Revision notes for Eduqas A Level Biology Adaptations for gas exchange. Open the guide for explanations and worked examples. Written against the Eduqas A Level Biology (A400QS) specification, so the content matches what's examinable rather than general Biology background.
Adaptations for gas exchange
What you'll learn
- Why increasing body size and metabolic rate creates a gas exchange problem.
- How small animals, fish, mammals and insects exchange gases in different environments.
- How leaves act as gas exchange organs while controlling water loss.
- How to approach the Eduqas specified practicals on fish gills, stomata and leaf microscopy.
The starting point: diffusion and surface area
Definition
Gas exchange
Gas exchange is the movement of respiratory gases between an organism and its environment: usually oxygen enters and carbon dioxide leaves. These gases move by diffusion, which is the net movement of particles from a higher concentration to a lower concentration.
Diffusion is only fast enough over short distances. As an organism gets larger, its volume increases faster than its surface area, so its surface area to volume ratio decreases. This means there is less exchange surface available for each unit of living tissue.
Metabolic rate means the rate of chemical reactions in cells. A large, active animal has many respiring cells, so it needs oxygen quickly and produces carbon dioxide quickly.
The cube model shows why larger organisms need specialised exchange surfaces rather than relying only on their outer body surface.
Key Idea
The gas exchange problem
Large size lowers surface area to volume ratio, while high metabolic rate increases oxygen demand and carbon dioxide production. Large active organisms therefore need specialised respiratory surfaces, transport systems and ventilation mechanisms.
Example
Calculating surface area to volume ratio
A cube-shaped model organism has sides of length 3.0 cm. Calculate its surface area to volume ratio.
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For a cube, calculate total surface area using 6s26s^26s2 and volume using s3s^3s3, where sss is side length.
surface area=6(3.0 cm)2=54 cm2volume=(3.0 cm)3=27 cm3\begin{aligned} \text{surface area} &= 6(3.0\ \text{cm})^2 = 54\ \text{cm}^2 \\ \text{volume} &= (3.0\ \text{cm})^3 = 27\ \text{cm}^3 \end{aligned}surface areavolume=6(3.0 cm)2=54 cm2=(3.0 cm)3=27 cm3 -
Compare surface area with volume.
SA:V=54:27\text{SA:V} = 54:27SA:V=54:27 -
Simplify the ratio by dividing both sides by 27, giving 2:1. This is lower than a 1 cm cube, so diffusion across the surface is less able to supply every cell.
Small animals: exchange across the general body surface
A general body surface is the outer surface of the organism, rather than a specialised structure such as a lung or gill. Small or thin animals can often exchange gases across this surface because diffusion distances are short.
Comparing Amoeba, flatworm and earthworm
| Organism | Gas exchange mechanism | Why it works |
|---|---|---|
| Amoeba | Oxygen and carbon dioxide diffuse across the cell surface membrane. | It is unicellular, so every part of the cytoplasm is close to the external environment. |
| Flatworm | Gases diffuse across the moist body surface. | The body is flattened, giving a large surface area and a short diffusion distance to internal cells. |
| Earthworm | Gases diffuse across moist skin, then are transported in blood. | The skin is kept moist by mucus and has many capillaries close to the surface. |
A capillary is a very small blood vessel with thin walls. Earthworms also use haemoglobin, a respiratory pigment that carries oxygen in the blood.
Common Mistake
Earthworms do not have lungs
Earthworms exchange gases through their moist skin. If the skin dries out, diffusion cannot occur effectively, which is why earthworms need damp conditions.
Specialised respiratory surfaces
A respiratory surface is a surface adapted for gas exchange. Larger animals need these surfaces because their outer body surface alone is not enough.
Common features include:
- large surface area, often by folding or branching
- thin barrier, giving a short diffusion distance
- moist, permeable surface so gases can dissolve before diffusing
- good blood supply or transport system to carry gases away
- ventilation, which moves the external medium over the exchange surface
Definition
Ventilation
Ventilation is the movement of air or water over a respiratory surface to maintain steep concentration gradients for oxygen and carbon dioxide.
Respiratory surfaces are also adapted to the environment. Fish have gills for aquatic environments. Mammals have lungs inside the body for terrestrial environments, reducing water loss from the moist exchange surface.
Fish gills: gas exchange in water
Water contains less oxygen than air and is denser, so fish need an efficient system. Bony fish use gills, supported by gill arches. Each arch has many gill filaments, and each filament has many thin lamellae, which greatly increase surface area.
Ventilation in bony fish
Bony fish use a buccal-opercular pump.
- The mouth opens and the buccal cavity floor lowers, reducing pressure so water enters.
- The mouth closes and the buccal cavity floor rises, increasing pressure.
- The operculum opens, so water is forced over the gills and out.
The operculum is the bony flap covering the gills.
Counter-current flow
In counter-current flow, water and blood flow in opposite directions across the lamellae. This keeps water oxygen concentration higher than blood oxygen concentration along the whole lamella, so oxygen continues to diffuse into the blood.
In parallel flow, water and blood move in the same direction. The gradient falls quickly, so less oxygen is absorbed.
Example
Comparing counter-current and parallel flow
Artificial gill data show that blood oxygen content rises from 20 to 82 units in a counter-current system, but from 20 to 55 units in a parallel-flow system.
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Calculate the oxygen gain in counter-current flow.
82−20=62 units82 - 20 = 62\ \text{units}82−20=62 units -
Calculate the oxygen gain in parallel flow.
55−20=35 units55 - 20 = 35\ \text{units}55−20=35 units -
Compare the gains: counter-current flow gives 27 more units of oxygen gain. This supports the explanation that counter-current flow maintains a diffusion gradient for longer.
Specified practical: fish head dissection
To observe the fish gas exchange system, place the fish head on a dissecting tray, lift or remove the operculum carefully, and identify the gill arches and filaments. A filament can be placed in water and viewed with a hand lens or microscope to see lamellae.
Use gloves, wash hands, disinfect the bench, and cut away from yourself. Keep the gills moist because dried filaments clump together and are harder to observe.
Human lungs and ventilation
Mammals use lungs because the exchange surface must stay moist but also protected from drying out. Air enters through the trachea, then bronchi, bronchioles and finally the alveoli.
An alveolus is a tiny air sac where gas exchange occurs. Alveoli provide a large surface area, a one-cell-thick epithelium, elastic fibres for recoil, and a dense capillary network.
What to recognise on microscope slides
In a transverse section, or T.S., of trachea, look for:
- C-shaped cartilage rings, which keep the airway open
- ciliated epithelium, which moves mucus upwards
- goblet cells, which produce mucus to trap particles
In a T.S. of lung, look for:
- many alveoli with thin walls
- capillaries close to alveolar walls
- elastic tissue
Human ventilation
During inspiration, the diaphragm contracts and flattens, and the external intercostal muscles contract to move the ribs up and out. Thoracic volume increases, pressure falls below atmospheric pressure, and air enters.
During normal expiration, these muscles relax. Elastic recoil reduces thoracic volume, pressure rises, and air leaves.
Gases diffuse according to partial pressure, which is the pressure contributed by one gas in a mixture. Oxygen diffuses from alveolar air into blood, while carbon dioxide diffuses from blood into alveolar air. Ventilation replaces alveolar air, and blood flow removes oxygenated blood, maintaining gradients.
Common Mistake
Ventilation is not gas exchange
Ventilation is the movement of air. Gas exchange is diffusion of oxygen and carbon dioxide across the alveolar surface.
Insects: the tracheal system
Insects have a waterproof exoskeleton, which reduces water loss but prevents gas exchange across the body surface. Instead, they use a tracheal system.
Air enters through spiracles, which are small openings that can close to reduce water loss. Air then passes through tracheae, tubes strengthened with chitin, and into tiny tracheoles that reach close to individual cells.
Oxygen diffuses directly from tracheoles into tissues, so insect blood does not usually transport oxygen. Some insects ventilate the system using abdominal pumping movements or air sacs.
Key Idea
Insect adaptation to land
The insect tracheal system brings air directly to respiring tissues while closable spiracles reduce water loss in a terrestrial environment.
Leaves as gas exchange organs
An angiosperm is a flowering plant. A dicotyledon leaf, such as Ligustrum privet, is adapted for photosynthesis and gas exchange.
Key leaf structures include:
- waxy cuticle, which reduces water loss
- upper epidermis, which is transparent to allow light through
- palisade mesophyll, packed with chloroplasts for photosynthesis
- spongy mesophyll, with air spaces for diffusion
- vascular bundles containing xylem and phloem
- stomata, which are pores for gas exchange
A stoma is a pore in the leaf epidermis. Guard cells are the two cells around each stoma that control its opening and closing.
Stomatal opening and closing
In the light, potassium ions move into guard cells. This lowers their water potential, so water enters by osmosis. The guard cells become turgid and curve apart, opening the stoma.
In darkness or water stress, potassium ions leave guard cells. Water leaves by osmosis, the guard cells become flaccid, and the stoma closes. This reduces water loss but also limits carbon dioxide entry.
Tip
Link structure to function
For leaf questions, always connect the structure to photosynthesis or diffusion: palisade cells absorb light, air spaces speed diffusion, stomata regulate carbon dioxide entry and water loss.
Specified practical: stomatal numbers
A common method is to make a clear nail varnish impression of the leaf surface. Once dry, peel it off with sticky tape, mount it on a slide, and view it under a microscope.
Use an eyepiece graticule or field of view calibration to find the area being counted. Count stomata in several fields of view on the upper and lower epidermis, avoiding the midrib. Calculate stomatal density in stomata per square millimetre.
Control variables include leaf species, leaf age, region sampled, magnification and environmental conditions. Improve reliability by sampling several leaves from several plants.
Example
Calculating stomatal density
A student counts 42, 38, 40, 44 and 36 stomata in five fields of view. The field of view diameter is 0.50 mm.
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Calculate the mean number of stomata.
42+38+40+44+365=40\frac{42 + 38 + 40 + 44 + 36}{5} = 40542+38+40+44+36=40 -
Calculate the field of view area using A=πr2A = \pi r^2A=πr2. The radius is 0.25 mm.
A=π(0.25 mm)2=0.196 mm2A = \pi(0.25\ \text{mm})^2 = 0.196\ \text{mm}^2A=π(0.25 mm)2=0.196 mm2 -
Calculate stomatal density.
density=400.196 mm2=204 stomata mm−2\text{density} = \frac{40}{0.196\ \text{mm}^2} = 204\ \text{stomata mm}^{-2}density=0.196 mm240=204 stomata mm−2
Specified practical: leaf drawing and magnification
For a low-power plan of a T.S. dicotyledon leaf, draw only the outlines of tissues, not individual cells. Use clear continuous lines, no shading, a title, labels and either a scale bar or magnification.
A stage micrometer is a microscope slide with a known scale. It is used to calibrate the eyepiece graticule so you can calculate actual size.
Example
Calculating drawing magnification
A leaf section is 1.2 mm wide in reality. In a student’s drawing, the same width is 96 mm. Calculate the drawing magnification.
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Use the magnification equation.
M=image sizeactual sizeM = \frac{\text{image size}}{\text{actual size}}M=actual sizeimage size -
Substitute values using the same units.
M=96 mm1.2 mm=80M = \frac{96\ \text{mm}}{1.2\ \text{mm}} = 80M=1.2 mm96 mm=80 -
State the magnification as 80 times. The drawing is 80 times larger than the actual leaf section.
Exam technique
In the exam
- For adaptation questions, link each feature to a diffusion principle: large surface area, short diffusion distance, steep gradient or reduced water loss.
- For comparisons, use named examples carefully: Amoeba, flatworm, earthworm, fish gills, mammalian lungs, insect tracheae and leaf stomata.
- In practical questions, include variables, repeats, calibration, risk control and how the data are processed.
Self review
Check yourself
- Why does counter-current flow extract more oxygen than parallel flow?
- How does a guard cell becoming turgid open a stoma?
- What features would you expect to identify in a T.S. trachea slide?
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