Components and Structure

The Fluid Mosaic Model

Components of the Cell Membrane

• The phosphate head groups of the phospholipids are hydrophilic and face the aqueous environment inside and outside the cell.
• The lipid tails are hydrophobic and face one another, away from the aqueous environment.
• Cholesterol is another lipid found in cell membranes.
• Glycoproteins are proteins modified with carbohydrates (prefix "glyco-" refers to sugars). Sometimes, the carbohydrates are attached directly to the lipids: these are glycolipids.
• These carbohydrates are always facing the outside of the cell, forming the glycocalix.

Membrane Proteins

• Proteins can be part of the membrane itself (integral) or only be on the outer edges of the lipid bilayer (peripheral).
• Integral proteins go through the lipid bilayer (transmembrane portion is not charged);
• Peripheral proteins can be located on the cell interior or exterior by associating with integral proteins or phospholipids.
• These proteins have many important functions, which include:
• Catalyzing reactions (enzymes);
• Transporting molecules through the lipid bilayer;
• Receptors for signaling between internal and external environment;
• Anchor internal structures to the cell membrane;
• Attach adjacent cells to one another;
• Serve as a marker for cell identification.

Factors Affecting Membrane Fluidity

• Temperature: at higher temperatures, the membrane has more fluidity than at lower temperatures.
• Tail length: longer fatty acid tails allow for more intermolecular interactions between phospholipids, leading to less fluidity.
• Degree of unsaturation: Unsaturated fatty acids have one or more double bonds in the fatty acid tails. Double bonds lead to a "bend", pushing the adjacent phospholipids further apart. The increased spacing reduces the number of intermolecular interactions and increases fluidity.
• Cholesterol: the presence of cholesterol in the phospholipid bilayer affects fluidity depending on the temperature; it acts as a buffer:
• High temperature: cholesterol decreases fluidity.
• Low temperature: cholesterol increases fluidity.