2.1.1Unit 2: Cells, Development, Biodiversity and Conservation
Properties of Gas Exchange Surfaces — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 26 Unit quiz
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 5Surface Area to Volume Ratio
The rate of gas exchange depends on the size of the exchange surface relative to the volume of the organism.
Gas exchange
- Gas exchange All aerobic organisms must take in oxygen and remove carbon dioxide across an exchange surface by diffusion.
Small organisms
- Small organisms They have a large surface area compared with their volume, so diffusion across the body surface is sufficient.
Large organisms
- Large organisms Their ratio is small and the diffusion distance to inner cells is large, so they need specialised exchange surfaces.
Increasing area
- Increasing area Alveoli in lungs, gill lamellae in fish and mesophyll air spaces in leaves greatly increase the surface area available.
Part 2 of 5Features of an Efficient Surface
Features of an Efficient Surface
Large surface area
- Large surface area A large area lets many molecules cross at the same time, speeding up the rate of diffusion.
Short diffusion pathway
- Short diffusion pathway The surface is often one cell thick and flattened, so gases only need to travel a very small distance.
Steep concentration gradient
- Steep concentration gradient Ventilation and blood flow constantly replace the gas on each side of the surface, so a high gradient is maintained.
Permeability
- Permeability The membranes are thin and permeable to the respiratory gases, and are kept moist so gases can dissolve.
Part 3 of 5Fick’s Law and Examples
1
Fick’s law states that the rate of diffusion is proportional to surface area × concentration difference ÷ diffusion distance.
2
In the lungs, the walls of the alveoli and capillaries are each one cell thick, giving a very short diffusion distance.
3
Breathing movements refresh the air in the alveoli, while the circulation carries oxygen away and brings carbon dioxide in.
4
In fish, blood flows in the opposite direction to water over the gills, which keeps a gradient along the whole lamella.
5
In plants, stomata open to allow carbon dioxide to diffuse into the leaf and into the air spaces around the mesophyll cells.
Part 4 of 5Worked Example
Question Compare the surface area to volume ratio of a cube of side 2 cm with one of side 4 cm.
- Cube of side 2 cm: surface area = 6 × 2² = 24 cm²; volume = 2³ = 8 cm³; ratio = 24 ÷ 8 = 3 : 1.
- Cube of side 4 cm: surface area = 6 × 4² = 96 cm²; volume = 4³ = 64 cm³; ratio = 96 ÷ 64 = 1.5 : 1.
- Doubling the side halves the ratio.
Answer The 2 cm cube has a ratio of 3 : 1 and the 4 cm cube 1.5 : 1, so the ratio falls as size increases.
Part 5 of 5Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Gas exchange surface A thin surface across which gases diffuse between an organism and its environment.
- Surface area to volume ratio Surface area divided by volume.
- Diffusion pathway The distance gases must diffuse across.
- Concentration gradient A difference in concentration between two regions.
Common Mistakes
- Saying that larger organisms have a higher surface area to volume ratio.
- Writing that a thicker exchange surface increases the rate of diffusion.
- Forgetting that the gradient must be maintained by ventilation and circulation.
- Stating that gases are actively transported across exchange surfaces; they diffuse.
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