Membranes (CH. 5)

Transmembrane proteins.

A major component of every membrane is a collection of proteins that float in the lipid bilayer. These proteins have a variety of functions, including transport and communication across the membrane. Many integral membrane proteins are not fixed in position. They can move about, just as the phospholipid molecules do. Some membranes are crowded with proteins, but in others, the proteins are more sparsely distributed.

The Structure of Membranes

Cellular membranes contain four components: (1) a phospholipid bilayer, (2) transmembrane proteins, (3) an internal protein network providing structural support, and (4) cell-surface markers composed of glycoproteins and glycolipids. The fluid mosaic model of membrane structure includes both the fluid nature of the membrane and the mosaic composition of proteins floating in the phospholipid bilayer. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have provided evidence supporting the fluid mosaic model.

Phospholipid bilayer

Every cell membrane is composed of phospholipids in a bilayer. The other components of the membrane are embedded within the bilayer, which provides a flexible matrix and, at the same time, imposes a barrier to permeability. Animal cell membranes also contain cholesterol, a steroid with a polar hydroxyl group (-OH). Plant cells have other sterols, but little or no cholesterol.

S. Jonathan Singer and Garth J. Nicolson

In 1972, S. Jonathan Singer and Garth J. Nicolson revised the model in a simple but profound way: They proposed that the globular proteins are inserted into the lipid bilayer, with their nonpolar segments in contact with the nonpolar interior of the bilayer and their polar portions protruding out from the membrane surface. In this model, called the fluid mosaic model, a mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond

Integral membrane

Integral membrane proteins are embedded in the membrane, and

The fluid mosaic model of cell membranes.

Integral proteins protrude through the plasma membrane, with nonpolar regions that tether them to the membrane's hydrophobic interior. Carbohydrate chains are often bound to the extracellular portion of these proteins, forming glycoproteins. Peripheral membrane proteins are associated with the surface of the membrane. Membrane phospholipids can be modified by the addition of carbohydrates to form glycolipids. Inside the cell, actin filaments and intermediate filaments interact with membrane proteins. Outside the cell, many animal cells have an elaborate extracellular matrix composed primarily of glycoproteins.

Lipidomics

Lipidomics, the field defining the number and biological function of lipids, is revealing a significant diversity in membrane lipids. Although there are over 1000 distinct lipids identified in cells, we can organize them into only three classes: glycerol phospholipids (see figure 5.1), sphingolipids (see figure 5.2), and sterols such as cholesterol. The classical phospholipid bilayer consists of a combination of glycerol phospholipids and sphingolipids.

Interior protein network.

Membranes are structurally supported by intracellular proteins that reinforce the membrane's shape. For example, a red blood cell has a characteristic biconcave shape because a scaffold made of a protein called spectrin links proteins in the plasma membrane with actin filaments in the cell's cytoskeleton. Membranes use networks of other proteins to control the lateral movements of some key membrane proteins, anchoring them to specific sites.

Sphingolipids

Sphingolipids usually contain saturated hydrocarbon chains. These components are not uniformly distributed in biological membranes, and different cellular compartments have distinct membrane lipid composition, as discussed later in this section.

glycerol phospholipids

The glycerol phospholipids are the most diverse with head groups that can have both positive and negative charge (zwitterionic), or primarily negative charge (anionic).

phospholipid

The lipid layer that forms the foundation of a cell's membranes is a bilayer formed of phospholipids. These phospholipids include primarily the glycerol phospholipids , and the sphingolipids such as sphingomyelin . An early model portrayed the membrane as a sandwich; a phospholipid bilayer between two layers of globular protein. Phospholipids can also vary in the length and composition of the fatty acid tail, with it being either saturated or cis-unsaturated (see section 3.5). The polar head groups are hydrophilic, whereas the nonpolar hydrocarbon tails are hydrophobic. The two nonpolar fatty acids extend in one direction, roughly parallel to each other, and the polar phosphate group points in the other direction. To represent this structure, phospholipids are often diagrammed as a polar head with two dangling nonpolar tails

lipid rafts

Theoretical work also showed that lipids can exist in either a disordered or an ordered phase within a bilayer. This led to the idea of lipid microdomains called lipid rafts that are heavily enriched in cholesterol and sphingolipids. These lipids appear to interact with each other, and with raft-associated proteins— together forming an ordered structure. This is now technically defined as "dynamic nanometer-sized, sterol and sphingolipid-enriched protein assemblies." There is evidence that signaling molecules, such as the B- and T-cell receptors, associate with lipid rafts and that this association affects their function

plasma membrane

We call the delicate skin of lipids with embedded protein molecules that encase the cell a plasma membrane

Cell-surface markers

membrane sections assemble in the endoplasmic reticulum, transfer to the Golgi apparatus, and then are transported to the plasma membrane. The ER adds chains of sugar molecules to membrane proteins and lipids, converting them into glycoproteins and glycolipids. Different cell types exhibit different varieties of these glycoproteins and glycolipids on their surfaces, which act as cell identity markers.

peripheral proteins

peripheral proteins are associated with the surface of the membrane.