Osmosis – GCSE Combined Science Biology

Start here

Osmosis is about how water moves into and out of cells. The word sounds hard, but the idea is small. We will build it up in short steps.

It matters because every cell in your body sits in a watery liquid. If too much water goes in or out, the cell cannot work properly. A plant wilts for the same reason.

By the end you will be able to say which way water moves. You will describe what happens to animal and plant cells. You will also work out a percentage change in mass from the potato practical.

What osmosis is

A solution is a liquid with something dissolved in it, like squash in water. A dilute solution has only a little dissolved, so it is mostly water. A concentrated solution has a lot dissolved, so it has less water.

A cell membrane is partially permeable, which means it lets some things through and blocks others. Water molecules are tiny, so they pass. Larger dissolved molecules cannot. Think of a tea bag: the water gets through, but the tea leaves stay in.

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution, through a partially permeable membrane. Diffusion means particles spreading out.

Predicting the direction

To predict which way water moves, ask one question. Which side has more dissolved substance? Water moves towards that side, because it has less water.

So water moves from the more dilute solution into the more concentrated solution. The concentrated side gains water and the dilute side loses it. Only the water moves.

If both sides have the same concentration, water still moves both ways at the same rate. Nothing changes overall. We say there is no net movement.

Animal cells

An animal cell has a cell membrane but no cell wall. A cell wall is a stiff outer layer. Without one, nothing strong holds the cell in shape.

Put an animal cell in pure water. The water outside is more dilute than the cell contents, so water moves in. The cell swells and may burst.

Put it in a very concentrated solution and water moves out. The cell shrinks and shrivels. This is why cells in the body sit in a fluid with a concentration close to their own.

Plant cells

A plant cell has a strong cell wall outside its membrane. It also has a vacuole, which is a bag in the cell filled with watery cell sap.

In pure water or a dilute solution, water moves in and the vacuole swells. The cell becomes turgid, which means firm and full of water. The wall stops it bursting. Think of a football pumped up hard. Turgid cells hold a plant upright.

In a concentrated solution, water moves out. The cell becomes flaccid, which means soft and limp, like a flat football. The plant wilts. If a lot of water leaves, the membrane pulls away from the wall. This is called plasmolysis.

The potato practical

Cut potato into cylinders that are all the same size. Find the mass of each one. Put each cylinder in a different sugar solution, with one in pure water.

Leave them for the same time at the same temperature. Take them out and blot them dry with a paper towel, to remove water on the surface. Then find the mass again.

The concentration of the solution is the independent variable, the one you change. The change in mass is the dependent variable, the one you measure. Everything else is kept the same, so the test is fair.

The solutions used are usually sucrose solutions of different concentrations, for example 0, 0.2, 0.4, 0.6 and 0.8 mol/dm³, with distilled water as the zero. Use a ruler or cork borer to make the cylinders the same size, so that the surface area is the same.

Percentage change and results

Percentage change tells you how big a change is compared with where you started. The equation is percentage change = (final mass − start mass) ÷ start mass × 100.

Try 2.0 g going to 2.3 g. The change is 0.3 g, and 0.3 ÷ 2.0 × 100 = 15%. Think of a £200 phone reduced by £50. You compare the £50 with the £200 you started with. A decrease gives a negative answer.

In pure water the potato gains mass. In a strong solution it loses mass. A change of 0 means the solution has the same concentration as the potato cells.

Recap

Osmosis is the diffusion of water through a partially permeable membrane, from a dilute solution to a more concentrated solution.

Animal cells can burst in pure water and shrink in a strong solution. Plant cells become turgid or flaccid.

In the potato practical you change the concentration and measure the change in mass.

Percentage change = (final − start) ÷ start × 100. A change of 0 means the concentrations match.