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3.1.6Exchange and Transport

Gas Exchange vs Water Loss — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 9 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

3.1.6 Gas Exchange vs Water Loss — AQA A Level Biology 7402 notes: Explain the conflict between gas exchange and water loss and describe the adaptations of insects and xerophytic plants that reduce water loss.Efficient gas exchange needs a large, thin, moist surface in contact with the air.

  • evaporation
  • waterproof exoskeleton
  • transpiration
  • waxy cuticle
  • Xerophytes
  • Exam-style question 1

Section 1 of 9The problem

Efficient gas exchange needs a large, thin, moist surface in contact with the air. Such a surface is also ideal for evaporation of water. Terrestrial organisms therefore face a conflict: the features that favour gas exchange also lead to dehydration. Organisms solve this by compromise, having mechanisms to reduce water loss and to open exchange surfaces only when needed.

Section 2 of 9Insects

Insects reduce water loss in several ways:

Waterproof exoskeleton
a waterproof exoskeleton with a layer of wax (cuticle) over the body surface;
spiracles
spiracles that can be closed to retain water,
  • and which are opened only when oxygen is needed or carbon dioxide has built up;
    • they may open and close in cycles;
hairs around spiracles trap moist air and reduce diffusion of water vapour.

The trade-off is that when spiracles are open for gas exchange, water vapour is lost. Insects that are very active therefore lose more water.

Section 3 of 9Plants: stomata and the leaf

In a leaf, water vapour is lost by transpiration through open stomata.

The plant balances this against its need for carbon dioxide.
Guard cells control the stomatal pore: when they become turgid by taking in water, they bend and open the stoma;
  • when they lose water, they become flaccid and the stoma closes.
Stomata generally open in the light, for photosynthesis,
  • and close in the dark or when the plant is short of water.
    • (abscisic acid is the hormone that triggers closure in drought)
Waxy cuticle
Features reducing water loss in most leaves include a waxy cuticle on the epidermis, and stomata mostly on the lower surface, out of direct sunlight.

Section 4 of 9Xerophytes

Xerophytes are plants adapted to arid conditions, such as marram grass, cacti and some conifers. Their adaptations reduce transpiration but still allow gas exchange.

AdaptationHow it reduces water loss
Thick waxy cuticleIncreases the diffusion path and is waterproof
Rolled leaves (marram grass)Trap a region of humid air so the water potential gradient to the outside is reduced
Hairs on the leaf surfaceTrap moist air close to the surface, reducing the gradient
Sunken stomata (in pits)Shelter stomata from air movement so humid air is retained
Reduced leaf surface area (spines, needles)Fewer stomata and a smaller surface for evaporation
Fewer stomataReduces the number of pores for water vapour to escape
Succulent tissues, deep or wide rootsStore water or access it

In each adaptation, the principle is to reduce the water potential gradient between the leaf and the air, to increase the diffusion path, or to reduce the surface area. In an exam, state the feature and its effect in terms of diffusion of water vapour.

Section 5 of 9Investigating transpiration

A potometer measures the rate of water uptake by a leafy shoot, which approximates transpiration.

1
Factors that increase the rate are higher temperature, lower humidity, greater air movement and light intensity (more stomata open).
2
Assumptions are that water uptake equals water loss, since some is used in photosynthesis.
3
Cutting the shoot underwater and sealing joints with grease prevents air bubbles from entering.
4
The rate of movement of the bubble over a measured distance, with the volume calculated from the capillary tube radius (volume = π r2 x distance), gives the rate in mm3 per minute.
5
Microscope slides of leaf sections, or peels, can be used to compare the stomatal density and structure.

Section 6 of 9Evaluating adaptations in desert and aquatic environments

For a data-based question, look at how a species of plant from a dry habitat compares with one from a wet habitat: the xerophyte may have fewer stomata per square millimetre, thicker cuticle and more hairs, and its rate of transpiration will be lower under the same conditions.

Evaluating adaptations in desert and aquatic environments
A mesophyte such as a garden plant is in between,
  • and a hydrophyte,
  • such as a water lily, has stomata on the upper surface only,
  • since the lower surface is in contact with water,
  • and large air spaces to help it float and distribute oxygen.
When asked to evaluate whether a feature is effective, compare the rate of water loss with and without it, for example leaves with petroleum jelly on the lower surface (which blocks stomata) and those without.
Remember that reducing water loss always reduces the uptake of carbon dioxide,
  • so growth of xerophytes is slower,
  • which is the cost of living in arid environments.

Section 7 of 9Exam-style questions

  1. Explain why insects lose water when they open their spiracles, and how they reduce this loss. [4]4 marks

  2. Explain how the structure of marram grass leaves reduces water loss. [4]4 marks

  3. A potometer bubble moved 36 mm in 12 minutes in a capillary tube of radius 0.5 mm. Calculate the rate of water uptake in mm3 per minute. [3]3 marks

Section 8 of 9Key terms

TermMeaning
TranspirationEvaporation of water from the leaves through the stomata
XerophytePlant adapted to live in dry conditions
CuticleWaxy waterproof layer covering the epidermis
Guard cellCell that controls the opening of a stoma
Water potential gradientDifference in water potential driving the diffusion of water vapour
PotometerApparatus for measuring the rate of water uptake by a shoot
SpiraclePore in the insect cuticle that can be closed to reduce water loss

Section 9 of 9Exam technique

1
Saying xerophytes have no stomata - they have fewer, or protected ones.
2
Forgetting that reduced water loss comes with a trade-off in gas exchange.
3
Stating that rolling leaves reduces the surface area available for photosynthesis only - it mainly traps moist air.
4
Writing that the cuticle is permeable to water.
5
Not stating that humid air reduces the water potential gradient.

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