Comparison of transport

Passive Processes
Movement of substances down the concentration gradient (from high to low until equilibrium is reached). No energy is required for any of this processes.

Active Processes
Cellular energy (ATP) is used to pump substances against (up) the concentration gradient – from low concentration of solute to high concentration of solute.

Channel Proteins
are responsible for the transport of water-soluble substances such as glucose and electrolytes. Since they are ion selective, they contain a pore through which the solutes pass at a very high flux rate (compared to the flux rate of carrier proteins). Channel proteins only do passive transport down the concentration gradient. Some channels are also gated and/or selective. Gated means it opens only when appropriately stimulated. Selective means it only lets certain substances through. It can be seen as a tunnel. Only some examples for channels are: Ca2+ channel protein, slow Na+ channel protein, fast Na+ channel proteins, Nicotinic Acetylcholine (nACh) receptor, N-methyl-D-aspartate.

Carrier Proteins
transport both water soluble and insoluble substances. When transporting the solutes, they bind them on one side of a membrane, undergo conformational changes, and release them on to the other side of the membrane. Both active and passive transport - active means transport ion of the solute up the concentration gradient, whereas passive is the transport down the concentration gradient where no energy (ATP) is needed. Carrier only transport specific substances. Because of the more complex functioning the flux rate is slower compared to channel proteins. Furthermore they can be compared with enzymes in their working method and there can also be saturation, in case all carriers are "occupied" with a substance. Examples: Glucose Transporter 4 (GLUT-4), Na+-K+ ATPase, Ca2+ ATPase.