IB MYP Integrated Science- Biology - Gas exchange-Study Notes - New Syllabus

Gas Exchange

🌟 Introduction

Gas exchange is the process by which organisms take in oxygen and release carbon dioxide.
It powers cellular respiration, which provides energy for all life processes.
Every organism performs gas exchange, but the structures differ with size, habitat, and complexity.

🧩 Why Gas Exchange Is Necessary

• All cells need oxygen for aerobic respiration.
• Respiration releases energy (ATP) for metabolism.
• Carbon dioxide is produced as waste and must be removed.
• Accumulation of CO₂ makes body fluids acidic and affects enzyme activity.

🌱 Basic Requirements of an Efficient Gas Exchange Surface

Think TMBL:

• Thin → short diffusion distance
• Moist → gases dissolve easily
• Broad surface area → faster diffusion
• Large blood supply → maintains concentration gradient

These features maximize diffusion, the main process in gas exchange.

🐟 Gas Exchange in Different Organisms

a) Unicellular Organisms (e.g., Amoeba)

• High surface area to volume ratio.
• Exchange gases directly through the cell membrane by diffusion.
• No specialized organs needed.

b) Gas Exchange in Plants

Plants perform both respiration and photosynthesis.

Where gas exchange occurs:

• Stomata
• Lenticels (woody stems)
• Roots (air spaces in soil)

Key points:

• Stomata open in light and close in dark.
• Guard cells regulate stomatal opening.
• CO₂ enters for photosynthesis; O₂ exits.
• During respiration (day and night), plants take in oxygen and release CO₂.

c) Gas Exchange in Fish (Gills)

• Gills are efficient due to thin filaments.
• Large surface area.
• Rich capillary network.
• Countercurrent flow (water and blood move in opposite directions).

Countercurrent mechanism maximizes oxygen absorption.

d) Amphibians

• Tadpoles: gills.
• Adult frogs: lungs and moist skin.
• Skin must remain moist for cutaneous respiration.

e) Gas Exchange in Humans (Lungs)

Humans have a highly specialized respiratory system.

Path of air:
Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

Features of alveoli:

• Very thin walls.
• Moist lining.
• Huge surface area (millions of alveoli).
• Surrounded by capillaries.
• Maintain steep concentration gradient.

What happens in alveoli?

• Oxygen diffuses into blood (binds to hemoglobin).
• CO₂ diffuses from blood into alveoli.

Breathing movements:

Inhalation:

• Diaphragm contracts and moves down.
• Ribcage expands.
• Air pulled into lungs.

Exhalation:

• Diaphragm relaxes and moves up.
• Ribcage falls.
• Air forced out.

🔬 Transport of Gases

Oxygen:

• Transported bound to hemoglobin.
• Forms oxyhemoglobin.

Carbon dioxide:

• Bicarbonate ions (major form).
• Dissolved CO₂.
• Carbaminohemoglobin.

⚠️ Gas Exchange Disorders

• Asthma: narrowed airways reduce oxygen intake.
• Emphysema: alveolar walls damaged.
• Bronchitis: airway inflammation.
• Pneumonia: fluid in alveoli.
• Covid-19: affects alveolar gas exchange surface.

📌 Summary Table

⚡ Quick Recap
Gas exchange supplies O₂ and removes CO₂.
Efficient surfaces are thin, moist, and have a large surface area.
Plants use stomata; fish use gills; humans use alveoli.
Alveoli are the main site of gas exchange in humans.
Gas transport involves hemoglobin and bicarbonate ions.
Disorders like asthma and pneumonia reduce gas exchange efficiency.