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Unit 3 - Water Loss in Plants

This GCSE Biology quiz is all about transpiration. Transpiration is the process by which plants transport water upwards against the pull of gravity and it is made possible by water loss due to evaporation.

The minerals that a plant needs are carried through it dissolved in water that has come from the ground. Plants have no pump like the heart that could move the water through them and so they have evolved a different method of transporting fluids - transpiration.

Leaves contain spongy and palisade cells. Water on the surface of these evaporates and leaves the leaves by diffusion. Water is then drawn out of the xylem cells within the leaves to replace water lost by this evaporation. The xylem cells form continuous tubes down to the roots and, as water is lost from the leaves, more is drawn up from the roots - rather like when you drink through a drinking straw. Transpiration is continuous and so there is a slow but continuous flow of water through the xylem tubes.

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You are expected to know the factors that affect transpiration rate for the exams. Most of it is just common sense - anything that changes the speed of water loss to evaporation will affect transpiration - changes in temperature, humidity and the speed of the wind blowing over the leaves. One factor that is less obvious is light. When the light is bright, the stomata open more so that more carbon dioxide can enter the leaves to enable more photosynthesis to take place. Because the stomata are open more, more water can escape from the leaf, increasing transpiration.

There are times when plants need to conserve water in order to survive e.g. when the roots are damaged or water is short. In such situations, gardeners can remove some leaves - fewer leaves mean less transpiration and less water loss. Plants can reduce transpiration for themselves by wilting - you may have noticed this happening for yourself on hot days. Thin needle-like leaves or thick waxy coatings on fleshy leaves and stems are other adaptations to reduce transpiration in plants which live in desert conditions or places where the ground water is frozen for long periods of time.

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  1. The process by which a plant loses water is called...
    It sounds like respiration but has nothing to do with the release of energy in cells
  2. The loss of water vapour from a surface is known as...
    Knowing the factors that affect evaporation should help you to answer most questions about the rate of transpiration
  3. What are the tiny pores found in plant leaves?
    They allow carbon dioxide into leaves and let oxygen and water out
  4. Which of the following are specialised cells found on either side of a stoma?
    Guard cells change shape and cause the stoma to open or close, depending on conditions
  5. During photosynthesis, what enters the leaves through the stomata?
    Carbon dioxide from the atmosphere enters the plant and is used in photosynthesis to make sugars
  6. During transpiration, what leaves the plant through stomata?
    Desert plants have reduced stomata or close them to avoid water loss
  7. Water is lost from a plant more rapidly on a ...
    Increased temperature increases the rate of water loss. If it is humid, there is less of a concentration gradient so less water is lost
  8. Plants can increase their water uptake by having...
    More roots provide the plant with a larger area for a greater water uptake
  9. Plants do which of the following to reduce water loss?
    Many plants close their stomata during the day when it is hot
  10. Plant roots have an enormous surface area for water absorption due to which of the following?
    Root hairs and root hair cells increase the water absorbing area of plant roots

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