Diffusion

• Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration (down a concentration gradient)
• Diffusion happens in both solutions and gases - that's because particles in these substances are free to move about randomly
• The simplest type of diffusion is when different gases diffuse through each other
• The bigger the concentration gradient (difference in concentration), the faster the diffusion rate

  
• A higher temperature will give a faster diffusion rate because the particles have more energy, so move around faster

  
• Cell membranes hold the cell together but they let substances 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), glucose, amino acids and water

  
• Big molecues like starch and proteins can't fit through the membrane

  
• The particles are moving about randomly so they go both ways - but if there are a lot more particles on one side of the membrane, there's a net (overall) movement from that side

  
• The larger the surface area of the membrane, the faster the diffusion rate, because more particles can pass through at once