Transpiration

• Transpiration is the loss of water from the leaf via diffusion and evaporation
• Transpiration stream is the movement of water from the roots to the leaves
• 1. The transpiration stream starts at the root. Here, water moves from the soil into the root tissues via the process of osmosis (minerals move by active transport because they are in a lower concentration in the soil)

  
• 2. Then water moves up the stem to the leaves. It is transported in the xylem which has strong walls. It moves in this direction because it is pulled upwards as water is lost in the leaf

  
• 3. The xylem flow into the leaves within the spongy mesophyll layer. Water then diffuses from here to the palisade mesophyll layer. In these cells, the water is used to perform photosynthesis.

  
• 4. Excess water diffuses out of the stomata which are created by two guard cells. The water then evaporates off the under surface of the leaf

  
• The higher the temperature, the faster transpiration happens because water evaporates quicker
• The windier it is, the faster transpiration happens because the wind pushes water off the surface
• The higher the humidity, the slower transpiration happens
• The higher the light intensity, the faster transpiration happens
• Why would an increase in light intensity have this effect on transpiration?

  Increased light intensity speeds up the rate of photosynthesis. More water is therefore used in the process and any excess needs to be removed
• Why would high humidity levels have this effect?

  High humidity means lots of water vapour (H2O) in the air, therefore the concentration gradient is less steep so it slows down the rate of evaporation.
• The bigger the surface area of the leaf, the faster transpiration happens
• The more stomata a leaf has, the faster transpiration happens
• Having more stomata can be both an advantage and disadvantage. Explain how.

  Disadvantage - Hot climate means water loss

  Advantage - Maximises CO2 intake for photosynthesis (e.g. even in low CO2 conditions)
• The apparatus is filled with water and a cutting plant is sealed in one end. An air bubble is introduced in the tube. The tube has a scale along it. The bubble will move as water is lost from the plant. If transpiration was happening quickly, we would observe the bubbles moving quickly

  
• The apparatus is filled with water and a cutting plant is sealed in one end. At the other end, the tube has a scale along it. The water level will move down as water is lost from the plant. If transpiration was happening quickly, we would observe the water level falling quickly

  
• The plant is placed on some scales and the mass is measured. Shows the speed of which mass is lost. As the plant loses water, the mass will decrease. If transpiration was happening quickly, we would observe a lot of mass being lost

  
• Vaseline (or an oily jelly) is smeared on one or both sides of the leaf. What would smearing the leaf in Vaseline do to the pores?

  This would block the pores
• How does smearing Vaseline on one or both sides of a leaf affect water loss from the plant?

  Less water would be lost