Biology AS OCR Unit 1 Module 2 Part 2

Attraction of water molecules to the walls of xylem

Epidermis root hair cells absorb mineral from soil by active transport using ATP.Minerals reduce water potential of cell cytoplasm.Water taken up across plasma membrane by osmosis

Movement is driven by an active process that occurs at the endodermis.Endodermis also known as starch sheathMoving from the apoplast pathway to symplast due to casparian strip.Low water potential in xylem due to solutes actively transported mean water moves from cortex through endodermal cells into xylem.Creates a water potential gradient across whole cortex.

Pathway through the cell walls.Cellulose cell walls have many water filled spaces which water can move through.Does not pass through any plasma membranes.Means solutes can be carried with water.

Pathway through plasma membrane into cell cytoplasm. Can then pass through plasmodesmata from one cell to another.

Like symplast but water is not confined to cytoplasm of cells.Able to enter and pass through vacuoles.

Blocks apoplast pathway between cortex and xylem.Forces dissolved nitrate ions to pass into cell membranes where they can be transported actively into xylem.This maintains a lower water potential in xylem so water follows.Blocks water from going back down the apoplast

Root pressureTranspiration pullCapillary action

The endodermis actively pulling water into xylem pushes water up the xylem. This root pressure can push water a few metres up a stem but not to top of tall trees.

Loss of water at top of leaf needs to be replaced.Water molecules are attracted to each other by cohesion. Cohesion forces keep molecules together in long column.Water lost at the top of column pulls up the chain of water.Creates transpiration stream.Lignin stops xylem collapsing under tension caused by water.THE MECHANISM (this is important)involves cohesion between water molecules and and tension in column of water - COHESION-TENSION THEORY. Relies on the plant maintaining an unbroken column of water all the way up the xylem.
If broken can be maintained by moving through a pit to another vessel.

Adhesion pulls water molecules up the side of the vessel.

Most leaves through stomata, pores in epidermis.Tiny amount leaves through cuticle.Water evaporates from cells lining cavity directly below guard cells.Lowers water potential in these cells causing more water from neighbouring cells to enter and then lost through stomata.

A plant that is adapted to reduce water loss so that it can survive in very dry conditions

Waxy cuticle on leaf -> reduce evaporationStomata under surface of leavesStomata close at night -> no light for photosynthesisDeciduous plants lose leaves in winter.

Smaller leaves -> needle shaped (Reduced SA)Densely packed spongy mesophyll reduces air spaces for water evaporationThicker wxy cuticleClosing stomata when low water availabilityHairs on surface, can become moist reducing gradient of water vapour potentialPits containing stomata at base can trap humid airRolling the leaves so lower epidermis not exposedMaintaining high salt concentration to reduce water potential -> gradient between cells and air space reduced.