What you'll learn
Cells must constantly take in what they need and remove what they do not, and they do it in only a few distinct ways. This topic covers diffusion, osmosis and active transport: how each works, how they differ, and what each one requires. It also covers the behaviour of plant and animal cells placed in solutions of different concentration, and the practical work on osmosis in potato tissue and visking tubing that CSEC examiners draw on year after year. The three processes are easy to confuse, so the single most valuable thing you can do is fix in your mind what each one moves, which way, and whether energy is needed.
Key terms and definitions
Diffusion — the net movement of particles of a gas or a dissolved substance from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Osmosis — the movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
Active transport — the movement of substances against a concentration gradient, using energy released by respiration.
Concentration gradient — the difference in concentration between two regions.
Partially permeable membrane — a membrane that allows some molecules through but not others, usually letting water pass while holding back larger solute molecules.
Turgid — describing a plant cell that is firm because its vacuole is full of water.
Flaccid — describing a plant cell that has lost water and is no longer firm.
Plasmolysis — the condition in which a plant cell has lost so much water that the cell membrane pulls away from the cell wall.
Haemolysis — the bursting of a red blood cell that has taken in too much water.
Isotonic — describing two solutions of equal concentration, between which there is no net movement of water.
Core concepts
Diffusion
Particles in a liquid or gas are in constant random motion. Where there are more particles in one region than another, that random motion produces a net movement from the crowded region to the less crowded one, until the particles are evenly spread. Diffusion is therefore passive: it needs no energy from the cell, because the movement is a consequence of motion the particles already have.
Oxygen reaching a respiring cell arrives by diffusion, because respiration keeps the concentration inside the cell low. Carbon dioxide leaves the same way. In the lungs, oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses in the opposite direction.
Three factors raise the rate of diffusion. A steeper concentration gradient increases it, because the imbalance driving the net movement is greater. A higher temperature increases it, because particles move faster. A larger surface area increases it, because more particles can cross at once. A shorter distance to travel also helps, which is why exchange surfaces such as the alveoli and the villi are thin as well as extensive.
Osmosis
Osmosis is a special case of diffusion that concerns water only. A partially permeable membrane lets water molecules through but holds back larger dissolved molecules such as sucrose. If a dilute solution is separated from a concentrated one by such a membrane, water moves from the dilute side to the concentrated side, because water molecules are more numerous, and so more free to move, on the dilute side.
It is worth being careful with the direction. Water moves from where there is more water to where there is less — which is the same thing as saying from a dilute solution to a concentrated one. Candidates who memorise only "high to low" often write the answer backwards, because the solute concentration runs the opposite way to the water concentration.
Like diffusion, osmosis is passive and requires no energy.
Active transport
Sometimes a cell needs a substance that is already more concentrated inside it than outside. Diffusion cannot help, because it only ever works down a gradient. Active transport moves substances against the concentration gradient using protein carriers in the cell membrane, and this requires energy released by respiration.
Two standard examples appear repeatedly. Root hair cells absorb mineral ions such as nitrates from soil water in which those ions are very dilute, so the uptake must be active. Cells lining the small intestine absorb the last of the glucose from the gut contents even when the concentration there has fallen below that in the blood.
Because active transport needs energy, cells that perform a great deal of it contain many mitochondria. This is the link examiners most often test: a question mentioning numerous mitochondria in a root hair cell or an intestinal cell is asking about active transport.
Cells in solutions of different concentration
An animal cell has no cell wall. Placed in pure water, water enters by osmosis, the cell swells and eventually bursts; for a red blood cell this is haemolysis. Placed in a concentrated solution, water leaves, and the cell shrinks and becomes crinkled. This is why the concentration of blood plasma must be kept steady, and why a solution used to store or inject cells must be isotonic with them.
A plant cell behaves differently because the cellulose cell wall is strong and fully permeable. In water, water enters the vacuole, which presses outwards on the wall; the wall resists, the cell becomes turgid, and it does not burst. In a concentrated solution, water leaves, the vacuole shrinks, the cell becomes flaccid, and if enough water is lost the membrane pulls away from the wall — plasmolysis. A plasmolysed cell can sometimes recover if returned to water; a burst animal cell cannot.
Turgor is what supports soft plant tissue. A wilting plant is one whose cells have become flaccid.
The practical work
Two experiments recur. In the visking tubing experiment, tubing filled with a sugar and starch mixture is placed in water. Sugar molecules are small enough to pass through and are detected in the surrounding water; starch molecules are too large and remain inside. This demonstrates that the tubing is partially permeable, modelling a cell membrane.
In the potato experiment, cylinders of potato of known mass are left in sucrose solutions of different concentration and reweighed. Cylinders in dilute solutions gain mass because water enters by osmosis; those in concentrated solutions lose mass. The concentration at which there is no change in mass is the concentration equal to that of the cell sap. Blotting each cylinder before weighing matters, because surface water would otherwise be counted as a gain.
Results are normally converted to percentage change in mass, calculated as the change in mass divided by the starting mass, multiplied by 100. This allows cylinders that did not start at exactly the same mass to be compared fairly, and it is the form in which examiners usually present or request the data.
Why exchange surfaces are shaped as they are
Because diffusion is faster over a short distance and across a large area, any surface an organism uses for exchange shows the same adaptations. The alveoli in the lungs are numerous and thin-walled, giving an enormous surface area and a short diffusion path, and they are surrounded by capillaries that carry oxygen away and so maintain a steep concentration gradient. The villi of the small intestine work on the same principles for absorbing digested food, with a large surface area, a thin lining and a dense blood supply.
Single-celled organisms need no such structures. Their surface area is large relative to their volume, and no part of the cell is far from the surface, so diffusion alone supplies their needs. As an organism becomes larger, its volume grows faster than its surface area, and this surface area to volume ratio falls — which is precisely why large organisms require specialised exchange surfaces and transport systems. This reasoning is a frequent extended-response question, and the marks lie in the ratio, not merely in saying that large animals are big.
Worked examples
Example 1. Potato cylinders are placed in four sucrose solutions and their change in mass recorded. In the 0.2 mol/dm³ solution the mass increases; in the 0.6 mol/dm³ solution it decreases. Explain both results.
In the 0.2 mol/dm³ solution the external solution is more dilute than the cell sap, so water moves into the cells by osmosis through the partially permeable membranes and the mass increases. In the 0.6 mol/dm³ solution the external solution is more concentrated than the cell sap, so water moves out of the cells and the mass decreases.
Example 2. Root hair cells absorb nitrate ions from soil water in which the nitrate concentration is lower than inside the cell. Name the process and state one requirement.
The process is active transport, and it requires energy released by respiration in the mitochondria. Diffusion cannot account for the uptake because the movement is against the concentration gradient.
Example 3. Explain why a red blood cell bursts in distilled water but a plant cell in the same water does not.
Water enters both cells by osmosis. The plant cell has a strong cellulose cell wall that resists the outward pressure, so the cell becomes turgid and stops taking in water. The animal cell has only a cell membrane, which cannot withstand the pressure, so the cell swells until it bursts.
Example 4. A potato cylinder of mass 5.0 g is left in a sucrose solution and afterwards has a mass of 4.6 g. Calculate the percentage change in mass and explain the result.
The change in mass is 4.6 − 5.0 = −0.4 g. As a percentage of the starting mass this is (−0.4 ÷ 5.0) × 100 = −8%. The negative value shows a loss of water, so the sucrose solution was more concentrated than the cell sap and water left the cells by osmosis.
Example 5. Explain why a large animal needs lungs whereas an amoeba does not.
An amoeba has a large surface area relative to its volume, and no part of it is far from the surface, so diffusion across the cell membrane supplies enough oxygen. As an animal becomes larger its volume increases faster than its surface area, so the surface area to volume ratio falls and the body surface can no longer supply the whole organism. Lungs provide a large, thin, well-supplied surface area to restore an adequate rate of diffusion.
Common mistakes and how to avoid them
Describing osmosis as the movement of solute. Osmosis is the movement of water. If your sentence has sugar moving, you are describing diffusion, not osmosis.
Getting the direction of osmosis backwards. Say "from a dilute solution to a more concentrated solution", or "from high water concentration to low water concentration". Mixing the two phrasings — "from high concentration to low" without saying concentration of what — is where marks are lost.
Saying active transport moves substances faster. It moves them against the gradient. Speed is not the distinguishing feature; direction and the energy requirement are.
Forgetting that diffusion and osmosis need no energy. Only active transport requires energy from respiration. A question asking which process needs energy has one answer.
Writing that a plant cell bursts in water. The cell wall prevents this. Bursting applies to animal cells; plant cells become turgid, and in concentrated solutions they plasmolyse.
Omitting "partially permeable" from the definition of osmosis. The membrane's selectivity is the reason osmosis happens at all, and the word is usually required for the mark.
Exam technique for Movement of Substances Into and Out of Cells
Decide first which of the three processes a question concerns, then write the definition that belongs to it. Most lost marks in this topic come from applying the right idea under the wrong name.
When explaining a result, always state the comparison that drives the movement: which side is more concentrated, and therefore which way the water or solute moves. An answer that says "osmosis occurred" without saying in which direction and why is incomplete.
For the potato practical, be ready to explain the control of variables — same length and mass of cylinder, same temperature, same time — and why each matters. Examiners award marks for the reason, not just the list.
Where data is given, read whether mass has been recorded as a change or a percentage change. Percentage change allows cylinders of different starting mass to be compared fairly, and saying so is often worth a mark in itself.
Quick revision summary
- Diffusion: net movement of particles down a concentration gradient; passive, no energy needed.
- Osmosis: movement of water from a dilute to a more concentrated solution through a partially permeable membrane; passive.
- Active transport: movement against the concentration gradient; requires energy from respiration and carrier proteins.
- Diffusion is faster with a steeper gradient, higher temperature, larger surface area and shorter distance.
- Cells doing much active transport have many mitochondria — root hair cells, intestinal lining cells.
- Animal cell in water: swells and bursts (haemolysis). In concentrated solution: shrinks.
- Plant cell in water: becomes turgid, does not burst, because of the cell wall. In concentrated solution: becomes flaccid, then plasmolysed.
- Isotonic solutions cause no net water movement.
- Visking tubing models a partially permeable membrane: sugar passes, starch does not.
- Potato cylinders gain mass in dilute solutions and lose mass in concentrated ones; blot before weighing.