Plasma Membranes are mostly Lipids and Proteins arranged in a Fluid Mosaic Model
A typical cell membrane has a composition of:
Lipids: 40-60% – arranged in a double lipid bilayer.
Protein: 30-50% – proteins which are inserted either partly or completely through bilayer.
Carbohydrate: 5-10% – carbohydrates which attach to extracellular fluid (ECF) side.
These percentages can vary significantly depending on the specific type of cell in the body.
Cytoplasm with organelles
Figure 1. This is the typical diagrammatic representation of a eukaryotic cell. The extracellular fluid (ECF) is kept separate from the intracellular fluid (ICF) by the plasma membrane.
General Function of Plasma Membranes
- Physical Barrier: The plasma membrane (PM) acts as a barrier; it separates the inside of the cell, containing ICF, from the outside of the cell, containing ECF. It creates the boundary of the cell and isolates it from other cells and structures.
- Regulation of Exchange: Anything that goes into or out of a cell must do so by crossing the plasma membrane. Exchange with the environment occurs across this membrane, either by slipping through the membrane or by being transported across by protein channels or protein carriers.
- Structural Support: Structural proteins are tethered to the internal or intracellular aspect of the plasma membrane in order to create the internal structural support for the cell. This internal framework is referred to as the cytoskeleton of the cell. For example, this helps create the shape of cells, like the distinctive biconcave disc shape of the red blood cell.
- Communication and Cell ID: Signals from the external environment of the cell are transferred into the internal compartment across the plasma membrane. This often involves receptors that sit on the external aspect of the plasma membrane to receive the signal. Signal molecules are called ‘ligands’ and they bind to receptors, much like substrates bind to enzymes. There are also molecules (glycoproteins and glycolipids) which attach to the external surface of the plasma membrane to help identify the cell as self. For example, these flags or markers are what make up the blood typing of a red blood cell (A, B, AB or O).
- Phospholipids – usually about 75% of lipid content.
The polar glycerol-phosphate head of a phospholipid is the hydrophilic end and a nonpolar fatty acid tail is the hydrophobic end. The entire molecule is amphiphilic, meaning it can mix with both water and lipid environments. The phospholipids are arranged in two rows, called the lipid bilayer and this functions as a barrier that only lipid-soluble molecules can penetrate. They also provide a framework for membrane proteins. Some lipids are involved in cellular communication. Some common phospholipids found in plasma membranes include phosphatidyl choline and sphingomyelin.
- Cholesterol – usually about 20 – 30% of lipid content.
This 4 ringed lipid structure inserts into the hydrophobic center with the nonpolar fatty acid tails. The more cholesterol in the plasma membrane the more insulative the membrane will be. For example, the myelin sheath membrane (which insulates axons of nerve cells) is about 30% cholesterol, while other mammalian cell membranes may be about 20% cholesterol.
Cholesterol helps to stabilize the plasma membrane. It functions to keep membranes impermeable and yet flexible. Membranes with higher cholesterol concentrations are less permeable to ions, water, and other small molecules. Presumably cholesterol blocks the openings between phospholipid tails through which these small molecules could otherwise pass. Mammals maintain a relatively constant Tb, so the “plasticizing” effect of cholesterol is not as important as it is in poikilothermic animals and plants that cannot maintain a constant body temperature.
- Glycolipids – usually about 5% of the lipid content.
The prefix glyco means ‘glucose’ or ‘sugar’, so a glycolipid is a small amount of a sugar attached to a large amount of lipid. Glycolipids are found on the external surface of the plasma membrane and act as a cell markers. This helps identify the cell as self to defense cells of the body.
Other Phospholipid Arrangements
- Micelles are small droplets with hydrophobic tails forming the interior; the hydrophilic heads form the exposed boundary. Important in digestion and absorption of fats in digestive tract.
- Liposomes are larger hollow spheres with phospholipid bilayer walls. Their hollow core can be loaded with water-soluble molecules. Can be used as a drug delivery system.
Plasma membrane carbohydrates attach to both lipids and proteins. The Glycocalyx is a protective layer on cell surface formed by Glycoproteins – when glucose attached to membrane proteins and Glycolipids – when glucose attached to membrane lipids. The carbohydrates of the glycocalyx play a critical role in identifying cells; for example, the carbohydrates of the glycocalyx in human blood cells differentiate the main ABO blood groups from one another.
- Associated Proteins
Also termed peripheral or extrinsic proteins. They are attached loosely to membrane-spanning proteins or to polar regions of phospholipids. They do not span the plasma membrane!
- Integral Proteins and Membrane-Spanning Proteins
Also termed intrinsic proteins. These are tightly bound into the phospholipid bilayer. Some integral proteins only extend partway into the membrane, others are membrane-spanning. Membrane-spanning proteins have segments that cross the membrane multiple times. Loops extend into extracellular and intracellular regions. Carbohydrates attach to extracellular loops and phosphates attach to intracellular loops. When amino acids are linked to each other, they can form an a-helix that has an exterior layer of nonpolar side groups and a central core composed of the polar amino and carboxyl groups. This ties the protein so firmly to the membrane that it can only be freed by disrupting the phospholipid bilayer with detergents.