2.6.1Inquiry Process
Gas Exchange in Organisms — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 63 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5What Gas Exchange Involves
Oxygen uptake
- Oxygen uptake Aerobic respiration needs oxygen as the final electron acceptor to release large amounts of ATP from glucose,
- so organisms must obtain it from their surroundings.
Carbon dioxide removal
- Carbon dioxide removal Respiration produces carbon dioxide as a waste gas that must be removed, otherwise it would lower pH and interfere with cell chemistry.
Photosynthetic organisms
- Photosynthetic organisms Plants and algae take in carbon dioxide and release oxygen when photosynthesis exceeds respiration,
- so the gas flow reverses in light.
Diffusion
- Diffusion Gases cross exchange surfaces passively, moving down their concentration gradient from high to low concentration without using ATP.
Part 2 of 5Properties of Good Exchange Surfaces
Properties of Good Exchange Surfaces
Large surface area
- Large surface area Folding, flattening or branching increases the area available for diffusion, so more molecules cross per second.
Thin barrier
- Thin barrier A surface that is one cell thick gives a short diffusion distance, which makes diffusion faster.
Steep gradient
- Steep gradient Ventilation and blood flow remove incoming oxygen and bring fresh supplies, keeping the concentration difference high.
Moist surface
- Moist surface Gases dissolve in a film of water before crossing the membrane, so exchange surfaces are kept damp.
| Factor | Effect on diffusion rate |
|---|---|
| Larger surface area | Increases rate |
| Longer distance | Decreases rate |
| Steeper gradient | Increases rate |
Part 3 of 5Size and the Need for Specialisation
1
A single-celled organism has a very high surface area to volume ratio, so diffusion across its membrane meets its needs.
2
As an organism grows, its volume increases faster than its surface area, so the ratio falls.
3
Oxygen needs rise with volume of living tissue, but supply across the body surface rises only with area, creating a shortage.
4
Cells deep inside a large body are too far from the surface for diffusion alone to deliver oxygen in time.
5
Large animals therefore evolved lungs or gills plus a transport system, and plants evolved leaves with stomata and air spaces.
Part 4 of 5Worked Example
Question
- A cube-shaped cell has sides of 2 µm and another has sides of 4 µm.
- Compare their surface area to volume ratios.
- Cell 1: surface area = 6 × 2² = 24 µm²; volume = 2³ = 8 µm³.
- Ratio 1 = 24 ÷ 8 = 3 : 1.
- Cell 2: surface area = 6 × 4² = 96 µm²; volume = 4³ = 64 µm³.
- Ratio 2 = 96 ÷ 64 = 1.5 : 1.
Answer Doubling the side halves the ratio from 3 : 1 to 1.5 : 1, so the larger cell is worse at exchanging materials.
Part 5 of 5Exam Checklist & Tips
2.6.1 – Key Points for the Exam
- Gas exchange The movement of respiratory gases between an organism and its environment across an exchange surface.
- Diffusion The net passive movement of particles from a region of higher concentration to lower concentration.
- Surface area to volume ratio
- Surface area divided by volume;
- it decreases as an object gets bigger.
- Surface area divided by volume;
- Concentration gradient A difference in concentration between two regions that drives diffusion.
Common Mistakes
- Saying that organisms 'breathe in oxygen to make energy'; oxygen is used in respiration, which makes ATP.
- Thinking that bigger organisms have smaller surface areas; they have a larger area but a smaller ratio.
- Forgetting that diffusion is passive and does not need ATP.
- Stating that plants only take in carbon dioxide; they also respire and use oxygen all the time.
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