3.3.4.2 Mass transport in plants

• Why do plants need a transport system?

  • Large, multicellular organism, has a smaller SA:V ratio
  • Move substances (water, minerals and sugars) around which are needed for survival
  • Get rid of waste substances produced by the cells of the plant
• Throughout the whole plant, the phloem is always adjacent to the xylem, since it uses water from the xylem to move it's sugars.
• Xylem structure:

  • Dead cells- no cytoplasm or organelles so more space for water flow
  • Lignin- tough, fibrous structure which creates a thick cell wall to withstand water pressure
  • No cell walls between cells- allows water to flow through
  • Pits in cell wall- allows water out into tissues
• Xylem- used to transport water and dissolved minerals from the roots to the rest of the plant, which requires strong forces to overcome gravity
• Transpiration pull- negative pressure at the top of the xylem pulls the water up, caused by the loss of water through evaporation in the mesophyll layers in the leaves
• Cohesion- water (polar) molecules form hydrogen bonds to each other, which helps to pull other water molecules up
• Adhesion- water forms hydrogen bonds with cellulose in the xylem lining
• Root pressure- pressure generated when water moves into the roots by osmosis
• Cohesion-tension- when water in the xylem moves up as a column of water
• The majority of the water leaves by transpiration, but some water is used for photosynthesis when the conditions are right.
• Factors that affect transpiration (Temperature):

  • When it's warmer, the higher the rate of evaporation of water particles
• Factors that affect transpiration (Wind):

  • More wind increases the wp gradient between the airspace and stomata- water vapour is blown away so more water leaves the stomata to maintain the wp gradient
• Factors that affect transpiration (Humidity):

  • Lower humidity increases transpiration rate- there is less water molecules outside the leaf so the diffusion of water increases
• Factors that affect transpiration (Sunlight):

  • High levels of light increases transpiration rate- higher levels of photosynthesis so more stomata are open and water is able to diffuse out of the leaf
• A potometer is used to measure the rate of transpiration.
• How water is moved through the plant:

  1. Water moves down the water potential gradient (wp), into the root hair cell via osmosis.
  2. As water moves through the root hair cell, there are differences in the wp gradient, causing osmosis and water moves into the xylem.
  3. At the top of the xylem, there is a transpiration pull. Cohesive forces pull up water using hydrogen bonds.
  4. Water is pulled up the xylem, and exits into cells with lower wp by osmosis.
  5. Water vapour in the airspace (from evaporation of liquid water in the cells) exits the stomata by transpiration, creating a water potential gradient.
• How rate of water flow/ transpiration is affected by the time of day:

  
• Changes in plant diameter (e.g. tree trunks):

  • Negative pressure at the top of the xylem causes water to move up by pulling the xylem inwards by adhesion
  • This causes the trunk to narrow as the xylem (by the circumference of the trunk) are pulled inwards
  • When water flow decreases, diameter increases as there is less negative pressure pulling the xylem inwards
• Transpiration- the loss of water vapour from the leaves or stem of the plant
• Translocation- the movement of dissolved substances (assimilates) throughout the plant
• The source- the part of the plant that loads assimilates into the phloem sieve tubes
• The sink- the part of the plant that removes assimilates from the phloem sieve tubes
• Phloem structure:

  • Phloem sieve tubes at the end of each cell
  • Companion cell containing organelles (mitochondria, nucleus of phloem, endoplasmic reticulum), which is always in contact with phloem cell
  • Cell wall containing cellulose, no lignin
  • Organelles in sieve tube pressed against cell wall- more space for movement of assimilates
• Plasmodesmata- gaps in the cell wall which connects the sieve tube to the companion cells
• The basic principle:

  • Water potential is dependent on the concentration of sugars in the phloem
  • Osmosis is used to move water to create changes in the water potential and pressure gradient
  • Active transport is indirectly involved in the movement of sucrose
• 1. In source (leaf cell), photosynthesis occurs, producing sucrose.
  2. Sucrose actively moves into companion cell, via plasmodesmata, into phloem sieve tube.
  3. As sucrose enters phloem, water potential decreases. Water moves into phloem from xylem, down the concentration gradient, by osmosis.
  4. Hydrostatic pressure in phloem increases, forcing sucrose towards areas of lower hydrostatic pressure (sink).
  5. Sucrose moves from phloem into companion cell and further towards sink. Water leaves phloem by osmosis, going back into xylem, while some water enters companion cell.
• Evidence of mass flow (aphids + isotopic tracer):

  • Plant used isotopic tracer to make sucrose during photosynthesis
  • Pressure of the liquid in the sieve tube forces sucrose down- liquid oozes out of the placed stylet (without aphid)
  • Demonstrates that pressure moves the sucrose, not the aphid
• Evidence of mass flow (tracing movement of sugars):

  • One leaf from a plant surrounded in a source of radioactive carbon dioxide
  • Radiography used to trace the radioactive sugars made during photosynthesis
  • Radioactive sugars made with the CO2 was traced from the leaves to the stem to the roots
• Evidence of mass flow (ringing experiments):

  • Phloem removed from the circumference of the stem
  • Stem swells above where the phloem was removed- due to the sugars accumulating where the phloem was removed and sealed off
  • Shows that sugars continue to move en masse but got blocked and had to accumulate
• Evidence FOR mass flow theory:

  • There is pressure within sieve tubes- shown by sap being released when theyre cut
  • Concentration of sucrose is higher in the leaves (source) than in the roots (sink)
  • Downward flow in phloem occurs during the day and stops at night or when the leaves are in the shade- photosynthesis isn't occurring so sugars aren't moved
• Evidence AGAINST mass flow theory:

  • Function of the sieve plates is unclear- would slow down mass flow, but might have a structural function to prevent high pressures damaging tubes
  • Not all solutes move at the same speed- but they should if theyre moving according to mass flow
  • Sucrose delivers to all regions at the same rate, not fastest to the regions with the lowest sucrose concentration