Exchange And Transport In Plants
Übersicht
A plant has to open itself to the air to live. Photosynthesis needs carbon dioxide, and carbon dioxide is out there in the atmosphere, so a leaf has to let the outside in. The trouble is that the inside of a leaf is wet, and the moment you open a wet space to dry air the water starts to leave. Every green plant you have ever seen is a working answer to that one problem.
This lesson follows the two things that cross the boundary of a leaf in opposite directions. Carbon dioxide drifts in through pores a few thousandths of a millimetre wide and reaches a cell that will build it into sugar. Water goes the other way: pulled out of the soil by roots covered in fine hairs, carried up to the leaves and lost from them as vapour. Along the way you will see why a leaf is flat and thin rather than fat and round, why the pores can shut, what a hot and windy afternoon does to a plant, and why a plant left on a sunny windowsill in August goes limp before evening.
Ziele
- In flowering plants: carbon dioxide enters leaves by diffusion through the stomata; most of the water and mineral ions are absorbed by roots.
- The surface area of roots is increased by root hairs, and the surface area to volume ratio of leaves is increased by the flattened shape and internal air spaces.
- Plants have stomata to obtain carbon dioxide from the atmosphere and to remove oxygen produced in photosynthesis. Stomata also help to control the rate of water loss.
- Plants mainly lose water vapour from their leaves. Most of the loss of water vapour takes place through the stomata.; Evaporation is more rapid in hot, dry and windy conditions.; If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.
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- Through which structures does carbon dioxide mainly enter a leaf? A. The stomata B. The root hairs C. The waxy cuticle D. The palisade cells Answer: A
- By which process does carbon dioxide enter a leaf from the air? A. Osmosis B. Diffusion C. Evaporation D. Photosynthesis Answer: B
- How does water enter a root hair cell from the soil? A. By osmosis B. By evaporation C. By diffusion of water vapour D. Through the stomata Answer: A
- Which set of conditions gives the highest rate of water loss from a plant? A. Cool, still, humid B. Warm, still, humid C. Cool, windy, dry D. Warm, windy, dry Answer: D
- Which two features increase the surface area to volume ratio of a leaf? A. Its flattened shape and its internal air spaces B. Its waxy cuticle and its green colour C. Its root hairs and its veins D. Its thickness and its water content Answer: A
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