Mass transport in plants

• Name and describe the adaptations of the xylem

  Lignin in the walls = provide strength to hold the xylem open

  Pits in the wall = allow lateral movement of water

  No organelles/cytoplasm in cells = allow continuous flow of water

  No end walls between fused cells = allow continuous flow of water
• Describe the cohesion-tension theory of water movement in xylem vessels

  -Water evaporates into their spaces through open stomata (transpiration)

  -Water potential of mesophyll cells is lowered

  -Water is drawn into the mesophyll cells from the xylem by osmosis, which creates suction (negative pressure). This forms tension as water is pulled up the xylem

  -Water is pulled up as a continuous column because of H bonds between molecules, creating cohesion

  -The pulling of water is helped by adhesion when water molecules stick to the xylem walls
• Describe the process of a potometer

  -A plant cutting takes up water from a capillary tube through its exposed xylems

  -An air bubble is introduced into the capillary tube first

  -Measure the rate of transpiration using $\pi$r2h

  r: radius of capillary tube

  h: distance air bubble in tube moves over time
• Why is a potometer only an estimate of the transpiration rate?

  -Water is used by plant cells in photosynthesis, vacuoles and hydrolysis

  -Water is made by plant cells in respiration and condensation reactions
• What are 4 precautions to ensure accuracy with a potometer?

  -Cut shoots underwater = prevent air from entering the xylem which would break the continuous column of water

  -Seal all joints with waterproof jelly = preventing water from leaking out

  -Dry off leaves beforehand = allows a water potential gradient for evaporation

  -Close reservoir tap before measurements = control variable - the volume of water in the apparatus
• Describe and explain the adaptations of the phloem for mass flow of solutes

  -Phloem cells, called sieve elements are stacked and have no organelles/cytoplasm = continuous flow of solutes

  -End walls between cells form sieve plates = continuous flow of solutes

  -Supported by companion cells = release energy for active transport
• What is actively transported into the phloem from source cells?

  Sucrose
• How is sucrose transported into the phloem?

  By companion cells through active transport
• What effect does the active transport of sucrose have on the phloem's water potential?

  It lowers the phloem's water potential
• What process allows water to enter the phloem from the xylem after sucrose is transported in?

  Osmosis
• What is the result of water entering the phloem from the xylem?

  It creates a high hydrostatic pressure at the source
• What causes the mass flow of phloem sap to respiring cells?

  The high hydrostatic pressure at the source
• How is sucrose transported out of the phloem at the sink?

  By companion cells through active transport
• What happens to the phloem's water potential when sucrose is transported out at the sink?

  It increases
• What occurs as a result of the increase in phloem water potential at the sink?

  Water leaves the phloem and enters the xylem by osmosis
• What is created at the sink due to the movement of water and sucrose?

  A low hydrostatic pressure
• What is established as a result of the low hydrostatic pressure at the sink?

  A hydrostatic pressure gradient from source to sink
• Describe evidence for mass flow - Ringing Experiment

  A ring of stem/bark which damages the phloem but leaves the xylem intact is removed, resulting in:

  -Swelling just above removed ring where hydrostatic pressure builds up

  -Death of cells below removed ring due to a lack of respiratory substrates

  -No effect on cells above the removed ring, showing the xylem is not involved in mass flow
• Describe evidence for mass flow - Radioactive Tracer Experiment

  One leaf is treated with radioactive CO2. The radioactive carbon is incorporated into glucose and then sucrose

  Autoradiography is used to trace the movement of the radioactive carbon from the source to sink at different time points