Materials in cells may be transported by passive or active processes, both of which may involve concentration gradients, the phospholipid bilayer, and membrane proteins. Part A: Compare the role of concentration gradients in passive and active transport. Part B: Compare the role of the phospholipid bilayer in passive and active transport. Part C: Compare the role of membrane proteins in passive and active transport.

Concentration gradients:

Phospholipid bilayer

Membrane proteins

Explanation:

Diffusion: Refers to the pass of some small polar hydrophilic molecules through the lipidic bilayer. There are momentary membrane openings that allow small molecules to freely move from one side to the other in favor of the electrochemical gradient. These openings are the result of the lipids movements. The membrane permeability depends on the size of the molecule. The bigger the molecule is, the less capacity to cross the membrane it has. Diffusion is a very slow process, and to be efficient requires short distances and pronounced concentration gradients.  

Facilitated diffusion: Occurs when big hydrophilic molecules cannot move freely across the membrane. In these situations, channel proteins are in charge of passive transport. Facilitated diffusion also occurs in favor of electrochemical gradients, meaning that molecules move from the more concentrated side to the less concentrated side.

Simple diffusion and facilitated diffusion are passive transport processes because the cell does not need any energy to make it happen.

Active transport is against the electrochemical gradient, so it does need energy to happen. Molecules move from a lower concentration side to a higher concentration side. This process is always in charge of carrier proteins. In primary active transport, the energy needed comes from the ATP molecule. In secondary active transport, the energy comes from the membrane electric potential.

Transmembrane proteins act in active and passive transport, connecting cells, and participating in signal transduction.    

Channel proteins act by passive transport (these proteins do not need energy to carry in or out substances. They do it according to an electrochemical gradient) and carrier proteins act during active transport (these proteins do need the energy to transport substances because they do it against an electrochemical gradient). These transmembrane proteins carry in those substances needed by the cells to properly perform their activities, and they can also carry out waste substances.

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