How do substances move into and out of cells by diffusion, osmosis and active transport?
Describe and compare diffusion, osmosis and active transport, and explain their importance in living organisms
A focused answer to the O-Level Combined Science outcome on movement of substances. Diffusion down a concentration gradient, osmosis of water across a partially permeable membrane, active transport against the gradient, and their importance.
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What this dot point is asking
SEAB wants you to describe diffusion, osmosis and active transport, to compare them (direction, what moves, and whether energy is needed), and to explain why each matters in living organisms. The definitions and the osmosis-in-cells effects are heavily tested, so be precise about concentration gradients and the partially permeable membrane.
The answer
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until they are evenly spread. It happens because particles move randomly. No energy from the cell is needed. Oxygen diffuses into cells and carbon dioxide diffuses out this way.
Osmosis
Osmosis is the net movement of water molecules from a region of higher water concentration (a more dilute solution) to a region of lower water concentration (a more concentrated solution), through a partially permeable membrane. The membrane lets water through but not the larger dissolved particles. Osmosis is how water enters and leaves cells.
Effects of osmosis on cells
- An animal cell in pure water gains water and may burst (no cell wall to stop it); in concentrated solution it loses water and shrinks.
- A plant cell in pure water gains water and becomes firm (turgid); the cell wall stops it bursting. In concentrated solution it loses water and becomes flaccid, and may undergo plasmolysis.
Active transport
Active transport is the movement of particles against a concentration gradient, from lower to higher concentration. Because this is "uphill", it requires energy from respiration (ATP). Cells doing a lot of active transport have many mitochondria. Root hair cells absorbing mineral ions, and the gut absorbing glucose, use active transport.
Comparing the three
- Diffusion: any particles, down the gradient, no energy.
- Osmosis: water only, down the water gradient, through a partially permeable membrane, no energy.
- Active transport: particles, against the gradient, energy from respiration needed.
Examples in context
Example 1. Absorbing the products of digestion. The small intestine absorbs glucose and amino acids into the blood, often against a concentration gradient by active transport, so that even the last traces are taken up. Water follows by osmosis, showing all three processes working together in digestion.
Example 2. Wilting and reviving plants. A plant wilts when its cells lose water and become flaccid in dry soil, and revives when watered as cells regain water by osmosis and become turgid again. This everyday observation is a direct demonstration of osmosis in plant cells.
Try this
Q1. Define osmosis. [2 marks]
- Cue. Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane.
Q2. State one similarity and one difference between diffusion and active transport. [2 marks]
- Cue. Similarity: both move substances across a cell membrane. Difference: diffusion is down the gradient with no energy; active transport is against the gradient and needs energy.
Q3. Explain why a red blood cell bursts when placed in pure water but a plant cell does not. [2 marks]
- Cue. Both gain water by osmosis, but the plant cell has a strong cell wall that resists the pressure and stops it bursting; the red blood cell has no wall.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA piece of potato is placed in pure water and another identical piece is placed in concentrated salt solution. Predict what happens to the mass of each piece, and explain your predictions in terms of osmosis.Show worked answer →
In pure water: the water outside is at a higher water concentration (more dilute) than the cell sap inside the potato cells. Water moves into the cells by osmosis, across the partially permeable cell membranes, down the water concentration gradient. The piece gains mass and becomes firm.
In concentrated salt solution: the solution outside has a lower water concentration than the cell sap. Water moves out of the cells by osmosis. The piece loses mass and becomes soft (flaccid).
Markers reward water moving in by osmosis in pure water (gain in mass), water moving out in salt solution (loss in mass), and reference to the water concentration gradient and the partially permeable membrane.
Original3 marksExplain why active transport is described as requiring energy, and give one example of active transport in a living organism.Show worked answer →
Active transport moves a substance against its concentration gradient, that is, from a region of lower concentration to a region of higher concentration. Moving particles "uphill" against the gradient requires energy, which is supplied by respiration (in the form of ATP). This is why cells carrying out active transport have many mitochondria.
Example: root hair cells absorbing mineral ions (such as nitrate ions) from the soil, where the ion concentration in the soil is lower than in the cell.
Markers reward movement against the concentration gradient, energy from respiration being needed, and a valid example such as mineral ion uptake by root hair cells.
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