Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intra- and Intercellular Communication
Membranes: Structure, Assembly, and Function
Introduction

Membranes are highly viscous yet pliable structures that surround all living Cells. Cell/30.html">The Plasma Membrane forms a closed compartment enclosing the Cytoplasm, which isolates individual cells from one another and defines their individuality. The plasma membrane is selectively permeable, acting as a barrier that maintains the distinct COMPOSITION OF THE extracellular and intracellular environments. This selective permeability is driven by channels and pumps that transport various ions and substrates, as well as by specific receptors, such as Hormone Receptors. In addition, Plasma Membranes facilitate the exchange of substances between The Cell interior and the external environment via exocytosis and endocytosis; specialized membrane structures known as Gap Junctions also allow adjacent cells to exchange metabolites.

Membranes also form specialized compartments within the cell. These intracellular membranes give rise to numerous morphologically distinct structures (Organelles) — Mitochondria, Endoplasmic reticulum, sarcoplasmic reticulum, Golgi apparatus, secretory granules, Lysosomes, and nuclear membranes. Furthermore, membranes harbor Enzymes that serve as integral components of excitation and response pathways, as well as enzymes involved in energy Transduction processes such as Photosynthesis and Oxidative Phosphorylation.