Transpiration And Translocation - OCR 21C - BBC Bitesize - BBC

In this guide

1. Revise
2. Test

Pages

1. Plant organisation
2. Epidermis, palisade mesophyll and spongy mesophyll
3. Plant transport tissues - xylem and phloem
4. Transpiration and translocation
5. Factors affecting transpiration
6. Scientific calculations
7. Measuring water uptake - potometers
8. Effects of environmental factors on water uptake
9. Investigate distribution of stomata and guard cell

Transpiration and translocation

Transpiration

When the plant opens its stomata to let in carbon dioxide, water on the surface of the cells of the spongy mesophyll and palisade mesophyll evaporates and diffuses out of the leaf. This process is called transpiration.

Water is drawn from the cells in the xylem to replace that which has been lost from the leaves.

A continuous column of water is therefore pulled up the stem in the transpiration stream by evaporation from the leaves.

As water travels through the xylem in the stem and leaf, it is being replaced by water taken up by the roots.

Transpiration is an unavoidable consequence of photosynthesis – only five per cent of the water taken up by the plant is used for photosynthesis – but does have its purposes:

• provides the water for photosynthesis
• transports mineral ions
• cools the leaf as water evaporates
• provides water that keeps the cells turgid, which supports leaves and herbaceous (non-woody) plants

Water uptake and transport across the root

Root hairs are single-celled extensions of epidermal cells in the root. They grow between soil particles and absorb water and minerals from the soil.

Water enters the root hair cells by osmosis. This happens because soil water has a higher concentration of water molecules than the cytoplasm of the root hair cell. Minerals enter by active transport.

A summary of water uptake, water transport and transpiration:

Translocation

Photosynthesis produces glucose in the green parts of plants, which are often leaves. This is then converted into sucrose. The sucrose is transported around the plant in phloem vessels. It needs to be able to reach all cells in the plant so that the sucrose can be converted back into glucose for respiration. The movement of sucrose and other substances like amino acids around a plant is called translocation. In general, this happens between where these substances are made (the sources) and where they are used or stored (the sinks):

• from sources in the root to sinks in the leaves in early spring time
• from sources in the leaves to sinks in the root in the summer

Comparison of xylem and phloem

Tissue	Process	What is moved	Structure
Xylem	Transpiration	Moves water and minerals from roots to leaves	Columns of hollow, dead reinforced cells
Phloem	Translocation	Moves food substances from leaves to rest of plant and from stores such as in the roots	Columns of living cells

Tissue	Xylem
Process	Transpiration
What is moved	Moves water and minerals from roots to leaves
Structure	Columns of hollow, dead reinforced cells

Tissue	Phloem
Process	Translocation
What is moved	Moves food substances from leaves to rest of plant and from stores such as in the roots
Structure	Columns of living cells

Next pageFactors affecting transpirationPrevious pagePlant transport tissues - xylem and phloem

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