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Bbc Biology Osmosis Coursework

Osmosis in cells

Osmosis takes place in all cells. The cell membrane is partially permeable.

Osmosis in red blood cells.

If a red blood cell is placed in water, water enters the cell by osmosis. Because the membrane is quite weak the cell will burst as the volume and therefore the pressure in the cell increases. Red blood cells shrink when placed in concentrated solutions of sugar as water moves out of them by osmosis. This makes the cells appear wrinkled when viewed through a microscope.

This does not happen inside the body because the kidneys make sure the concentration of the blood stays about the same as the concentration of the solution inside the red blood cell.

Osmosis in plant cells

Plant cells have a strong rigid cell wall on the outside of the cell membrane. This stops the cell bursting when it absorbs water by osmosis. The increase in pressure makes the cell rigid. This is useful as plants do not have a skeleton. Instead the leaves and shoots can be supported by the pressure of water in their cells. If plant cells lose too much water by osmosis they become less rigid and eventually the cell membrane shrinks away from the cell wall.

Important vocabulary - higher tier

  • Lysis – bursting an animal cell by osmosis
  • Crenation – shrinking an animal cell by osmosis
  • Turgid – a plant cell fully inflated with water
  • Plasmolysed – a plant cell that has lost water causing the cell membrane to be pulled away from the inside of the cell wall
  • Flaccid – a plant cell that is limp through a reduction of pressure inside the cell

Water entering the cell by osmosis inflates the cell and makes it rigid

Loss of water makes the cell limp and shrinks the cell membrane away from the cell wall.

Back to It’s a green world index

Cylinders or discs of fresh potato are often used to investigate osmosis in living cells. To carry out this type of experiment, you need to:

  1. cut equal-sized pieces of potato
  2. blot with tissue paper and weigh
  3. put pieces into different concentrations of sucrose solution for a few hours
  4. remove, blot with tissue paper and reweigh

The percentage change in mass can be calculated for each piece of potato:

Example 1

A piece of potato has a mass of 2.5 g at the start and 3.0 g at the end.

percentage change in mass = (3.0 – 2.5) ÷ 2.5 × 100 = 0.5 ÷ 2.5 × 100 = +20%

The plus sign shows that it has gained mass - it will have gained water by osmosis.

Example 2

A piece of potato has a mass of 2.5 g at the start and 2.0 g at the end.

percentage change in mass = (2.0 – 2.5) ÷ 2.5 × 100 = –0.5 ÷ 2.5 × 100 = –20%

The minus sign shows that it has lost mass - it will have lost water by osmosis.

Example results

A graph of change in mass (vertical axis) against concentration of sucrose (horizontal axis) can be plotted.

Where the line crosses the horizontal axis at 0% change in mass, the sucrose concentration is equal to the concentration of the contents of the potato cells. The sucrose concentration is isotonic with the cells' cytoplasm, so there is no net movement of water by osmosis.

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