What Is Osmosis? KS3 Biology Explained

Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane. It is a passive process — no energy is required. This definition appears in the KS3 national curriculum for biology and is assessed at GCSE.

What does "partially permeable membrane" mean?

A partially permeable membrane (sometimes called a semi-permeable membrane) has tiny pores that allow small molecules — such as water — to pass through, but block larger dissolved molecules such as sugar or salt ions. Think of it like a kitchen sieve: water passes freely, but larger particles are held back.

In living cells, the cell membrane acts as the partially permeable membrane. Water molecules move across it all the time, but bigger molecules such as glucose cannot pass without specific transport proteins.

Which way does water move during osmosis?

Water moves from the side with a higher water concentration (a dilute solution) to the side with a lower water concentration (a concentrated solution). Another way to say this: water moves down its own concentration gradient.

A concentrated sugar solution has fewer free water molecules per unit volume because so many are associated with the dissolved sugar. A dilute solution — or pure water — has more free water molecules. Water therefore flows from the dilute side to the concentrated side across the membrane until concentrations equalise or an opposing pressure stops the flow.

A worked example: potato strips in sugar solutions

This is a classic Year 8 practical experiment used in schools across England.

Method

Cut three equal strips of raw potato (each about 3 cm long, same width). Place them in:

Beaker A — pure water (distilled)

Beaker B — 0.5 mol/l sugar solution

Beaker C — 1.0 mol/l sugar solution

Leave for 30 minutes, then measure and weigh each strip.

Results (typical)

Strip | Solution | Change in mass | Change in length | Observation
A | Pure water | +8% | Longer | Firm and turgid
B | 0.5 mol/l | Little change | Little change | Slightly firm
C | 1.0 mol/l | −12% | Shorter | Limp and flaccid

Explanation

In Beaker A, the potato cells have a lower water concentration than the surrounding pure water, so water enters the cells by osmosis. The cells swell and become turgid, making the strip longer and heavier.

In Beaker C, the sugar solution has a lower water concentration than the potato cells, so water leaves the cells by osmosis. The cells become flaccid (floppy), making the strip shorter and lighter.

This experiment appears regularly in KS3 assessments and is the foundation for understanding plant cell turgor pressure.

Why is osmosis important in plant cells?

Plant cells have a rigid cell wall outside the cell membrane. When water enters by osmosis, the cell swells against this wall, building up turgor pressure. Turgid cells push against one another and give non-woody plants their upright structure. If plants lose too much water — on a hot, dry day — cells become flaccid and the plant wilts.

Turgor pressure is also what drives the opening and closing of guard cells, which control the tiny pores (stomata) on leaves through which carbon dioxide enters for photosynthesis and water vapour exits.

Why is osmosis important in animal cells?

Animal cells do not have a cell wall. If they are placed in pure water, water enters by osmosis and the cells can burst — this is called lysis. If they are placed in a very concentrated solution, water leaves and the cells shrink and shrivel — this is called crenation.

This is why the fluid in a hospital drip (intravenous saline) must match the concentration of blood cells almost exactly. A solution with the same solute concentration as a cell is called isotonic. Blood plasma is roughly isotonic to red blood cells at a concentration of about 0.9% salt (sodium chloride).

How is osmosis different from diffusion?

Both osmosis and diffusion involve particles moving from a region of higher concentration to a region of lower concentration without using energy. The key difference is:

Diffusion refers to the movement of any substance (solid, liquid, gas) down its concentration gradient — including oxygen, carbon dioxide, and glucose.

Osmosis refers specifically to the movement of water across a partially permeable membrane.

All osmosis involves diffusion of water, but not all diffusion is osmosis. The membrane is what makes osmosis a distinct process.

How do you explain osmosis to someone who has never heard of it?

Picture a teabag in a cup of hot water. The paper of the teabag acts like a partially permeable membrane. Water moves freely in and out, but the tea molecules are too large to pass back out quickly — so the tea gradually spreads into the water. The water is effectively moving from where it is more concentrated (outside the bag) towards where it is less concentrated (inside the bag where tea chemicals are dissolving). That movement is analogous to osmosis.

Osmosis and the KS3 national curriculum

According to the Department for Education's Science Programmes of Study for Key Stage 3 (published 2013, updated 2014), pupils should be taught about "the role of diffusion in the movement of materials in and between cells" — and osmosis is specifically taught as a special case of diffusion involving water and a membrane. Schools typically cover osmosis in Year 8 or Year 9, depending on their scheme of work.

BBC Bitesize KS3 Biology covers osmosis as part of the Cells and Organisation topic, defining it consistently as the diffusion of water through a partially permeable membrane from a dilute to a concentrated solution (see bbc.co.uk/bitesize/subjects/z9ddmp3).

Frequently asked questions

What is osmosis in simple terms for KS3?

Osmosis is when water moves from a weak (dilute) solution to a strong (concentrated) solution through a membrane with tiny holes. The water moves to even out the concentrations on both sides.

Does osmosis require energy?

No. Osmosis is a passive process — it happens automatically because water molecules move down their own concentration gradient. No ATP or cell energy is needed, which distinguishes it from active transport.

What is the difference between a dilute and a concentrated solution?

A dilute solution has a lot of water compared to dissolved substances — the water concentration is high. A concentrated solution has more dissolved substances and relatively less water — the water concentration is low. Osmosis moves water from the dilute side to the concentrated side.

Why do plants wilt when they lack water?

Without enough water, the cells cannot take in water by osmosis, so they lose turgor pressure. The cells become flaccid and cannot push against neighbouring cells. Without that structural support, the whole plant droops.

Can osmosis happen in animal cells?

Yes. Animal cells are surrounded by a partially permeable cell membrane, so osmosis occurs across it. Because animal cells have no cell wall, uncontrolled osmosis can cause them to burst (if placed in pure water) or shrivel (if placed in a very concentrated solution).

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