In 1727 the English clergyman and scientist Stephen Hales published Vegetable Staticks, a book of experiments on plants. In one of them he grew a sunflower in a covered pot and weighed it again and again. Because the soil was covered, almost all the weight it lost had to be water escaping from its leaves. He was measuring what we now call transpiration, using little more than a balance.
This lesson covers plant tissues, organs and systems in AQA GCSE Biology Topic 2 (Organisation). It's also useful for Combined Science.
The leaf as an organ
A leaf is an organ made of several tissues. AQA limits you to these:
- Epidermis: a thin outer layer. The upper epidermis is transparent, so light reaches the cells below, and is often coated in a waxy cuticle.
- Palisade mesophyll: tall cells packed with chloroplasts, where most photosynthesis happens.
- Spongy mesophyll: loosely packed cells with air spaces, so gases diffuse quickly.
- Xylem and phloem: the transport tissues, in the veins of the leaf.
- Guard cells around stomata: they control gas exchange and water loss.
The other plant tissue on the list is meristem, found at the growing tips of shoots and roots.
Stomata and guard cells
Stomata let carbon dioxide in for photosynthesis, and let oxygen and water vapour out. In many plants there are more of them on the lower surface of the leaf. When guard cells take in water and swell, they bend apart and the stoma opens. When they lose water, it closes. Closing stomata saves water, but it also cuts off the carbon dioxide supply.
You can investigate the distribution of stomata by looking at the epidermis under a microscope, counting stomata in several fields of view, and calculating a mean.
Xylem and the transpiration stream
The roots, stem and leaves form an organ system for transport. Root hair cells have a large surface area for taking up water by osmosis and mineral ions by active transport.
Xylem carries water and mineral ions from the roots up to the stem and leaves. It is made of hollow tubes of dead cells, strengthened by lignin. As water evaporates from the leaf and diffuses out through the stomata, more water is pulled up the xylem behind it. This flow is the transpiration stream.
What changes the rate of transpiration
| Factor | Effect | Why |
|---|---|---|
| Higher temperature | Faster | Water evaporates and diffuses faster |
| Higher humidity | Slower | Smaller concentration gradient of water vapour |
| More air movement | Faster | Vapour is carried away, keeping the gradient steep |
| Brighter light | Faster | More stomata open for photosynthesis |
A potometer measures how fast a cut shoot takes up water. If the air bubble moves 36 mm in 6 minutes, the rate is 36 ÷ 6 = 6 mm per minute. You may also be asked to turn that distance into a volume using the cross-sectional area of the tube.
Phloem and translocation
Phloem carries dissolved sugars from the leaves to the rest of the plant, for immediate use or storage, for example in growing tips, roots and fruits. This is translocation. Phloem is made of tubes of elongated living cells, and cell sap moves from one cell to the next through pores in the end walls. AQA doesn't require the detailed structure of phloem or how the transport works.
Common exam mistakes
- Swapping xylem and phloem. Xylem: water and mineral ions. Phloem: dissolved sugars.
- "Humidity speeds up transpiration." It slows it, by making the concentration gradient smaller.
- "A potometer measures water loss." It measures uptake. A little of the water is used by the plant, so uptake is only an estimate.
- "Translocation is the movement of water." Translocation is the movement of food molecules in the phloem.
- "Plants take in sugar through their roots." Roots take in water and mineral ions. Sugars are made in the leaves by photosynthesis.
Frequently asked questions
What is the difference between transpiration and translocation?
Transpiration is the loss of water vapour from the leaves, which draws water up the xylem. Translocation is the movement of dissolved sugars through the phloem.
What affects the rate of transpiration at GCSE?
Temperature, humidity, air movement and light intensity. Humidity is the one that slows it down as it increases.
Why is xylem made of dead cells?
The cells form hollow tubes with no contents in the way, so water can flow in a continuous column. Lignin keeps the tubes strong.
Is the potometer a required practical for AQA?
No. The specification lists "measure the rate of transpiration by the uptake of water", which is what a potometer does, as a skills opportunity, but it isn't one of the ten required practicals.
Try it
Say each answer before you flip the card. Then put yourself in Hales's place: you know the sunflower loses water from its leaves, but not exactly which part of the leaf it escapes from. What would you have tested next?
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