The Transport System
All multicellular organisms must have a transport system in place, as the distance between the cells and the external environment is too large to be dealt with by diffusion alone.
Xylem: Carry water and soluble minerals upwards
Phloem: Carry sugars both upwards and downwards.
How are xylem vessels adapted to their function?
- As a young xylem vessel develops, it's walls are "waterproofed" by lignin, killing off the cells
- As the cells are dead, they have no contents, they become tubes with no end walls- xylem vessels
- The lignin continues to gather, strengthening the xylem vessel and preventing it's collapse.
- The lignin can form spiral lignin or rings of lignin
How does water travel up a xylem vessel?
- Cohesion/adhesion- Water molecules are attracted to other water molecules and other polar molecules.
- Root pressue: (involves ATP) Mineral ions are moved into the root, followed by water through osmosis, pushing the water up the stem in a constant cycle.
- Transpiration pull: Water molecules are lost through the leaves through transpiration, and a pull is created in the xylem.
- Capillary action- Water molecules adhere to xylem vessels, they are very thin so collective adhesion is great.
How is phloem tissue adapted to its role?
- Phloem tissue is living and made up of sieve tubes and companion cells.
- Sieve tubes- Sieve tube elements are joined up end to end to form tubes, they contain cross-walls at intervals.
- Companion cells are filled with mitochondria to produce the ATP needed for the active process of sugar transport in the sieve tubes, they are joined to the sieve tubes through many plasmodesmata to allow the exchange of substances and communication.
How does transport occur in the phloem tissue?
- Sucrose is loaded at a source, for example the leaves, where it is produced by photosynthesis.
- The water potential inside the sieve tubes is reduced, as a result water moves into the phloem from the xylem by osmosis.
- Sucrose is then unloaded at a sink, for example the root, where it can be converted into starch and stored. The water potential inside the sieve tubes increases, and so water returns to the xylem vessels by osmosis.
Water Transport Pathways
The Symplast Pathway: Sym- meaning with or in and -plast meaning main part, the Symplast pathway involves the flow of water between the cytoplasm of cells, through the plasmodesmata.
The Apoplast Pathway: Apo- meaing away from and -plast meaning main part, the Apoplast pathway involves the flow of water away from the cytoplasm and through the cell walls of the parenchyma, which are essentially dead. It eventually meets the casparian strip, and must briefly pass through a selectively permeable membrane.
The Vacuolar Pathway: Similar to the Symplast pathway, but the water can flow through the vacuoles as well as the cytoplasm.
Xerophytes are plants adapted to live in areas with lower amounts of water, this isn't strictly limited to desert plants.
Adaptations of xerophytes:
- Smaller leaves
- Stomata close when there is a low level of water to prevent diffusion
- The leaves are rolled up
- Hair on the surface of the leaf traps air
- Low water potential gradient inside cells