Plasma Membrane Structure and Transport

Defining the Outer Border: Establishes the boundary for all cells and organelles.
Managing Entry and Exit: Controls the movement of substances into and out of the cell.
Receiving External Signals: Detects signals from the environment and initiates appropriate cellular responses.
Adhering to Neighboring Cells: Facilitates cell-to-cell attachment.

Description: A model proposed by S.J. Singer and G.L. Nicolson in 1972. It describes the plasma membrane as a flexible, fluid structure composed of a mosaic of various components.
Components: Phospholipids, cholesterol, proteins, and carbohydrates, all contributing to the membrane's fluid character.

Main Fabric: Phospholipids are amphiphilic lipid molecules that form the primary structure of the cell membrane.
Structure: Each phospholipid molecule consists of:
- 2 fatty acid chains: These are nonpolar and considered the hydrophobic tails.
- A glycerol molecule.
- A phosphate group: This is polar and forms the hydrophilic head.
Fatty Acid Types:
- Saturated: Carbon atoms are fully saturated with hydrogen atoms, meaning all carbon-carbon (C-C) bonds are single bonds.
- Unsaturated: Contains at least one double carbon-carbon (C=C) bond, which creates kinks in the fatty acid chain.
Phospholipid Bilayer: Due to their amphiphilic nature, phospholipids spontaneously arrange into a bilayer in aqueous environments:
- Polar heads: Face outward, interacting with the aqueous intracellular and extracellular fluids.
- Hydrophobic tails: Face inward, forming a nonpolar core, shielded from water.

Second Major Component: Proteins are crucial elements of cell membranes.
Functions: They serve various roles, including:
- Transporters of molecules.
- Receptors for signals.
- Enzymes catalyzing reactions.
- Involved in binding and adhesion to other cells or structures.
Types of Membrane Proteins:
- Integral proteins: Fully integrated into the lipid bilayer, often spanning the entire membrane.
  - Possess one or more regions that are hydrophobic (composed of hydrophobic amino acids) and other regions that are hydrophilic.
  - Their arrangement within the bilayer is determined by the specific locations and number of these hydrophobic/hydrophilic regions.
- Peripheral proteins: Occur only on the surfaces of the membrane, not integrated into the hydrophobic core.

Integral membrane proteins utilize amino acids with nonpolar, aliphatic R groups (e.g., Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline) and nonpolar, aromatic R groups (e.g., Phenylalanine, Tyrosine, Tryptophan) for anchors within the hydrophobic lipid bilayer.
Polar, uncharged R groups (e.g., Serine, Threonine, Cysteine, Asparagine, Glutamine) and charged R groups (Negatively charged: Aspartate, Glutamate; Positively charged: Lysine, Arginine, Histidine) would typically face the aqueous environments or form channels within the protein.

Third Major Component: Oligosaccharide carbohydrates are found on the exterior surface of the plasma membrane.
Attachment: They are covalently bound to:
- Proteins: Forming glycoproteins.
- Lipids: Forming glycolipids.
Function: Primarily involved in cell-cell recognition and attachment processes.

HIV and T Cells: The immune system's T cells have CD4 receptor glycoproteins. These receptors mistakenly recognize HIV as a