Exchange And Transport In Plants
Overview
A tall forest tree lifts hundreds of litres of water from the soil to its highest leaves every hot day, and it does it with no heart, no muscle and no moving part of any kind. The whole system is powered by water evaporating from the inside of a leaf. Once you can see how that works, a plant stops looking like a passive green thing in the ground and starts looking like the most efficient piece of plumbing in biology.
This lesson follows two journeys. The first is a gas: carbon dioxide drifting in through a pore only a few thousandths of a millimetre wide, reaching a cell that will build it into sugar. The second is water, pulled from between soil particles, through a root hair, up a column of dead hollow cells and out into the air. Along the way you will meet the guard cells that decide how far the pore opens, the two transport tissues that carry very different cargo in different directions, and the reason a plant left on a windowsill in August goes limp by the afternoon.
Objectives
- 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.
- The size of stomata is controlled by guard cells, which surround them.
- Flowering plants have separate transport systems: xylem tissue transports water and mineral ions from the roots to the stem and leaves; the movement of water from the roots through the xylem and out of the leaves is called the transpiration stream; phloem tissue carries dissolved sugars from the leaves to the rest of the plant, including the growing regions and the storage organs. This process is called translocation; the structure of the xylem and the phloem is related to its function.
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Lesson Note
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Lesson Evaluation
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You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- Through which structures does carbon dioxide mainly enter a leaf? A. The stomata B. The xylem vessels C. The root hairs D. The waxy cuticle Answer: A
- Which statement about xylem vessels is correct? A. They are living cells with sieve plates B. They carry sucrose from the leaves to the roots C. They are dead, hollow tubes with lignified walls D. They transport water downwards from the leaves to the roots Answer: C
- How are mineral ions taken up by root hair cells from the soil? A. By osmosis B. By active transport C. By transpiration D. By translocation Answer: B
- Which set of conditions gives the highest rate of transpiration? A. Cool, still, humid B. Warm, still, humid C. Cool, windy, dry D. Warm, windy, dry Answer: D
- A stoma opens when: A. Water leaves the guard cells by osmosis and they straighten B. Water enters the guard cells by osmosis and they curve apart C. The guard cells lose their chloroplasts D. The thick outer wall of each guard cell stretches less than the inner wall Answer: B
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