Mass-flow hypothesis

• What is produced in the mesophyll cells during photosynthesis?

  Sucrose
• How do companion cells transport hydrogen ions (H⁺)?

  They use ATP for active transport
• What effect does the H⁺ concentration gradient have on sucrose transport?

  It causes H⁺ ions to diffuse back into companion cells
• How does sucrose move from companion cells to sieve tube elements?

  It diffuses via plasmodesmata
• What happens to water potential in sieve tubes when sucrose concentration is high?

  Water potential decreases
• What process causes water to move into the sieve tubes from the xylem?

  Osmosis
• What is the result of increased hydrostatic pressure at the source?

  Sucrose moves down the phloem
• What is the term for the movement of sap due to pressure gradient?

  Bulk flow
• How is sucrose removed from the phloem at the sink?

  Through active transport or diffusion
• What happens to water potential in the sieve tube after sucrose is removed?

  Water potential increases
• What causes water to move back into the xylem from the sieve tube?

  Osmosis
• What is the effect of lowering hydrostatic pressure at the sink?

  It maintains the pressure gradient for mass flow
• What are the key points regarding mass flow in phloem transport?

  • Mass flow is passive
  • Requires active transport at source and sink
  • Hydrostatic pressure differences drive sap movement
  • Companion cells are crucial for loading and unloading sucrose
• What is the main function of leaves in plants?

  Photosynthesis
• What is the basic word equation for photosynthesis?

  Carbon dioxide plus water creates glucose and oxygen
• What type of substances are glucose and other sugars?

  Organic substances
• Why do plant cells require glucose?

  For respiration
• What is the role of the phloem in plants?

  Transporting organic substances
• What are the two key cell types in phloem tissue?

  Sieve tube elements and companion cells
• What is a feature of sieve tube element cells?

  They have perforated end walls
• Do sieve tube elements contain a nucleus?

  No, they do not contain a nucleus
• What is required for active transport in sieve tube elements?

  ATP produced by companion cells
• What is the mass flow hypothesis related to?

  Transport of organic substances from source to sink
• What is the source in the mass flow hypothesis?

  Where organic substances are created
• What is the sink in the mass flow hypothesis?

  Where sucrose is transported to and used
• How does osmosis affect the source cell during translocation?

  Water moves into the source cell
• What happens to the hydrostatic pressure in the source cell?

  It increases due to water influx
• What occurs at the sink cell during respiration?

  Soluble sugars are used up
• How does the water potential change in the sink cell?

  It becomes more positive
• What is the result of sucrose being actively transported into the sink cell?

  Water moves into the sink cell by osmosis
• What happens to the hydrostatic pressure in the sink cell?

  It decreases due to water loss
• What is the purpose of tracer experiments in studying translocation?

  To track the location of radioactively labeled carbon
• How do ringing experiments demonstrate the role of phloem?

  They show swelling above the ring due to sugar accumulation
• What is the process of translocation in plants?

  • Movement of organic substances from source to sink
  • Involves phloem tissue
  • Driven by hydrostatic pressure changes
  • Active transport of sucrose into sieve tubes
• What are the key steps in the mass flow hypothesis?

  1. Photosynthesis occurs in source cells
  2. Sucrose is actively transported into sieve tubes
  3. Water enters sieve tubes by osmosis
  4. Increased hydrostatic pressure pushes solution to sink
  5. Sucrose is used in sink cells for respiration
• What are the methods used to investigate translocation?

  • Tracer experiments with radioactively labeled carbon
  • Ringing experiments to observe sugar accumulation