Diffusion

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.1 Investigation ➡ Lesson 1: Diffusion

Learning Intentions

By the end of this topic, you should be able to:

Explain what diffusion is and describe how particles move.
Link diffusion to real-life examples.
Explain how molecules move into and out of cells by diffusion through a semi-permeable membrane.

Glossary

Key terms and definitions.

barrier: A structure or layer that slows or blocks movement.

cell: The smallest living unit that makes up all living things.

cell membrane: A semi-permeable barrier around a cell that controls what enters and leaves.

concentration: A measure of mass (of solute) per volume (of solution). How crowded particles are in a specific space or volume.

concentration gradient: A difference in the concentration of a substance between two areas. This characteristic is the sole driver of diffusion.

diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration.

permeable: Allows substances to pass through.

semi-permeable: Ability to allow some substances to pass through but not others.

How do molecules in a substance move?

The molecules that make up substances are constantly moving about in a random way. This random motion causes them to disperse from areas of high to low concentration. This dispersal, called diffusion, does NOT require energy.

Each type of molecule moves down its own concentration gradient until evenly dispersed and an equilibrium is reached. Each molecule does this independent of the concentration of other types of molecule.

What factors affect the rate of diffusion?

Several factors affect the rate of diffusion, one of the most important being the concentration gradient. The rate of diffusion is higher when there is a greater difference between the concentrations of two regions.

Other factors that affect diffusion are:

Diffusion distance

Diffusion over shorter distances occur at a greater rate than over a large distance.

Temperature

Particles at a high temperature diffuse at a greater rate than at a low temperature.

Surface area

The larger the are across which diffusion occurs, the greater the rate of diffusion.

Solubility

Lipid-soluble or non-polar molecules pass across membranes more easily than polar materials, so their rates of diffusion are faster.

Barriers

Thick barriers have a slower rate of diffusion than thin barriers.

Solvent density

As the density of a solvent increases, the rate of diffusion decreases. Cellular dehydration adversely affects diffusion rates within cells.

What are some examples of diffusion in living things?

Diffusion is essential for keeping ALL cells alive. It plays an important role in passively transporting molecules into and out of cells, down their concentration gradient until an equilibrium is reached.

When you breathe in, there is a high concentration of oxygen in the air inside your lungs and a lower concentration in your blood. So Oxygen diffuses into your bloodstream. At the same time, carbon dioxide moves the opposite way (down its concentration gradient) so you can breathe it out.

Waste substances made inside cells, such as urea, diffuse out of the cell into the blood so they can be carried to the kidneys and removed.

Plants take in carbon dioxide through tiny openings in their leaves called stomata. There is usually more carbon dioxide outside the leaf than inside, so it diffuses into the leaf, where it is used for photosynthesis. Oxygen produced by photosynthesis diffuses out.

Small molecules like glucose can diffuse from the bloodstream into cells where they are needed for energy. There is often more glucose in the blood than inside the cell, so it diffuses across.

How do molecules diffuse into cells?

Cells are surrounded by a cell membrane. This membrane acts like a very thin barrier that controls what can enter and leave the cell. It is semi-permeable, which means some particles can pass through easily, while others cannot.

Diffusion occurs freely across membranes, as long as the membrane is permeable to that molecule. Semi-permeable membranes allow the passage of some molecules but not others. Think of it as a molecular sieve.

Large molecules (like proteins) and many charged particles cannot diffuse directly through. They need special transport systems, which is why the membrane is so important — it protects the cell while still allowing the right substances to move in and out.

More on Diffusion

📕 article 📸 image 🎮 interactive 🎦 video

Glossary

Key terms and definitions.

barrier: A structure or layer that slows or blocks movement.

cell: The smallest living unit that makes up all living things.

cell membrane: A semi-permeable barrier around a cell that controls what enters and leaves.

concentration: A measure of mass (of solute) per volume (of solution). How crowded particles are in a specific space or volume.

concentration gradient: A difference in the concentration of a substance between two areas. This characteristic is the sole driver of diffusion.

diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration.

equilibrium: When particles (or water) are balanced and there is no net movement.

permeable: Allows substances to pass through.

semi-permeable: Ability to allow some substances to pass through but not others.

Practice Tasks for Different Thinking Skills

Need help? Consider getting Private Tutoring or Personalised Feedback for your work from Lemonade-Ed's Mrs. Heald.

Applying

Topic: Diffusion and Membrane Transport

Applying your knowledge to PhET simulations to understand particle motion during diffusion.

Applying

Topic: Effect of Temperature on KMNO₄ Diffusion

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Remembering

Topic: Vocabulary Flash Cards

Test yourself using flash cards with prompts on the front and answers on the back. A printable version is available!

Remembering

Topic: Vocabulary Quiz

A series of multiple choice questions about the vocab list. A printable version is available!

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