Rasvakalvo

Rasvakalvo, commonly referred to as the lipid bilayer, is the fundamental structure of biological membranes in cells and organelles. It is formed by two layers of amphipathic lipid molecules, mainly phospholipids, with hydrophilic heads facing the aqueous environment and hydrophobic tails facing inward. This arrangement creates a semi-permeable barrier that separates the interior of the cell or organelle from its surroundings and defines membrane boundaries.

The bilayer includes other lipids such as cholesterol in animal membranes, which modulates fluidity and stability, and a variety of proteins that can be embedded in the membrane (integral proteins) or attached to its surface (peripheral proteins). Carbohydrate groups may be present on lipids (glycolipids) or proteins (glycoproteins) on the outer surface, contributing to cell recognition and signaling.

Membranes are described by the fluid mosaic model: lipids diffuse laterally within the bilayer, and many proteins move within this lipid matrix. The inner and outer leaflets are typically asymmetric in composition. The physical state of the membrane depends on temperature, lipid saturation, and cholesterol content, allowing transitions between more ordered and more fluid states as needed.

Functions are diverse: they act as selective barriers controlling the movement of ions and molecules, host transport proteins such as channels and pumps, and house receptors and enzymes for signal transduction. They organize membrane microdomains and enable processes like endocytosis and exocytosis. In mitochondria and chloroplasts, specialized membranes with unique lipid compositions support energy conversion and other essential functions.

Rasvakalvo is thus central to cellular organization, communication, and metabolism.