Question

Compare osmosis, simple diffusion, facilitated diffusion and active transport: A) describe their differences and B) explain when cells have to use each one.

Answer 1

Osmosis: It is the movement of solvent molecules through a semi-permeable membrane from a dilute to a concentrated region. It takes place along the concentration gradient.

Cells use osmosis to maintain the interanl fluid concentration. It can be done by either removing water by exosmosis or taking in water by endosmosis.

Facilitated diffusion: It is a process of spontanoeus transport of molecules through a membrane by the help of transmembrane integral proteins. It takes place along the concentration gradient.
This process is required for the diffusion of large or polar molecules across the membrane, which normally would not be able to directly diffuse across the membrane.

Active transport: It is the energy-dependent movement of ions across a semi-permeable membrane. Enzymes are required for this process.
Cells perform active transport when they need to transport molecules against their concentration gradient across a membrane.