Revise Fluid mosaic membranes for Cambridge 9700 (4.1)

The fluid mosaic model, proposed by Singer and Nicolson in 1972, is the currently accepted model for the structure of the cell membrane. It describes the membrane as a dynamic and flexible structure. The 'fluid' aspect refers to the constant lateral movement of phospholipid and protein components, preventing the membrane from being static. The 'mosaic' aspect describes the patchwork of various proteins embedded within or attached to the phospholipid bilayer, similar to tiles in a mosaic.

The cell surface membrane is primarily composed of a phospholipid bilayer, with proteins, cholesterol (in animal cells), and carbohydrates integrated within it. Each component has a specific structure and plays a vital role in the membrane's overall function.

The foundation of the membrane is the phospholipid bilayer. Each phospholipid molecule is amphipathic, meaning it has a hydrophilic (polar) phosphate head and two hydrophobic (non-polar) fatty acid tails. In an aqueous environment, they spontaneously arrange into a bilayer with the hydrophilic heads facing the watery cytoplasm and extracellular fluid, and the hydrophobic tails pointing inwards, creating a non-polar core. This structure forms a barrier to water-soluble substances and is responsible for the membrane's basic integrity and self-sealing properties.

Found only in animal cell membranes, cholesterol molecules are lipids that fit between the phospholipids. They play a crucial role in regulating membrane fluidity. At high temperatures (e.g., body temperature), cholesterol restricts the movement of phospholipids, reducing fluidity and stabilizing the membrane. At low temperatures, it prevents the phospholipids from packing too closely, thereby increasing fluidity and stopping the membrane from becoming too rigid.

Proteins are the functional workhorses of the membrane. They are classified based on their position:

• Intrinsic (or Integral) Proteins: These are embedded within the bilayer. Transmembrane proteins are a type of intrinsic protein that spans the entire membrane. They often function as channels (e.g., for ions) or carriers (e.g., for glucose) to transport substances across the membrane.
• Extrinsic (or Peripheral) Proteins: These are loosely attached to the outer or inner surface of the membrane, often bound to intrinsic proteins. They can act as enzymes, provide structural support by linking to the cytoskeleton, or be involved in cell signalling.

On the exterior surface of the cell membrane, carbohydrate chains are attached to proteins (forming glycoproteins) or lipids (forming glycolipids). This carbohydrate-rich layer is called the glycocalyx. The glycocalyx is highly specific to each cell type and is crucial for:

• Cell-cell recognition: Allowing cells to identify each other (e.g., immune system distinguishing self from non-self).
• Cell adhesion: Helping cells stick together to form tissues.
• Receptors: Acting as binding sites for hormones and neurotransmitters.

The fluidity of the cell membrane is crucial for its function and is influenced by several factors.

• Temperature: Higher temperatures increase the kinetic energy of phospholipids, making the membrane more fluid. Conversely, lower temperatures decrease fluidity, making it more viscous and rigid.
• Cholesterol: In animal cells, cholesterol acts as a fluidity buffer. At high temperatures, it restrains phospholipid movement, reducing fluidity. At low temperatures, it prevents phospholipids from packing too tightly, maintaining fluidity and preventing solidification.
• Fatty Acid Tails: The nature of the phospholipid tails is critical. Longer tails have stronger intermolecular forces (van der Waals forces), which restricts movement and reduces fluidity. The presence of unsaturated fatty acids, which have C=C double bonds causing 'kinks' in their tails, prevents close packing and therefore increases membrane fluidity.