Mass Transport

Mass Transport systems are needed when the distance is too large for diffusion.

It needs a:

• Suitable Medium to carry the materials eg blood
• A form of Mass Transport to carry the materials in bulk
• A Closed System of tubular vessels eg capillaries
• A Mechanism for moving the medium eg the heart acts as a pump
• A way to maintain a One way Flow eg valves in veins

Heart -->Artery -->Arterioles -->Capillaries -->Venules -->Vein -->Heart

Arteries:

• Tough Outer Layer - Protection eg prevents aneurysms
• Muscle Tissue - Contracts to constrict artery, reducing blood flow as it narrows lumen/Dilates to widen lumen and increase blood flow (Atreioles)
• Elastic Tissue - Expands to take the surge of the pulse, then recoils to push the blood on, smoothening the flow.
• Squamous Endothelium
• Lumen

Veins:

• Valves - controls the blood flow, keeping it going in one direction.

Capillaries:

• Squamous Endothelium - for exchange of substances (1 cell thick for thin diffusion distance, large networks for large surface area and the cells use up oxygen/glucose maintaining the diffusion gradient)

Water leaves the ateriole end due to high hydrostatic pressure. Then at the venule end, the hydrostatic pressure is reduced due to water loss. The osmotic effect is greater and causes water to move back into the cappillary down a water potential gradient, caused by the proteins in the plasma, which are too large to leave.

Excess tissue fluid goes into the lymphatic vessel into the lymphatic system which eventually returns it to the blood.

Apoplast Pathway - Water soaks through the cell walls, until it hits the casparian *****, where it joins the Symplast Pathway.

Symplast Pathway - Water enters the root by osmosis down the water potential gradient, then the water moves from one cell to the next by osmosis down the water potential gradient through the cytoplasm [(sss)-ytoplasm & (sss)-ymplast]. The water potential gradient is maintained by the endodermis actively transporting ions into the xylem. - requires oxygen and ATP

Water mainly moves up the stem by Cohesion Tension, caused by Transpiration - the evaporation of water through the stomata of the leaves, which puts tension on the xylem, creating negative pressure, pulling more water up.
So, water moves up the xylem by pulling each molecule in a continuous, unbroken column.

Evidence:
1. Diameters of tree trunks - the diameter of trees is smaller in the day, when there is sunlight so stomata open to allow in CO2 for photosynthesis, meaning chance for more transpiration putting tension on the xylem, slimming the trunk.
2. If the xylem vessel is broken and air enters, then water is no longer pulled up, suggesting that the water is pulling each other up in an unbroken column.
3. If the xylem vessel is broken, the water does not leak out, suggesting the xylem is under tension rather than pressure.

Water also moves up the stem by capillary movement by adhesion - molecules stick to the walls and creep up that way.