Notes on Membrane Structure and Function

Extracellular Matrix (ECM) and Cell Membrane

  • Fibers of ECM: Provide structural and biochemical support to surrounding cells.
  • Components: Glycoproteins, glycolipids, and carbohydrates integrated into the membrane structure.

Structure of Cell Membrane

  • Plasma Membrane:
  • Selectively Permeable: Allows certain substances to cross more easily than others.
  • Fluid Mosaic Model:
    • Fluid: Membrane components move freely past one another.
    • Mosaic: Comprised of various proteins, phospholipids, and carbohydrates.

Composition of Cell Membranes

  • Phospholipids:
  • Amphipathic Nature: Hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
  • Creates a bilayer that forms the fundamental structure of membranes.
  • Membrane Proteins:
  • Integral Proteins: Span the membrane; involved in transport and communication.
  • Peripheral Proteins: Attached to the membrane surface; involved in strengthening membrane structure.

Membrane Fluidity

  • Factors Affecting Fluidity:
  • Temperature: Low temperatures increase viscosity; unsaturated fatty acids prevent tight packing.
  • Cholesterol: Maintains membrane fluidity by preventing close packing at low temperatures and restraining too much movement at high temperatures.

Passive and Active Transport

  • Passive Transport:
  • No Energy Required: Moves substances down their concentration gradient (high to low concentration).
  • Types:
    • Diffusion: Movement of small nonpolar molecules (oxygen, carbon dioxide).
    • Osmosis: Diffusion of water across a selectively permeable membrane.
  • Active Transport:
  • Requires Energy (ATP): Moves substances against their concentration gradient (low to high concentration).
  • Examples: Na+/K+ pump, proton pump.

Osmosis and Water Potential

  • Water Potential (ψ):
  • Equation: ψ = ψS (solute potential) + ψP (pressure potential).
  • Movement of Water: From regions of higher water potential (less negative) to lower water potential (more negative).
  • Influence of Solutes: Addition of solutes decreases water potential (more negative values).

Types of Solutions Relative to Cell Internal Environment

  • Hypotonic: Lower solute concentration outside; causes cells to swell (animal cells may lyse).
  • Isotonic: Equal solute concentration; cells remain stable.
  • Hypertonic: Higher solute concentration outside; causes cells to shrivel (crenation in animal cells).

Facilitated Diffusion

  • Mechanism: Transport proteins assist hydrophilic substances across membranes without using energy.
  • Examples: Aquaporins (water transport), glucose transport proteins.

Bulk Transport Mechanisms

  • Endocytosis: Cell takes in macromolecules by forming vesicles.
  • Types:
    • Phagocytosis: “Cellular eating” of solids.
    • Pinocytosis: “Cellular drinking” of fluids.
    • Receptor-Mediated Endocytosis: Specific uptake of molecules based on receptor-ligand interactions.
  • Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

Conclusion

  • Understanding the structure and function of cell membranes is essential for grasping how cells interact with their environment, regulate internal conditions, and maintain homeostasis.