Transpiration in Plants (OCR A Level Biology)

Water uptake

• Plant roots are responsible for the uptake of water and mineral ions from the soil
  • Root hair cells are adapted to maximise water and ion uptake
• 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 transport across the root

• Once water has entered the root hair cells it must be transported across the root cortex to the xylem
• There are two pathways that water can take across the cortex:
  • The apoplast pathway
  • The symplast pathway

Apoplast pathway

• Water moving on the apoplast, or apoplastic, pathway travels by diffusion within the cell walls and intercellular spaces of plant tissue
  • Note that this is not osmosis because the water does not cross any cell membranes
• Water is drawn across the root via the apoplast pathway due to cohesive forces between water molecules:
  • Water moves upwards in the xylem due to transpiration
  • Cohesion between water molecules means that more water is drawn along the apoplast pathway within the root to replace the water that has moved upwards

Symplast pathway

• Water moving on the symplast, or symplastic, pathway travels via cell cytoplasm and vacuoles
  • Water enters the symplast pathway and moves between cells, and into cell vacuoles, by osmosis
  • Water can also move from cell to cell by diffusion via the plasmodesmata
• Water is drawn across the root via the symplast pathway as follows:
  • water moves into root hair cells from the soil by osmosis, increasing the water potential of the root hair cell
  • water moves down its water potential gradient into neighbouring root cells, increasing their water potential
  • water continues to move across the root from high to low water potential

Water movement into the xylem

• When water reaches the centre of the root it must cross the endodermis to enter the xylem
• The cells of the endodermis are surrounded by a waxy band known as the Casparian strip, which forms an impassable barrier to water
  • The waxy material is known as suberin
• The Casparian strip blocks the cell walls of the endodermis cells, preventing water from entering the xylem via the apoplast pathway and instead forcing it into the symplast pathway
  • it is thought that this may help the plant control which mineral ions reach the xylem

Water enters root hair cells by osmosis and then crosses the root via the apoplast or symplast pathway; once it reaches the endodermis it is forced into the symplast pathway by the Casparian strip

Water movement in the xylem

• Water is drawn upwards in the xylem due to transpiration as follows:
  • water evaporates from the surface of cells in the leaves, lowering the water potential of leaf cells
  • water is drawn out of the xylem and into leaf cells by osmosis down its water potential gradient
  • more water molecules are drawn upwards in the xylem in a continuous column due to forces of cohesion between water molecules
    • attractive forces of adhesion between water molecules and the sides of the xylem aid this process
• The upward movement of water in the xylem is known as the transpiration stream

Water is drawn upwards in the xylem due to evaporation of water in the leaves; forces of cohesion and adhesion ensure that water moves in a continuous column

Movement of water in the leaves

• Water moves through the leaves of plants due to transpiration as follows:
  • Water vapour diffuses out of leaf air spaces and into the surrounding environment down a water vapour potential gradient
  • The loss of water vapour from the air spaces creates a water potential gradient between leaf mesophyll cells and the leaf air spaces, so more water moves from the leaf mesophyll cells into the air spaces
    • Water first moves from the cell cytoplasm to the cell surface, before evaporating into the air space
  • Losing water lowers the water potential of the leaf mesophyll cells, so water moves into the cells by osmosis from neighbouring cells and the xylem
• Note that water movement through the leaf also occurs via the apoplast and symplast pathways

Water moves across leaves and into the leaf air spaces by osmosis in the symplast or apoplast pathway