Overview
This lecture covers the structure and functions of the cell membrane, emphasizing its main components—membrane lipids, proteins, and the glycocalyx—and explaining their roles in cellular function and transport.
Structure of the Cell Membrane
- The cell membrane acts as a barrier separating intracellular and extracellular fluids.
- It is primarily composed of membrane lipids, membrane proteins, and the glycocalyx.
- Membrane lipids include phospholipids (with hydrophilic heads and hydrophobic fatty acid tails), sphingolipids, and cholesterol.
- The phospholipid bilayer forms a hydrophilic outer and inner surface, with hydrophobic fatty acids in the center.
- Cholesterol is embedded in the membrane and stabilizes its structure.
- Membrane proteins are either integral (transmembrane, spanning the bilayer) or peripheral (attached to the membrane surface).
- The glycocalyx is a carbohydrate-rich layer formed by glycoproteins and glycolipids on the outer membrane surface.
Functions of Cell Membrane Components
Glycocalyx
- Regulates water movement, reducing cell dehydration.
- Plays an antigenic role by helping the immune system distinguish self from foreign cells (e.g., MHC I complexes, blood typing).
Membrane Lipids
- Contribute to membrane fluidity, influenced by temperature, cholesterol content, and type of fatty acids (saturated vs. unsaturated).
- Allow simple diffusion of small, nonpolar, or lipid-soluble molecules (e.g., Oâ‚‚, COâ‚‚, steroid hormones).
- Phospholipids can move laterally (lateral diffusion) or flip between layers (transverse diffusion), aided by enzymes (flippase, floppase).
Membrane Proteins
- Facilitate transport of large, polar, or water-soluble molecules through channels or carriers.
- Participate in exocytosis and endocytosis for large molecule movement.
- Serve as receptors for signal transduction, initiating cellular responses.
- Mediate cell-to-cell adhesion (tight junctions, desmosomes, adherens junctions).
- Possess enzymatic activity, catalyzing chemical reactions at the membrane.
- Enable cell communication via structures like gap junctions.
- Anchor cells to the extracellular matrix (e.g., hemidesmosomes).
Key Terms & Definitions
- Phospholipid bilayer — Double layer of phospholipids with hydrophilic heads and hydrophobic tails forming the membrane.
- Integral protein — Membrane protein that spans the entire bilayer (transmembrane).
- Peripheral protein — Protein attached to the membrane surface with weak bonds.
- Glycocalyx — Outer layer of carbohydrates (glycoproteins & glycolipids) involved in cell recognition and protection.
- Fluidity — Ability of membrane components to move, affecting flexibility and function.
- Simple diffusion — Passive movement of small, nonpolar, or lipid-soluble molecules across the membrane.
- Lateral diffusion — Side-to-side movement of lipids within the same membrane layer.
- Transverse diffusion (flip-flop) — Movement of lipids between the inner and outer layers of the membrane.
Action Items / Next Steps
- Review the structure and roles of the membrane lipids, proteins, and glycocalyx.
- Memorize factors affecting membrane fluidity.
- Understand transport types and protein functions.
- Prepare for further study on cell junctions and membrane transport mechanisms.