Limitations

• What does Mass Flow suggest about sugar movement?

  It should move from source to sink.
• What does research show about solute movement in sieve tubes?

  Different solutes can move in opposite directions.
• What is the alternative theory to Mass Flow regarding solute movement?

  Cytoplasmic Streaming or Active Transport Mechanisms.
• How do individual solutes move according to the alternative theory?

  They move independently due to active transport.
• What problem does Mass Flow have regarding the transport rates of substances?

  It suggests all substances move at the same rate.
• What do studies show about the movement of solutes like amino acids compared to sucrose?

  Amino acids move at different speeds than sucrose.
• What is the alternative theory for different transport rates of solutes?

  Selective Active Transport.
• What does Selective Active Transport imply about solutes?

  They are actively loaded and unloaded at varying rates.
• What is the problem with sieve plates in relation to Mass Flow?

  Sieve plates should slow down phloem sap flow.
• What does Mass Flow suggest about the movement of phloem sap?

  It moves freely under pressure.
• What is the alternative theory regarding sieve plates and flow resistance?

  Electro-Osmotic Flow Hypothesis.
• What does the Electro-Osmotic Flow Hypothesis suggest about sieve plates?

  They have charged proteins that assist flow.
• What problem does Mass Flow have regarding the speed of sucrose movement?

  It predicts passive pressure differences for movement.
• What do experiments show about the speed of sugar movement?

  It moves much faster than diffusion allows.
• What do some scientists suggest about protein filaments in sieve tubes?

  They actively direct solutes to different sinks.
• What are the limitations of the Mass Flow Hypothesis in phloem transport?

  • Sugars move in opposite directions
  • Different rates of transport for substances
  • Sieve plates obstruct flow
  • Transport speed exceeds passive flow
• What is the conclusion regarding the Mass Flow Hypothesis?

  It is the best-supported theory but has limitations.
• What do alternative theories help explain about phloem transport?

  They explain gaps in the Mass Flow Hypothesis.
• What links companion cells to sieve tube elements?

  Plasmodesmata
• What is exchanged between companion cells and sieve tube elements?

  Sugars, ATP, and other nutrients
• How do companion cells transport sucrose into sieve tubes?

  By using active transport
• What role do cytoplasmic strands play in sucrose movement?

  They help move sucrose efficiently
• How does selective transport differ from mass flow in phloem transport?

  It allows control over what enters and exits
• What limitation of the Mass Flow Hypothesis is addressed by cytoplasmic strands?

  Different solutes move at different speeds
• What are the main functions of cytoplasmic strands in phloem transport?

  • Actively loading sucrose
  • Regulating transport between cells
  • Maintaining communication between companion cells and sieve tubes