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Cellular Respiration: How Cells Harvest Chemical Energy

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Cellular Respiration: Aerobic Harvesting of Energy

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce adenosine triphosphate (ATP). This process is essential for powering cellular activities in almost all living organisms.

  • Definition: Cellular respiration is a series of metabolic pathways that break down sugars to generate ATP, the energy currency of the cell.

  • Importance: Provides energy for all cellular processes, from muscle contraction to active transport.

  • Relationship to Photosynthesis: Energy in most ecosystems originates from sunlight, captured by photosynthesis and released by cellular respiration.

  • Example: Brown fat in mammals generates heat through a 'short circuit' in cellular respiration, producing heat instead of ATP.

Overview of Cellular Respiration

  • General Equation:

  • Stages: Cellular respiration occurs in three main stages: Glycolysis, Pyruvate Oxidation & Citric Acid Cycle, and Oxidative Phosphorylation.

  • Location: Glycolysis occurs in the cytoplasm; the other stages occur in the mitochondria of eukaryotic cells.

  • ATP Yield: Up to 32 ATP molecules can be produced per glucose molecule (about 34% of the energy stored in glucose).

Key Concepts in Cellular Respiration

  • Redox Reactions: Cellular respiration involves oxidation (loss of electrons) and reduction (gain of electrons) reactions. Electrons are transferred from glucose to oxygen, releasing energy.

  • Electron Carriers: Molecules like NAD+ and FAD accept electrons during oxidation and become NADH and FADH2, which carry electrons to the electron transport chain.

  • ATP Formation: ATP can be formed by substrate-level phosphorylation (direct transfer of a phosphate group) or oxidative phosphorylation (using energy from the electron transport chain).

Stages of Cellular Respiration

Stage I: Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down one molecule of glucose into two molecules of pyruvate, generating a small amount of ATP and NADH.

  • Location: Cytoplasm

  • Inputs: 1 Glucose, 2 NAD+, 2 ATP

  • Outputs: 2 Pyruvate, 2 NADH, 4 ATP (net gain: 2 ATP)

  • Phases:

    • Energy Investment Phase: 2 ATP are used to phosphorylate glucose intermediates.

    • Energy Payoff Phase: 4 ATP and 2 NADH are produced.

  • ATP Formation: By substrate-level phosphorylation.

Stage II: Pyruvate Oxidation and the Citric Acid Cycle (Krebs Cycle)

After glycolysis, pyruvate is transported into the mitochondria, where it is oxidized to acetyl-CoA. Acetyl-CoA enters the citric acid cycle, which completes the breakdown of glucose derivatives, releasing CO2 and transferring electrons to NAD+ and FAD.

  • Pyruvate Oxidation: Each pyruvate is converted to acetyl-CoA, producing 1 NADH and releasing 1 CO2 per pyruvate.

  • Citric Acid Cycle: Each acetyl-CoA enters the cycle, generating 3 NADH, 1 FADH2, 1 ATP (by substrate-level phosphorylation), and 2 CO2 per turn.

  • Location: Mitochondrial matrix

  • Key Products (per glucose): 6 NADH, 2 FADH2, 2 ATP, 4 CO2

Stage III: Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis)

Oxidative phosphorylation is the final stage, where most ATP is produced. Electrons from NADH and FADH2 are transferred through the electron transport chain, creating a proton gradient that drives ATP synthesis via chemiosmosis.

  • Electron Transport Chain (ETC): Series of protein complexes in the inner mitochondrial membrane that transfer electrons to oxygen, the final electron acceptor, forming water.

  • Proton Gradient: Electron transfer pumps protons (H+) into the intermembrane space, creating an electrochemical gradient.

  • ATP Synthase: Protons flow back into the matrix through ATP synthase, driving the phosphorylation of ADP to ATP.

  • ATP Yield: About 28 ATP per glucose from oxidative phosphorylation.

Summary Table: Stages of Cellular Respiration

Stage

Main Location

Inputs

Outputs

ATP Produced

Glycolysis

Cytoplasm

Glucose, 2 NAD+, 2 ATP

2 Pyruvate, 2 NADH, 4 ATP (net 2 ATP)

2

Pyruvate Oxidation

Mitochondrial Matrix

2 Pyruvate, 2 NAD+

2 Acetyl-CoA, 2 NADH, 2 CO2

0

Citric Acid Cycle

Mitochondrial Matrix

2 Acetyl-CoA, 6 NAD+, 2 FAD

4 CO2, 6 NADH, 2 FADH2, 2 ATP

2

Oxidative Phosphorylation

Inner Mitochondrial Membrane

10 NADH, 2 FADH2, O2

H2O, ATP

~28

Additional Key Points

  • Energy Use in the Body: ATP produced by cellular respiration powers all cellular and bodily activities, from movement to biosynthesis.

  • Sudden Energy Release: The controlled release of energy in cells contrasts with the explosive release seen in combustion (e.g., rocket launch).

  • Role of Oxygen: Oxygen is the final electron acceptor in the electron transport chain, essential for efficient ATP production.

Glossary of Key Terms

  • ATP (Adenosine Triphosphate): The primary energy carrier in cells.

  • Glycolysis: The metabolic pathway that breaks down glucose into pyruvate.

  • Citric Acid Cycle (Krebs Cycle): A series of reactions that generate electron carriers from acetyl-CoA.

  • Oxidative Phosphorylation: The process of ATP formation driven by the transfer of electrons through the electron transport chain and chemiosmosis.

  • NAD+/NADH: Electron carrier molecules involved in redox reactions.

  • Substrate-level Phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP.

  • Electron Transport Chain: A sequence of proteins that transfer electrons and pump protons to generate a proton gradient.

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