Gaseous Exchange and Respiration

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your WAEC exam.

Key Definitions:

• Gaseous Exchange: Diffusion of gases (O₂ and CO₂) across a respiratory surface
• Respiration: Release of energy from glucose through oxidation (with or without oxygen)
• Breathing: Physical movement of air in and out of lungs

Equation - Aerobic Respiration: $$\text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP (energy)}$$

Equation - Anaerobic Respiration (Plants): $$\text{C}6\text{H}{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + \text{ATP}$$

Equation - Anaerobic Respiration (Animals): $$\text{C}6\text{H}{12}\text{O}_6 \rightarrow 2\text{Lactic acid} + \text{ATP}$$

Gas Diffusion Principles:

• Gases diffuse from HIGH to LOW concentration
• Rate ∝ Surface area × (Difference in concentration)/Thickness
• O₂ diffuses into blood; CO₂ diffuses out

Features of Efficient Respiratory Surfaces:

1. Thin (for short diffusion path)
2. Large surface area
3. Moist (gases dissolve before diffusing)
4. Good blood supply (maintains concentration gradient)
5. Ventilated (fresh air replaces used air)

⚡ WAEC Tip: In humans, alveoli provide all these features. The alveoli are surrounded by capillary networks, giving large surface area (~75m² total) with very thin walls (~0.5μm).

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Human Respiratory System:

Structure	Function
Nasal cavity	Filters, warms, moistens air
Pharynx	Common passage for air and food
Larynx	Contains vocal cords
Trachea	Windpipe, air passage to bronchi
Bronchi	Primary airways to each lung
Bronchioles	Smaller airways in lungs
Alveoli	Gas exchange site
Diaphragm	Muscle for breathing movement

Mechanism of Breathing:

Inhalation (Inspiration):

1. Diaphragm contracts and flattens
2. External intercostal muscles contract
3. Rib cage moves upward and outward
4. Thoracic cavity volume increases
5. Intrapulmonary pressure decreases below atmospheric
6. Air rushes into lungs

Exhalation (Expiration):

1. Diaphragm relaxes and domes upward
2. External intercostal muscles relax
3. Rib cage moves downward and inward
4. Thoracic cavity volume decreases
5. Intrapulmonary pressure increases above atmospheric
6. Air rushes out of lungs

Forced expiration (e.g., coughing): Internal intercostal muscles contract to pull ribs down more forcefully.

Gas Exchange in Alveoli:

Gas	Alveolar Air	Blood entering lungs	Blood leaving lungs
O₂	High (~104 mmHg)	Low (~40 mmHg)	High (~100 mmHg)
CO₂	Low (~40 mmHg)	High (~46 mmHg)	Low (~40 mmHg)

O₂ diffuses from alveoli into blood. CO₂ diffuses from blood into alveoli.

Transport of Gases:

Oxygen Transport:

• ~97% bound to haemoglobin as oxyhaemoglobin (HbO₂)
• ~1% dissolved in plasma
• Each Hb can bind 4 O₂ molecules
• Hb + 4O₂ → Hb(OC)₄

Carbon Dioxide Transport:

• ~70% as bicarbonate ions (HCO₃⁻) in plasma
• ~23% bound to haemoglobin as carbaminohaemoglobin
• ~7% dissolved in plasma

Bicarbonate formation: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

⚡ Common Mistake: Confusing breathing with respiration. Breathing is mechanical (movement of air), respiration is chemical (energy release in cells). You can breathe but not respire if cells die.

Differences Between Aerobic and Anaerobic Respiration:

Feature	Aerobic	Anaerobic
Oxygen	Required	Not required
Products	CO₂ + H₂O	Ethanol/lactic acid + CO₂
Energy yield	38 ATP per glucose	2 ATP per glucose
Location	Cytoplasm + mitochondria	Cytoplasm only
Efficiency	High	Low
By-products	Non-toxic	Toxic (lactic acid causes fatigue)

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for serious exam preparation.

Respiratory Surfaces in Different Organisms:

Organism	Respiratory Surface
Amoeba	Plasma membrane (entire cell surface)
Hydra	Body surface
Earthworm	Moist skin (capillary network)
Insects	Tracheal system (internal tubes)
Fish	Gills (water flow opposite to blood flow = countercurrent)
Mammals	Lungs with alveoli
Plants	Stomata (mainly), lenticels

Gills - Countercurrent Flow:

In fish gills:

• Water flows in one direction; blood flows in opposite direction
• This maintains maximum concentration gradient throughout
• ~80-90% of oxygen in water is extracted (vs ~50% in parallel flow)

Insect Tracheal System:

• Air enters through spiracles
• Tracheae branch into tracheoles
• Tracheoles deliver air directly to cells
• No respiratory pigment needed
• Gas exchange by diffusion only

Plant Gaseous Exchange:

• Stomata: pores mainly on lower leaf surface
• Guard cells control opening/closing
• Opening = K⁺ ions pumped in, water follows by osmosis
• Closing = K⁺ ions pumped out, water leaves by osmosis
• Light stimulates stomatal opening (blue light activates H⁺ pumps)

Condition	Stomata Open/Closed
Light	Open
Dark	Closed
CO₂ concentration low	Open
CO₂ concentration high	Closed
Water stress	Closed

Lenticels allow gaseous exchange in roots, stems, bark.

Factors Affecting Respiratory Quotient (RQ):

$$RQ = \frac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}}$$

Substrate	RQ	Reason
Carbohydrate	1.0	Equal CO₂ and O₂ in equation
Fat	~0.7	Fat is more reduced (less O₂ in structure)
Protein	~0.8	Variable, complex composition

Effect of Exercise on Breathing:

Factor	Resting	During Exercise
Breathing rate	~15/min	~40-50/min
Tidal volume	~500 cm³	~2000-3000 cm³
Minute ventilation	~7.5 L/min	~80-100 L/min
O₂ consumption	~250 mL/min	~4000 mL/min

Recovery After Exercise:

• Lactic acid is converted back to glucose in the liver (Cori cycle)
• Oxygen debt is repaid
• This explains why breathing rate remains elevated after exercise stops

Disorders of the Respiratory System:

Disorder	Cause	Effect
Asthma	Allergic reaction, bronchoconstriction	Wheezing, difficulty breathing
Emphysema	Smoking, alveolar wall breakdown	Reduced surface area for gas exchange
Bronchitis	Inflammation of bronchi	Mucus accumulation, coughing
Pneumonia	Bacterial/viral infection	Fluid in alveoli
Tuberculosis	Mycobacterium tuberculosis	Lung damage, infectious

⚡ WAEC Previous Year Pattern:

Year	Question	Concept
2023	Mechanism of breathing	Diaphragm and intercostal muscles
2022	Gaseous exchange in alveoli	Diffusion gradient
2021	Difference between aerobic and anaerobic	Products and energy yield

Experimental Questions:

To investigate CO₂ production:

1. Set up respirometer with germinating seeds
2. Potassium hydroxide (KOH) absorbs CO₂
3. Manometer measures oxygen consumption
4. Blue-brown solution in capillary tube shows gas change

Note: KOH absorbs CO₂, so any volume change = oxygen consumed only.

⚡ Exam Strategy: Draw a labelled diagram of the human respiratory system. Include all structures and indicate direction of air flow. For gas exchange questions, always mention concentration gradients.

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