Overview
This lecture introduces the plasma (cell) membrane, emphasizing its phospholipid bilayer structure, component molecules, and the role of membrane proteins, carbohydrates, and cholesterol in membrane function.
Structure of the Plasma Membrane
- The plasma membrane is mainly composed of phospholipids arranged in a bilayer.
- Each phospholipid contains a phosphate group (polar head), glycerol molecule, and two fatty acid tails (nonpolar).
- Polar heads are hydrophilic (water-loving) and face the aqueous environment inside and outside the cell.
- Nonpolar tails are hydrophobic (water-fearing) and orient away from water, forming the membrane's interior.
Phospholipid Details
- The phosphate group (polar head) carries positive and negative charges, making it water-attractive.
- The tail region consists of one saturated fatty acid (no double bonds) and one unsaturated fatty acid (has a double bond).
- Glycerol connects the head and tail regions of the phospholipid.
Fluid Mosaic Model & Components
- The fluid mosaic model describes the membrane as flexible, with phospholipids and proteins able to move laterally.
- Integral proteins are embedded within the membrane; peripheral (surface) proteins are attached to the exterior or surface.
- Carbohydrates can attach to proteins (glycoproteins) or to lipids (glycolipids) on the membrane surface.
Role of Cholesterol
- Cholesterol is embedded in the membrane with a hydroxyl group and four fused rings.
- Cholesterol maintains membrane fluidity: increases fluidity at low temperatures (prevents rigidity) and decreases fluidity at high temperatures (prevents excess movement).
Key Terms & Definitions
- Phospholipid — a molecule with a phosphate head, glycerol backbone, and two fatty acid tails, forming the cell membrane bilayer.
- Hydrophilic — attracted to water.
- Hydrophobic — repelled by water.
- Saturated fatty acid — a fatty acid with no double bonds.
- Unsaturated fatty acid — a fatty acid with at least one double bond.
- Integral protein — a protein embedded in the membrane.
- Peripheral protein — a protein attached to the outer or inner surface of the membrane.
- Glycoprotein — a membrane protein with attached carbohydrates.
- Glycolipid — a membrane lipid with attached carbohydrate.
- Cholesterol — a lipid molecule that regulates membrane fluidity.
- Fluid mosaic model — describes the flexible, moving structure of the cell membrane.
Action Items / Next Steps
- Review the structure and function of membrane components.
- Study diagrams of the phospholipid bilayer and label its parts.