What is the difference between 'diffusion' and 'net movement' in a concentration gradient?

Diffusion refers to the process of random particle spread, while 'net movement' describes the overall direction of the majority of particles from high to low concentration until equilibrium is reached.

How does the surface area to volume ratio (SA:V) change as an object's size increases?

As size increases, the SA:V ratio decreases. This is because volume increases at a much faster rate (cubed) than surface area (squared), making diffusion less efficient for larger objects.

Compare the diffusion of a gas in a vacuum versus the diffusion of a gas in air.

Diffusion in a vacuum is much faster because there are no air particles to collide with, allowing for an unobstructed path. In air, frequent collisions with other molecules slow down the net progress.

What is a common error when interpreting a 'Time vs. Temperature' graph for diffusion?

A common error is assuming a downward-sloping line means the rate is decreasing. In fact, a decrease in time taken indicates an increase in the rate of diffusion.

Why is it incorrect to say that particles stop moving once they are evenly distributed?

Particles possess kinetic energy and are in constant random motion. At equilibrium, they continue to move, but the number of particles moving in one direction equals the number moving in the opposite direction.

What mistake is made when comparing diffusion rates of different sized cubes without calculating SA:V?

One might incorrectly assume a larger cube diffuses 'faster' because it absorbs more total substance, whereas the 'rate' relative to its volume is actually much slower than a smaller cube.

Define a 'Concentration Gradient'.

A concentration gradient is the difference in the concentration of a substance between two areas. Particles naturally move 'down' this gradient from high to low concentration.

What is 'Dynamic Equilibrium' in the context of diffusion?

It is the state where particles are spread out evenly and there is no net change in concentration, although individual particles continue to move randomly.

State Fick's Law in terms of the factors that increase diffusion rate.

The rate of diffusion is increased by increasing the surface area, increasing the concentration gradient, or decreasing the diffusion distance (thickness of the membrane).

How does increasing temperature affect the rate of diffusion at a molecular level?

Increasing temperature increases the kinetic energy of particles, causing them to move faster and collide more often, which speeds up the overall spread of the substance.

Investigating Diffusion | Cambridge International Examinations AS-Level Biology

Revision Notes

AS-Level

Cambridge International Examinations

Biology

4. Cell Membranes & Transport

Investigating Diffusion

Summary

Diffusion is the passive net movement of particles from a region of higher concentration to a region of lower concentration. Investigating this process involves quantifying how physical variables—such as temperature, surface area, and concentration gradients—influence the rate of particle spread in both biological and non-biological systems.

1. Definition & Core Concepts

Diffusion is defined as the net movement of molecules or ions from a region of their higher concentration to a region of their lower concentration, moving down a concentration gradient.
This process is passive, meaning it does not require an external energy source like ATP; instead, it is driven by the intrinsic kinetic energy of the particles.
The end state of diffusion is dynamic equilibrium, where particles are uniformly distributed but continue to move randomly with no further change in net concentration.

Diagram showing particles moving from a high concentration region to a low concentration region across a semi-permeable boundary.

2. Underlying Principles

3. Methods & Techniques

Investigating Surface Area

Use agar cubes of varying dimensions (e.g., 1cm, 2cm, 3cm) containing an indicator like phenolphthalein and an alkali. When placed in an acid, the acid diffuses inward, neutralizing the alkali and changing the color.
Measure the distance moved by the color change in a fixed time or the time taken for the entire cube to change color to calculate the rate.

Investigating Temperature

Place a set volume of a colored solute (like a dye) into water baths at different temperatures. Measure the time taken for the dye to reach a specific marker or for the solution to reach a uniform color intensity.

Investigating Concentration

Create a series of solutions with known concentrations of a solute. Observe the rate of diffusion into a solvent or across a model membrane (like dialysis tubing) to establish the relationship between gradient steepness and speed.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Equilibrium Myth: Students often think particles stop moving at equilibrium. In reality, they continue to move randomly, but there is no net change in concentration.
Rate vs. Time: A common error is stating that a higher temperature results in a 'higher time'. Correct terminology is 'shorter time' or 'higher rate'.
Surface Area Confusion: Larger organisms have more total surface area than smaller ones, but they have a smaller surface area relative to their volume (SA:V ratio).