📖 Notes
LESSON 04 OF 16
Cell Membrane — Fluid Mosaic Model & Transport
The Fluid Mosaic Model
Singer & Nicolson (1972). The membrane is a phospholipid bilayer with embedded proteins. Controls what enters and exits the cell.
Structure
Phospholipid Bilayer & Proteins
- Hydrophilic heads face outward, toward water
- Hydrophobic tails face inward, away from water
- Integral proteins are embedded in the bilayer; peripheral proteins sit on the surface
- Glycoproteins handle cell recognition; channel proteins allow passive transport; carrier proteins drive active transport
Five Transport Mechanisms
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Diffusion
High → low concentration. Passive, no ATP required.
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Osmosis
Water movement across a semipermeable membrane.
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Active Transport
Low → high concentration. Requires ATP.
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Endo/Exocytosis
Endocytosis brings material in (phagocytosis); exocytosis pushes it out.
Osmosis in Cells
Transport Comparison
REFERENCEPassive diffusion: no protein, no ATP Facilitated diff: protein channel, no ATP Active transport: carrier protein, needs ATP Osmosis: water only, no ATP
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Exam tip
MDCAT often asks you to identify whether a scenario is diffusion, facilitated diffusion, or active transport — the presence of a protein AND the direction relative to the gradient are what distinguish them.