Mass transport in plants

• Describe translocation of sucrose in the phloem.

  Sucrose starts in a source cell, moves into a companion cell and moves into a sieve tube element. Sieve tube elements are responsible for transporting the sucrose solution throughout the plant. They contain sieve plates, which have holes the sucrose solution can travel through. There, the sucrose solution moves up or down through the sieve tube elements. When it gets to where it is needed, the sucrose solution moves into a second companion cell and then into the desired sink cell.
• what is hydrostatic pressure ?


  pressure exerted by a fluid
• Explain the mass flow hypothesis for the translocation of sucrose in the phloem

  First, sucrose moves into the sieve tube element. This causes the water potential in the sieve tube element to decrease. Water moves from the xylem into the sieve tube element by osmosis. This causes the hydrostatic pressure in the sieve tube element to increase. At the sink cell, water moving out of the sieve tube element decreases the hydrostatic pressure here. As a result, the sucrose solution moves down the hydrostatic pressure gradient. Finally, sucrose moves into the sink cell .
• Explain how sucrose moves through companion cells from a source cell to the sieve tube elements

  First, a carrier protein moves H+ ions from the cytoplasm of the companion cell to the cell wall, using active transport. This creates a concentration gradient, and the H+ ions are returned to the cytoplasm via a co-transport protein, which also transports sucrose. Once the sucrose is inside the companion cell, it diffuses into the sieve tube element.
• What is the first step in the translocation of sucrose in the phloem?

  A carrier protein moves H+ ions
• How do H+ ions move in the first step of sucrose translocation?

  They move using active transport
• What is created by the movement of H+ ions to the cell wall?

  A concentration gradient
• What type of protein returns H+ ions to the cytoplasm?

  A co-transport protein
• What does the co-transport protein also transport along with H+ ions?

  Sucrose
• What happens to sucrose once it is inside the companion cell?

  It diffuses into the sieve tube element
• How does the addition of sucrose affect the water potential in the sieve tube element?

  It decreases the water potential
• How does water move into the sieve tube element?

  By osmosis
• What effect does water moving into the sieve tube element have?

  It increases hydrostatic pressure
• What happens at the sink cell regarding hydrostatic pressure?

  Sucrose moving out decreases hydrostatic pressure
• What drives the movement of the sucrose solution in the phloem?

  The hydrostatic pressure gradient
• What is the final destination of sucrose in the translocation process?

  The sink cell
• What are the steps involved in the translocation of sucrose in the phloem?

  1. H+ ions are moved by a carrier protein.
  2. A concentration gradient is created.
  3. H+ ions return via a co-transport protein with sucrose.
  4. Sucrose diffuses into the sieve tube element.
  5. Water potential in the sieve tube decreases.
  6. Water moves from the xylem by osmosis.
  7. Hydrostatic pressure in the sieve tube increases.
  8. Sucrose moves out at the sink cell, decreasing pressure.
  9. Sucrose solution moves down the pressure gradient.
  10. Sucrose enters the sink cell.
• Explain fully the process of translocation of sucrose in the phloem. Include details about movement of sucrose through the companion cells in your answer

  First, a carrier protein moves H+ ions from the cytoplasm of the companion cell to the cell wall, using active transport. This creates a concentration gradient, and the H+ ions are returned to the cytoplasm via a co-transport protein, which also transports sucrose. Once the sucrose is inside the companion cell, it diffuses into the sieve tube element. This causes the water potential in the sieve tube element to decrease. Water moves from the xylem into the sieve tube element by osmosis. This causes the hydrostatic pressure in the sieve tube element to increase. At the sink cell, sucrose moving out of the sieve tube element decreases the hydrostatic pressure here. As a result, the sucrose solution moves down the hydrostatic pressure gradient. Finally, sucrose moves into the sink cell.
• Explain fully the process of translocation of sucrose in the phloem

  First, a carrier protein moves H+H+ ions from the cytoplasm of the companion cell to the cell wall, using active transport. This creates a concentration gradient, and the H+H+ ions are returned to the cytoplasm via a co-transport protein, which also transports sucrose. Once the sucrose is inside the companion cell, it diffuses into the sieve tube element. This causes the water potential in the sieve tube element to decrease. Water moves from the xylem into the sieve tube element by osmosis. This causes the hydrostatic pressure in the sieve tube element to increase. At the sink cell, sucrose moving out of the sieve tube element decreases the hydrostatic pressure here. As a result, the sucrose solution moves down the hydrostatic pressure gradient. Finally, sucrose moves into the sink cell.
• key features of mass flow hypothesis


  • water moves in and out the phloem by osmosis
  • pressure moves sucrose solution
  • it involves active processes that use ATP