Cell membrane and transport mechanisms

In short: The cell membrane surrounds every cell and determines which substances enter or leave. It consists of a lipid bilayer embedded with numerous proteins that enable targeted transport.

In more detail: The so-called fluid mosaic model describes the membrane as a mobile structure in which lipids and proteins can move freely within the layer.

In detail

• Lipid bilayer: Phospholipids arrange themselves so their water-loving heads point outward and inward, while their water-repelling tails meet in the middle.
• Passive diffusion: Small, uncharged molecules like oxygen or carbon dioxide can move directly through the membrane without expending energy (osmose-und-diffusion).
• Channel proteins: Special proteins form pores through which certain ions or molecules can selectively pass.
• Active transport: Against a concentration gradient, substances are pumped into the cell by transport proteins while consuming energy (usually ATP).
• Endocytosis and exocytosis: Larger particles or entire cell parts are taken in or released by the membrane pinching inward or outward.
• Sodium-potassium pump: This well-known transport protein maintains the unequal ion distribution between the cell interior and surroundings, important for nerve impulses.
• Membrane fluidity: The membrane’s mobility depends strongly on temperature and the proportion of cholesterol in the lipid layer (zelle-aufbau-und-funktion).