PRELIMS
Human Respiratory System and Cellular Respiration
Respiration involves the exchange of gases ( and ) to release energy from food.
I. Respiratory Anatomy
- Pathway: Nasal passage Pharynx Larynx (Voice box) Trachea (Bronchi/Bronchioles) Lungs.
- Epiglottis: Thin door at the larynx entrance that prevents food from entering the windpipe.
- Pleural Membrane: Double-walled membrane surrounding the lungs.
II. Breathing Mechanism
- Inspiration: Air enters the lungs (low pressure).
- Expiration: Air leaves the lungs.
- Gas Composition:
- Nitrogen: Inhaled: 79% (No change). Exhaled: 79%
- Oxygen: Inhaled: 21%. Exhaled: 17%
- Carbon dioxide: Inhaled: 0.03%. Exhaled: 4%
- Note: ~400ml of water is lost daily via breathing.
III. Cellular Respiration
- Anaerobic Respiration: Absence of oxygen.
- In Animal Tissues: Produces Lactic acid and 2 ATP.
- In Yeast: Produces Ethanol and .
- Aerobic Respiration: Presence of oxygen. Complete oxidation of glucose.
- In Mitochondria: .
The Two Metabolic Steps
- Glycolysis (EMP Path): Occurs in Cytoplasm. Converts Glucose Pyruvic acid. Net gain: 2 ATP.
- Kreb's Cycle (Citric Acid Cycle): Occurs in Mitochondria. Discovered by Hans Krebs (1937). Net gain: 36 ATP.
- Total Energy: 38 ATP per Glucose molecule (classical NCERT figure; some modern biochemistry texts revise this down to ~30–32 ATP due to updated proton-pumping ratios — both figures appear in exam sources).
III(a). Lung Capacity Terms
- Tidal Volume: Volume of air inhaled/exhaled in normal breathing (~500 ml).
- Vital Capacity: Maximum air that can be exhaled after maximum inhalation (Tidal + Inspiratory Reserve + Expiratory Reserve).
- Residual Volume: Air remaining in lungs even after forceful exhalation (prevents lung collapse).
- Total Lung Capacity: Vital Capacity + Residual Volume.
IV. Gas Transport in Blood
- Oxygen: Transported by Haemoglobin ( ).
- Carbon Dioxide: 70% as Bicarbonates in plasma ( / ); 20% by ; 7% dissolved in plasma.
UPSC Relevance
The biology of respiration explains athletic performance, respiratory health, and metabolic science.
- ATP Tally: Understanding that Glycolysis (2 ATP) + Kreb's Cycle (36 ATP) leads to 38 ATP is a recurring Prelims fact.
- Acid-Base Balance: The role of transport as bicarbonates for maintaining blood pH.
- Respiratory Pathology: Linking anaerobic respiration to fatigue (Lactic acid) and the importance of the Epiglottis.
- Confused Pair — Aerobic vs Anaerobic Respiration: Aerobic needs , occurs in mitochondria, complete oxidation, high ATP yield; Anaerobic needs no , occurs in cytoplasm, incomplete oxidation, low ATP yield (2 ATP), and produces lactic acid (muscle) or ethanol+ (yeast fermentation).