Cell membranes are lipid bilayers.

Functions of Cell membrane

• Compartmentalization: Cell membranes define the internal compartments of eukaryotic cells, including the nucleus and cytoplasmic organelles
• Cell membranes provide a selectively permeable barrier.
• The plasma membrane plays a role in transporting solutes.
• Responding to external signals: The plasma membrane plays a critical role in the response of a cell to external stimuli, a process known as signal transduction.
• Intracellular interaction: The plasma membrane mediates the interactions between the cells of a multicellular organism.

Cell membranes are composed of phospholipid bilayer and proteins.

There are three main types of membrane lipids: phosphoglycerides, shingolipids, and cholesterol. All lipids are amphipathic, that is, they have hydrophilic and hydrophobic regions.

• Phosphoglycerides. Most membrane lipids contain a phosphate group, which makes them phopholipids. Because most membrane phospholipids are built on a glycerol backbone, they are called phosphoglycerides. Some of the phosphoglycerides present in the membrane are phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol.
• Sphingolipids are less abundant that the phopholipids. Sphingolipids are derivative of sphingosine, an amino alcohol that contains a long hydrocarbon chain. Ceramide, sphingomyelin, glycolipid, cerebroside and gangliosides are sphingosine based lipids.
• Cholesterol although absent in plasma membranes from most plant cells and bacteria, constitutes up to 50% of the lipid molecules in the plasma membrane of certain animal cells.

Three classes of membrane proteins exist in the cell membranes:

• Integral proteins are proteins that contain one or more transmembrane helices.
• Peripheral proteins are membrane proteins of the cytoplasmic surface of the membrane that are noncovalently bonded to the polar head group of the lipid bilayer or to an integral membrane protein.
• Lipid anchored proteins are membrane proteins that are covalently bonded either to a phospholipid or a fatty acid that is embedded in one of the leaflets of the lipid bilayer.
• Membrane proteins may act as receptors that allow the cell to respond to external signals; may be involved in selective transport of molecules across the membrane; may participate in electron transport and oxidative phosphorylation; may control interactions between cells of multicellular organisms

Most membranes including the plasma membrane in eukaryortes are synthesized in the endoplasmic reticulum.

Lipids and Proteins may be glycosilated. Carbohydrate content of plasma membrane is between 2-10% by weight. A small percentage of these carbohydrates are linked to lipids to form GLYCOLIPIDS. However, the great majority of carbohydrates are covalently linked to protein to form GLYCOPROTEINS.

All carbohydrates of the plasma membrane face outward into the extracellular space.

The lipid bilayer is asymmetrical:

• Lipids with choline head groups: phosphatidylcholine and sphingomyelin are neutral lipids which are present predominantely on the outer half of the lipid bilayer.
• Phospholipids with terminal primary amino groups: phophatidylethanolamine (a neutral lipid) and phosphatidylserine (a negatively charged lipid) are present predominantely in the inner half of the membrane.
• Cholesterol is equally distributed in both lipid layers.

Thus, there's a difference in charge between the two halves of the membrane.

Lipid fluidity

An important property of lipid bilayers is that they behave as two-dimensional fluids in which molecules can rotate and move in lateral directions. With the aid of flippases, the lipids can also "flip-flop." The movement of the lipids and proteins within the plasma membrane bilayer has been coined the name of "THE FLUID MOSAIC MODEL."

Such fluidity is a critical property and is determined by temperature and lipid composition.

• Fluidity can be determined by chain length of the fatty acid of the phospholipid. The shorter the fatty acyl chains of a phospholipid, the lower its melting temperature, the more fluid at 37oC, compared to a saturated lipid at 37oC.
• Fluidity can be determined by the number of unsaturated bonds within the fatty acid of the phospholipid. Unsaturated bonds prevent phospholipids from packing together. Therefore, the greater the degree of unsaturation of phospholipid fatty acid chains, the lower the melting point.
• Cholesterol also affects fluidity. Cholesterol molecules, which align themselves with the fatty acyl chains of the phospholipids in a bilayer, affect membrane fluidity by preventing the tight packing of the long hydrocarbon tails. As a result, cholesterol tends to abolish sharp transition temperatures and may also increase the stability and decrease the permeability of the membrane.

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