A) Compare and contrast diffusion, passive transport (diffusion & facilitated), primary active transport and secondary active transport. Be sure that your answer clearly differentiates between each mechanism and includes specific examples of the way in which specific molecules are transported across the cell membrane by each mechanism. B

Passive transport: It does not need any energy to occur. Happens in favor of an electrochemical gradient. Simple diffusion and facilitated diffusion are kinds of passive transport.

Simple diffusion: molecules freely moves through the membrane.

Facilitated diffusion: molecules are carried through the membrane by channel proteins or carrier proteins.

Active transport needs energy, which can be taken from the ATP molecule (Primary active transport) or from a membrane electrical potential (Secondary active transport).

Explanation:

Diffusion: This is a pathway for some small polar hydrophilic molecules that can freely move through the membrane. Membrane's 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. An example of diffusion is osmosis where water is the transported molecule. Facilitated diffusion: Refers to the transport of hydrophilic molecules that are not able to freely cross the membrane. Channel protein and many carrier proteins are in charge of this passive transport. If uncharged molecules need to be carried this process depends on concentration gradients and molecules are transported from a higher concentration side to a lower concentration side. If ions need to be transported this process depends on an electrochemical gradient. The glucose is an example of a hydrophilic protein that gets into the cell by facilitated diffusion.

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

Active transport occurs against the electrochemical gradient, so it does need energy to happen. Molecules go from a high concentration side to a lower concentration side. This process is always in charge of carrier proteins. In primary active transport the energy needed comes from the ATP molecule. An example of primary active transport is the Na-K bomb. In secondary active transport, the energy comes from the membrane electric potential. Examples of secondary active transport are the carriage of Na, K, Mg metallic ions.