Membranavgrensede

Membranavgrensede, a Danish term meaning "membrane-bound," describes cellular structures that are enclosed by a lipid bilayer and thus form distinct compartments within or around the cell. These membranes create a barrier between the interior of the compartment and the cytosol and regulate transport, chemical conditions, and signaling across the boundary.

Most eukaryotic cells contain numerous membranavgrensede organelles. Examples include the nucleus, mitochondria, chloroplasts (in plants and algae), the endoplasmic reticulum, the Golgi apparatus, lysosomes, peroxisomes, endosomes, and vesicles. Some organelles have a double limiting membrane (e.g., nucleus, mitochondria, chloroplasts); others have a single membrane. Membranes bud to form vesicles and can fuse with other membranes to exchange material.

Membranes are primarily composed of a phospholipid bilayer with embedded proteins, and they often contain cholesterol or other lipids that modulate fluidity. They support essential functions such as selective transport through channels and pumps, maintenance of ion and pH gradients, metabolic reactions within defined spaces, and processing and trafficking of proteins and lipids (as in the endoplasmic reticulum and Golgi). They also participate in signaling and environmental sensing.

In contrast to membranavgrensede structures, membranefrie structures lack a surrounding lipid envelope. In many prokaryotes, internal membrane-bound compartments are limited, and the rich, compartmentalized organization typical of eukaryotic cells is largely absent. Some bacteria do possess internal membranes, but these are not as extensive or uniform as in eukaryotes.