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.