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Diffusion and Factors That Affect the Rate of Diffusion

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Contents

Diffusion

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Recall Questions

This topic requires prior knowledge of cell membranes and particle movement to understand how substances passively move into and out of cells.

What is the function of the cell membrane?

To control the movement of substances into and out of the cell

What is the function of the cell wall?

To strengthen and maintain the shape of a plant cell

Why are red blood cells bi-concave in shape?

To maximise surface area to absorb oxygen

Topic Explainer Video

Check out this @JoeDoesBiology video that explains diffusion and factors that affect the rate of diffusion, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!

Diffusion

Definition:

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
Diffusion is passive, it does not require energy from respiration.

Diagram showing diffusion across a partially permeable membrane, with particles moving from a region of higher concentration to lower concentration until more evenly distributed.

Examples of Diffusion in Biology:

  • Oxygen diffuses into cells for respiration.
  • Carbon dioxide diffuses out of cells as a waste gas.
  • Glucose diffuses into the blood from the small intestine.

Factors Affecting the Rate of Diffusion (with detailed explanations)

  • Concentration Gradient - The difference in concentration between two areas.
    • The greater the difference in concentration between two areas, the faster the rate of diffusion.
    • A steep gradient means particles move quickly down the gradient.
    • As diffusion continues, the gradient becomes shallower and diffusion slows down, an equilibrium (the same concentration) is reached.

  • Temperature

    • Higher temperatures give particles more kinetic energy, so they move faster.

    • Faster movement increases the rate at which particles collide with the membrane and diffuse.

  • Surface Area of the Membrane
    • A larger surface area allows more particles to pass through at the same time.
    • This is why cells adapted for diffusion (e.g., alveoli) have folded or extended surfaces to maximise surface area.

  • Thickness of the Exchange Surface
    • 1 cell thick exchange surfaces (eg. the walls of the alveoli) means a shorter diffusion path, so substances can move across more quickly.

  • Movement of the Medium (Ventilation or Circulation)
    • In living organisms, ventilation (lungs) and circulation (blood flow) help maintain a steep concentration gradient, keeping the diffusion rate higher.

Key Terms

  • Concentration gradient – difference in concentration between two areas.
  • Diffusion – net movement of particles from an area of higher concentration to an area of lower concentration.
  • Surface area - total area of the outer surface of an object or structure.
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Exam Tips

  • Don’t confuse diffusion with osmosis or active transport.
  • Always keep the factors that affect diffusion in your mind. Lots of questions will link back to these.
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Into which cell will glucose enter the fastest. Explain your answer. (2 marks)

Diagram comparing cells in different solute concentrations, showing how water movement by osmosis causes cells to swell or shrink depending on the surrounding solution.

  • Cell A
  • This cell has the steepest / largest concentration gradient, with a higher concentration of glucose outside the cell.

More Pracatice

Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!

Calculating surface area to volume ratio