Translocation

• Define Phloem:

  A living plant transport vessel that carries assimilates (mainly sucrose and amino acids) to all parts of the plant
• Name the features of phloem vessels:

  • sieve plates
  • sieve tube elements
  • companion cells
• Define Companion Cells:

  Small active cells of the phloem located in between the sieve tube elements
• Name the living features of the phloem:

  1. companion cells
  2. sieve tube elements
• Outline the features of the companion cells:

  • Large Nucleus
  • Dense Cytoplasm
  • Lots of mitochondria = for active transport, produces ATP for metabolic processes e.g loading sucrose into sieve tubes
• Define Sieve Plates:

  The perforated end walls of sieve tube elements that allow assimilates to flow from one element to the next in the phloem
• Define Sieve Tube Elements:

  Elongated cells lined up end to end with sieve plates in between to form long continuous columns
• Outline the features of sieve tube elements:

  • Bidirectional - up and down
  • No nucleus = provides space for the mass flow of sap
  • very little cytoplasm = provides space for the mass flow of sap
  • Thin wall
• The cytoplasm of sieve tube elements and companion cells is linked by plasmodesmata
• Define Translocation:

  The transport of assimilates throughout the plant
• Define Sink:

  The regions of a plant that remove assimilates
• Define Source:

  The regions of a plant that produce assimilates
• Active Loading:

  • Via active transport, H+ ions are pumped out into surrounding tissue of the companion cell using energy from ATP = increases the concentration of H+ ions outside the companion cell while decreasing the concentration inside the cell
  • creates a concentration gradient
  • Through the process of facilitated diffusion, it causes H+ ions to diffuses back accompanied by sucrose through co-transport proteins= increases the concentration of sucrose in the companion cell which diffuses out through plasmodesmata into sieve tube elements
• Movement of sucrose in the phloem

  1. Mass flow of assimilates caused by difference in hydrostatic pressure produces a pressure gradient

  2. Source loads sucrose which enters the sieve tube elements via active transport= reduces the water potential in the sieve tube elements

  3. Via osmosis, water molecules from the xylem enter the sieve tube elements= increases hydrostatic pressure

  4. The sap moves down to a lower hydrostatic pressure

  5. The sink removes the sucrose which enters the surrounding tissues via active transport= increases the water potential in the sieve tube elements

  6. Water molecules move out into the xylem via osmosis= reduces the hydrostatic pressure
• Translocation occurs through the sieve elements by mass flow
• By what process is sugars produced in plants?

  Photosynthesis
• During translocation, what type of cell is sucrose actively transported into?

  Companion cell