membranesurrounded

Membrane-surrounded is a descriptive term used in biology to denote structures that are enclosed by one or more lipid bilayer membranes. This enclosure creates a defined internal environment that is chemically distinct from the surrounding cytoplasm or extracellular space. The lipid bilayer, often interspersed with proteins and sterols, forms a semi-permeable barrier that regulates the movement of ions, molecules, and information signals across the boundary, supporting selective transport, signaling, and energy transactions.

In cellular biology, most cellular components are membrane-surrounded. Examples include the plasma membrane that encloses a cell, the nucleus with its double membrane, mitochondria and chloroplasts, endoplasmic reticulum, Golgi apparatus, vacuoles, and transport vesicles. Many viruses acquire a host-derived membrane envelope, forming membrane-surrounded viral particles during part of their life cycle. The presence of membranes distinguishes organelles and compartments from the surrounding cytosol or extracellular space.

Membranes provide structural integrity and enable compartmentalization, allowing distinct chemical environments and metabolic processes to occur in parallel. Double membranes, such as those of the nucleus or mitochondria, offer additional regulatory control and spatial separation of processes like transcription and translation or oxidative phosphorylation. The concept of being membrane-surrounded helps explain how cells organize complex functions and maintain homeostasis despite changing external conditions.

The term is descriptive rather than a formal taxonomic category, and it is often used interchangeably with membrane-bound in general discussions of cell biology. See also cell membrane, lipid bilayer, organelle, and vesicle.