Cambridge Syllabus Notes

Topic 3.1 – Diffusion (Passive Transport)

Learning Objective

State that some substances move into and out of cells by diffusion through the cell membrane.

Definition (syllabus wording)

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient). The energy driving this movement comes from the kinetic energy of the particles themselves; no cellular ATP is required.

Key Features of Diffusion

Occurs down a concentration gradient.
Spontaneous – does not require cellular energy (passive transport).
Can take place in gases, liquids and across semi‑permeable membranes.
Does not involve carrier proteins or active‑transport mechanisms.

Core Factors that Influence the Rate of Diffusion (required by the syllabus)

Concentration gradient – a steeper gradient increases the rate.
Temperature – higher temperature raises kinetic energy, speeding diffusion.
Surface area of the membrane – a larger area allows more particles to pass at once.
Distance (or thickness) of the membrane – a shorter distance shortens the path particles must travel.

Extension (optional for deeper study)

Particle size and polarity – small, non‑polar molecules diffuse more readily than large or charged particles.

Investigative Requirement

Design a simple experiment to investigate **one** of the four core factors listed above. The investigation must keep the other three factors constant.

Example Investigation

Factor: Surface area
Method: Cut pieces of dialysis tubing to give three different surface‑area values (e.g., 5 cm², 10 cm², 20 cm²). Fill each tube with the same concentration of a coloured solute (e.g., potassium permanganate). Immerse the tubes in identical beakers of distilled water at the same temperature. Measure the time taken for the colour to become visible in the external water (or record the concentration change with a spectrophotometer). Plot “Rate of diffusion” (e.g., change in concentration per minute) against surface area and explain the trend in terms of molecular motion.

Substances That Diffuse Through the Cell Membrane

Substance	Typical Direction of Movement	Reason for Diffusion
Oxygen (O₂)	Outside → Inside	Needed for cellular respiration; higher concentration in extracellular fluid/blood than inside the cell.
Carbon dioxide (CO₂)	Inside → Outside	Waste product of respiration; higher concentration inside the cell than in the surrounding medium.
Water (H₂O)	Variable – follows the water‑potential gradient	Moves to equalise osmotic pressure (osmosis is a special case of diffusion). See also: Osmosis (3.2).
Small non‑polar gases (e.g., N₂, NO)	Both directions, following concentration gradients	Non‑polar nature allows easy passage through the lipid bilayer.
Small ions (e.g., Na⁺, Cl⁻)	Down their individual concentration gradients	When the membrane is sufficiently permeable (e.g., brief channel openings) these ions can diffuse passively.

Why Diffusion Is Important for Cells

Provides a rapid, energy‑free way to obtain essential gases (O₂) and to remove waste gases (CO₂).
Maintains water balance through osmosis, crucial for cell turgor in plants and volume regulation in animal cells.
Allows passive movement of small nutrients and ions, supporting metabolism and helping to maintain electrochemical gradients.

Suggested Diagram

Cross‑section of a phospholipid bilayer showing arrows for O₂ entering the cell, CO₂ leaving the cell, water moving by osmosis, and a small ion (e.g., Na⁺) diffusing down its gradient.