Respiration and Gaseous Exchange

🟢 Lite — Quick Review (1h–1d)

Rapid summary of respiration and gaseous exchange for NABTEB biology.

Cellular Respiration is the process that releases energy from glucose inside cells: $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP (energy)}$$

Two Types:

1. Aerobic Respiration (requires oxygen):

• Occurs in mitochondria
• Complete breakdown of glucose
• Produces maximum ATP (36–38 per glucose molecule)
• End products: CO₂, H₂O, ATP

2. Anaerobic Respiration (without oxygen):

• Occurs in cytoplasm
• Incomplete breakdown of glucose
• Produces far less ATP (2 per glucose molecule)
• In muscles (when oxygen runs out): Glucose → Lactic acid + small ATP
• In yeast and some plants: Glucose → Ethanol + CO₂ + small ATP

Gaseous Exchange is the process of swapping oxygen and carbon dioxide between an organism and its environment.

In Humans:

Respiratory System Parts:

1. Nasal cavity — warms, moistens, filters air
2. Pharynx — throat passage
3. Larynx — voice box (contains vocal cords)
4. Trachea — windpipe (held open by C-shaped cartilage rings)
5. Bronchi — two tubes, one per lung
6. Bronchioles — smaller branching tubes
7. Alveoli — tiny air sacs (~300 million in human lungs; ~70 m² surface area)
8. Diaphragm — dome-shaped muscle below lungs; contracts during inspiration

Gaseous Exchange in Alveoli:

• Oxygen diffuses from alveoli → blood (into red blood cells, binds to haemoglobin)
• Carbon dioxide diffuses from blood → alveoli (to be exhaled)

⚡ NABTEB Exam Tip: During inhalation, the diaphragm contracts and flattens, increasing chest volume and reducing air pressure in lungs (below atmospheric). Air rushes in. During exhalation, diaphragm relaxes, chest volume decreases, air pressure increases, air is pushed out.

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🟡 Standard — Regular Study (2d–2mo)

For NABTEB students who want thorough understanding.

Mechanism of Breathing (Ventilation):

Inhalation (Inspiration):

1. Diaphragm contracts and flattens
2. External intercostal muscles contract, ribs move upward and outward
3. Chest cavity volume increases
4. Intrapulmonary pressure drops below atmospheric pressure
5. Air flows INTO lungs

Exhalation (Expiration):

1. Diaphragm relaxes and domes upward
2. External intercostal muscles relax, ribs move downward and inward
3. Chest cavity volume decreases
4. Intrapulmonary pressure rises above atmospheric pressure
5. Air flows OUT of lungs

During forced exhalation (e.g., running): Internal intercostal muscles and abdominal muscles also contract to actively reduce chest volume further.

Gas Exchange Principles:

Gas exchange occurs by diffusion. For diffusion to be efficient:

1. Surface area must be large (alveoli provide huge surface area)
2. Distance between blood and air must be thin (alveolar wall = 1 cell thick; capillary wall = 1 cell thick)
3. Concentration gradient must be maintained (blood continuously refreshed)

Haemoglobin:

• Iron-containing protein in red blood cells
• Each haemoglobin binds 4 oxygen molecules: $Hb + 4O_2 \rightarrow Hb(O_2)_4$
• Oxyhaemoglobin: bright red (oxygenated blood)
• Deoxyhaemoglobin: dark red/purple (deoxygenated blood)

The oxygen-haemoglobin dissociation curve shows that haemoglobin releases oxygen more readily:

• When CO₂ levels are high (active tissues)
• When temperature is higher
• When pH is lower (more acidic)

The Bohr Effect: Lower pH (from CO₂) shifts the curve to the right, promoting oxygen release to active tissues.

Respiratory Pigments:

Organism	Pigment	Metal	Where Found
Mammals	Haemoglobin	Iron (Fe)	Red blood cells
Earthworm	Haemoglobin	Iron	Blood plasma
Insects	None	—	—
Scorpion	Haemocyanin	Copper (Cu)	Blood plasma (blue)

Factors Affecting Breathing Rate:

• Exercise: increases breathing rate (more CO₂ produced, need more O₂)
• Emotions: fear/excitement increases rate
• Altitude: higher altitude = lower O₂, so increased breathing rate
• Respiratory diseases: reduce efficiency

⚡ NABTEB Exam Tip: In anaerobic respiration in muscles, lactic acid builds up causing oxygen debt. After exercise, you continue breathing heavily to repay this debt — supplying oxygen to convert lactic acid back to glucose in the liver.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage of respiration for thorough NABTEB preparation.

Cellular Respiration — Detailed Stages:

Glycolysis (in cytoplasm):

• 1 glucose (6C) → 2 pyruvate (3C each)
• Net gain: 2 ATP, 2 NADH
• Does not require oxygen

Link Reaction (mitochondrial matrix):

• 2 pyruvate → 2 acetyl CoA + 2 CO₂ + 2 NADH

Krebs Cycle (mitochondrial matrix):

• 2 acetyl CoA → 4 CO₂ + 6 NADH + 2 FADH₂ + 2 ATP

Electron Transport Chain (inner mitochondrial membrane):

• NADH and FADH₂ donate electrons
• Electrons pass along chain, releasing energy
• Energy used to pump H⁺ across membrane
• H⁺ flows back through ATP synthase, driving ATP synthesis
• Final electron acceptor is oxygen (forms water)
• If oxygen unavailable, ETC stops → aerobic respiration ceases

Total ATP from 1 glucose:

• Glycolysis: 2 ATP + 2 NADH (= ~3-5 ATP)
• Link reaction: 2 NADH (= ~5 ATP)
• Krebs cycle: 2 ATP + 6 NADH + 2 FADH₂ (= ~15 + 3 ATP)
• Total: ~36–38 ATP (aerobic); 2 ATP (anaerobic)

Respiration in Plants:

Plants respire all the time (day and night). During photosynthesis:

• In light, photosynthesis produces more O₂ than respiration consumes
• Net gas exchange depends on light intensity

Gaseous Exchange in Plants:

• Stomata: Pores on leaf underside; allow gas exchange (CO₂ in, O₂ out)
• Guard cells: Regulate stomatal opening/closing
• Stomata close at night (conserve water) and in dry conditions
• Lenticels: Small openings in stem for gas exchange

Gas Exchange in Different Organisms:

Organism	Medium	Gas Exchange Surface	Mechanism
Amoeba	Water	Plasma membrane	Diffusion (small, high surface area)
Earthworm	Air	Moist skin	Diffusion (skin must stay moist)
Fish	Water	Gills	Countercurrent flow (maximises O₂ uptake)
Insects	Air	Tracheal system	Tubes directly to cells (no circulatory transport)
Humans	Air	Alveoli	Negative pressure breathing
Tadpole (frog)	Water	Gills	External gills
Adult frog	Air	Lungs + moist skin	Both

Gills — Countercurrent Flow:

In fish gills:

• Blood flows in capillaries OPPOSITE direction to water flow
• This maintains a concentration gradient along the entire length of the gill
• Maximum oxygen extraction achieved (up to 80% of dissolved O₂)
• Countercurrent multiplier principle

Adaptations for Gas Exchange:

• Large surface area (gills, alveoli, flattened bodies)
• Thin membranes (1 cell thick)
• Moist surfaces (dissolves gases)
• Ventilation mechanism (moves medium over surface)
• Rich blood supply (maintains concentration gradient)

Disorders of the Respiratory System:

1. Asthma: Bronchioles constrict; caused by allergens; wheezing, breathlessness
2. Bronchitis: Inflammation of bronchi; caused by smoking/infections; chronic cough
3. Emphysema: Alveoli break down; caused by smoking; reduced surface area for gas exchange
4. Pneumonia: Alveoli fill with fluid; caused by infection; reduced gas exchange

Lung Capacities:

Measurement	Value	Description
Tidal volume	0.5 L	Air breathed in/out at rest
Inspiratory reserve	2.5 L	Additional air that can be inhaled
Expiratory reserve	1.5 L	Additional air that can be exhaled
Residual volume	1.5 L	Air remaining after maximal exhalation
Vital capacity	5.0 L	Total usable air (tidal + reserves)
Total lung capacity	6.0 L	Vital capacity + residual

⚡ NABTEB Quick Reference:

• Aerobic: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36-38 ATP$
• Anaerobic (muscles): Glucose → Lactic acid + 2 ATP
• Anaerobic (yeast): Glucose → Ethanol + CO₂ + 2 ATP
• Glycolysis: 1 glucose → 2 pyruvate + 2 ATP
• Inspiration: Diaphragm flattens, intercostals contract, air IN
• Expiration: Diaphragm relaxes, intercostals relax, air OUT
• Alveoli: 300 million, 70 m² surface area
• Haemoglobin: 4 O₂ binding sites; iron is the metal
• Bohr effect: High CO₂ → lower pH → haemoglobin releases O₂ more readily