Diffusion - Biology Past Paper PDF

# Topic 1b Transport in Cells ## 1. Diffusion **Particles tend to move about randomly and end up evenly spaced. This is important when it comes to getting substances in and out of cells** ### What is diffusion? `"Diffusion" is simple. It's just the gradual movement of particles from places where...

# Topic 1b Transport in Cells ## 1. Diffusion **Particles tend to move about randomly and end up evenly spaced. This is important when it comes to getting substances in and out of cells** ### What is diffusion? `"Diffusion" is simple. It's just the gradual movement of particles from places where there are lots of them to places where there are fewer of them. That's all it is - just the natural tendency for stuff to spread out.` **Unfortunately you also have to learn the fancy way of saying the same thing, which is this:** Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration. **Diffusion happens in both solutions and gases - that's because the particles in these substances are free to move about randomly. The simplest type is when different gases diffuse through each other.** ### Example When you spray perfume, the smell of perfume diffuses through the air in a room: * A diagram is shown of the perfume particles moving randomly from an area of high concentration to an area of low concentration. ## Diffusion across cell membranes **Cell membranes are clever because they hold the cell together but they let stuff in and out as well. Dissolved substances can move in and out of cells by diffusion.** **Only very small molecules can diffuse through cell membranes though - things like oxygen (needed for respiration - see page 173), glucose, amino acids and water. Big molecules like starch and proteins can't fit through the membrane (see Figure 2 on the next page).** ## Learning Objectives: * Know the definition of 'diffusion'. * Be able to describe how dissolved substances (such as oxygen) can move in and out of a cell, through the cell membrane, by diffusion. * Understand that during diffusion, the net movement of substances will be from an area of higher concentration to an area of lower concentration. * Be able to explain how the rate of diffusion is affected by the difference in the concentration of particles, the temperature and the surface area available for diffusion. **Specification Reference 4.1.3.1** * A picture of a flask is shown, the ink is diffusing from the bottom to the top. **Figure 1: The ink particles in this flask are diffusing into the water - they're moving from an area of high concentration (at the bottom of the flask) to an area of low concentration (higher up).**