Translocation

• What is transported in the phloem?

  Products of photosynthesis
• What is the source in the context of translocation?

  The leaves where photosynthesis occurs
• What is the sink in the context of translocation?

  Regions that store or use products of photosynthesis
• What is translocation in plants?

  Transport of soluble organic materials
• What are the four types of cells in phloem tissue?

  • Sieve tubes
  • Companion cells
  • Phloem fibres
  • Phloem parenchyma
• What is the function of sieve plates in phloem?

  Allow bidirectional flow of materials
• Why is the cytoplasm of sieve tubes thin?

  To allow unobstructed flow of materials
• What are plasmodesmata?

  Channels that allow transport between cells
• What do companion cells provide for sieve tube elements?

  Proteins and ATP
• What is the main function of sieve tubes?

  Transport organic materials like sucrose
• How are sieve tubes formed?

  From sieve elements placed end to end
• What happens to the organelles in sieve tubes during development?

  Most disintegrate, leaving a thin cytoplasm
• What evidence supports translocation in the phloem?

  • Ringing experiments show sucrose accumulation above cut rings.
  • Aphids feeding on sieve tubes reveal sugary sap.
  • Radioactive carbon tracing shows sucrose movement.
• What occurs during ringing experiments?

  Phloem is removed, causing sucrose accumulation above
• What happens to water potential when sucrose accumulates?

  It decreases, allowing water to enter cells
• What do aphids use to feed on phloem sap?

  Hollow, needle-like mouthparts called stylets
• How is radioactive carbon used in translocation studies?

  It traces sucrose movement in plants
• What does autoradiography reveal in translocation studies?

  Presence of radioactivity in plant tissues
• What does the mass flow hypothesis explain?

  Passive mass flow of sugars from source to sink
• What happens to water potential at the source during sugar production?

  It becomes more negative, drawing in water
• What causes hydrostatic pressure to increase in source cells?

  Water entering the cells by osmosis
• How does mass flow occur in the phloem?

  Products are forced along by water flow
• Why is hydrostatic pressure lower at the sink?

  Because sugars are stored or used
• What happens to water at the sink?

  It passes from sink cells to the xylem
• What are the arguments against the mass flow theory?

  1. Translocation rate is much faster than diffusion.
  2. Sieve plates impede flow.
  3. Different rates and directions for sucrose and amino acids.
  4. High oxygen consumption stops translocation with poisons.
  5. Companion cells' role is not explained.