Transpiration (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Transpiration

An overview of transpiration

• Transpiration is caused by the evaporation and diffusion of water from the surfaces of the plant

  • Transpiration primarily occurs within the leaves

•  As evaporation and diffusion of water creates a net loss of water in the leaf, water is drawn up through the xylem to replace it

  • Consequently, more water is absorbed by the roots

  • This creates a constant transpiration stream

Water uptake in the root

• Water and mineral ions are taken into the plant via the roots and root hair cells

• Root hairs are single-celled extensions of epidermis cells in the root which increase the surface area of the cells significantly; this increases the rate of the absorption of water by osmosis and mineral ions by active transport

• They grow between soil particles and are adapted for the efficient uptake of water and mineral ions from the soil

• Water and minerals enter the roots as follows:

  • minerals are taken up from the soil by either active transport or diffusion, depending on soil mineral concentrations

  • mineral ions lower the water potential of the root hair cells, and water enters the cells from the soil by osmosis

Water movement in the xylem

• Water moves into the xylem by osmosis and is drawn upwards, towards the leaves, due to transpiration

•  Movement in the xylem only takes place in one direction - from roots to leaves 

• The role of the xylem is to transport water and mineral ions from the roots to other parts of the plant

• The upward movement of water in the xylem is known as the transpiration stream

Structure of the xylem

• The xylem is formed from a hollow tube of dead cells, reinforced by lignin, which provides a route for the column of water to move through the plant by transpiration

Water loss in the leaves

• In the leaves, water moves from the xylem into spongy mesophyll

• Water evaporates from the surface of the mesophyll cells and is lost through the stomata as water vapour

Stomata

• Stomata are spaces found between two guard cells predominantly on the lower epidermis of the leaf

• The guard cells open and close the stomatal pore, controlling gas exchange and water loss

• Stomata open when water moves (by osmosis) into the guard cells causing them to become turgid

  • This allows gases to diffuse in and out of the leaf through the stomatal pore

  • Stomata tend to open when there is plenty of water and sunlight

• Stomata close when the guard cells lose water (by osmosis) to the neighbouring epidermal cells and they become flaccid

  • This prevents any diffusion into or out of the leaf

  • Stomata tend to close due to low water availability or low sunlight

Factors Affecting Transpiration

• Air movement, humidity, temperature and light intensity all have an effect on the rate at which transpiration occurs

• The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low