How do substances move in and out of cells by diffusion, osmosis and active transport, and how does surface area affect this?
Diffusion of gases and solutes, osmosis as the movement of water across a partially permeable membrane, active transport against a concentration gradient, the factors affecting these, and the importance of surface area to volume ratio and exchange surfaces.
A focused answer to AQA GCSE Biology 4.1.3, covering diffusion, osmosis and active transport, the factors that affect them, surface area to volume ratio, and the required practical on osmosis.
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What this dot point is asking
AQA wants you to explain diffusion, osmosis and active transport, compare them, describe the factors that affect each, and explain why a large surface area to volume ratio and specialised exchange surfaces help transport.
Diffusion
Diffusion is faster when the concentration gradient is steeper (a bigger difference in concentration), the temperature is higher (particles have more kinetic energy and move faster), and the surface area of the membrane is larger. Examples include oxygen and carbon dioxide moving across the alveoli in the lungs, and urea diffusing from cells into the blood plasma to be removed by the kidneys.
Osmosis
The direction of osmosis matters for cells. A plant cell in pure water gains water and becomes firm (turgid), which supports the plant. In a concentrated solution it loses water and becomes flaccid, and the cell membrane can pull away from the wall (plasmolysis). Animal cells, which have no wall, can burst in pure water or shrink (crenate) in a concentrated solution.
Active transport
Active transport moves substances against a concentration gradient, from a lower to a higher concentration, so it requires energy from respiration. It lets root hair cells absorb mineral ions from dilute soil solutions, and lets the small intestine absorb glucose into the blood even when the blood glucose concentration is already higher. Cells that carry out a lot of active transport contain many mitochondria to supply the energy.
Surface area to volume ratio
Single-celled organisms have a large surface area compared with their volume, so substances diffuse in and out fast enough across the whole surface. As an organism gets bigger, its volume grows faster than its surface area, so the surface area to volume ratio falls. Larger organisms therefore need exchange surfaces that are adapted to be efficient: a large surface area, thin walls (a short diffusion path), and a good blood supply or ventilation to maintain a steep concentration gradient. Examples include the alveoli in lungs, villi in the small intestine, and gills in fish.
Try this
Q1. State two factors that increase the rate of diffusion. [2 marks]
- Cue. A steeper concentration gradient and a larger surface area (or a higher temperature).
Q2. Explain why active transport is needed to absorb mineral ions into root hair cells. [2 marks]
- Cue. Ions move against the concentration gradient (soil is more dilute than the cell), so energy from respiration is required.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA student investigated osmosis using cylinders of potato in sugar solutions of different concentrations. Describe how the student could find the concentration of sugar solution that has the same water concentration as the potato cells, and explain how this is shown by the results.Show worked answer →
A 4-mark required-practical question rewards a clear method and the use of percentage change in mass.
Cut potato cylinders of equal size, measure and record the mass of each, then place each in a different known concentration of sugar solution for a set time. Remove, dry and reweigh each cylinder. Calculate the percentage change in mass for each solution and plot it against concentration.
Where the percentage change in mass is zero (the line crosses the x axis), there is no net movement of water, so that solution has the same water concentration as the potato cells. Drying the cylinders and using percentage change (not raw mass) make the comparison fair.
Markers reward measuring mass before and after, using percentage change in mass, and identifying zero change as the point of equal concentration.
AQA 20223 marksExplain why root hair cells use active transport to absorb mineral ions from the soil, and why this process requires energy.Show worked answer →
A 3-mark explain question wants the concentration gradient idea linked to energy.
The concentration of mineral ions is usually higher inside the root hair cell than in the dilute soil solution. To absorb more ions the cell must move them from a lower concentration (soil) to a higher concentration (cell), which is against the concentration gradient. Moving substances against a gradient requires energy, which is supplied by respiration, so root hair cells contain many mitochondria.
Markers reward against the concentration gradient, energy required, and energy coming from respiration.
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Sources & how we know this
- AQA GCSE Biology (8461) specification — AQA (2016)