Osmosis

Definition of Osmosis

The net movement of particles from an area of high water potential (concentration) to an area of low water potential (concentration) across a partially permeable membrane.

Osmosis

Osmosis is a specific type of diffusion.

• Osmosis is a passive process.

Water potential

Water potential is a measure of waters ability to move through the cell membrane.

• Water potential can have a value between 0 and a negative number.

• HIGHEST WATER POTENTIAL - 0

• LOWER WATER POTENTIAL - negative number

• Pure water moves most freely, so has a water potential of 0.

• The more solute you dissolve in the water, the less freely water can move. The water potential becomes more negative.

Permeable

Small molecules diffuse through the cell membrane.

Partially/ semi permeable

Only small/certain molecules/particles can diffuse through the cell membrane..

Impermeable

Particles are too large to diffuse through the semi/partially permeable membrane.

Turgid

When you put cells into pure water OR a solution that is not as concentrated as the cell’s cytoplasm…

• Water moves via osmosis into the cells vacuole.

• The pressure of the added water causes the cell to swell.

• The plant cell membrane becomes turgid.

Flaccid

When you put cells into a solution with the same concentration as the cells cytoplasm…

• Water moves via osmosis in and out of the cells vacuole at the same rate.

• There is no added pressure caused by the movement of water. The cell doesn’t swell.

• The plant cell is flaccid.

Plasmolysed

When you put cells into a solution with a higher concentration than the cells cytoplasm…

• Water moves via osmosis out of the cells vacuole.

• The loss of water from the vacuole decreases pressure in the cell.

• The cell membrane pulls away from the cell wall.

• The plant cell is plasmolysed.

Crenation

When you put cells into a solution with a higher concentration than the cells cytoplasm…

• Water moves via osmosis out of the cell membrane.

• The loss of water from the cell decreases pressure in the cell.

• The cell begins to shrink.

• The animal cells are crenated.

Normal

When you put cells into a solution with the same concentration as the cells cytoplasm…

• Water moves via osmosis in and out of the cell at the same rate.

• There is no added pressure caused by the movement of water, and the cell doesn’t swell.

Haemolysed

When you put cells into pure water, OR a solution that’s not as concentrated a the cells cytoplasm…

• Water moves via osmosis into the cell.

• The pressure of the added water causes the cell to swell.

• The cell becomes turgid and (eventually) the cell membrane splits.

• The cell is haemolysed (cell dies)

Isotonic

a solution having the same osmotic pressure as another solution.

Hypotonic

a solution having a lower osmotic pressure than another solution.

Hypertonic

a solution with a higher osmotic pressure than another solution.

describe an experiment to investigate diffusion in non-living organisms

phenolphthalein:

alkaline = pink

acidic = colourless

• agar jelly → phenolphthalein and dilute sodium hydroxide.

• measure hydrochloric acid into a beaker.

• cut out a few cubes from the jelly and put them in the beaker of acid.

• the cubes will eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide.

describe how to make 10 cm3 of a 5% sucrose solution from a 10% sucrose solution and water.

• use a measuring cylinder.

• dilute the solution by mixing

  • 5cm3 pure water

  • 5cm3 of 10% sucrose

explain the change in appearance of the onion cells shown in the images after being in the 5% sucrose solution for one hour.

• water molecules leave the cell by osmosis from an area of higher concentration, to an area of lower concentration.

describe an experiment to investigate osmosis in living organisms

1. cut potato into even cylinders.

2. place in a range of sugar solutions (with varying concentrations) and some in pure water.

3. measure initial length of the cylinders.

4. leave in beakers for 30 minutes.

5. osmosis = longer in length

6. water left the cylinders = shorter in length.

describe an experiment to investigate osmosis in non-living organisms

1. visking tube over the end of a thistle funnel.

2. pour sugar solution down glass tube into thistle funnel.

3. put funnel → beaker of pure water.

4. measure where the sugar solution comes up to on the beaker.

5. leave apparatus overnight, then measure where solution is in the glass tube.

6. water should be drawn through the visking tube by osmosis, which forces the solution up the glass tube.