Plant organisation - AQATranspiration

Plant cells, tissues and organs are adapted to their functions. The stem, root and leaves form an organ system that transports substances into, around and out of a plant.

Part of Combined ScienceOrganisation

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.

Water molecules inside the xylem cells are strongly attracted to each other. There is strong cohesion between the molecules because of . 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
  • cools the leaf as water
  • provides water that keeps the cells , which supports herbaceous 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 water potential than the cytoplasm of the root hair cell. Minerals enter by .

Diagram of water entering the root hair cells by osmosis
Figure caption,
Soil water moves into the root hair cell by osmosis and across the root cortex to the xylem

A summary of water uptake, water transport and transpiration:

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