B2.1.10 Fluid mosaic model of membrane structure Notes

The Fluid Mosaic Model Helps Us Understand Membrane Structure

1. The fluid mosaic model describes the structure of cell membranes as a flexible, dynamic arrangement of molecules.
   1. Fluid: Membrane components can move laterally within the layer.
   2. Mosaic: Proteins and other molecules are distributed throughout the membrane in a varied pattern, like tiles in a mosaic.
2. This model explains how membranes function as selective barriers while remaining flexible and adaptable.

Components of the Membrane

1. Phospholipid Bilayer

1. The foundation of the membrane is a double layer of phospholipids.
2. Each phospholipid has two parts:
   1. Hydrophilic head: Contains a phosphate group. Attracted to water. Faces outward toward aqueous environments (inside and outside the cell).
   2. Hydrophobic tails: Two fatty acid chains. Repelled by water. Face inward, away from water, forming the membrane's core.
3. Arrangement:
   1. Heads face the watery environments on both sides.
   2. Tails face each other in the middle, creating a hydrophobic barrier.
4. Function: The hydrophobic core prevents most polar and charged molecules from crossing the membrane freely, making it selectively permeable.

2. Cholesterol

1. Cholesterol molecules are interspersed among phospholipids in the bilayer.
2. Structure:
   1. Has both hydrophobic regions (fit between fatty acid tails) and hydrophilic regions (near phospholipid heads).
3. Function:
   1. Regulates membrane fluidity:
   2. At high temperatures, cholesterol restricts phospholipid movement, preventing the membrane from becoming too fluid.
   3. At low temperatures, cholesterol prevents phospholipids from packing too tightly, maintaining fluidity.

3. Proteins

1. Proteins are embedded in or attached to the membrane.
2. They perform various functions like transport, signaling, and structural support.

Integral Proteins

1. Embedded within the phospholipid bilayer.
2. Many are transmembrane proteins, they span the entire bilayer, with regions exposed on both sides.
3. Have hydrophobic regions that interact with the hydrophobic tails of phospholipids.
4. Have hydrophilic regions that extend into the aqueous environments inside and outside the cell.

Peripheral Proteins

1. Attached to the surface of the membrane (inner or outer side).
2. Not embedded in the bilayer, they associate with integral proteins or phospholipid heads.
3. Generally hydrophilic.
   1. Functions: Structural support, signaling, anchoring proteins, they span the entire bilayer, with regions exposed on both sides.
   2. Have hydrophobic regions that interact with the hydrophobic tails of phospholipids.
   3. Have hydrophilic regions that extend into the aqueous environments inside and outside the cell.

4. Glycoproteins and Glycolipids

1. Glycoproteins: Proteins with attached carbohydrate chains.
2. Glycolipids: Lipids with attached carbohydrate chains.
   1. Located on the outer surface of the membrane (extracellular side).
   2. Carbohydrate chains extend into the extracellular space.
3. Functions: Cell recognition, cell signaling, immune response.

Fluidity of the Membrane

1. Phospholipids can move laterally (side-to-side) within their layer.
2. Proteins can also shift position within the membrane.
3. This fluidity allows the membrane to be flexible, self-repair, form vesicles, and enable proteins to cluster for specific functions.
4. Factors affecting fluidity:
   1. Temperature (higher = more fluid)
   2. Cholesterol content (regulates fluidity)
   3. Fatty acid saturation (unsaturated = more fluid)

Drawing the Fluid Mosaic Model

1. Phospholipid bilayer:
   1. Draw two layers of phospholipids.
   2. Show circular hydrophilic heads facing outward (top and bottom).
   3. Show wavy hydrophobic tails facing inward (middle).
2. Cholesterol:
   1. Draw small molecules interspersed between phospholipids in the bilayer.
   2. Show them fitting within the hydrophobic region.
3. Integral proteins:
   1. Draw proteins that span across both layers (transmembrane).
   2. Show hydrophobic regions in contact with the hydrophobic core.
   3. Show hydrophilic regions extending beyond the heads on both sides.
4. Peripheral proteins:
   1. Draw proteins attached to the surface (inner or outer).
   2. Show them in contact with phospholipid heads or integral proteins, not embedded.