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

3.1 Diffusion

What is Diffusion?

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration until the concentrations equalise. It happens without the cell using energy (passive transport). Think of a drop of food colouring in a glass of water – the colour spreads out until the whole glass looks the same. 🍹

Key Points

• Only molecules that can cross the lipid bilayer (e.g., gases, small non‑polar molecules) diffuse easily.
• Water moves by osmosis, a special type of diffusion through a semi‑permeable membrane.
• Diffusion is driven by the concentration gradient, not by the cell’s energy.
• Rate of diffusion depends on temperature, concentration difference, and membrane surface area.

Analogy: The Sponge

Imagine a sponge soaked in water. The water inside the sponge moves out into the air because the air has a lower water concentration. The sponge’s pores act like the cell membrane, letting water pass through. The same principle works for gases like oxygen and carbon dioxide in our bodies. 🧽➡️💨

Diffusion in the Body

Oxygen (O₂) diffuses from the air in the lungs into the blood because the concentration of O₂ is higher in the alveoli than in the blood. Carbon dioxide (CO₂) moves the opposite way, from the blood into the alveoli to be exhaled. This is how we breathe! 🫁

Fick’s Law (Quick Math)

For those who like a bit of math, the flux (J) of a substance across a membrane is given by:

$J = -D \frac{dC}{dx}$

Where D is the diffusion coefficient, dC/dx is the concentration gradient, and the negative sign shows movement from high to low concentration.

Exam Tip Box

Quick Quiz

1. What drives diffusion? (Answer: concentration gradient)
2. Is energy required for diffusion? (Answer: No)
3. Give an example of a substance that diffuses into a cell. (Answer: O₂, CO₂, water)

Summary

Diffusion is the simple, energy‑free movement of molecules across the cell membrane, driven by concentration differences. It’s essential for gas exchange, nutrient uptake, and waste removal in all living cells. Keep the key terms in mind, and you’ll ace any exam question on this topic! 🚀