A&P Lecture 1.1: Cell Membrane

This lecture note on Anatomy and Physiology is linked to A&P Lecture 1: The Cell to further explain the physiology of the Cell Membrane. 

ANATOMY AND PHYSIOLOGY LECTURE NOTE 1.1

CELL MEMBRANE

The outermost component of the cell is the plasma membrane. It is a boundary that separates the substances inside the cell, which are intracellular , from substances outside the cell, which are extracellular.It determines what moves into and out of cells. As a result, the intracellular contents of cells is different from the extracellular environment. The membrane encloses and supports the cell contents. It attaches cells to the extracellular environment or to other cells. The cells’ ability to recognize and communicate with each other takes place through the plasma membrane.

The regulation of ion movement by cells results in a charge difference across the plasma membrane called the membrane potential. The outside of the plasma membrane is positively charged, compared with the inside, because there are more positively charged ions immediately on the outside of the plasma membrane and more negatively charged ions and proteins inside. This concept is paramount in understanding the Sodium Potassium Pump that controls the electrophysiology of the heart, transmission of nerve impulses and contraction of muscles.

The plasma membrane consists of 45%–50% lipids, 45%– 50% proteins, and 4%–8% carbohydrates ( figure 3.2 ). The carbohydrates combine with lipids to form glycolipids and with proteins to form glycoproteins. The glycocalyx (glı¯-ko¯-ka¯ liks) is the collection of glycolipids, glycoproteins, and carbohydrates on the outer surface of the plasma membrane. The glycocalyx also contains molecules absorbed from the extracellular environment, so there is often no precise boundary where the plasma membrane ends and the extracellular environment begins.

Indepth Medical Physiology

The membrane that surrounds cells is a notable structure. It is made up of lipids and proteins and is semipermeable meaning- it allows some substances to pass through it and excluding others. However, its permeability can also be varied because it contains numerous regulated ion channels and other transport proteins that can change the amounts of substances moving across it. It is generally referred to as the plasma membrane. The nucleus and other organelles in the cell are bound by similar membranous structures.

Although the chemical structures of membranes and their properties vary considerably from one location to another, they have certain common features. They are generally about 7.5 nm thick. The major lipids are phospholipids such as phosphatidylcholine, phosphotidylserine, and phosphatidylethanolamine. The shape of the phospholipid molecule reflects its solubility properties: the “head” end of the molecule contains the phosphate portion and is relatively soluble in water  and the “tail” ends are relatively insoluble. The possession of both hydrophilic and hydrophobic properties makes the lipid an amphipathic molecule. In the membrane, the hydrophilic ends of the molecules are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane. In prokaryotes, the membranes are relatively simple, but in eukaryotes, cell membranes contain various glycosphingolipids, sphingomyelin, and cholesterol in addition to phospholipids and phosphatidylcholine.

Many different proteins are embedded in the membrane. They exist as separate globular units and many pass through or are embedded in one leaflet of the membrane- integral proteins, whereas others- peripheral proteins, are associated with the inside or outside of the membrane. The amount of protein varies significantly with the function of the membrane but makes up on average 50% of the mass of the membrane; that is, there is about one protein molecule per 50 of the much smaller phospholipid molecules.

The proteins in the membrane carry out many functions. Some are cell adhesion molecules that anchor cells to their neighbors or to basal laminas. Some proteins function as pumps, actively transporting ions across the membrane. Other proteins function as carriers, transporting substances down electrochemical gradients by facilitated diffusion. Still others are ion channels, which, when activated, permit the passage of ions into or out of the cell. The role of the pumps, carriers, and ion channels in transport across the cell membrane is discussed below. Proteins in another group function as receptors that bind ligands or messenger molecules, initiating physiologic changes inside the cell. Proteins also function as enzymes, catalyzing reactions at the surfaces of the membrane. 

The uncharged, hydrophobic portions of the proteins are usually located in the interior of the membrane, whereas the charged, hydrophilic portions are located on the surfaces. Peripheral proteins are attached to the surfaces of the membrane in various ways. One common way is attachment to glycosylated forms of phosphatidylinositol. Proteins held by these glycosylphosphatidylinositol (GPI) anchors include enzymes such as alkaline phosphatase, various antigens, a number of CAMs, and three proteins that combat cell lysis by complement. Over 45 GPI-linked cell surface proteins have now been described in humans. Other proteins are lipidated, that is, they have specific lipids attached to them. Proteins may be myristoylated, palmitoylated, or prenylated (ie, attached to geranylgeranyl or farnesyl groups).

The protein structure—and particularly the enzyme content—of biologic membranes varies not only from cell to cell, but also within the same cell. For example, some of the enzymes embedded in cell membranes are different from those in mitochondrial membranes. In epithelial cells, the enzymes in the cell membrane on the mucosal surface differ from those in the cell membrane on the basal and lateral margins of the cells; that is, the cells are polarized. Such polarization makes directional transport across epithelia possible. Th e membranes are dynamic structures, and their constituents are being constantly renewed at different rates. Some proteins are anchored to the cytoskeleton, but others move laterally in the membrane.

Underlying most cells is a thin, “fuzzy” layer plus some fibrils that collectively make up the basement membrane or, more properly, the basal lamina. Th e basal lamina and, more generally, the extracellular matrix are made up of many proteins that hold cells together, regulate their development, and determine their growth. These include collagens, laminins, fibronectin, tenascin, and various proteoglycans.