Start with the definition of diffusion, which is the tendency for molecules to spread out into available space, noting that it only applies to a population of molecules. Cover the factors that affect diffusion, such as the concentration gradient (where molecules move from a higher to a lower concentrated region), resistance, size of the molecule, temperature and pressure. Include pictures in the lecture so students will have a better understanding of how the factors work.
Introduce the two types of diffusion: passive and active. There are two forms of passive: facilitated diffusion and osmosis. In facilitated diffusion, molecules enter and exit the cell by passing through proteins. Make sure the students recognize that no work is being done. For osmosis, make sure the students understand this applies only to the movement of water. Cover the three main types: isotonic, hypertonic and hypotonic.
Next, discuss active transport. Emphasize that this process does require work (unlike passive diffusion) and the cell must utilize its own energy for the process to occur. Talk about sodium-potassium pumps and how these are an excellent example of active transport. Talk about how reverse osmosis is another example of active transport. It is used in labs to filter sterile solutions and is used to purify water.
Add a slide at the end that shows the important points about each of these processes. When viewing the information side by side, students will get a better understanding of the differences in each process.