Cell-Cell Recognition

• Many membrane proteins contain attached carbohydrates on the outside of the lipid bilayer, allowing it to form hydrogen bonds with water.

Carbohydrates

• Properties of carbohydrates include being short and having branched chains of fewer than 15 sugar units.
• Some carbohydrates are covalently bonded to lipids, forming glycolipids.
• Carbohydrates are involved in cell-cell recognition by bonding to cells.
• Cell-cell recognition with carbohydrates is crucial to organism functioning.
• It's also the basis for the rejection of foreign cells, including transplanted organs, by the immune system.
• The carbohydrate layer on the external side of the plasma membrane varies greatly.
• It differs from species to species, among individuals of the same species, and even from one cell type to another in a single individual, such as human blood type A, AB, B, or O.

Phospholipids

• When the head of a phospholipid is hydrophilic, it can dissolve in water.
• The fatty acid tail is hydrophobic and non-polar, avoiding water because the tail is insoluble.
• Phospholipids form a double layer called a lipid bilayer spontaneously.
• The tails are sandwiched between the heads.
• Heads are exposed to water, while tails interact with each other.
• The phospholipid bilayer is critical to the cell membrane because it acts as a barrier to the passage of molecules and ions in and out.
• The selective passage of substances is also an important function.

Bio Membrane

• The main components of the plasma membrane include:
  • Lipids.
  • Proteins.
  • Carbohydrates.
• This helps the cell transport substances.
• Most lipids in membranes are phospholipids which are amphipathic molecules.
• The head of phospholipids is hydrophilic.
• A pure phospholipid bilayer membrane surface adheres less strongly to water.
• Proteins in the membrane have both hydrophilic and hydrophobic regions.

Sandwich Model

• Membranes were coated on both sides with hydrophilic proteins, with lipids in the middle.
• It's accepted at all the internal membranes of the cell.
• Two problems were recorded within the sandwich model.
  • Membranes with different functions differs in chemical composition and structure.
  • Solubility and chemical properties of membrane proteins, which are often not water-soluble unlike proteins in the cytosol.
  • If such proteins were layered on the surface of the membrane, their hydrophobic parts would be in an aqueous environment.

Fluid mosaic model

• Membrane proteins are dispersed and individually inserted into the phospholipid bilayer.
• Only their hydrophilic regions protrude far enough from the bilayer to be exposed to water.
• Molecular arrangement maximizes contact of hydrophilic regions of proteins and phospholipids with water.
• Also provides a non-aqueous environment for their hydrophobic parts.

Phospholipids

• Phospholipids are lipids that contain a phosphate group and is a major component of cell membranes.
  • Head: Has one phosphate group, and is polar/hydrophilic.
  • Tail: Has two fatty acids and is non-polar/hydrophobic.