Membrane Components
High-Yield Summary
- Membrane lipids: fatty acids/triacylglycerols (storage), glycerophospholipids (glycerol backbone), sphingolipids (sphingosine backbone: ceramide → sphingomyelin/cerebroside/ganglioside), steroids/cholesterol, waxes.
- Cis double bonds bend the fatty acid chain (increase fluidity); trans double bonds keep it straight (pack tightly like saturated, reduce fluidity).
- Cholesterol is a two-way fluidity buffer: limits fluidity at high temp, prevents rigidity at low temp. Also precursor to bile acids and vitamin D.
- Membrane proteins: transmembrane (span entire bilayer — transporters/channels/receptors), embedded (one side only — signaling/anchoring), peripheral (don't insert, attach loosely — signaling via lipid rafts, cytoskeleton linkage). Transmembrane + embedded = integral proteins.
- Membrane receptors: enzyme-linked (e.g. receptor tyrosine kinases — catalytic intracellular domain), ligand-gated ion channels (open on ligand binding), GPCRs (initiate signaling cascades).
- Membrane carbohydrates (glycoproteins, glycolipids) sit on the extracellular surface, drive cell recognition (immune self/non-self, blood type antigens), form a protective hydrated layer.
- Cell-cell junctions: tight junctions (seal, zipper-like), desmosomes (cell-to-cell anchor via cadherins + intermediate filaments), hemidesmosomes (cell-to-matrix anchor via integrins), gap junctions (connexon channels for direct cytoplasmic communication). Organized by cell adhesion molecules (CAMs).
Key Terms
- Integral protein
- Transmembrane or embedded protein interacting closely with the hydrophobic membrane interior; needs detergent to remove.
- Peripheral protein
- Protein attached loosely to the membrane surface, not inserted into the bilayer.
- Glycoprotein / Glycolipid
- Protein or lipid with attached carbohydrate chains, usually on the extracellular surface; drives cell recognition.
- Desmosome
- Cadherin-based junction linking neighboring cells to intermediate filaments — mechanical strength.
- Hemidesmosome
- Integrin-based junction anchoring a cell to the basement membrane.
- Connexon
- Channel of six connexin subunits; two connexons form one gap junction pore.
Transmembrane vs. Embedded vs. Peripheral Proteins
| Type | Spans Bilayer? / Example Function |
|---|---|
| Transmembrane | Yes, entire bilayer / Transporters, channels, receptors |
| Embedded | No, only one side / Signaling, anchoring |
| Peripheral | Doesn't insert / Signaling (lipid rafts), cytoskeleton linkage |
Cell-Cell Junctions
| Junction | Structure / Tissue Example |
|---|---|
| Tight junction | Zipper-like seal near apical surface / Epithelial sheets |
| Desmosome | Cadherins + intermediate filament plaques / Skin, cardiac muscle |
| Hemidesmosome | Integrins binding laminin/matrix / Epithelial basal surface |
| Gap junction | Two connexons (6 connexins each) / Cardiac and smooth muscle |
Common MCAT Trap
- Desmosomes anchor cell-to-cell; hemidesmosomes anchor cell-to-matrix (basement membrane) — easy to mix up given the similar names and shared use of intermediate filaments.
- Cholesterol's fluidity buffering is bidirectional and temperature-dependent — limits fluidity when hot, prevents rigidity when cold. Don't state it as a one-directional effect.
- Cis vs. trans unsaturation have opposite fluidity effects: cis kinks the chain (more fluid, natural form); trans keeps it straight (packs like saturated, less fluid).
Quick Recall
What two membrane protein categories together make up "integral proteins"?
What structural feature of gap junctions allows direct cytoplasmic communication between cells?
What is the functional difference between cis and trans double bonds in a fatty acid chain?
What role do cell adhesion molecules (CAMs) play beyond forming junctions?