water relations of cells

Define water potential

The measure of the concentration of free water molecules in a solution

Explain what you know about water potential

• highest water potential is pure water = 0kpa

• W.p decreases when solutes are added

• All water potential values are negative (apart from pure water)

Define Solute potential

The measure of the concentration of solute molecules in a solution

Explain what you know about solute potential

• always negative

• More solute molecules present = more negative value

Define pressure potential

Measure of the pressure exerted by the cell wall of a plant cell that resists the further entry of water by osmosis into the cell

Explain what you know about pressure potential

• usually positive - can be negative in the xylem

• When water enters a plant it doesn’t burst - turgor pressure is exerted which stops the water molecules from entering - plant cell is turgid

• Cell contents are always of lower w.p even when cell is turgid - only a small amount of water is needed for the pressure potential to not allow anymore water To enter

• Animal cells cannot exert a pressure potential - no cell wall, so will burst hence osmoregulation is very important

Describe the process of plasmolysis

1. Cell is placed in a hypertonic solution

2. Water leaves Tonoplast (membrane of vacuole)

3. Protoplast shrinks and pulls away from cell wall

Incipient plasmolysis

• point where plasmolysis is just about to happen

• Protoplasm stops exerting pressure - neither pulling nor pushing, it just fills up cell volume

• Also known as flaccid

• I’m experiments it is reached when 50% of cells are plasmolysed

• SINCE AT INCIPIENT PLASMOLYSIS P = 0, W.P = SOLUTE POTENTIAL

Mention reasons why Osmotic phenomena are or great importance to plants

1. Keeps plant from wilting

2. Over large distances, pressure potential drives the movement of water

3. Movement of water from cell to cell is driven by a water potential gradient.