AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    Factors which affect the rate of diffusion are: • the difference in…

    Full specification wording

    Diffusion: Factors which affect the rate of diffusion are: • the difference in concentrations (concentration gradient) • the temperature • the surface area of the membrane.

    Three named factors control how quickly substances diffuse.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 5 of 11
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Three named factors control how quickly substances diffuse. The difference in concentrations, or concentration gradient, is the difference between the concentration on each side of a membrane; a larger difference gives a faster net rate because more particles move from the higher to the lower concentration region. Temperature affects the kinetic energy of particles, so a higher temperature makes them move faster and increases the rate. The surface area of the membrane is the area available for particles to cross; a larger area lets more particles pass per unit time, increasing the rate. In the lungs, a steep oxygen gradient, body temperature and the large alveolar surface area together give rapid gas exchange.

    What earns the marks

    • 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.

    Common mistakes and how to put them right

    • Treating concentration itself, rather than the difference in concentration, as the controlling factor.

      Put it right by comparing the two concentrations on either side of the membrane.

    • Assuming temperature affects only reactions and not diffusion.

      Put it right by explaining that temperature changes particle kinetic energy and therefore movement.

    • Confusing surface area with volume.

      Put it right by referring specifically to the area of membrane available for exchange.

    Exam tips

    • 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.

    Try the lesson

    The start of the Diffusion: Factors which affect the rate of diffusion are: • the difference in concentrations (concentration gradient) • the temperature • the surface area of the membrane. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which of the following is a factor that affects the rate of diffusion across a cell membrane?

    Listen

    Three named factors control how quickly substances diffuse. The difference in concentrations, or concentration gradient, is the difference between the concentration on each side of a membrane; a larger difference gives a faster net rate because more particles move from the higher to the lower concentration region. Temperature affects the kinetic energy of particles, so a higher temperature makes them move faster and increases the rate. The surface area of the membrane is the area available for particles to cross; a larger area lets more particles pass per unit time, increasing the rate. In the lungs, a steep oxygen gradient, body temperature and the large alveolar surface area together give rapid gas exchange. A strong answer earns marks for these points. 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. A common slip: Treating concentration itself, rather than the difference in concentration, as the controlling factor. Instead, comparing the two concentrations on either side of the membrane. Exam tip: List all three factors when asked to state them, then select the one the question asks you to explain in detail.

    See it

    A top-down flowchart with three parallel branches that all lead to a single outcome. The first branch starts at a node labelled Concentration gradient, which leads to Larger difference in concentration, which leads to Faster net rate of diffusion. The second branch starts at Temperature, which leads to Higher temperature, which leads to Particles have more kinetic energy, which also leads to Faster net rate of diffusion. The third branch starts at Surface area of the membrane, which leads to Larger surface area, which leads to More particles pass per unit time, which also leads to Faster net rate of diffusion.
    Shows the three factors that affect the rate of diffusion and how each one increases the rate.

    Key words

    Say what each word means before you turn the card.

    Concentration gradient

    Try answering aloud or in your head before turning the card.

    Temperature

    Try answering aloud or in your head before turning the card.

    Surface area of the membrane

    Try answering aloud or in your head before turning the card.

    See every definition
    Concentration gradient
    The difference in concentration between two regions separated by a membrane; a larger difference gives a faster net rate because more particles move from the higher to the lower concentration region.
    Temperature
    Affects the kinetic energy of particles, so a higher temperature makes them move faster and increases the rate of diffusion.
    Surface area of the membrane
    The area available for particles to cross; a larger area lets more particles pass per unit time, increasing the rate.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart with three parallel branches that all lead to a single outcome. The first branch starts at a node labelled Concentration gradient, which leads to Larger difference in concentration, which leads to Faster net rate of diffusion. The second branch starts at Temperature, which leads to Higher temperature, which leads to Particles have more kinetic energy, which also leads to Faster net rate of diffusion. The third branch starts at Surface area of the membrane, which leads to Larger surface area, which leads to More particles pass per unit time, which also leads to Faster net rate of diffusion.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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