Transport in and out of Cells
Diffusion, osmosis and active transport are the three ways substances move in and out of cells. Diffusion is the net movement of particles from an area of higher to lower concentration, down a concentration gradient; osmosis is the diffusion of water through a partially permeable membrane from a dilute to a concentrated solution; and active transport moves substances against the concentration gradient using energy from respiration.
Method
- Learn the three definitions word for word, because each mark point is a specific phrase: 'net movement', 'down a concentration gradient', 'partially permeable membrane', 'against the concentration gradient', 'requires energy from respiration'.
- For any rate-of-diffusion question, name the three factors: the difference in concentration (the steeper the gradient, the faster), the temperature (higher temperature means faster-moving particles), and the surface area of the membrane (a larger area means faster diffusion).
- For osmosis questions, decide the direction first: water moves towards the more concentrated solution (the one with the lower water concentration). Then say what happens to the cell: an animal cell may swell and burst or shrink; a plant cell becomes turgid or plasmolysed, because the cell wall stops it bursting.
- For the osmosis practical, always work in percentage change in mass, not raw mass, so cylinders of different starting mass can be compared: percentage change = (change in mass / starting mass) x 100. A negative value means water was lost.
- For active transport, give a named example and say why the concentration gradient is the wrong way round: root hair cells absorb mineral ions from a dilute soil solution, and the small intestine absorbs glucose when the concentration in the gut is lower than in the blood.
- For surface area to volume ratio, calculate both quantities before dividing, and then link the answer to the biology: as an organism gets larger the ratio falls, so diffusion across the outer surface is too slow, and specialised exchange surfaces such as lungs, gills and villi are needed.
Worked example
A potato cylinder has a mass of 4.20 g at the start of an osmosis investigation. After 30 minutes in a 0.5 mol per cubic decimetre sugar solution its mass is 3.78 g. Calculate the percentage change in mass and explain what it shows.
- Find the change in mass: 3.78 - 4.20 = -0.42 g, so the cylinder lost mass.
- Divide by the starting mass: -0.42 / 4.20 = -0.1.
- Multiply by 100 to get a percentage: -0.1 x 100 = -10 percent.
- Interpret the sign: a loss of mass means water moved out of the potato cells.
- Explain with osmosis: the sugar solution was more concentrated than the cell contents, so water moved out of the cells by osmosis through the partially permeable cell membranes, down a water concentration gradient.
- Final answer: a 10 percent decrease in mass, showing the external solution was more concentrated than the cell sap.
Practice questions
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Q1Define osmosis.Show answer
Answer: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Q2Name the process that moves substances against a concentration gradient.Show answer
Answer: Active transport.
Q3Why does active transport require energy?Show answer
Answer: Because substances are moved from a dilute to a more concentrated solution, against the concentration gradient, which does not happen spontaneously. The energy is transferred by respiration.
Q4State three factors that increase the rate of diffusion.Show answer
Answer: A greater difference in concentration, a higher temperature, and a larger surface area of the membrane.
Q5Calculate the surface area to volume ratio of a cube of side 2 cm.Show answer
Answer: Surface area = 6 x 2 x 2 = 24 square centimetres; volume = 2 x 2 x 2 = 8 cubic centimetres; ratio = 24 : 8 = 3 : 1.
Q6Explain why a plant cell placed in pure water does not burst.Show answer
Answer: The cellulose cell wall is strong and resists the pressure as water enters, so the cell becomes turgid rather than bursting.
Q7Give one adaptation of a root hair cell for absorbing mineral ions.Show answer
Answer: A long extension giving a large surface area, and many mitochondria to transfer the energy needed for active transport.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain why the small intestine is adapted for the efficient absorption of digested food.
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A student places identical potato cylinders in five sugar solutions of increasing concentration and records the percentage change in mass. Describe the pattern the student should expect, and state what the point where the line crosses zero represents.
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Single-celled organisms such as amoeba exchange gases across their outer surface, but a mammal needs lungs. Explain why, referring to surface area to volume ratio.
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