Transpiration and Translocation

• Transpiration (part 1):

  • 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.
• Transpiration (part 2):

  • 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 (part 3):

  • 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 (part 1):

  • 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.
• The role of stomata (part 2):

  • 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.
• The role of stomata (part 3):

  • 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 (part 1):

  
• The guard cells control the opening and closing of the stomata (part 2):

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

  
• Translocation (part 1):

  • 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).
• Translocation (part 2):

  • 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 (part 1):

  
• Translocation through the phloem (part 2):

  
• The palisade mesophyll is where photosynthesis takes place. Xylem tissue transports water and dissolved mineral ions from the roots to the leaves. Waters enters the roots through root hair cells. Like this...

  
• Water is constantly evaporating from the surfaces of leaves. This process is called transpiration. Transpiration starts with the evaporation of water from cells inside the leaf. Like this...

  
• The water vapour then diffuses through the air spaces in the spongy mesophyll and out of the leaf, through the stomata. Like this...

  
• Now water passes from the xylem into the leaf to replace the water that has been lost. Finally, water is drawn into the root hair cells and up the xylem vessels to the leaf, this is called transpiration stream. Like this...

  
• Transpiration brings water to the leaf. Water is required for photosynthesis. The transpiration stream transports dissolved mineral ions such as magnesium, which play important roles in the plant. The evaporation of water from the leaf cools the leaf down, especially in warm weather.
• The rate of transpiration is greater at higher temperatures. That is because evaporation is faster when temperatures are higher. Also transpiration is also faster under dry conditions when the air is not humid. That is because evaporation takes place more quickly under dry conditions.
• The rate of transpiration increases in windy conditions. That is because wind removes any water vapour, allowing more water to evaporate. The rate of transpiration increases when the light intensity increases. That is because high light intensity increases the rate of photosynthesis. The stomata now opens to allow carbon dioxide to enter. Once the stomata have opened, water vapour can now pass out of the leaf.
• Stomata are surrounded by two guard cells. When the light intensity is high for example during the day, the guard cells swell and they change their shape. like this...

  
• This causes the stomata to open. Now carbon dioxide can diffuse into the leaf and be used in photosynthesis. Like this...

  
• Under hot conditions, the plant closes its stomata to reduce water loss by transpiration. That now means that the plant cannot photosynthesise. Like this...