Mass transport in plants

• Xylem transports water and mineral salts up the stem from roots to leaves
• Phloem transports sugars and amino acids to all other parts of the plant.
• Vascular tissue diagrams

  A) Xylem

  B) Phloem

  C) Vascular bundle

  
• Movement of water out of the stomata

  1. Water moves out of stomata if humidity of atmosphere is lower than that of air spaces next to stomata
  2. There is a water potential gradient from stomata air spaces to air
  3. Water vapor diffuses out of air spaces into air if stomata are open
  4. Water that is lost is replaced with water that diffuses from cell walls of surrounding mesophyll cells
  5. Rate of transpiration is controlled by changing size of stomata
• Symplast - water moves between cytoplasm of neighbouring cells
• Apoplast - water moves directly through permeable cell walls
• How water moves across a leaf

  1. Water is lost from mesophyll cells by evaporation, now has a lower water potential
  2. Replaced by water coming from neighbouring cells via osmosis or through cytoplasm (symplast), now the neighbouring cells water potential is lowered
  3. They in turn take water from neighbouring cells by osmosis
  4. Water potential pulls water from xylem across leaf mesophyll and into atmosphere.
• Xylem is composed of dead cells joined together by long narrow tubes.
• Cohesion-tension theory

  1. Water evaporates from leaf as a result of transpiration
  2. Water evaporates from mesophyll cells into air spaces beneath stomata, more water is drawn up as a result of cohesion. Reducing water potential
  3. Water molecules stick to sides of xylem vessels, known as adhesion
  4. Water is hence pulled up the xylem, transpiration pull
  5. Transpiration pull puts xylem under tension (negative pressure)
• 3 pieces of evidence for the cohesion-tension theory

  1. Diameter of tree trunk varies depending on transpiration rate
  2. Broken xylem vessels cause tree to have difficulty drawing up water
  3. Broken xylem vessels do not leak water
• Potometer is used to measure rate of transpiration

  
• 3 environmental factors that affect the rate of transpiration

  1. Temperature - increase in temperature means increase in evaporation of water from the surface of the leaf
  2. Light intensity - affects stomatal opening
  3. Co2 concentration - low Co2 concentration means stomata don't open as much
• Describe how you would use a potometer to investigate rate of transpiration

  1. Cut a sample plant underwater to prevent air from entering xylem (negative pressure means constantly taking up water)
  2. Fill potometer with water to remove all air bubbles
  3. Attach plant to potometer using a rubber seal and use petroleum jelly around seal to make air tight.
  4. Introduce one air bubble by lifting capillary tube out of water
  5. Start stop watch and see how far air bubble moves along capillary tube.
  6. Distance air bubble moves can be used to work out volume
  7. Volume / time = transpiration rate

  
• What is the transport of phloem called?

  Translocation
• Phloem is made of 4 tissue types

  1. Sieve tube cell
  2. Companion cell
  3. Fibres
  4. Parenchyma
• Transfer of sucrose into sieve elements from photosynthesising tissue

  1. Sucrose is made from the products of photosynthesis in cells with chloroplasts
  2. Sucrose moves down a concentration gradient by facilitated diffusion into companion cells
  3. H+ ions are actively transported from companion cells into spaces within cell walls using ATP
  4. These H+ ions then diffuse down a concentration gradient through carrier proteins into the sieve tube
  5. Sucrose molecules are transported along with the H+ ions in a process called co-transport. The protein carriers are called co-transport proteins.

  
• Movement of phloem is too fast to be explained by diffusion

  Mass flow theory is the favoured theory

  Mass flow theory has 3 stages:

  1. Transfer of sucrose into sieve elements from photosynthesising tissue
  2. Mass flow of sucrose through sieve tube elements
  3. Transfer of sucrose from sieve tube elements into storage of other sink cells
• Where are source cells found?

  1. Leaves and stems
  2. Storage organs e.g. potato
  3. Food stores in seeds which are germinating
• Translocation is the movement of organic molecules from source to sink
• Where in the plant are sink cells found?

  1. meristems that are actively dividing
  2. Roots that are growing or actively absorbing mineral ions
  3. Any part of the plant where organic molecules or mineral ions are found.
• Sucrose is actively transported from sieve tube elements into sink cells
• This active transport causes the water potential of the sieve tube to become more negative
• Xylem has a much higher water potential, so water follows by osmosis, creating a high hydrostatic pressure
• In sink cells, sucrose is either used for respiration or converted to starch
• Sink cells have low sucrose content, so sucrose is actively transported into them from sieve tubes, lowering the water potential
• Due to the low water potential, water moves into sink cells and back into the xylem by osmosis
• The hydrostatic pressure in the sieve tubes is lowered in the sink area
• This results in high hydrostatic pressure in the source area of the phloem and low pressure in the sink area
• There is a mass flow of sucrose down the hydrostatic gradient in the sieve tubes (phloem)