Topic 3 of IGCSE Biology (Cambridge 0610) is movement in and out of cells. You need to define diffusion, osmosis, and active transport accurately, describe and explain practical results (e.g. potato chips in different concentrations), and state how factors such as temperature and concentration gradient affect the rate of diffusion. This revision guide takes you through each process, the factors that affect them, and how to answer “define”, “describe”, “explain”, and practical questions for full marks.

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What is diffusion?

Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration down a concentration gradient. It does not require energy (it is a passive process) and it continues until the particles are evenly distributed (equilibrium). Diffusion happens in gases and liquids (and in solids only very slowly). Everyday examples include the smell of perfume spreading across a room, or oxygen moving from the air in the lungs into the blood.

In living organisms, diffusion is important for gas exchange (e.g. oxygen into cells, carbon dioxide out) and for the movement of some dissolved substances across cell membranes. In your definition, always include: net movement, higher to lower concentration, down a concentration gradient, and that it does not require energy.

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Factors that affect the rate of diffusion

The rate of diffusion (how fast particles spread) is affected by several factors. A steeper concentration gradient (bigger difference between high and low concentration) increases the rate because more particles move from the high to the low region per second. Higher temperature increases the rate because particles have more kinetic energy and move faster. Larger surface area (e.g. a thin, flat shape) increases the rate because more particles can cross the surface in a given time. Shorter distance (e.g. a thin membrane) also increases the rate because particles reach the other side more quickly.

In “explain” questions, link each factor to movement of particles (e.g. “Higher temperature means particles move faster, so they diffuse more quickly.”).

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What is osmosis?

Osmosis is the net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (more concentrated solution) through a partially permeable membrane. A partially permeable membrane allows water molecules to pass through but blocks or restricts larger molecules (e.g. sugar, salt). Osmosis is a type of diffusion (of water only) and does not require energy.

It is important to get the wording right. Do not say “water moves from low to high concentration” if you mean solute: a dilute solution has high water concentration; a concentrated solution has low water concentration. So water moves from dilute to concentrated (from high water concentration to low water concentration). Always mention the partially permeable membrane in your definition.

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Osmosis in practice: potato chips

A common practical is to place identical potato chips in different concentrations of sugar or salt solution (e.g. distilled water, 0.2 M, 0.4 M, 0.6 M) and measure the change in mass or length after a set time. In distilled water (very dilute), the potato chip gains mass because water enters the cells by osmosis (the cytoplasm is more concentrated than the surrounding solution). In concentrated solution, the chip loses mass because water leaves the cells by osmosis (the surrounding solution is more concentrated than the cytoplasm). There may be one concentration where there is little or no change: the concentration inside the cells is similar to that outside.

When you describe the result, state what you observe (e.g. “The chip in distilled water increased in mass.”). When you explain, use osmosis (e.g. “Water entered by osmosis because the water concentration outside was higher than inside the cells.”). Identify variables: independent = concentration of solution; dependent = change in mass or length; control = same type of potato, same size of chip, same time, same temperature.

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What is active transport?

Active transport is the movement of substances against the concentration gradient (from lower to higher concentration). It requires energy (from respiration) and carrier proteins in the cell membrane. Because it goes against the gradient, it allows cells to take in substances even when the concentration outside is lower than inside (e.g. root hair cells absorbing mineral ions from dilute soil solution; gut cells absorbing glucose from the gut into the blood).

Do not say active transport goes “down” the gradient or that it does not need energy. Always state: against the concentration gradient; requires energy (respiration); carrier proteins involved.

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Comparing diffusion, osmosis, and active transport

Process	Direction of movement	Energy needed?	Membrane?	Example
Diffusion	High → low concentration	No	Not always	Oxygen into cells
Osmosis	High → low water conc.	No	Yes, partially permeable	Water into root hair cells
Active transport	Low → high concentration	Yes	Yes, carrier proteins	Mineral ions into root

In exams you may be asked to compare the three in terms of gradient, energy, and an example in plants or animals.

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Exam tips and command words

• Define: Use the full idea (e.g. osmosis: net movement of water, higher to lower water concentration, through a partially permeable membrane).
• Describe the result: State what you observe (e.g. chip gained mass; chip lost mass).
• Explain: Use the concept (e.g. water entered by osmosis because the solution was more dilute than the cytoplasm).
• Identify variables: Independent (what you change), dependent (what you measure), control (what you keep the same).

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Common mistakes to avoid

• Defining osmosis without mentioning the partially permeable membrane.
• Confusing water concentration with solute concentration (dilute = high water concentration).
• Saying active transport does not need energy, or that it goes “down” the gradient.
• Not stating that active transport uses carrier proteins and energy from respiration.

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Revision checklist

• Define diffusion, osmosis, and active transport accurately (include gradient, energy, membrane where relevant).
• Describe a simple osmosis experiment (e.g. potato chips in different concentrations) and the expected results.
• Explain the effect of temperature and concentration gradient on the rate of diffusion.
• Compare the three processes (direction of gradient, energy, membrane, one example each).

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Next steps

Book a free trial with an IGCSE Biology tutor to practise describing and explaining diffusion and osmosis experiments, or explore Tutopiya’s learning portal for more revision resources.