Biology · Class 9-10 · CBSE
Respiration: Aerobic, Anaerobic and Gas Exchange in Lungs
Learn cellular respiration pathways: breakdown of glucose (glycolysis, aerobic respiration in mitochondria vs anaerobic in yeast and muscle cells), ATP energy currency, human respiratory anatomy, and gas exchange in alveoli.
1Cellular Respiration and the Breakdown of Glucose
Respirationis a catabolic biochemical process where digested food (glucose) is oxidised inside living cells to release energy stored in high-energyATP (Adenosine Triphosphate)molecules. The first step,Glycolysis, occurs in the cytoplasm where 1 molecule of 6-carbon glucose is cleaved into two 3-carbonPyruvatemolecules.
Three Fates of Pyruvate in Living Organisms
| Pathway | Location | Presence of Oxygen? | End Products Formed | ATP Energy Yield |
|---|---|---|---|---|
| Aerobic Respiration | Mitochondria (all higher organisms) | Rich in Oxygen (O₂) | CO₂ + H₂O + Energy | 36 – 38 ATP per glucose (High yield) |
| Anaerobic: Alcoholic Fermentation | Cytoplasm of Yeast / Bacteria | Absence of Oxygen (Anoxic) | Ethanol (C₂H₅OH) + CO₂ + Energy | 2 ATP per glucose (Low yield) |
| Anaerobic: Lactic Acid Fermentation | Human skeletal muscle cells (vigorous sprint) | Lack / Deficiency of Oxygen | Lactic Acid (C₃H₆O₃) + Energy | 2 ATP per glucose (causes muscle cramps) |
Aerobic Respiration
- Location
- Mitochondria (all higher organisms)
- Presence of Oxygen?
- Rich in Oxygen (O₂)
- End Products Formed
- CO₂ + H₂O + Energy
- ATP Energy Yield
- 36 – 38 ATP per glucose (High yield)
Anaerobic: Alcoholic Fermentation
- Location
- Cytoplasm of Yeast / Bacteria
- Presence of Oxygen?
- Absence of Oxygen (Anoxic)
- End Products Formed
- Ethanol (C₂H₅OH) + CO₂ + Energy
- ATP Energy Yield
- 2 ATP per glucose (Low yield)
Anaerobic: Lactic Acid Fermentation
- Location
- Human skeletal muscle cells (vigorous sprint)
- Presence of Oxygen?
- Lack / Deficiency of Oxygen
- End Products Formed
- Lactic Acid (C₃H₆O₃) + Energy
- ATP Energy Yield
- 2 ATP per glucose (causes muscle cramps)
2Aerobic vs Anaerobic Respiration Comparison
Aerobic vs Anaerobic Respiration: Comprehensive Comparison
| Parameter | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen Requirement | Always requires atmospheric oxygen (O₂) | Occurs in the complete absence or deficiency of oxygen |
| Cellular Site | Cytoplasm (Glycolysis) + Mitochondria (Krebs cycle) | Occurs entirely within the Cytoplasm |
| Completeness of Oxidation | Complete breakdown into inorganic CO₂ and H₂O | Incomplete oxidation into organic ethanol or lactic acid |
| Energy Output | High: 38 ATP (2870 kJ) per mole of glucose | Low: 2 ATP (150 kJ) per mole of glucose |
| Toxicity of By-products | Non-toxic by-products (CO₂ and H₂O easily exhaled) | Ethanol is toxic to yeast; lactic acid causes muscle fatigue and cramps |
Oxygen Requirement
- Aerobic Respiration
- Always requires atmospheric oxygen (O₂)
- Anaerobic Respiration
- Occurs in the complete absence or deficiency of oxygen
Cellular Site
- Aerobic Respiration
- Cytoplasm (Glycolysis) + Mitochondria (Krebs cycle)
- Anaerobic Respiration
- Occurs entirely within the Cytoplasm
Completeness of Oxidation
- Aerobic Respiration
- Complete breakdown into inorganic CO₂ and H₂O
- Anaerobic Respiration
- Incomplete oxidation into organic ethanol or lactic acid
Energy Output
- Aerobic Respiration
- High: 38 ATP (2870 kJ) per mole of glucose
- Anaerobic Respiration
- Low: 2 ATP (150 kJ) per mole of glucose
Toxicity of By-products
- Aerobic Respiration
- Non-toxic by-products (CO₂ and H₂O easily exhaled)
- Anaerobic Respiration
- Ethanol is toxic to yeast; lactic acid causes muscle fatigue and cramps
3Human Respiratory System and Alveolar Gas Exchange
Pathway of Inhaled Air in the Human Respiratory Tract
- 1
1. Nostrils: Air is filtered by fine nasal hairs and moistened/warmed by mucus lining.
- 2
2. Pharynx & Larynx (Voice box) with Epiglottis preventing food entry during swallowing.
- 3
3. Trachea (Windpipe): Kept open by C-shaped rings of cartilage so air passage does not collapse.
- 4
4. Bronchi & Bronchioles: Trachea divides into two bronchi, branching into fine bronchioles.
- 5
5. Alveoli: Balloon-like thin-walled sacs surrounded by rich network of blood capillaries.
- 6
6. Gas Exchange: O₂ diffuses across alveolar wall into blood (binds to Hemoglobin); CO₂ diffuses from blood into alveoli to be exhaled.
4Exam Corner: Tips, Mistakes & Board Questions
High-Yield Board Revision Takeaways
- Alveoli provide massive surface area (~80 m², equivalent to a tennis court) for efficient gas diffusion between air and pulmonary capillaries.
- Hemoglobin has high affinity for oxygen; respiratory pigment is essential in humans because diffusion alone would take 3 years for oxygen to reach toes from lungs!
- Muscle cramps during vigorous sprinting occur due to accumulation of lactic acid produced by anaerobic respiration in muscle fibers.
Exam question · CBSE Class 10 Board
- A.Due to accumulation of alcohol in muscles
- B.Due to lack of oxygen causing pyruvate conversion into lactic acid in muscle cells
- C.Due to excess production of carbon dioxide
- D.Due to dehydration and loss of blood
- During sudden vigorous muscular activity, demand for oxygen exceeds supply by blood.
- Muscle cells switch to anaerobic respiration: Pyruvate (3C) is converted into Lactic Acid (3C) + energy.
- Accumulation of lactic acid in muscle tissue induces painful cramps.
- Correct answer is option B.
Answer: B. Due to lack of oxygen causing pyruvate conversion into lactic acid in muscle cells
Remember
- Glycolysisin cytoplasm splits glucose into pyruvate without oxygen.
- Aerobic respirationin mitochondria yields 36–38 ATP; anaerobic yields only 2 ATP.
- Lactic acid accumulationin skeletal muscle causes athletic cramps.
- Alveoliprovide an extensive, paper-thin, capillary-rich surface for gas diffusion.
Check yourself
Question 1
What keeps the human trachea (windpipe) from collapsing when there is less air inside it?