2.2.7 Transpiration & Translocation

• Plants possess two specialist transport vessels called the xylem and phloem which are integral to the processes of transpiration and translocation
• Water travels up the xylem from the roots into the leaves of the plant to replace the water that has been lost due to 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
  •  Movement in the xylem only takes place in one direction - from roots to leaves 

Water uptake, transport and transpiration

• Transpiration is a side-effect of how leaves are adapted to maximise gas-exchange for photosynthesis
• Plants contain tiny pores (openings) called stomata that allow for gas exchange
  • Stomata are typically found on plant leaves but can be found on some stems

The role of stomata

• Stomata are formed by two kidney-shaped guard cells which open and close the stomatal pore
  • Stomata can be opened or closed depending on the conditions the plant is in
• The role of stomata and guard cells is to control gas exchange and water loss
• Guard cells have cell walls with unevenly distributed cellulose – the inner wall is thicker and the outer wall is thinner to aid opening and closing of the stomata
• When the availability of water is high, guard cells become turgid as a result of osmosis
  • This causes the stomatal pore to open which allows gases to diffuse in and out of the leaf
  • Water is consequently lost via transpiration
• When less water is available, the guard cells lose water by osmosis and become flaccid, pulling them together
  • This closes the stomatal pore and reduces water loss via transpiration
• Stomata are sensitive to light and open in the day and close during the night
  • This allows water to be conserved whilst no photosynthesis is occurring 

The guard cells control the opening and closing of the stomata

The guard cells control whether or not the stomata are open or closed, directly affecting how much transpiration can occur

• The transport of the soluble products of photosynthesis (mainly sucrose) in the plant is called translocation
  • Translocation is an active process that requires energy
• Sucrose (and amino acids) are transported around the plant in the phloem tubes 
• In general, translocation happens between where the substances are made (sources) and where they are used or stored (sinks)
• The direction of transport can vary depending on the season and requirements of the plant 
  • During early spring, sucrose is transported from sources in the root to sinks in the leaves (which are starting to grow following winter)
  • During summer, sucrose is transported from sources in the leaves to sinks in the roots (where it can be used or stored as starch)

Translocation through the phloem