Respiration: aerobic and anaerobic
Respiration breaks down glucose to release energy as ATP. Aerobic respiration uses oxygen in three stages; anaerobic respiration gives ethanol or lactic acid and little energy.
Aerobic vs anaerobic respiration
Muscles respire anaerobically during hard exercise. Lactic acid builds up and causes muscle fatigue (tiredness and cramps).
Stages of aerobic respiration
The stages in detail
- 1★Glycolysis (glykos: sugar, lysis: splitting). One glucose splits into two pyruvic acid molecules in the cytoplasm. Net gain: 2 ATP and 2 NADH₂. It is the first stage of both aerobic and anaerobic respiration.
- 2Link step. Each pyruvic acid enters the mitochondrion, loses one CO₂ and becomes acetyl CoA (2-carbon).
- 3★Krebs cycle (citric acid cycle or TCA cycle), in the mitochondrial matrix. Acetyl CoA joins oxaloacetic acid (4C) to form citric acid (6C). Citric acid is broken down step by step back to oxaloacetic acid, releasing CO₂, NADH₂ and FADH₂.
- 4★Electron transport chain, on the inner mitochondrial membrane. NADH₂ and FADH₂ pass on electrons, and the energy released makes ATP from ADP (oxidative phosphorylation). Oxygen is the final electron acceptor and becomes water.
★ key step
Kinds of anaerobic respiration
| Where | What happens | Seen in daily life |
|---|---|---|
| Yeast (alcoholic fermentation) | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy | Bread dough rises because yeast gives off CO₂ |
| Muscle cells (lactic acid fermentation) | C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + energy | Leg cramps after a long, fast run |
| Lactic acid bacteria | Milk sugar is turned into lactic acid | Milk sets into curd |
How to draw the flow chart of aerobic respiration
- 1Write Glucose at the top. Draw an arrow labelled glycolysis (cytoplasm) down to 2 pyruvic acid, and write '2 ATP, 2 NADH₂' beside it.
- 2Draw a box marked mitochondrion. Inside it, draw an arrow from pyruvic acid to acetyl CoA, with CO₂ leaving.
- 3Draw a circle for the Krebs cycle: acetyl CoA + oxaloacetic acid → citric acid, with arrows for CO₂, NADH₂ and FADH₂ coming out.
- 4Below it, draw a staircase for the electron transport chain, ending with O₂ → H₂O, and write ATP at each step.
Idli and dosa batter rising overnight, curd set at home and bread from a bakery all depend on anaerobic respiration by microbes. The burning and cramps you feel after a fast race come from lactic acid in your muscles.
2 marks: “What is glycolysis? Where does it occur?” The splitting of one glucose into two pyruvic acid molecules. It occurs in the cytoplasm and is the first stage of both kinds of respiration.
4 marks: “Give four differences between aerobic and anaerobic respiration.” Needs oxygen vs not; complete vs incomplete breakdown; CO₂ + H₂O vs ethanol + CO₂ or lactic acid; much energy vs little energy (2 ATP); cytoplasm and mitochondria vs cytoplasm only.
7 marks: “Explain the stages of aerobic respiration with a flow chart.” Definition and equation; flow chart; glycolysis (cytoplasm, 2 pyruvic acid, 2 ATP); link step; Krebs cycle (matrix, citric acid, CO₂); ETC (inner membrane, oxidative phosphorylation, O₂ → water, most ATP made here).
'Girls Keep Energy': Glycolysis (cytoplasm), Krebs (matrix), ETC (inner membrane). Anaerobic: 'Yeast makes Ethanol, Muscle makes Lactic acid'.