Translocation

• What is translocation in plants?

  Movement of soluble products through phloem
• What are the soluble products of photosynthesis that are translocated?

  Sucrose and amino acids
• What are sieve tube elements?

  Components of phloem for transporting substances
• Why is sucrose preferred over glucose for transport?

  Sucrose is less osmotically reactive
• How does the structure of sieve tube elements facilitate transport?

  They have perforated end walls for flow
• What is the role of companion cells in phloem?

  • Control metabolism of sieve tube elements
  • Highly metabolically active
  • Contain many mitochondria and rough ER
• What is the purpose of ringing experiments?

  To analyze phloem contents above and below a ring
• What did ringing experiments reveal about sucrose?

  Sucrose accumulates above the ring
• What is the significance of radioactive tracers in studying phloem transport?

  They show the movement of sucrose in phloem
• How do aphid experiments demonstrate phloem transport?

  Sap exudes under pressure from aphid's stylet
• What speed does sucrose move in phloem according to aphid experiments?

  0.5-1 m h$^{-1}$
• What is the mass flow hypothesis in translocation?

  • Passive mass flow of sugars from source to sink
  • Sugars move from high to low concentration
  • Water potential changes drive the process
• What limitations exist for the mass flow hypothesis?

  Translocation is much faster than diffusion
• Why is the mass flow hypothesis insufficient to explain translocation?

  It does not account for high oxygen consumption
• What happens to sucrose and amino acids in the same tissue?

  They move at different rates
• What role do protein filaments play in phloem transport?

  They may carry different solutes through sieve tubes
• What is translocation in plants?

  Movement of organic solutes through phloem
• What are the two main components involved in translocation?

  Source and sink
• What is the role of the phloem in translocation?

  Transport organic solutes from source to sink
• What are the two types of cells in phloem tissue?

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

  Allow solutes to pass through phloem
• Why do sieve tube elements lack a nucleus?

  To reduce resistance to solute flow
• What is the role of companion cells in phloem?

  Provide ATP for active transport
• What are plasmodesmata?

  Cytoplasmic connections between cells
• What defines a source in plant translocation?

  Produces more sugar than it needs
• What defines a sink in plant translocation?

  Requires or stores sugar
• How does sugar move in translocation?

  In both upward and downward directions
• What does the Mass Flow Hypothesis explain?

  How sucrose is transported in phloem
• What is the first step in the Mass Flow Hypothesis?

  Loading of sucrose at the source
• How do companion cells create a concentration gradient?

  By actively pumping H⁺ ions out
• What happens to H⁺ ions after they are pumped out?

  They diffuse back into companion cells
• What effect does sucrose loading have on water potential?

  It lowers water potential in sieve tubes
• What causes water to move into the sieve tubes?

  Osmosis from the xylem
• What is the result of increased water volume in sieve tubes?

  High hydrostatic pressure at the source
• What drives the mass flow of sucrose solution?

  Pressure differences in the phloem
• What happens to sucrose at the sink?

  It is actively transported or diffuses into cells
• What occurs to water potential at the sink after sucrose unloading?

  It increases, causing water to move out
• How does water loss affect hydrostatic pressure at the sink?

  It reduces hydrostatic pressure at the sink
• What are the stages of the Mass Flow Hypothesis?

  1. Loading of sucrose at the source
  2. Mass flow (bulk transport)
  3. Unloading of sucrose at the sink
• What evidence supports and challenges the Mass Flow Hypothesis?

  Supporting Evidence:

  • Aphid experiments show pressure in sieve tubes
  • Radioactive carbon tracing shows sugar transport
  • Ringing experiments indicate blocked sugar transport

  
  

  Challenges:

  • Sieve plates may slow down flow
  • Not all solutes move at the same speed
  • Sugars can move to multiple sinks