Mass transport in plants

• The xylem transports water and mineral salts. It has dead cells which are strengthened by lignin
• Phloem transports sugars and amino acids. They have companion cells
• Transpiration stream is the movement of water and solutes taken in by the roots, transported by the xylem to the leaves from which the water evaporates from
• Tracheids are dead long and tapered lignified cells in the xylem for conducting water transport
• Apoplast is the space outside the plasma membrane of a plant that allows free movement of material
• Symplast is the interconnected cytoplasm of plant cells and therefore includes everything within the plant that is bound within the cell plasma membrane
• Lignin is impermeable so nothing can pass into the xylem
• The plasmodesmata are strands of cytoplasm that extended through pores in adjacent plant cell walls that connect the cytoplasm of one cell to another.
• Adhesion is the hydrogen bonds between water molecules and the surface they are attaching to
• Cohesion is hydrogen bonds between water molecules
• Use a potometer to measure rate of transpiration
• Environmental facts that affect rate of transpiration

  • light intensity
  • temperature
  • humidity
  • wind speed
• The rate of translocation is described with mass flow theory
• Sucrose moves from a source to companion cell by facilitated diffusion down concentration gradient
• Sucrose then is co transported into phloem tube with hydrogen ions through co transporter protein
• Contents in the phloem can move up and down simultaneously but not by the same tube
• Source Cells are usually photosynthetic cells
• Sink Cells are usually respiring cells that need sucrose for respiration
• Water potential in phloem decreases as it becomes saturated with sucrose. Water moves from xylem with higher water potential by osmosis to the phloem creating hydrostatic pressure.
• Sucrose moves into sink cells by active transport
• Movement of sucrose and amino acids is called translocation
• Evidence for Translocation

  • ringing of bark
  • radioactive tracers carbon 14 isotope in CO2
• Companion cells and phloem sieve tube elements connected by plasmodesmata