Membranebounded

Membranebounded describes cellular structures that are enclosed by a lipid bilayer, forming distinct internal compartments. In biology, the term most often refers to membrane-bound organelles within eukaryotic cells, which are separated from the surrounding cytosol by one or more membranes.

Key membranebounded organelles include the nucleus, which is enclosed by a double membrane called the nuclear envelope; mitochondria, with inner and outer membranes and a highly folded inner membrane; chloroplasts in plants and algae; the endoplasmic reticulum (rough and smooth); the Golgi apparatus; lysosomes and peroxisomes; and vacuoles, particularly large central vacuoles in plant cells. Vesicles and transport intermediates are also membranebounded, enabling trafficking of proteins and lipids between compartments. The membranes create distinct environments, regulate ion gradients, and control the passage of molecules through embedded transport proteins and channels.

The presence of membranes around these organelles reflects evolutionary history, including the endosymbiotic origins of mitochondria and chloroplasts, and the development of the endomembrane system through vesicle trafficking. Protein targeting mechanisms direct newly synthesized proteins to their correct membranes, via signal peptides, sorting receptors, and transmembrane domains.

Non-membranebound cell components such as ribosomes, cytoskeletal elements, and many enzymes operate in the cytosol or on surfaces that lack enclosing membranes. In virology, some viral particles are enveloped by host-derived membranes, though such envelopes are not equivalent to cellular organelles.