Consignment

The penetration of a substance or compound through a membrane is called transport. Transport that does not require energy is called passive, while that which requires energy is called active.

Passive transport involves certain molecules passing through the plasma membrane according to the concentration gradient. Small polar molecules (water, urea, ethanol), gases, and lipophilic molecules simply diffuse through the lipid bilayer of membranes. This type of transport is called diffusion.

Diffusion is a slow process, and its speed, at a constant temperature, depends on the concentration gradient of the transported compound. The greater the concentration differences between the outside and inside of the cell, the faster the diffusion.

Active transport also occurs according to the concentration gradient of the transported compound, but with the involvement of certain proteins present in the membrane. Through facilitated transport, monosaccharides (glucose, fructose) and amino acids enter cells from the blood. There are two types of transport proteins: carrier proteins and channel proteins. Carriers are integral membrane proteins whose spatial structure changes when the transported compound binds, allowing the compound to pass through the membrane and be released into the cytoplasm. Channel proteins do not bind the transported compound but form a channel through which the compound enters the cell. Channel activity is regulated. They are activated (opened) or inhibited (closed) by electrical potential or regulatory molecules called ligands or ions.

Transport mediated by channel proteins is faster than by carriers.

Active transport allows compounds and ions to be transported against the concentration gradient. This type of transport requires carrier proteins and energy, which can be ATP or ion gradients. Carrier proteins participate in both facilitated and active transports, while channel proteins are only involved in facilitated transport. Active transport is similar to enzymatic catalysis, and carriers act like enzymes. The speed of transport is highest when all carrier sites on the membrane are bound to a specific compound or ion. Carriers can be characterized by the same kinetic parameters as enzymes, i.e., affinity for the transported compound or ion (K_m) and maximum velocity (v_max). Active transport can occur in three ways: the carrier transports only one compound or ion through the membrane, i.e., unidirectional transport; the carrier exchanges an intracellular compound or ion for an extracellular one, i.e., counter transport; when the carrier transports a compound or ion along with another compound or ion, i.e., conjugate transport. Carrier proteins in active transport, whose energy source is ATP, are called pumps.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė