8: Plant physiology

• What is the process of transporting organic solutes in plants called?

  Translocation
• Through which tissue does translocation occur?

  Phloem tissue
• What are the important components of phloem?

  Sieve tube elements and companion cells
• What is the function of sieve tube elements?

  Transport organic solutes like sucrose
• How do companion cells assist sieve tube elements?

  They control the functions of sieve tubes
• What is the direction of translocation in plants?

  From source to sink
• What typically acts as a source in translocation?

  Leaves making food
• What typically acts as a sink in translocation?

  Roots or storage organs
• What is the composition of phloem sap?

  10-25% dry matter, mostly sucrose
• What is the "pressure flow" theory in translocation?

  It explains how organic solutes move in plants
• What are the steps in the pressure flow mechanism of translocation?

  1. Sugars loaded into phloem by companion cells
  2. Increased sugar concentration reduces water concentration
  3. Water enters phloem via osmosis, raising hydrostatic pressure
  4. Pressure difference causes translocation from source to sink
  5. Organic solutes absorbed by sink cells
  6. Water flows back into xylem tubes
• How does the loading of sucrose into sieve tubes affect water movement?

  It causes water to move in by osmosis
• What happens to the phloem sap when it reaches the sink?

  Organic solutes are actively absorbed
• What is the effect of solute loss in phloem sap?

  Increases solute and water potential
• What is required for the active loading and unloading of sucrose?

  ATP
• How does the pressure flow mechanism resemble a hose?

  It pushes sugary solution through pressure differences
• What is the role of root cells in translocation?

  They actively absorb organic solutes
• What happens to water after solutes are absorbed at the sink?

  Water flows back into the xylem tubes
• How do leaves and roots function in translocation?

  Leaves are sources; roots are sinks
• What is the significance of the pressure flow mechanism in plants?

  It facilitates efficient transport of nutrients