Membrane Biology – Study Guide (Alberts
et al., Essential Cell Biology 6th Ed.)
Major Macromolecules
● Carbohydrates, Lipids, Proteins, Nucleic acids.
Membrane Lipids
Phospholipid structure:
● Head group (polar, hydrophilic), phosphate (polar), glycerol backbone, fatty acid chains
(nonpolar, hydrophobic).
● Amphipathic = hydrophilic + hydrophobic regions.
Cholesterol:
● Hydroxyl group (polar, hydrophilic), rigid steroid rings + short hydrocarbon tail (nonpolar,
hydrophobic).
Bilayer properties:
● Phospholipids self-assemble in water → minimize hydrophobic exposure →
bilayer/liposome forms.
● Bilayer is ~5 nm thick, hydrophobic core blocks polar/charged molecules.
Permeability (most → least):
1. Hydrophobic/nonpolar (O₂, CO₂, NO, steroids, benzene).
2. Small uncharged polar (H₂O, ethanol, glycerol – slow).
, 3. Large uncharged polar (glucose, sucrose – need transporter).
4. Ions/charged (Na⁺, K⁺, Cl⁻, Ca²⁺, amino acids, acetate – require transporter).
Fluidity factors:
● Fatty acid chain length (shorter = more fluid).
● Saturation (unsaturated = kinks, ↑fluidity).
● Temperature (↑T = ↑fluidity).
● Cholesterol: ↓fluidity at high T (stiffens), prevents freezing at low T.
Lipid assembly:
● New lipids made in ER cytosolic leaflet.
● Symmetry maintained by scramblases (randomize lipids).
● Plasma membrane asymmetry maintained by flippases/floppases (move specific lipids).
Membrane Proteins
Secondary structures:
● α-helix: side chains project outward; hydrophobic chains stabilize in bilayer.
● β-sheet (β-barrel): side chains alternate orientation; hydrophobic outside, hydrophilic
inside pore.
Crossing bilayer:
● Backbone peptide bonds are polar → hidden via H-bonding (α-helix or β-barrel).
● Transmembrane segment side chains are hydrophobic, exposed to lipid core.
Protein types:
et al., Essential Cell Biology 6th Ed.)
Major Macromolecules
● Carbohydrates, Lipids, Proteins, Nucleic acids.
Membrane Lipids
Phospholipid structure:
● Head group (polar, hydrophilic), phosphate (polar), glycerol backbone, fatty acid chains
(nonpolar, hydrophobic).
● Amphipathic = hydrophilic + hydrophobic regions.
Cholesterol:
● Hydroxyl group (polar, hydrophilic), rigid steroid rings + short hydrocarbon tail (nonpolar,
hydrophobic).
Bilayer properties:
● Phospholipids self-assemble in water → minimize hydrophobic exposure →
bilayer/liposome forms.
● Bilayer is ~5 nm thick, hydrophobic core blocks polar/charged molecules.
Permeability (most → least):
1. Hydrophobic/nonpolar (O₂, CO₂, NO, steroids, benzene).
2. Small uncharged polar (H₂O, ethanol, glycerol – slow).
, 3. Large uncharged polar (glucose, sucrose – need transporter).
4. Ions/charged (Na⁺, K⁺, Cl⁻, Ca²⁺, amino acids, acetate – require transporter).
Fluidity factors:
● Fatty acid chain length (shorter = more fluid).
● Saturation (unsaturated = kinks, ↑fluidity).
● Temperature (↑T = ↑fluidity).
● Cholesterol: ↓fluidity at high T (stiffens), prevents freezing at low T.
Lipid assembly:
● New lipids made in ER cytosolic leaflet.
● Symmetry maintained by scramblases (randomize lipids).
● Plasma membrane asymmetry maintained by flippases/floppases (move specific lipids).
Membrane Proteins
Secondary structures:
● α-helix: side chains project outward; hydrophobic chains stabilize in bilayer.
● β-sheet (β-barrel): side chains alternate orientation; hydrophobic outside, hydrophilic
inside pore.
Crossing bilayer:
● Backbone peptide bonds are polar → hidden via H-bonding (α-helix or β-barrel).
● Transmembrane segment side chains are hydrophobic, exposed to lipid core.
Protein types:
