Transpiration and translocation

Transpiration

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

Cross-section of a leaf showing water travelling through xylem cells to the spongy mesophyll cells, where it evaporates and exits through the stoma as water vaper

Water is drawn from the cells in the 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 – 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
cools the leaf as water evaporates
provides water that keeps the cells , which supports leaves and herbaceous (non-woody) plants

Water uptake and transport across the root

Root hairs are single-celled extensions of 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 .

Diagram of water entering the root hair cells by osmosis

A summary of water uptake, water transport and transpiration:

Diagram illustrating how water enters the plant and is transported across the root

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 around a plant is called . 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

How do producers transport substances they need? - OCR 21st CenturyTranspiration and translocation