19.2: Movement of substances in multicellular organisms

Diffusion

Passive movement of substances down their concentration gradients; requires no energy.

Diffusion rate across membrane

Depends mostly on the molecule's relative hydrophobicity = the more lipid-soluble and non-polar, the more readily it will diffuse.

Osmosis

Movement of only water across semipermeable membranes down its concentration gradient.

Tonicity

Describes how the extracellular solute concentration changes plant volume by making the cell lose or gain water.

Hypotonicity

Solute concentration outside cell is lower than inside = water enters cell via osmosis = causes plasma membrane to push against the cell creating turgor pressure.

Hypertonicity

Extracellular surroundings have a higher solute concentration than inside cell = water leaves the cell = vacuoles decrease in size and plasma membrane detaches from the cell wall as cytoplasm shrinks.

Isotonicity

Solute concentrations outside and inside the cell are equal = no net movement of water = cells become flaccid and plant starts to droop.

Water potential (ψ)

tendency of water molecules to move from an area of higher to lower potential, influenced by solute concentration and pressure = determines water movement

Golden rule of water potential

Water always moves across a selectively permeable membrane towards regions of lower (more negative) water potential.

Unit of measurement for Water potential

Expressed in mega pascals (MPa) - unit of pressure.

Two components of Water potential

Solute potential (Ψs) and pressure potential (Ψp).

Solute potential (Ψs)

As solutes are added to pure water, the tendency to take up water increases, resulting in lower (more negative) solute potential.

• usually negative as adding solutes decreases water potential

Pressure potential (Ψp)

When any closed compartment takes up water, it tends to swell, but the walls of this compartment resist that swelling = increase in internal pressure of compartment.

• positive pressure potential increases water potential

Turgor pressure

internal pressure exerted by the vacuole on the cell walls

how does high solute concentration affect the movement of water

High solute concentration lowers the water potential, causing water to move out of cells through osmosis to balance solute concentrations.

high vs low water potential

high = low solute concentration and high free water availability

low = high solute concentration and low free water availability.