There are several different ways substances can be transported:
1) simple diffusion, i.e. substances that tend to be lipid soluble/ non-polar and can pass easily across the phospholipid bilyar of cells. 2) facilitated diffusion, which involves ihe roles of carrier proteins and channel proteins. E.g. oxygen and glucose 3) osmosis ( water is transported from an area of higher water potential to a lower water potential across a membrane. 4) active transport- involves the transport of substances against a concentration gradient and uses carrier proteins and ATP hydrolysis is required. An example of this is a sodium-potassium pump after an action potential. 5) co-transport- this is when substances are simultaneously transported across a membrane by one protein e.g. the absorption of sodium ions and glucose by cells lining the mammalian ileum.