Processes In Plants

• Transpiration


  • The loss of water by evaporation from plants
  • Plants lose water when they open the stomata in the leaves to let in carbon dioxide
  • Air around the plant usually contains less water than the cells of the plant, so water evaporates into the air
• Transpiration moves water from the roots to the top of the plant without using energy
• Rate of transpiration


  • The speed at which a plant loses water
  • This varies depending on the plant's environment
• Environmental factors affecting rate of transpiration

  • Light intensity
  • Temperature and humidity (amount of moisture in the air)
  • Air movement (wind)
• Measuring transpiration rate


  Using a potometer - A cut plant stem is sealed into the potometer, an air bubble is introduced to the capillary tube, the distance the bubble travels shows how much water the stem has taken up
• How to use a potometer


  Cut leafy shoot under water at a slant

  2. Assemble potometer under water, insert shoot

  3. Remove from water, keep capillary tube submerged

  4. Make apparatus watertight and airtight

  5. Dry leaves, allow to acclimatise, shut tap to reservoir

  6. Remove capillary tube until one air bubble forms, then replace

  7. Record starting position of bubble, measure distance it moves over time
• Translocation


  Transports substances like sugars

  Starts at the source (usually leaves)

  Sugars move into phloem by active transport

  Water is then drawn into the phloem by osmosis, increasing pressure to push substances along

  Sugar is unloaded at the sink (roots, stems, flowers) for respiration and growth
• Apparatus setup

  1. Apparatus removed from water but end of capillary tube kept submerged in beaker of water

  2. Apparatus must be watertight and airtight

  3. Leaves on leafy shoot dried, shoot acclimatised, tap to reservoir shut

  4. Capillary tube removed from beaker until one air bubble forms then placed back in beaker

  5. Starting position of bubble recorded, distance bubble moves over time recorded
• Translocation


  • Transports substances like sugars
  • Starts at the source (usually leaves)
  • Sugars move into phloem by active transport
  • Water drawn into phloem by osmosis
  • Increases pressure in phloem tubes, pushing substances along
  • Sugar unloaded at the sink (roots, stems, flowers)
  • Sugar used at sink for respiration and growth
• Increasing light intensity


  Increases the rate of transpiration, but eventually levels out as light intensity becomes a limiting factor
• Increasing air movement

  Increases the rate of transpiration due to larger gradient for diffusion
• Increasing temperature

  Increases the rate of transpiration until optimum, then decreases as enzymes denature