Answer to Question #208044 in Cell Biology for Mahi

Question

Write a comprehensive note on the transport of various substance across the plasma membrane?

Answer 1

Transport of molecules into (and out of) the cell can take three main forms: diffusion, active transport, vesicle transport.

Diffusion:

Passive ("simple") diffusion: occurs along a concentration gradient directly through the lipid bilayer. Example: Oxygen and carbon dioxide molecules.

Facilitated diffusion: occurs along a concentration gradient, but requires a protein channel as a conduit. Example: aquaporins

Ion channels:  selective conduit proteins, usually gated, which only allow the passage of specific ions, usually in response to a triggering stimulus. Example: voltage-gated sodium channels.

Active transport:

Primary active transport: mediated by a "pump" protein which uses chemical energy stored in ATP to facilitate the transport of molecules (usually against their concentration gradient). Example: sodium and potassium transport by Na⁺/K⁺ ATPase.

Secondary active transport: mediated by an exchaner or co-transporter which facilitates the movement of molecules using the energy of a concentration gradient set up by another (primary) ATP-powered transport process. Example: sodium and glucose co-transport.

Vesicle transport

Endocytosis: where the transport of substances into the cell occurs by formation membrane-bounded vesicles containing the substance. Example: catecholamine neurotransmitter reuptake.

Exocytosis: the opposite of endocytosis, where vesicles transport molecules to the cell surface and empty their contents into the extracellular fluid. Example: catecholamine neurotransmitter release.