SB6d- Transpiration and translocation

• What does the phloem transport?

  Transports sugars (food) for translocation
• What does the xylem transport?

  Transports water for transpiration
• Describe the phloem

  • It has seive tube elements which have very little cytoplasm so that there is a lot of space to transport sucrose
  • It has companion cells which have lot of mitochondria, which supplies energy for active transport. Active transport moves sucrose in and out of the sieve tubes -It has pores to enable movement of cell sap in either direction
• Describe the xylem

  • It is made out of dead cells, which means there is more space for water containing mineral ions to pass through
  • They have pits (holes) to allow water and mineral ions to move out
  • Walls are strengthened by lingin rings, making them very strong and prevents collapsing
  • It has no end walls so they form a long tube that water can flow through easy (this tube transports water and mineral ions from the roots up the stem to leaves where water is used for photosynthesis)
• How does the phloem transport?

  Transports sugars up and down the stem from photosynthetic tissues through translocation
• How does the xylem transport?

  Transports water and minerals up the plant, from the roots to the leaves through the transpiration stream
• What is transpiration?

  The process of losing water by evaporation from the leaves
• Transpiration stream

  The movement of water from the roots to the leaves
• What is the transpiration stream?

  1. Water enters the roots by osmosis
  2. Water is drawn up the stem through the xylem from the roots
  3. Water is drawn up out of the leaf cells and xylem
  4. Water vapour evaporates from leaves mainly through the stomata
• What can affect the rate of transpiration?

  1. Temperature
  2. Light intensity
  3. Air flow
  4. Humidity
• How can temperature affect the rate of transpiration?

  The warmer it is, the higher the rate of transpiration because water particles have more energy, therefore more likely to evaporate
• How can light intensity affect transpiration?

  The brighter light intensity, the more photosynthesis, therefore the longer the stomata is open to let carbon dioxide in. More water evaporates because of this.

  At night, the stomata is closed so there is little transpiration
• How can air flow affect transpiration?

  When airflow is high, water particles are blown away, keeping the concentration gradient and increasing transpiration
• How can humidity affect transpiration?

  The more humid air decreases the rate of transpiration because large amounts if water in the air decreases the concentration gradient
• Guard cells and stomata

  • When there is water in the leaf it can diffuse into guard cells by osmosis
  • When guard cells vacuoles are full of water (turgid), the cell expands and opens the stoma.
  • Stoma= a gap between two guard cells where water can evaporate and carbon dioxide can diffuse in.
  • When there is not enough water in the leaf, water will leave the vacuoles of the guard cells and makes then flaccid, closing the stoma and preventing more water from evaporating
• What is used to measure the rate of transpiration?

  A potometer
• How is a potometer used to measure the rate of transpiration?

  1. Note the position of the air bubble on the ruler at the start of the investigation
  2. Note the position of the bubble on the ruler after a known number of minutes
  3. Divide the distance moved by the bubble by the time taken
• What is the formula for the rate of transpiration?

  distance moved by bubble / time taken
• What assumptions are made with the transpiration equation?

  That the rate of water uptake = rate of transpiration