Cell membrane
Cell Membrane — High-Yield Summary
Based on your lecture PDF.
MED103 Cell Membrane Dr. Atakan Tevlek.pdf
1. Basic structure and functions
The plasma membrane is a dynamic, selectively permeable phospholipid bilayer containing proteins, cholesterol, and carbohydrates.
Its main functions are:
Boundary: separates the cell interior from the external environment.
Transport: controls nutrient entry, waste removal, and ion movement.
Communication: receptors receive extracellular signals.
Adhesion: supports cell–cell and cell–matrix interactions.
Flexibility: allows budding, fusion, and changes in cell shape.
2. Fluid Mosaic Model
Singer and Nicolson, 1972.
Fluid: lipids and many proteins move laterally within the membrane.
Mosaic: different lipids, proteins, and carbohydrates form a functional mixture.
The membrane is flexible, asymmetric, and selectively permeable.
Structure → function: lateral movement and flexibility allow membrane reorganization, vesicle formation, and rapid responses to signals.
3. Phospholipids: the structural backbone
Phospholipids are amphipathic: they contain both hydrophilic and hydrophobic regions.
Component | Property | Position in bilayer |
Polar head | Hydrophilic | Faces extracellular fluid or cytosol |
Fatty acid tails | Hydrophobic | Face inward, toward each other |
The hydrophobic interior creates a barrier to the free passage of ions and many polar molecules.
Structures formed in water:
Structure | Arrangement |
Micelle | Single-layer sphere; hydrophobic regions inward |
Bilayer | Two layers; tails face each other |
Liposome | Closed bilayer vesicle enclosing an aqueous compartment |
4. Membrane lipids and asymmetry
Lipid | Main point |
Phospholipids | Form the bilayer |
Sphingolipids | Stability, recognition, and signaling |
Cholesterol | Buffers membrane fluidity |
The two membrane leaflets have different lipid compositions:
Outer leaflet—exoplasmic | Inner leaflet—cytosolic |
Phosphatidylcholine (PC) | Phosphatidylserine (PS) |
Sphingomyelin | Phosphatidylethanolamine (PE) |
Phosphatidylinositol (PI) |
Major exam point: during apoptosis, PS becomes exposed on the outer leaflet, signaling macrophages to remove the dying cell.
5. Factors affecting membrane fluidity
Fluidity = ease of movement of lipids and proteins within the bilayer.
Factor | Effect on fluidity | Reason |
Shorter fatty acid chains | ↑ | Weaker interactions between tails |
Longer fatty acid chains | ↓ | Stronger interactions between tails |
More unsaturation | ↑ | Kinked tails pack less tightly |
More saturation | ↓ | Straight tails pack tightly |
Higher temperature | ↑ | Greater molecular movement |
Lower temperature | ↓ | Less molecular movement |
Cholesterol has a temperature-dependent effect:
High temperature: restrains lipid movement → limits excessive fluidity.
Low temperature: prevents tight packing → prevents solidification.
Memorize: short + unsaturated + warm → more fluid.
6. Membrane curvature
Curvature allows:
Vesicle budding: endocytosis and Golgi transport.
Membrane fusion: exocytosis and viral entry.
Organelle shaping: endoplasmic reticulum tubules and mitochondrial cristae.
Factor | Effect |
Phosphatidylcholine: cylindrical shape | Favors flat bilayers |
Phosphatidylethanolamine: cone shape | Favors curvature |
BAR-domain proteins and clathrin | Help bend or scaffold membranes |
7. Types of membrane proteins
Type | Attachment | Key roles |
Transmembrane proteins | Span the bilayer; hydrophobic regions contact lipid tails | Channels, transporters, receptors |
Peripheral proteins | Noncovalently attached to membrane surfaces or other proteins | Enzymes, cytoskeletal anchoring |
Lipid-anchored proteins | Covalently attached to a lipid embedded in the membrane | Combine lipid mobility with protein function |
Precision point: transmembrane proteins are a subset of integral membrane proteins; not every integral protein spans the entire bilayer.
Protein content reflects function:
Inner mitochondrial membrane: ~76% protein → many functional protein complexes.
Myelin: ~18% protein → lipid-rich insulation.
8. Functions of membrane proteins
Function | Examples / mechanism |
Transport | Channels, carriers, pumps |
Signal transduction | Receptors such as G-protein-coupled receptors (GPCRs) |
Adhesion | Cadherins, integrins, selectins |
Anchoring / enzymatic activity | Cytoskeletal attachments and membrane-associated enzymes |
Transport distinction:
Channels: passive movement down an electrochemical gradient.
Carriers: can mediate passive or active transport.
Pumps: active transport using an energy source, often ATP.
Exam trap: a carrier does not necessarily use ATP or perform active transport.
9. Clinical connections
Condition | Membrane connection | Main consequence |
Cystic fibrosis | Defective CFTR chloride channel | Abnormal chloride and water transport → thick mucus → lung infections and digestive problems |
Atherosclerosis | Cholesterol accumulation and inflammation in arterial walls | Plaque formation → arterial narrowing and cardiovascular complications |
HIV infection | Binding to CD4 + CCR5 or CXCR4 | Viral envelope fuses with the host membrane, permitting entry |
For atherosclerosis, prioritize cholesterol accumulation → inflammation → plaque formation; reduced membrane flexibility alone does not explain the disease.
Must-memorize checklist
Fluid Mosaic Model → Singer and Nicolson, 1972.
Amphipathic phospholipids → hydrophilic heads outward, hydrophobic tails inward.
Outer leaflet → PC + sphingomyelin.
Inner leaflet → PS + PE + PI.
Externalized PS → apoptosis and macrophage recognition.
Shorter chains, more unsaturation, higher temperature → increased fluidity.
Cholesterol → fluidity buffer.
PC favors flat membranes; PE favors curvature.
Peripheral = noncovalent attachment; lipid-anchored = covalent lipid attachment.
CFTR → chloride channel; HIV → CD4 + CCR5/CXCR4.