Mass transport in plants

water transport in plants

• Plants need a constant supply of water and minerals

  • Water is required for photosynthesis and to maintain cell structure

  • Minerals are needed for production of important biological molecules, e.g. proteins and chlorophyll

xylem tissue

• Water and dissolved minerals are transported from the soil to the rest of a plant in the xylem

  • Xylem are tubes that form part of the mass transport system of plants

  • Xylem cells are specialised for water transport

    • Hollow tubes with no end walls allow the continuous flow of water

    • Lignin provides waterproofing to prevent loss of water by evaporation

    • Lignin strengthens the xylem to reduce breakages

• Xylem cells together form xylem tissue which, together with phloem tissue, makes up plant vascular tissue

movement of water in the xylem

• The upward movement of water in the xylem is driven by the process of transpiration

  • Transpiration is the loss of water from the leaves of plants by evaporation

• Transpiration drives water transport as follows:

  1. water diffuses out of leaves into the surrounding air via the stomata

  2. the loss of water vapour lowers the water potential in the air spaces surrounding the mesophyll cells

  3. water within the mesophyll cell walls evaporates into the leaf air spaces, lowering the water potential of the mesophyll cells

  4. water is drawn from the xylem into the mesophyll cells by osmosis

  5. water moves up the xylem vessels in a continuous column to replace this lost water; this upward movement is the transpiration stream

     • Water molecules are pulled upwards due to forces of cohesion and adhesion

the cohesion-tension theory

• The upward pulling force acting on water in the xylem can be so great that the water is under tension, exerting an inward pull on the walls of the xylem vessels; this is known as cohesion tension

  • The mechanism of water movement described above is sometimes known as the cohesion-tension theory of water transport

movement of organic substances in the phloem

• In addition to water and minerals from the soil, plants also need to transport organic substances

• Organic substances, also known as assimilates, are transported in the phloem; examples include

  • sucrose

  • amino acids

  • plant hormones

phloem specialised cells - phloem sieve tube cells

• Reduced cell contents to reduce resistance to flow of assimilates

• Sieve plates allow passage of assimilates between cells

phloem specialised cells - companion cells

Contain many mitochondria to produce ATP for the active loading of sucrose into the phloem tubes

what is translocation

• Movement of assimilates through the phloem is translocation

• Translocation moves assimilates either upwards or downwards from cells in the source to cells in the sink

  • A source of assimilates is the place in which it has been produced or stored, e.g.:

    • cells in photosynthesising leaves are a source of sugars

    • cells in storage organs during the early spring may be a source of carbohydrates for new growth

  • A sink is the part of a plant where assimilates are required, e.g.:

    • cells in parts of a plant that are actively growing

    • cells in plant storage organs

• Translocation is an active process, dependent on energy from ATP

transport of carbohydrates

• The transport of carbohydrates in plants, e.g. sucrose, in the phloem is known as translocation

• The process by which phloem sap moves in one direction along phloem sieve tubes is known as the mass flow hypothesis

  • The direction may differ depending on the location of sources and sinks in the plant

stages of translocation

1. The sucrose loading mechanism uses active transport to load sucrose into the phloem at the source

   • Companion cells use ATP to actively pump hydrogen ions out of the cytoplasm into their cell walls

   • Hydrogen ions move down their concentration gradient back to the cytoplasm via a cotransporter protein, carrying sucrose molecules

   • Sucrose molecules then move into the sieve tubes via plasmodesmata

2. the high concentrations of solutes in the phloem lower the water potential and cause water to move into the phloem vessels by osmosis

   • Water can move in from the neighbouring xylem vessels

3. this results in increased hydrostatic pressure and generates a hydrostatic pressure gradient between the source and the sink; the contents of the phloem move towards the sink down a pressure gradient

4. at the same time sucrose is being unloaded from the phloem at the sink, lowering the water potential of the cells of the sink

5. water follows by osmosis, maintaining the hydrostatic pressure gradient between the source and the sink

evidence for the mass flow hypothesis

• When the phloem sieve tube is punctured phloem sap oozes out —> The contents of the phloem exert pressure on the phloem walls

• Phloem sap extracted near a source has a higher sucrose concentration than sap extracted near a sink —> Water would move into the phloem by osmosis near a source and out of the phloem by osmosis near a sink

• Metabolic inhibitors stop translocation —> ATP is required for mass flow to occur; it is an active process

• Removal of a ring of bark from trees results in a bulge above the ring, and fluid from the bulging region has a higher sugar concentration than fluid from below the ring —> Removal of the phloem tissue (which is just below the bark) prevents the passage of sugars

evidence against the mass flow hypothesis

• Amino acids appear to travel more slowly than sucrose in the phloem —> The mass flow hypothesis states should be flowing at the same rate

• Experiments have detected different substances, within the same sieve tube, moving in opposite directions —> The mass flow hypothesis states everything should be flowing in one direction

• Sieve plates are present in the phloem —> These create a barrier to mass flow, so there is no reason for them to have evolved

tracers

• are chemicals that can be traced as they move through an organism; a common example of a tracer used in studies on plants is radioactive carbon dioxide, ¹⁴CO₂

  • It is readily absorbed by the leaves and used in photosynthesis to produce sucrose

  • The sucrose formed will be radioactive so its movement around the plant via translocation can be traced

• Tracers can be used in ringing experiments that involve removing a ring of tissue from the outside of a plant stem

  • As the phloem is located towards the outside of the stem and the xylem towards the centre, the ring removes the phloem only with the xylem remaining intact

a ringing experiment with radioactive carbon dioxide

• An example of a ringing experiment is described below:

  1. A series of plants were ringed at different locations on the stem

     • A control plant did not have a ring of tissue removed

  2. The plants were then supplied with radioactive carbon dioxide (¹⁴CO₂)

  3. After a period of time the levels of radioactive carbon in the different parts of the plant were measured

• The results from the experiment show that:

  • the phloem is involved in the transport of sucrose, and not the xylem

    • There is no radioactive sucrose detected past the ringing point on the stems, due to the break in the phloem at this point

  • in the phloem the transport of sucrose occurs both upwards and downwards

    • Sucrose is translocated from the source tissues in the leaves to the sink tissues above and below