Membránátjutás

Membránátjutás refers to the process by which substances cross biological membranes. These membranes, primarily composed of lipid bilayers, act as selective barriers, controlling the passage of molecules into and out of cells and organelles. There are several mechanisms by which membránátjutás can occur, broadly categorized as passive and active transport.

Passive transport does not require the cell to expend energy. It includes simple diffusion, where small, lipid-soluble molecules move down their concentration gradient directly across the membrane. Facilitated diffusion also moves substances down their gradient but requires the assistance of membrane proteins, such as channel proteins or carrier proteins, to help transport molecules that cannot easily cross the lipid bilayer. Osmosis, a specific type of diffusion, refers to the movement of water across a semipermeable membrane.

Active transport, on the other hand, requires the cell to use metabolic energy, typically in the form of ATP, to move substances against their concentration gradient. This process also relies on specific membrane proteins, often called pumps, that bind to the transported molecule and undergo conformational changes to move it across the membrane. Endocytosis and exocytosis are bulk transport mechanisms where the membrane itself engulfs or expels larger particles or even whole cells, forming vesicles.