence of oxygen (e.g., deep water, muddy sediments).

  • Aerobic Respiration: Occurs in the presence of oxygen.

Process of Cellular Respiration

  • Main goal: Transfer energy from glucose to form ATP.

  • Combines reactions that lead to energy extraction from carbohydrates.

  • Aerobic Respiration Equation:

    • C₆H₁₂O₆ (Glucose) + 6 O₂ → 6 CO₂ + 6 H₂O + ATP + Heat

Electron Transfer in Respiration

  • Redox Reactions: Transfer of electrons between molecules.

    • Oxidation: Loss of electrons (and hydrogen).

    • Reduction: Gain of electrons (and hydrogen).

    • Overall reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O with energy release.

Electron Transport Chain

  • Function: Series of molecules that transfer electrons and release energy.

  • Uses this energy to synthesize ATP through chemiosmosis.

  • Key components: NADH, NAD+, H+, O₂ (eventually reduced to water).

Coenzyme NAD+

  • Function: Organic molecule derived from vitamin niacin.

  • Accepts electrons during redox reactions, becoming NADH.

    • NAD⁺ + 2H → NADH + H⁺

  • Plays a vital role in cellular oxidation and energy transfer.

Stages of Aerobic Respiration

  1. Glycolysis: Breaks down glucose into pyruvate.

  2. Krebs Cycle (Citric Acid Cycle): Processes acetyl CoA derived from pyruvate, generating NADH and ATP.

  3. Oxidative Phosphorylation: Involves the electron transport chain and chemiosmosis; most ATP is produced here.

Glycolysis

  • Overview:

    • Series of reactions occurring in the cytosol that converts glucose to 2 pyruvate (3-carbon molecules).

    • ATP is formed by substrate-level phosphorylation.

  • Energy Investment Phase:

    • Initial use of 2 ATP to activate glucose.

  • ATP-Generating Steps:

    • Redox reactions convert G3P into pyruvate while generating NADH.

    • ATP produced in substrate-level phosphorylation.

  • Total Outputs from 1 molecule of glucose:

    • 2 NADH, 4 ATP (net gain = 2 ATP), 2 pyruvate.

Pyruvate Processing

  • Occurs in mitochondria:

    • Pyruvate oxidized to yield CO₂, ATP, and reduced coenzymes.

    • Key reactions: Pyruvate → Acetyl CoA, CO₂ released, NAD+ reduced to NADH.

Citric Acid Cycle

  • Steps:

    1. Acetyl CoA combines with oxaloacetate to form citrate.

    2. Redox reactions generate NADH and ATP; CO₂ is released.

    3. Cycle regenerates oxaloacetate to continue the Krebs Cycle.

  • Output Counts:

    • From each cycle: Reduced coenzymes (NADH, FADH₂) and ATP.

Totals and Summary of Cellular Respiration

  • From Glycolysis: 2 pyruvate, 2 ATP, 2 NADH.

  • From Pyruvate Processing and Citric Acid Cycle: 8 NADH, 2 FADH₂, 2 ATP, and release of CO₂.

Oxidative Phosphorylation

  • Process occurs in the inner mitochondrial membrane.

  • Involves the use of NADH and FADH₂ which donate electrons to the electron transport chain, driving ATP synthesis through chemiosmosis.

Overview of Cellular Respiration Stages

  • Stage 1: Glycolysis (cytosol) breaks glucose into pyruvate.

  • Stage 2: Pyruvate oxidation and the citric acid cycle (mitochondria) process pyruvate and produce electrons.

  • Stage 3: Oxidative phosphorylation (inner mitochondrial membrane) where ATP is synthesized and oxygen is used to form water.

Anaerobic Respiration: Fermentation

  • Type: Does not require oxygen and begins with glycolysis.

  • Types of Fermentation:

    • Alcoholic Fermentation: Converts pyruvate to ethanol, CO₂ is released.

    • Lactic Acid Fermentation: Converts pyruvate to lactate; occurs in muscle cells when ATP demand exceeds O₂ supply.

Obligate and Facultative Anaerobes

  • Obligate Anaerobes: Require anaerobic conditions; oxygen is toxic (e.g., certain prokaryotes).

  • Facultative Anaerobes: Can switch between fermentation and oxidative phosphorylation based on oxygen availability.

Alternative Energy Sources

  • Include complex carbohydrates, fats, and proteins which can be metabolized through glycolysis or Krebs Cycle.

  • Interaction with coenzymes leads to energy reduction and ATP synthesis.