Learning Objectives
- Understand the process of cellular respiration in plants
- Learn glycolysis, Krebs cycle, and electron transport chain
- Differentiate between aerobic and anaerobic respiration
- Calculate the energy yield from one glucose molecule
- Understand the concept of respiratory quotient
Key Concepts
Cellular Respiration Overview
The process of breaking down organic molecules to release energy (ATP). Overall equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP). Three main stages: Glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), Electron Transport Chain/Oxidative Phosphorylation (inner mitochondrial membrane).
Glycolysis (Embden-Meyerhof-Parnas Pathway)
Occurs in cytoplasm. Does not require oxygen — common to both aerobic and anaerobic respiration. Glucose (6C) → 2 Pyruvate (3C). Net gain: 2 ATP (4 produced, 2 consumed) and 2 NADH. Key enzymes: hexokinase (step 1), phosphofructokinase (rate-limiting enzyme), pyruvate kinase. Glucose is first phosphorylated using 2 ATP, then split into 2 triose phosphates (G3P), which are oxidized to pyruvate.
Fermentation (Anaerobic Respiration)
In absence of O2, pyruvate is converted to: Ethanol + CO2 (alcoholic fermentation — yeast, by enzyme pyruvate decarboxylase and alcohol dehydrogenase) or Lactic acid (lactic acid fermentation — muscles during vigorous exercise, by lactate dehydrogenase). Fermentation produces only 2 ATP per glucose (very inefficient). Incomplete oxidation of glucose.
Aerobic Respiration
Pyruvate Oxidation (Link Reaction): In mitochondrial matrix. Pyruvate (3C) → Acetyl CoA (2C) + CO2 + NADH. Enzyme: pyruvate dehydrogenase complex. This is an irreversible step.
Krebs Cycle (TCA/Citric Acid Cycle): In mitochondrial matrix. Discovered by Hans Krebs. Acetyl CoA (2C) + OAA (4C) → Citrate (6C), which is progressively oxidized back to OAA. Per turn: 3 NADH, 1 FADH2, 1 GTP (equivalent to ATP), 2 CO2. Per glucose (2 turns): 6 NADH, 2 FADH2, 2 GTP, 4 CO2. Krebs cycle is amphibolic (both catabolic and anabolic).
Electron Transport Chain (ETC) and Oxidative Phosphorylation: Inner mitochondrial membrane. Electron carriers: Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc1), Complex IV (cytochrome c oxidase). Electrons pass through complexes, releasing energy to pump H+ ions from matrix to intermembrane space. O2 is the final electron acceptor, forming H2O. Proton gradient drives ATP synthesis through ATP synthase (Complex V) — chemiosmotic hypothesis by Peter Mitchell.
Energy Yield
Per glucose molecule (aerobic): Glycolysis: 2 ATP + 2 NADH. Pyruvate oxidation: 2 NADH. Krebs cycle: 2 GTP + 6 NADH + 2 FADH2. Total NADH = 10 (each → ~2.5 ATP in ETC) = 25 ATP. Total FADH2 = 2 (each → ~1.5 ATP) = 3 ATP. Total GTP = 2. Direct ATP = 2. Grand total: ~30-32 ATP (theoretical maximum 36-38 ATP).
Respiratory Quotient (RQ)
RQ = CO2 evolved / O2 consumed. Carbohydrates: RQ = 1.0. Fats: RQ less than 1 (~0.7). Proteins: RQ ~0.8. Organic acids: RQ greater than 1. Anaerobic respiration: RQ = infinity (no O2 consumed).
Summary
Respiration involves glycolysis (cytoplasm), Krebs cycle (matrix), and ETC (inner membrane). Glycolysis is common to aerobic and anaerobic pathways. Fermentation yields only 2 ATP. Aerobic respiration yields 30-32 ATP per glucose. The respiratory quotient indicates the substrate being respired.
Important Terms
- Glycolysis: Breakdown of glucose to pyruvate in cytoplasm (Embden-Meyerhof pathway)
- Krebs Cycle: Cyclic pathway oxidizing acetyl CoA in mitochondrial matrix
- Oxidative phosphorylation: ATP synthesis coupled to electron transport in ETC
- Chemiosmotic hypothesis: ATP synthesis driven by proton gradient (Peter Mitchell)
- Fermentation: Anaerobic breakdown of pyruvate to ethanol or lactic acid
- Respiratory Quotient: Ratio of CO2 released to O2 consumed
- Amphibolic pathway: Pathway with both anabolic and catabolic functions (Krebs cycle)
- Cytochrome: Iron-containing electron carrier proteins in ETC
Quick Revision
- Glycolysis: cytoplasm, 2 ATP + 2 NADH (net), no O2 needed
- Krebs cycle: matrix, per turn — 3 NADH, 1 FADH2, 1 GTP, 2 CO2
- ETC: inner membrane; O2 = final electron acceptor
- 1 NADH → ~2.5 ATP; 1 FADH2 → ~1.5 ATP
- Total ATP per glucose (aerobic): ~30-32
- Fermentation: only 2 ATP; anaerobic
- RQ: carbohydrates = 1, fats < 1, organic acids > 1
- Krebs cycle is amphibolic (catabolic + anabolic)