BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis
Biology 9.7 Water and mineral transport in plants — Transpiration Stream
5 min read 6 parts Lesson 7 of 9
Lesson summary
9.7 Water and mineral transport in plants — Transpiration Stream — Alhussan International Academy — Biology (Stages 7–9) notes: Describe how water and mineral ions move from the soil through the plant and out of the leaves, and the factors that affect transpiration.Roles — plants…
- Root hair cells
- Xylem vessels
- Loss control
- Celery stem experiment
- Roles — plants need water for photosynthesis, for support and to carry substances around the plant body
- Wilting — if a plant does not get enough water its stems and leaves droop
Section 1 of 6Why plants need water
- Roles
- plants need water for photosynthesis, for support and to carry substances around the plant body.
- Wilting
- if a plant does not get enough water its stems and leaves droop.
Section 2 of 6Uptake by roots
- Root hair cells
- they have long, microscopic hairs that give a large surface area for absorbing water and minerals.
- Water
- it moves from the soil into root hair cells by osmosis, because the soil solution is more dilute than the cell sap.
- It then crosses the root cortex to the xylem.
- Mineral ions
- nitrate and other ions enter root hair cells by active transport, using energy against a concentration gradient.
- They are carried dissolved in the water.
Section 3 of 6Xylem and the transpiration stream
1Xylem vessels2 points
- they are long, dead, hollow tubes strengthened by lignin that form a continuous channel from root to leaf.
- They carry water and dissolved minerals upwards and also support the plant.
2Transpiration
= the evaporation of water from the leaves and its loss through the stomata by diffusion.
3Transpiration stream5 points
- water moves up the plant in steps.
- Water evaporates from the surface of mesophyll cells into the air spaces of the leaf.
- Water vapour diffuses out through the open stomata into the drier air.
- As water is lost, more is pulled up the xylem to replace it, rather like sucking on a drinking straw.
- Water molecules stick to each other (cohesion) and to the xylem walls, so a continuous column is pulled up from the roots.
4Stomata2 points
- tiny holes on the underside of leaves, each surrounded by cells that open and close it.
- Carbon dioxide moves in and spare oxygen moves out through them.
Section 4 of 6Benefits of transpiration
- It brings water and minerals to the leaves.
- It keeps cells firm (turgid), which supports the plant.
- It supplies water for photosynthesis.
- Evaporation helps to cool the leaves.
- Loss control
- a plant that loses water faster than it takes it up wilts, so stomata close at night or in dry conditions.
Section 5 of 6Factors affecting the rate
| Factor | Effect | Reason |
|---|---|---|
| Higher temperature | Faster | Faster evaporation and diffusion |
| Greater light intensity | Faster | Stomata open wider |
| Higher humidity | Slower | Smaller concentration gradient |
| More wind | Faster | Moist air is blown away, keeping the gradient steep |
Section 6 of 6Measuring and showing water movement
1Potometer4 points
- it measures water uptake by a cut shoot, since a bubble in a capillary tube moves along as the shoot takes up water.
- The distance moved in a set time gives the rate.
- Keep the apparatus airtight and cut the shoot under water so air does not block the xylem.
- Change one factor at a time and repeat readings.
2Celery stem experiment2 points
- a celery stem stood in coloured water draws the dye up the xylem, which can be seen in the stem and leaves.
- Dipping the stem end in wax blocks the xylem and no dye is taken up, which shows that water travels in the xylem and enters through the cut end.
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