Topic 2: Cells: Osmosis

Solute

What is getting dissolved

Solvent

Dissolves the solute

Solution

What is the result of the combination of solute and solvent

Highest water potential is 0

e.g. Water that is pure with no dissolved solute

Water potential is the pressure exerted by water molecules on a membrane surrounding a solution

Measured in kPa

Any other water potential is negative

Values get more negative as more solute is added

Water always moves from higher water potential to lower water potential

(Where water potential is more negative)

Solutes are attracting water molecules

Reducing their potential to move across the partially permeable membrane

Osmosis

The net movement of water from an area of higher water potential to an area of lower water potential across a semi permeable membrane. Down a water potential gradient until water potential is equal on both sides of the membrane (equilibrium)

What is it called when red blood cells get turgid then bursts?

Haemolysis

Isotonic

Solution has same water potential as cells (no net osmosis)

Hypotonic

Solution has a higher water potential than cells

Hypertonic

Solution has lower water potential than cells

Plasmolysis

Water has moved out of a plant cell by osmosis to a lower water potential outside - this causes the vacuole to shrink and the cell membrane moves away from the cell wall

Incipient plasmolysis

When the membrane is about to move away from the cell wall (is starting to lose water and no longer turgid)

Open stoma

Turgid guard cells. Water moves in by osmosis. Potassium ions ( K+) move into guard cell via active transport. Decreases water potential

Stoma closing

Flaccid guard cells. Potassium ions (K+) move out of guard cell via active transport. Increase water potential in guard cell. Water moves out by osmosis

High water potential

Solution has a higher water concentration (so not very much solute dissolved in it)

Low water potential

Solution has a low water concentration (so lots of solute dissolved in it)

Difference in osmosis between animal and plant cells

Animal cells can burst whereas plant cells cannot due to the presence of a cell wall

First factor that affects the rate of osmosis (faster)

Temperature - at higher temperatures, water molecules have more kinetic energy and diffuse faster

Second factor that affects the rate of osmosis (faster)

Water potential gradient - steeper the gradient, the faster the rate of osmosis

Third factor that affects the rate of osmosis (faster)

Thinner membrane - water molecules travel shorter distances through thin exchange surfaces, so diffuse faster

Fourth factor that affects the rate of osmosis (faster)

Larger surface area - means more water molecules can cross the membrane at once, making osmosis faster