Rucete ✏ AP Biology In a Nutshell
7. Cellular Respiration
This chapter explores how cells break down glucose and other molecules to release energy, using a series of metabolic pathways. Key processes include glycolysis, the oxidation of pyruvate, the Krebs cycle, oxidative phosphorylation, and fermentation.
Overview
• Cellular respiration releases energy stored in organic molecules.
• Overall equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
• Main stages: 1. Glycolysis 2. Oxidation of Pyruvate 3. Krebs Cycle 4. Oxidative Phosphorylation 5. (If no oxygen) → Fermentation
• Aerobic respiration (with O₂) produces more ATP than anaerobic processes.
Glycolysis
• Occurs in cytosol of all cells—evolutionarily ancient and universal.
• Glucose (6C) is broken into 2 pyruvate (3C) molecules.
• Inputs: glucose, 2 NAD⁺, 2 ATP
• Outputs: 2 pyruvate, 2 NADH, 4 ATP (net gain = 2 ATP)
• NAD⁺ is reduced to NADH as glucose is oxidized.
Oxidation of Pyruvate
• Occurs in the mitochondria (matrix).
• Each pyruvate (3C) loses 1 carbon as CO₂ → becomes 2C acetyl group.
• NAD⁺ is reduced to NADH.
• Coenzyme A binds to form acetyl-CoA, which enters the Krebs cycle.
• Per glucose: 2 pyruvate → 2 acetyl-CoA + 2 NADH + 2 CO₂
Krebs Cycle (Citric Acid Cycle)
• Occurs in mitochondrial matrix.
• Each acetyl-CoA (2C) joins a 4C compound → forms 6C citrate → releases 2 CO₂ → returns to 4C.
• Per cycle turn: – 3 NAD⁺ → 3 NADH – 1 FAD⁺ → 1 FADH₂ – 1 ATP (by substrate-level phosphorylation)
• Per glucose: cycle turns twice → yields 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂
Oxidative Phosphorylation
• Occurs in the inner mitochondrial membrane.
• NADH and FADH₂ donate electrons to the electron transport chain (ETC).
• Electrons move through protein complexes in the ETC, releasing energy.
• This energy pumps H⁺ ions from the matrix into the intermembrane space → creates a proton gradient.
Chemiosmosis and ATP Synthase
• H⁺ ions flow back into the matrix through ATP synthase.
• This flow drives the phosphorylation of ADP → ATP.
• Final electron acceptor is O₂ → forms H₂O.
• This is oxidative phosphorylation: using redox reactions to power ATP production.
• Produces about 26–28 ATP per glucose molecule.
Total ATP Yield
• Glycolysis: 2 ATP (net), 2 NADH
• Pyruvate oxidation: 2 NADH
• Krebs cycle: 2 ATP, 6 NADH, 2 FADH₂
• Each NADH ≈ 2.5 ATP, each FADH₂ ≈ 1.5 ATP
• Total yield: about 30–32 ATP per glucose (depending on shuttle system used)
Fermentation
• Occurs in the cytosol when no oxygen is present.
• Regenerates NAD⁺ to allow glycolysis to continue producing ATP.
• Lactic acid fermentation (in animals): – Pyruvate is reduced to lactate, NADH is oxidized to NAD⁺.
• Alcohol fermentation (in yeast): – Pyruvate → acetaldehyde → ethanol + CO₂; NADH is oxidized to NAD⁺.
• Fermentation produces only 2 ATP per glucose (from glycolysis).
Other Fuels for Respiration
• Proteins: deaminated and converted into Krebs cycle intermediates.
• Fats: broken into glycerol and fatty acids; fatty acids undergo beta-oxidation to form acetyl-CoA.
• Fats yield more than twice the ATP of carbohydrates per gram.
In a Nutshell
Cellular respiration is the stepwise breakdown of glucose to produce ATP. Glycolysis, the Krebs cycle, and oxidative phosphorylation generate up to 32 ATP per glucose. In the absence of oxygen, fermentation allows ATP production to continue anaerobically. The process is tightly regulated and can use multiple fuel sources beyond glucose.