Module 2 Exchange and transport 2

Plants use photosynthesis to convert light energy to chemical energy in compounds such as carbohydrates, fats and proteins. The raw materials in this process of assimilation are simple inorganic substances, such as carbon dioxide, water and ions.

• in xylem tissue water ions move from roots to stems, leaves, flowers and fruit.
• in phloem tissue sucrose and other assimilates travel upwards and downwards.

Movement in xylem and phloem is by mass flow. Everything travels in the same direction within each column of xylem or phloem cells.

Xylem vessels

• Structure:
  • long cells with think walls that have been impregnated by lignin.
  • as the xylem develops, the lignin waterproofs the walls of the cells.
  • As a result cells die, leaving a long column of dead cells with no contents.
  • a tube with no end walls.
• Function
  • transports water from the root to the rest of the cell plant.

Sieve tube elements (phloem)

• Structure
  • not true cells, contain very little cytoplasm and no nucleus.
  • lined up end-to-end to form a tube
  • tubes contain cross walls (sieve plates)at intervals, cross walls perforated by many pores to allow sap to flow.
  • have very thin walls
• Function
  • transports sugar (usually sucrose)
  • Sucrose is dissovled in water to form sap.

Companion cells:

• Structure:
  • samll cells, large nucleus and dense cytoplasm.
  • have numerous mitochondria to produce ATP
• Function:
  • Carry out metabolic processes needed by the sieve tube elements
  • This includes using ATP as a source of energy to load sucrose into the sieve tubes.
  • Cytoplasm of comanion cells and the sieve tubes are linked through many plasmodesmata. (gaps in cell wall)
  • 

Pathways that water molecules can take between cells:

• The apoplast pathway:
  • The pathway along cell walls and in the spaces between cells. Wter passes along the apoplast pathway when crossing the cortex of the root.
• The symplast pathway:
  • The pathway through cells. Water flows from cell to cell through plasmodesmata, microscopic cytoplasmic connections between cells, without crossing cell walls.
• The vacuolar pathway:
  • This is similaur to the symplast pathway, but the water is not confined to the cytoplasm of the cells. It is able to pass and pass through the vacuoles as

    

    well.

Movement across the root:

• Root hair cells absorb minerals from the soil by active transport, this reduces the water potential inside the cell.
• So water can enter root hair cell by osmosis.
• Water moves across cortex by osmosis through the apoplast pathway.
• Minerals actively transported into xylem through the endodermis. This leaves water potential in the xylem and water follows by osmosis through symplast pathway.
• This is because the Casparian ***** blocks the apopast pathway, forcing water into the symplast pathway.

• Root pressure:
  • Water driven into xylem by endodermal cells by osmosis
    • water forced up the xylem
    • accounts for a column of a few metres only.
• Transpiration pull
  • Cohesion:
    • Attraction of water molecules for one another
      • Holds water molecules together in a long column
  • Molecules lost from the top of xylem by evaporation
    • whole column is pulled up
    • Replaced by water from xylem
  • Causes tension in water column
    • Requires lignification of xylem to prevent collapse
    • ie ‘cohesion-tension theory’ :
      • Requires unbroken column of water.
      • Maintained via pits if break in column in a xylem vessel.
• Capillary action:
  • Adhesion- attraction of water molecules to walls of xylem
  • Xylem vessels very narrow, can pull water up the sides of the vessel

Transpiration is the loss of water vapour from the upper parts of the plant - particulary the leaves

Factors that affect the transpiration rate:

• Number of leaves:
  • More leaves = larger surface area for water vapour to be lost
• Number, size and position of stomata:
  • Many large stomata = faster water vapour loss. Stomata on lower surface slows rate
• Presence of cuticle:
• • Reduces evaporation from leaf surface

• Light:
  • Stomata open to allow gas exchange for photosynthesis
• Temperature
  • Increase rate of evaporation = increase water vapour potential in leaf
  • Increase rate of diffusion through stomata (water molecules have more kinetic energy)
  • Decrease relative water vapour potential in air, increasing water vapour diffusion gradient
  • Higher relative humidity in air will decrease rate of water loss. Smaller water vapour potential gradient
• Relative humidity:

• Air movement or wind:
  • Carries away water vapour that has diffused out of leaf. Maintains high water vapour potential gradient
• Water availability:
  • If limited, plant can’t replace water that is lost. Reduce water loss by closing stomata, or when leaves are shed in winter

Potometer:

A potometer (sometimes known as a 'transpirometer') is a device used for measuring the rate of water uptake of a leafy shoot.

To reduce water vapour potential gradient:

• Hairs on leaf surface
  • Trap layer of saturated air
• Pits with stomata at their base
  • Trap saturated air
• Rolling leaves
  • so lower epidermis not in contact with air
  • Trap saturated air
• Maintain high salt concentration in leaf cells
  • Low water potential inside leaf cells
  • Reduce evaporation from cell surfaces

To reduce water vapour potential gradient:

• Hairs on leaf surface
  • Trap layer of saturated air
• Pits with stomata at their base
  • Trap saturated air
• Rolling leaves
  • so lower epidermis not in contact with air
  • Trap saturated air
• Maintain high salt concentration in leaf cells
  • Low water potential inside leaf cells
  • Reduce evaporation from cell surfaces

Active loading

• H+ ions are pumped out of the companion cell by active transport using ATP.
• H+ ions diffuse back in with sucrose using cotransporter proteins.
• Sucrose diffuses into sieve tube element through plasmodesmata.

Ringing a tree

• Area above cut act as sink where sugar collects.
• No further growth under cut.

Evidence (mechanism of translocation

• If plant is supplied with radioactive carbon dioxide. Carbon radioactive isotope appears in phoem
• Ringing a tree ; removing the phloem ; sugar collects
• aphids feed on sugar in phloem