3.4.2 Mass transport in plants Flashcards
What is transpiration?
Transpiration is the evaporation of water from the leaves. Water molecules evaporate from the mesophyll cells into the air spaces inside the leaf, and then diffuse out of the leaf through the stomata.
Describe the cohesion-tension theory of water transport in the xylem.
As water molecules evaporate from leaf cells, they lower water potential of the mesophyll (cells in the leaf), drawing water out of the pits in the leaf xylem. The water molecules are cohesive - they are connected in an unbroken chain by hydrogen bonds - so the pulling of water molecules out of the pits creates tension in the water column inside the xylem. This force of tension pulls water up the xylem. Adhesion of water molecules to the walls of the xylem helps the water molecules to not move back down. New water enters the xylem by osmosis in the roots.
The xylem are under negative pressure, because as water leaves by osmosis, it pulls on the water molecules inside the xylem, which in turn pull on the walls. The walls don’t collapse because they are lignified.
Explain how xylem tissue is adapted for its function.
Xylem is the tissue that transports water in the stem and leaves of plants.
Xylem tissue is made up of hollow tubes with no end walls, with no cytoplasm or organelles to impede the flow of water, allowing the transpiration stream to flow quickly and as a continuous column.
Xylem walls are thickened with lignin. Lignin provides strength, preventing the xylem collapsing under the negative pressure generated by the tension in the water column, and providing support to the plant.
Water molecules can adhere to the lignin, which helps maintain the column of water.
Lignin is waterproof, preventing water loss, but the xylem have pits in the walls, allowing lateral movement (so water can get round blocked vessels, and get in or out of the xylem).
Xylem is a non-living tissue, but the mass transport of water is a passive process, no energy from respiration is needed.
Describe the mass flow hypothesis for the mechanism of translocation in plants.
At the source sucrose is actively loaded (by co-transport with hydrogen ions) into sieve tube elements by companion cells, lowering the water potential. Water from the xylem enters the sieve tube element by osmosis down this water potential gradient, raising the hydrostatic pressure.
Meanwhile, at the sinks, sucrose is unloaded from the phloem by active transport and facilitated diffusion, increasing the water potential, so water from the phloem returns to the xylem by osmosis down this water potential gradient, lowering the hydrostatic pressure.
Sap (the fluid in the phloem, composed of sucrose and other assimilates dissolved in water) moves down the pressure gradient from source to sink by mass flow.
Sources can be any photosynthesising tissue, or any tissue releasing sugars from starch (eg bulbs).
Sinks can be any respiring cells (eg growing buds, young leaves, growing fruit, shoot tips, root tips, any meristems) or any storage tissue (eg bulbs, seeds).
The phloem are under positive pressure, as water enters by osmosis. The cells don’t burst due to the cellulose cell walls.
Explain how phloem tissue is adapted for its function.
Phloem is the tissue that transports organic substances in plants.
Phloem tissue is made up of two cell types - sieve tube elements, which form the actual vessels, and companion cells. Sieve tube elements have few to no organelles, and a reduced cytoplasm which is only found at the periphery of the cell. They have no nucleus. Their end walls are composed of sieve plates, which allow flow of sap from one element to the next, and can be quickly sealed in the event of damage to prevent loss of sap or infection. The sieve tube element plasma membrane is continuous with that of their companion cells - they are joined by tiny pores (plasmodesmata) through the adjoining cell walls, which allow sucrose to pass from the companion cell into the sieve tube element without having to cross any cell membrane. Companion cells do have a nucleus, and many mitochondria which provide the ATP for the active loading of sucrose.
Describe the use of tracers to investigate transport in plants.
Tracers are molecules that can be traced, ie they emit some sort of signal that can be detected. Radioactive isotopes, such as Carbon-14 , can be used to ‘radio-label’ molecules. If a plant is provided with carbon dioxide radio-labelled with C-14, it will incorporate it that traceable isotope into the sugars it makes from photosynthesis, so scientists can then track the movement of that sugar through the plant.
Tracer investigations have shown that the phloem tissue needs to be alive for the radio-labelled sugars to pass through them - when sections of phloem are killed (using heat for example) the sugar cannot pass through. This provides evidence that the (companion) cells need to be actively respiring.
Describe the use of ringing experiments to investigate transport in plants.
In woody plants (not all plants have wood - trees and bushes do) the bulk of the stem is made of wood, which is dead tissue. Each spring, when growth begins after the winter, a new layer of xylem and phloem is made, outside of the last year’s layer. This is why trees have rings across their cut trunks - each ring represents one year’s growth. This means that all the living phloem in a trunk or branch exists only just below the surface, in the newest ring. That new ring is protected by a tough tissue called bark.
‘Ringing’ is the term given to the practice of removing that layer, by stripping a complete ring of tissue off, all the way around the circumference of the branch or trunk. After a while swelling can be seen on the source side of that ‘ring’ (so usually above the damage). This is because sugars continue to be actively loaded into the phloem, but accumulate at the edge of the ring as they cannot progress any further. This build up of sugars lowers the water potential so water moves into the area by osmosis, causing the swelling.
If you ‘ring’ the main trunk of a tree it is usually fatal.
