Transport in plants

• what is transpiration?

  loss of water vapour from the stomata by evaporation
• what factors affect rate of transpiration?

  temperature

  humidity

  light intensity

  wind
• how does light intensity affect rate of transpiration ?

  more light causes more stomata to open, increasing the surface area for evaporation
• how does temperature affect the rate of transpiration?

  more heat means more kinetic energy so more evaporation
• how does wind effect rate of transpiration?

  more wind will blow away water vapour, maintaining the water potential gradient
• how does humidity affect rate of transpiration?

  increased humidity reduced the water potential gradient so decreases rate of transpiration.
• what is cohesion?

  when water molecules stick together as a chain & pull one another up the xylem
• what is adhesion?

  water sticks to the xylem walls & sticks to other water molecules pulling them up the xylem\
• what is root pressure?

  as water moves into the roots by osmosis it increases the pressure inside the root.

  • this increase in pressure forces water upwards.
• describe movement of water up the xylem?

  -water vapour evaporates from the leaves via the stomata.

  • this creates a lowered pressure.
  • when water is lost by transpiration, more water is pulled up to replace it.
  • due to hydrogen bonds between water molecules they are cohesive & form a column.
  • water sticks to the walls of the xylem helping pull it upwards.
• what are sieve tube elements?

  living cells

  no nucleus

  few organelles
• what are companion cells?

  provide ATP required for active transport of organic substances.
• describe how sucrose is transported from the source to the sieve?

  -photosynthesis in the leaves creates organic products.

  • this creates a high concentration of sucrose.
  • sucrose moves down its concentration gradient by facilitated diffusion into the companion cell.
  • active transport of H+ from the companion cell into the cell wall spaces occurs using ATP.
  • this creates a concentration gradient so H+ now moves down it into the sieve tube via co-transport proteins.
  • carrying sucrose with it into the sieve tube element.
• describe the movement of sucrose within the sieve tube?

  -increase of sucrose lowers the water potential.

  • water enters the sieve tube from the xylem by osmosis.
  • this increases hydrostatic pressure, forcing liquid down to the sink.
• describe the transport of sucrose to the sink?

  -sucrose is used in respiration at the sink cell or is stored as starch.

  • more sucrose is actively transported into the sink, decreasing water potential.
  • this results in osmosis of water from the sieve into the sink cell or xylem.
  • the removal of water decreases the hydrostatic pressure.