cell communication

BIO 151 – Lecture 3 Notes

Topic: Plasma Membrane Structure & Function

Cell Membranes: Basics

  • Shared features (prokaryotes & eukaryotes): DNA, plasma membrane, ribosomes.

  • Key difference: DNA inside nucleus (eukaryotes) vs. not (prokaryotes).

  • Membrane composition: Phospholipids (amphipathic → hydrophilic head, hydrophobic tails).

  • Fluid Mosaic Model: Membrane is fluid, dynamic, and embedded with proteins.

Membrane Fluidity

  • Lateral movement: Very frequent (~10⁷ times/sec).

  • Flip-flop (across bilayer): Rare (~once/month).

  • Why fluidity matters: Affects protein rotation, diffusion, signaling, and cell function.

Factors affecting fluidity:

  1. Fatty acid structure:

    • Unsaturated (double bonds, kinks) → more fluid.

    • Saturated (straight chains) → less fluid.

    • Shorter tails → more fluid; longer tails → less fluid.

  2. Temperature:

    • Cold → membrane solidifies.

    • Hot → membrane becomes too fluid.

  3. Sterols:

    • Cholesterol (animals): Buffer – prevents solidifying at low temp, prevents excess fluidity at high temp.

    • Sitosterol (plants) and Hopanoids (bacteria): similar role.

Adaptations:

  • Seasonal environments → cells change fatty acid saturation to adjust membrane fluidity.

  • Enzymes: Saturases (add hydrogens, more saturation), Desaturases (remove hydrogens, add double bonds).

Membrane Proteins

  • Integral proteins: Span bilayer, cannot be removed without damaging membrane.

  • Peripheral proteins: Loosely attached, removable without damage; often organize lipid rafts.

  • Functions: signaling, transport, structural organization.

Selective Permeability

  • Can diffuse freely: small, nonpolar molecules (O₂, CO₂).

  • Limited diffusion: small polar molecules (H₂O).

  • Cannot diffuse freely: large polar molecules (sugars), ions (Na⁺, K⁺, Cl⁻).

Transport types:

  1. Passive Transport (no energy, down gradient):

    • Simple diffusion (small, hydrophobic).

    • Facilitated diffusion (via channel/carrier proteins).

    • Osmosis (water movement via aquaporins).

    • Movement stops when concentrations equalize (no net movement).

  2. Active Transport (requires energy, against gradient):

    • Primary active transport: Direct ATP use (e.g., Na⁺/K⁺ pump).

    • Secondary active transport: Uses electrochemical gradient created by primary transport to move other molecules.

✅ Key Takeaways

  • Membrane = phospholipid bilayer, dynamic & selectively permeable.

  • Fluidity regulated by fatty acid composition, sterols, and temperature.

  • Transport occurs via passive (diffusion/osmosis) or active (ATP-driven) mechanisms.

  • Proteins are essential for selective transport and signaling.