Chapter 6 – Cell Membranes

What are the main components of biological membranes and their functions?

• Lipids: Maintain integrity & act as barrier to hydrophilic molecules

• Proteins: Perform most membrane functions (transport, signaling, etc.)

• Carbohydrates: Involved in recognition & signaling

• Membrane structure determines function and organization.

What does the Fluid Mosaic Model describe?

• “Mosaic” → different components (lipids, proteins, carbohydrates)

• “Fluid” → components move freely within the bilayer

• Bilayer = hydrophilic heads (outward) + hydrophobic tails (inward)

• All membranes share a basic structure but differ in lipid composition.

How does cholesterol and lipid composition affect membrane fluidity?

• Cholesterol (20–50%) maintains integrity & modulates fluidity.

• ↑ Cholesterol / long saturated FA → ↓ fluidity

• ↑ Unsaturated / short FA → ↑ fluidity

• Cold temps → ↓ fluidity; organisms adapt by increasing unsaturated FA.

What are the types of membrane proteins and their properties?

• Integral: Embedded in bilayer; hydrophobic & hydrophilic regions

• Transmembrane: Span entire membrane

• Peripheral: Loosely attached to membrane surface or other proteins

• Anchored: Covalently bonded to lipid tails

• Asymmetric: Inner and outer surfaces differ in proteins

• Some move freely; others are anchored by the cytoskeleton.

What are glycolipids, glycoproteins, and proteoglycans?

• Found only on the outer surface; key for recognition & signaling

• Glycolipids: Carb + lipid → recognition signals

• Glycoproteins: Carb + protein → short chains (<20 sugars)

• Proteoglycans: Protein + long carb chains → heavily glycosylated

How do cells recognize and adhere to each other?

• Recognition: Cells identify each other via surface molecules

• Adhesion: Strengthens connections between cells

• Involves carbohydrate–carbohydrate, carbohydrate–protein, or protein–protein interactions

• Plant cells use membrane + cell wall carbohydrates.

What are homotypic/heterotypic bindings and types of cell junctions?

• Homotypic: Same molecules bind on both cells

• Heterotypic: Different molecules interact

• Tight junctions: Seal spaces; prevent leakage & maintain polarity

• Desmosomes: Rivet-like; provide mechanical strength

• Gap junctions: Channels for ion/molecule exchange → communication

What is the extracellular matrix (ECM) and its role?

• ECM: Collagen fibers + proteoglycan matrix → structural support

• Cells attach or move via integrins (transmembrane proteins)

• Integrins connect cytoskeleton to ECM and transmit signals.

What are the main types of membrane transport?

• Passive transport: No energy; moves down gradient

• Active transport: Uses ATP; moves against gradient

• Selective permeability: Only certain substances can cross.

How do diffusion and osmosis work?

• Diffusion: Random motion → equilibrium (high → low conc.)

• Simple diffusion: Small, nonpolar molecules cross freely

• Osmosis: Water diffuses toward higher solute concentration

  • Isotonic: Equal solute

  • Hypotonic: Water enters → swelling

  • Hypertonic: Water leaves → shrinkage

  • Turgor pressure: Water pressure in plant cells.

What are channel and carrier proteins? What do aquaporins do?

• Channel proteins: Tunnels for ions/water (can be gated)

• Carrier proteins: Bind molecule → shape change → release

• Specific & saturable

• Aquaporins: Water channels → ↑ water permeability, exclude ions

  • Shown via CHIP28 protein experiments in frog oocytes.

What is active transport and what are its types?

• Moves molecules against gradient; requires energy (ATP)

• Uniporter: 1 substance, one direction

• Symporter: 2 substances, same direction

• Antiporter: 2 substances, opposite directions

• Primary active transport: Direct ATP use

• Secondary active transport: Uses energy from ion gradients.

How do large molecules enter and leave cells?

• Endocytosis: Brings substances into cell

  • Phagocytosis: “Eating” large particles

  • Pinocytosis: “Drinking” fluids/solutes

  • Receptor-mediated: Specific uptake via receptor–ligand binding (clathrin-coated pits)

• Exocytosis: Releases materials out of cell

  • Vesicle fuses with membrane → contents released

  • “Kiss and run” = brief pore opening

  • Used for secretion (enzymes, hormones, neurotransmitters).