Diffusion — AQA GCSE Biology

Diffusion is the spreading out of particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration. This passive process is essential in living organisms for gas exchange, nutrient absorption, and waste removal across partially permeable cell membranes.

Diffusion is a fundamental passive transport mechanism whereby substances move down a concentration gradient due to the random thermal motion of particles. In AQA GCSE Biology, mastering diffusion is vital for understanding how essential substances like oxygen, glucose, and carbon dioxide enter and exit cells.

This topic links closely with cellular structure, organ system adaptations such as the alveoli and gills, and comparative transport processes like osmosis and active transport. Developing a clear grasp of factors affecting diffusion rates provides the foundation for excelling in transport systems across plants and animals.

• →Net movement: Diffusion describes the overall (net) movement of particles down a concentration gradient, even though individual particles move randomly in all directions.
• →Three main rate factors: The rate of diffusion increases when the concentration gradient is steeper, the temperature is higher (more kinetic energy), or the surface area of the exchange surface is larger.
• →Diffusion pathway distance: A thinner membrane or barrier decreases the diffusion distance, thereby accelerating the rate of substance exchange across the surface.
• →Biological significance: Essential substances such as oxygen and glucose enter cells via diffusion, while metabolic waste products like carbon dioxide and urea diffuse out into body fluids.

• Substances can enter cells across the cell membrane by diffusion.
• Substances can leave cells across the cell membrane by diffusion.
• Diffusion across a membrane depends on a concentration difference between the two sides.
• The net movement is from higher concentration to lower concentration until the concentrations become equal.
• Diffusion is passive and does not require energy from respiration.
• Examples include oxygen entering cells and carbon dioxide leaving cells.
• Diffusion involves the spreading out of particles of a substance in solution or particles of a gas.
• Particles move randomly, which causes them to spread from where they are more concentrated.
• There is a net movement from an area of higher concentration to an area of lower concentration.
• Net movement means the overall movement, even though individual particles move in all directions.
• Diffusion continues until the concentration is uniform, after which there is no net movement.
• Diffusion is a passive process and does not require energy from respiration.
• Oxygen diffuses into cells and is used in respiration.
• Carbon dioxide is produced by respiration and diffuses out of cells.
• In gas exchange, oxygen and carbon dioxide diffuse between an organism and its environment, for example in the lungs.
• Urea is a waste product formed from the breakdown of excess amino acids.
• Urea diffuses from cells into the blood plasma.
• Urea is transported to the kidney and excreted, so diffusion is part of the route to excretion.
• State that diffusion is the net movement of particles down a concentration gradient, from higher to lower concentration, caused by random particle motion.
• Explain that a steeper concentration gradient increases the rate because more particles move in one direction than the other, giving a greater net movement per unit time.
• Explain that a higher temperature increases the rate because particles have more kinetic energy and move faster, so collisions and net movement are more frequent.
• Explain that a larger surface area of membrane increases the rate because more particles can cross the membrane at the same time.
• Apply the factors to a named biological context, such as oxygen uptake across alveoli or absorption of mineral ions by root hair cells.
• Use the relationship rate proportional to surface area multiplied by concentration difference divided by distance, treating it as a qualitative guide rather than a calculation requirement.
• Identify the concentration gradient as the difference in concentration between two regions separated by a membrane.
• State that a larger concentration difference increases the rate of diffusion.
• State that a higher temperature increases the rate of diffusion because particles have more kinetic energy.
• State that a larger surface area of membrane increases the rate of diffusion.
• Link each factor to the number of particles crossing the membrane per unit time.
• Recognise that these factors can act together, as in the alveoli where a steep gradient and large surface area both raise the rate.
• Define surface area to volume ratio as surface area divided by volume.
• Explain that as size decreases, volume decreases more rapidly than surface area, so the ratio increases.
• State that a single-celled organism is small and therefore has a relatively large surface area to volume ratio.
• Link the large ratio to a large area of cell surface membrane relative to the volume of cytoplasm that must be supplied.
• Contrast this with a large multicellular organism, which has a smaller surface area to volume ratio and needs specialised exchange surfaces.
• Use a simple numerical example, such as a cube of side 1 unit having a ratio of 6 compared with a cube of side 2 units having a ratio of 3.
• State that the large surface area to volume ratio provides sufficient membrane area for exchange.
• Explain that oxygen and glucose diffuse into the cell while carbon dioxide and waste products diffuse out.
• Link the short diffusion distance in a small cell to the speed of supply to the cytoplasm.
• State that diffusion alone meets the needs of a single-celled organism without a transport system.
• Contrast this with large multicellular organisms, where a small surface area to volume ratio and long diffusion distance make diffusion alone insufficient.
• Recognise that the rate of diffusion must match the organism's metabolic demand for the transport to be sufficient.
• Calculate surface area correctly using the appropriate formula for the shape, such as 6 × L² for a cube.
• Calculate volume correctly using the appropriate formula, such as L³ for a cube.
• Divide surface area by volume to obtain the ratio, cancelling units correctly.
• Compare two or more ratios and state which object has the larger or smaller surface area to volume ratio.
• Relate a larger surface area to volume ratio to more effective diffusion over the available surface.
• State that multicellular organisms have a small surface area to volume ratio.
• Explain that diffusion over the outer surface alone would be too slow or insufficient to supply all cells.
• Describe how exchange surfaces increase surface area and provide a short diffusion path.
• Explain that a transport system carries substances between exchange surfaces and body cells.
• Link these features to meeting the metabolic demands of a large organism.
• Describe how villi and microvilli in the small intestine increase surface area and how a thin wall and blood supply aid absorption.
• Explain how alveoli, thin walls, capillaries and ventilation adapt lungs for gas exchange.
• Describe gill filaments and lamellae, thin surfaces and counter-current flow in fish gills.
• Explain how root hair cells increase surface area for uptake of water and mineral ions.
• Describe leaf adaptations such as a flat blade, thin lamina, stomata and air spaces for gas exchange.
• Multicellular organisms need specialised exchange surfaces because diffusion over large distances is too slow to supply all cells.
• A large surface area increases the number of molecules that can cross at the same time, increasing the rate of exchange.
• A thin membrane provides a short diffusion path, so molecules diffuse across more quickly.
• In animals, an efficient blood supply maintains a steep concentration gradient by transporting substances to and from the exchange surface.
• Ventilation for gaseous exchange maintains concentration gradients by replacing air, keeping oxygen concentration high and carbon dioxide concentration low.

• 💡Always name the membrane as the barrier crossed and state the direction of net movement.
• 💡Use the phrase concentration gradient to explain why diffusion happens.
• 💡When giving an example, name the substance and the two regions it moves between.
• 💡Use the words net movement and concentration gradient to make the definition precise.
• 💡State that particles are in solution or in a gas, because this matches the specification wording.
• 💡If asked to explain, link random particle motion to the overall net direction.
• 💡Name the substance, its starting region and its destination to show the direction of diffusion clearly.
• 💡Link oxygen and carbon dioxide to respiration so the reason for their movement is clear.
• 💡For urea, state that it moves into the blood plasma and is then excreted by the kidney.
• 💡Name the factor, state whether the rate increases or decreases, then give the particle-level reason in one linked sentence.
• 💡When a question supplies data, quote a relevant value or trend from the table or graph before explaining it.
• 💡Use a named example such as gas exchange in alveoli to show application rather than describing diffusion in the abstract.
• 💡List all three factors when asked to state them, then select the one the question asks you to explain in detail.
• 💡Use comparative language such as 'steeper', 'higher' and 'larger' to show the direction of the effect.
• 💡Where a diagram is given, identify the membrane and the two regions before naming the gradient.
• 💡Write the ratio as surface area divided by volume and show the substitution if values are given.
• 💡Use the phrase 'relative to its volume' whenever you describe the ratio of a small organism.
• 💡Compare a small and a large organism in the same answer to demonstrate understanding of the trend.
• 💡Use the phrase 'sufficient to meet the needs of the organism' and then justify it with the ratio and diffusion distance.
• 💡Name at least one substance entering and one leaving the cell to show the transport is bidirectional.
• 💡When comparing with a multicellular organism, refer to both surface area to volume ratio and diffusion distance.
• 💡Write the formula, substitute values, then give the final ratio with units cancelled.
• 💡Show each step of working so method marks can be awarded even if the final value is wrong.
• 💡When comparing, quote both calculated ratios and then make the comparative statement explicit.
• 💡Use the phrase surface area to volume ratio explicitly in your answer.
• 💡Link each adaptation to a consequence, such as a short diffusion path increasing the rate of diffusion.
• 💡Refer to named examples, such as alveoli or villi, to support your explanation.
• 💡Name the organism and organ, then state the adaptation and its benefit.
• 💡Use comparative language such as increases surface area or shortens the diffusion path.
• 💡Include at least one adaptation linked to maintaining a concentration gradient for each example.
• 💡Name a specific exchange surface, such as alveoli or villi, and link each feature to faster diffusion.
• 💡Use the phrase concentration gradient when explaining the roles of blood supply and ventilation.
• 💡For calculation questions, show the surface area and volume values clearly and compare the ratio to explain exchange efficiency.
• 💡Always state 'concentration gradient' rather than just 'gradient' to secure technical precision marks in structured explanation questions.
• 💡When explaining adaptations of exchange surfaces, explicitly link structure to function by referencing short diffusion distance, large surface area, or maintaining a steep concentration gradient.
• 💡Check units carefully in surface area to volume ratio calculations and simplify ratios to the form 'X : 1' when requested.

• Saying that diffusion requires energy; the correction is that diffusion is passive and relies on the random movement of particles.
• Describing movement only into cells; the correction is that substances move both into and out of cells by diffusion.
• Confusing diffusion with active transport; the correction is that active transport moves substances against a concentration gradient and requires energy.
• Defining diffusion as movement only from high to low concentration without mentioning random particle movement; the correction is to include both the random motion and the net direction.
• Saying particles stop moving when concentrations are equal; the correction is that particles continue to move randomly but there is no net movement.
• Applying diffusion only to gases; the correction is that particles of substances in solution also diffuse.
• Saying carbon dioxide diffuses into cells for respiration; the correction is that carbon dioxide is a waste product of respiration and diffuses out of cells.
• Stating that urea is excreted by diffusion in the kidney; the correction is that urea diffuses into the blood plasma and the kidney removes it from the blood for excretion.
• Confusing gas exchange with respiration; the correction is that gas exchange is the diffusion of oxygen and carbon dioxide, while respiration is the chemical process that uses oxygen and produces carbon dioxide.
• Saying particles 'want' to move or move 'to be equal', which is teleological; correct by stating that random motion produces net movement down a concentration gradient.
• Confusing diffusion with active transport; correct by noting that diffusion is passive and requires no energy from respiration.
• Claiming that a higher temperature slows diffusion because particles 'stick together'; correct by explaining that increased kinetic energy speeds up particle movement and raises the rate.
• Treating concentration itself, rather than the difference in concentration, as the controlling factor; correct by comparing the two concentrations on either side of the membrane.
• Assuming temperature affects only reactions and not diffusion; correct by explaining that temperature changes particle kinetic energy and therefore movement.
• Confusing surface area with volume; correct by referring specifically to the area of membrane available for exchange.
• Saying a single-celled organism has a large surface area in absolute terms; correct by saying it has a large surface area relative to its volume.
• Inverting the ratio by dividing volume by surface area; correct by dividing surface area by volume.
• Believing that all small organisms have a large surface area; correct by specifying that the ratio, not the absolute area, is large.
• Claiming that single-celled organisms do not need to exchange gases; correct by stating that they still respire and must take in oxygen and remove carbon dioxide.
• Saying diffusion is 'fast' without linking it to the large ratio and short distance; correct by explaining both reasons.
• Assuming a large organism can rely on diffusion alone; correct by explaining that its small surface area to volume ratio and long diffusion distance require specialised surfaces and transport systems.
• Dividing volume by surface area instead of surface area by volume; correct this by always writing the ratio as surface area first, then volume.
• Forgetting to square or cube the side length; correct this by checking each formula before substituting numbers.
• Assuming the larger object always has the larger ratio; correct this by calculating both ratios, since larger objects usually have a smaller ratio.
• Saying large organisms have a large surface area to volume ratio; correct this by stating that the ratio decreases as size increases.
• Confusing exchange surfaces with transport systems; correct this by describing exchange surfaces as sites of diffusion and transport systems as the means of moving substances around the body.
• Claiming diffusion stops in large organisms; correct this by explaining that diffusion still occurs but is too slow or limited to meet the organism's needs without additional adaptations.
• Describing villi as increasing volume rather than surface area; correct this by stating that villi increase the surface area for absorption.
• Saying fish gills exchange gases with air; correct this by stating that gills exchange gases with water.
• Forgetting that a concentration gradient must be maintained; correct this by linking blood flow or ventilation to keeping the gradient steep.
• Thinking a large organism can rely on its outer surface for exchange: correct this by explaining that a small surface-area-to-volume ratio makes simple diffusion too slow, so specialised surfaces are needed.
• Confusing surface area with volume: correct this by stating that exchange depends on a large surface area, not a large volume.
• Believing a thin membrane makes diffusion happen without a concentration gradient: correct this by explaining that a thin membrane shortens the diffusion path, but a concentration gradient is still required.
• Believing particles stop moving at dynamic equilibrium: When concentrations equalize, particles continue moving randomly at equal rates in both directions, resulting in zero net movement.
• Confusing diffusion with active transport: Students often assume moving into cells always requires cellular energy (ATP), forgetting that diffusion is completely passive.
• Thinking larger organisms have higher SA:V ratios: As an organism increases in size, its volume grows much faster than its surface area, resulting in a lower surface area to volume ratio.

1. 1Day 1: Memorise the exact AQA definition of diffusion, focusing on 'net movement', 'concentration gradient', and 'passive transport'.
2. 2Day 2: Create a comparison table detailing the three factors that affect diffusion rate (gradient, temperature, surface area) and why each impacts particle movement.
3. 3Day 3: Practice surface area to volume ratio calculations for cubes of varying sizes and explain the biological necessity of specialised exchange systems in multicellular organisms.
4. 4Day 4: Review biological examples of diffusion (alveoli, villi, leaf stomata, fish gills), making flashcards connecting specific anatomical features to Fick's law principles.
5. 5Day 5: Complete past paper exam questions on diffusion, specifically tackling 4-mark and 6-mark extended response questions under timed conditions.

• 📋Definition and Recall: 1-2 mark questions asking for the definition of diffusion or naming substances transported by diffusion (e.g. urea, carbon dioxide, oxygen).
• 📋Data Analysis and Calculations: Questions requiring students to calculate surface area to volume ratios or interpret graphs showing rates of diffusion across different temperatures or concentrations.
• 📋Extended Response (4-6 marks): Evaluative or explanatory questions on how specialised exchange surfaces (e.g. alveoli, villi, gills, root hair cells) are adapted for efficient diffusion.