Exchange and transport in plants

• Carbon dioxide enters leaves by diffusion through the stomata
• Most of the water and mineral ions are absorbed by roots
• The surface area of roots is increased by root hair
• The surface area to volume ratio of leaves is increased by flattened shape and internal air spaces
• Stomata also help to control the rate of water loss
• To open stomata, water moves in by osmosis. The cells become turgid opening a pore between the two guard cells
• To close stomata, water moves out via osmosis. The cells become flaccid closing the pore between the two guard cells
• Plants mainly lose water vapour from their leaves
• Most of the loss of water vapour takes place through the stomata
• Evaporation is more rapid in hot, dry and windy conditions
• Plant adaptations to conserve water:-

  Rolled leaves and small hairs coating a leaf trap a layer of water increasing humidity

  Reduced number of stomata and thick waxy cuticle and reduced leaves/spines reduces evaporation

  Thick stems store water
• Xylem tissue transports water and mineral ions from the roots to the stem and leaves
• Transpiration is the loss of water vapour from the leaf
• Transpiration stream the movement of water from the roots through the xylem and out of the leaves
• Translocation is when phloem tissue carries dissolved sugars from the leaves to the rest of the plant, including the growing regions and the storage organs