2.1.1Topic 2 · Genes and Health
Properties of Gas Exchange Surfaces — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 9 parts Lesson 3 of 35 Unit quiz
2.1.1 Properties of Gas Exchange Surfaces — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how the surface area to volume ratio of an organism limits gas exchange and describe the features that make an exchange surface efficient.Every living cell must take in oxygen for aer…
- surface area to volume ratio (SA:V)
- Large surface area
- Thin barrier
- Steep concentration gradient
- Good blood supply
- Moist surface
Section 1 of 9Why exchange surfaces are needed
Every living cell must take in oxygen for aerobic respiration and get rid of carbon dioxide. In a tiny single-celled organism these gases simply diffuse across the cell surface membrane and through the cytoplasm fast enough to meet demand. In larger organisms this stops working, and the reason is geometry.
As an object gets bigger, its volume (which determines how much metabolic demand it has) increases faster than its surface area (which determines how much it can exchange).
- so it needs a specialised gas exchange surface and usually a transport system and a ventilation mechanism as well.
Section 2 of 9Worked idea: SA:V of cubes
Imagine a cube of side 1 cm: surface area 6 cm2, volume 1 cm3, SA:V = 6:1. A cube of side 4 cm has surface area 96 cm2 and volume 64 cm3, so SA:V = 1.5:1. The larger cube has a quarter of the ratio. Organisms are not cubes, but the trend is the same, which is why a large body needs an internal exchange organ with a vastly folded surface.
Other factors alter the need. Animals in cold climates tend to be larger and more compact (smaller SA:V) to reduce heat loss, whereas small, active animals have a high SA:V and a high metabolic rate, so they need very efficient gas exchange.
Section 3 of 9Features of an efficient exchange surface
Whatever the organism, good exchange surfaces share a set of properties that all increase the rate of diffusion:
- Large surface area folding, branching or flattening gives more room for molecules to cross.
- (alveoli, gill filaments and lamellae, leaf air spaces)
- Thin barrier often one cell thick, so the diffusion distance is very short.
- (squamous epithelium)
- Steep concentration gradient maintained by blood flow carrying gases away and by ventilation bringing fresh air or water to the surface.
- Moist surface gases dissolve in a film of water before diffusing across the membrane.
- Permeability the membranes allow the gases to pass freely.
Section 4 of 9Examples across organisms
| Organism | Exchange surface | Key adaptations |
|---|---|---|
| Single-celled amoeba | Cell surface membrane | Very high SA:V; short diffusion path |
| Fish | Gill filaments and lamellae | Huge surface area; thin lamellae; rich capillaries; counter-current flow |
| Insect | Tracheae and tracheoles | Spiracles; air-filled tubes reach cells directly; tracheole ends contain fluid |
| Mammal | Alveoli | Millions of thin-walled sacs; capillary network; ventilation |
| Dicotyledonous leaf | Spongy mesophyll and stomata | Air spaces; stomata open and close; thin, flat leaf |
Section 5 of 9Conflict with water loss
A surface that is thin, moist and permeable to gases is also permeable to water vapour.
- and mammals keep the lungs inside the body so that the moist surface is protected and only a small amount of water is lost in exhaled air.
This theme links to the next lessons: Fick's law gives the quantitative relationship between these features and the rate of diffusion, and the mammalian lung is the main worked example.
Section 6 of 9Scaling up: why transport systems follow
Gas exchange does not work alone.
- because diffusion through several centimetres of tissue would take hours or days.
Plants show the same logic in a different form.
- giving a large SA:V, and the air spaces inside the spongy mesophyll let carbon dioxide diffuse in the gas phase,
- which is about ten thousand times faster than diffusion through water.
Section 7 of 9Exam-style questions
A cube-shaped organism has sides of 2 cm. Calculate its surface area to volume ratio. [3]3 marks
Surface area = 6 × 2 × 2 = 24 cm2 [1]. Volume = 2 × 2 × 2 = 8 cm3 [1]. SA:V = 24 / 8 = 3 : 1 [1].
Explain why a large, active mammal needs a specialised gas exchange system but an amoeba does not. [4]4 marks
The amoeba has a large SA:V ratio [1], so oxygen diffuses across its membrane quickly enough to supply all of its small volume [1]. A mammal has a small SA:V ratio and a high metabolic demand for oxygen [1], so diffusion across the body surface would be far too slow and the diffusion distance to inner cells too long; a specialised, folded exchange surface and transport system is required [1].
Describe three features of the alveolar epithelium that make it an efficient gas exchange surface. [3]3 marks
Any three: it is only one cell thick (squamous), giving a short diffusion distance [1]; there are millions of alveoli giving a large surface area [1]; it is covered by a dense network of capillaries which maintains a steep concentration gradient [1]; it is moist, so gases dissolve before diffusing [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Surface area to volume ratio | The surface area of an organism or structure divided by its volume; it falls as size increases. |
| Diffusion | Net passive movement of particles from a region of higher concentration to lower concentration down a gradient. |
| Concentration gradient | A difference in concentration of a substance between two regions. |
| Squamous epithelium | A flat, single-cell-thick tissue that gives a short diffusion distance. |
| Ventilation | Movement of air or water over a gas exchange surface to maintain a steep gradient. |
| Metabolic rate | The rate at which an organism uses energy, often measured by oxygen consumption. |
| Diffusion distance | The distance molecules must travel across the exchange surface. |
Section 9 of 9Exam technique
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