In practice, given the structure of known membrane proteins , these holes are only large enough to allow the passage of small molecules through the plasma membrane, almost always simple ions like hydrogen, potassium or sodium. The ions may pass through the hole or orifice by passive diffusion, in which case the protein that allows this transport is called an ion channel. Alternatively, the transmembrane protein may invest energy, usually derived from ATP, to actively force ions from one side of the plasma membrane to the other, in which case it will be an ion pump. Given the importance of membrane transport, cells utilize a wide range of transport mechanisms. Diffusion Simple diffusion means that the molecules can pass directly through the membrane.
Diffusion is always down a concentration gradient.
The speed of facilitated transport is limited by the number of protein channels available, whereas the speed of diffusion is dependent only on the concentration gradient. Active Transport Active transport requires the expenditure of energy to transport the molecule from one side of the membrane to the other, but active transport is the only type of transport that can actually take molecules up their concentration gradient as well as down. Similarly to facilitated transport, active transport is limited by the number of protein transporters present. We are interested in two general categories of active transport, primary and secondary. Secondary active transport involves using energy to establish a gradient across the cell membrane, and then utilizing that gradient to transport a molecule of interest up its concentration gradient.