Step by Step
PL
Phospholipid — the building block
Each phospholipid has a hydrophilic (water-loving) head — a phosphate group attached to glycerol — and two hydrophobic (water-fearing) fatty acid tails. This amphipathic nature drives self-assembly: heads face the aqueous environment, tails hide from water in the interior.
Memory trick: Phospholipid = lollipop with two sticks. Head = candy (loves water). Tails = sticks (hate water).
Bi
The bilayer — heads out, tails in
Two layers of phospholipids arrange tail-to-tail — hydrophilic heads face outward (toward extracellular fluid and cytoplasm), hydrophobic tails face inward (away from water). This creates a stable barrier between the cell and its environment. The bilayer is ~7-10 nm thick.
FM
Fluid Mosaic Model (Singer & Nicolson, 1972)
The membrane is a fluid structure — phospholipids and proteins move laterally (sideways) within the bilayer. It is a mosaic because it contains a variety of proteins embedded in or attached to the lipid bilayer. Integral proteins span the membrane; peripheral proteins attach to the surface.
Memory trick: Fluid mosaic = icebergs (proteins) floating in an ocean (lipid bilayer). The icebergs drift; the ocean flows.
SP
Selective permeability
The membrane is selectively permeable — small nonpolar molecules (O₂, CO₂, lipids) cross freely. Small uncharged polar molecules (H₂O, ethanol) cross slowly. Large polar molecules and ions cannot cross without protein channels or carriers. This is the basis of all membrane transport.
Applied Walkthrough
1
Phospholipids in aqueous solution spontaneously form bilayers — the hydrophobic effect drives the tails together to minimize water contact.
2
Membrane proteins float laterally within this bilayer — shown by cell fusion experiments (FRAP: Fluorescence Recovery After Photobleaching).
3
Oxygen diffuses freely across the membrane (small, nonpolar). Glucose cannot — it requires a transporter protein (GLUT1-4).
4
Cholesterol inserts between phospholipids — preventing crystallization at low temperatures and reducing fluidity at high temperatures (covered next lesson).
Exam Application
Exams test the fluid mosaic model (Singer & Nicolson), the amphipathic nature of phospholipids, which molecules cross freely vs which require transport, and integral vs peripheral proteins. Know that flip-flop (moving from one leaflet to the other) is rare and requires enzymes (flippases), but lateral movement is fast and constant.
⚠ Common Trap
Students think the membrane is a rigid structure — it is fluid. Phospholipids move laterally very rapidly. Also: water crosses the membrane slowly by simple diffusion but much faster through aquaporin channels — don't say water "cannot" cross; it can, just slowly. The key distinction is between simple diffusion and facilitated transport.
✓ Quick Self-Check
1. What are the two components of a phospholipid and how do they behave in water?
Hydrophilic head (phosphate + glycerol — water-loving, faces outward) and hydrophobic tails (fatty acids — water-fearing, face inward).
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2. What does "fluid mosaic model" mean?
Fluid = phospholipids and proteins move laterally. Mosaic = the membrane contains a variety of different proteins embedded in the lipid bilayer.
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3. What types of molecules cross the membrane freely?
Small nonpolar molecules (O₂, CO₂, lipids) cross freely. Small uncharged polar molecules (H₂O) cross slowly. Large polar molecules and ions cannot cross without transport proteins.
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4. What is the difference between integral and peripheral membrane proteins?
Integral proteins are embedded within the lipid bilayer (often spanning it). Peripheral proteins are attached to the membrane surface without penetrating the hydrophobic core.
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5. Why do phospholipids spontaneously form bilayers in water?
The hydrophobic effect — the hydrophobic tails are driven together to minimize contact with water, while the hydrophilic heads face the aqueous environment.
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