Biochemistry 1 Comprehensive Notes

Biochemistry 1 Notes

Introduction to Cellular Membranes

  • The biological membrane, also known as the plasma membrane, is a lipid bilayer embedded with proteins performing various functions (enzymes, transporters).
  • Key Components:
  • Lipids (e.g., phospholipids, glycolipids)
  • Proteins (e.g., transmembrane proteins, peripheral proteins)
  • Membrane forms a thin barrier isolating cytoplasm from the external environment.

Importance of the Plasma Membrane

  • Acts as a selective barrier deciding what enters/exits the cell.
  • Found in both eukaryotic and prokaryotic cells.
  • Functions:
  • Isolates cell contents
  • Regulates substance exchange
  • Affords communication with other cells.
  • Structure stabilized by hydrophobic interactions among lipids.

Composition of Membranes

  • Lipid Bilayer Structure:
  • Composed of hydrophilic heads and hydrophobic tails.
  • Lipid aggregates, such as micelles and vesicles, form in aqueous environments.
  • Dynamic Composition:
  • Functions within cells where organelles (ER, Golgi, lysosomes) carry proteins/lipids.
  • Membrane trafficking alters lipid compositions over time, affecting functions.

Membrane Proteins

  • Types of Membrane Proteins:
  • Integral Proteins: Firmly embedded, can span bilayer (e.g., transmembrane proteins).
  • Peripheral Proteins: Attached loosely, interact with integral proteins or lipids.
  • Amphitropic Proteins: Can associate reversibly.
  • Posttranslational modifications like glycosylation can affect proteins.

Membrane Fluidity

  • Fluidity affected by:
  • Fatty acid composition (saturated vs. unsaturated fatty acids)
  • Cholesterol concentration (increases fluidity at lower temperatures, decreases fluidity at higher temperatures).
  • States of Lipid Bilayer:
  • Liquid-ordered (Lo) state: gel-like, constrained motion
  • Liquid-disordered (Ld) state: individual chains in constant motion.

Transport Across Membranes

  • Types of Transport:

  • Passive Transport: No energy required, movement down concentration gradient (e.g., diffusion, facilitated diffusion, osmosis).

  • Active Transport: Requires energy to move substances against a gradient (e.g., sodium-potassium pump).

  • Endocytosis/Exocytosis: Mechanisms for transport of large particles into/out of cells.

  • Diffusion and Concentration Gradients:

  • Molecules move from high to low concentration until equilibrium is achieved.

  • Factors affecting diffusion rates: size of molecules, concentration gradient, and lipid solubility.

Endocytosis Variants

  • Pinocytosis: Cell drinking, uptake of fluids.
  • Receptor-mediated Endocytosis: Uptake of specific substances via receptor interactions.
  • Phagocytosis: Cell eating mechanism for large particles.

Membrane Proteins and Signals

  • Transmembrane Receptors: Allow cells to respond to external signals (hormones, neurotransmitters).
  • Recognition Proteins (e.g., glycoproteins): Allow cells to recognize and interact with each other.

Lipid Rafts and Membrane Organization

  • Lipid rafts: microdomains comprised of cholesterol and sphingolipids, enriched with specific proteins for signaling.
  • Membrane curvature important for various cellular processes including vesicle formation and fusion.

Active Transport Mechanisms

  • ABC Transporters: Use ATP to transport substrates across membranes against gradients. Common in drug resistance mechanisms.
  • Primary vs. Secondary Transport:
  • Primary: directly linked to energy release (exergonic reactions).
  • Secondary: couples with another molecule's transport that is initially moved uphill.

Free Energy Changes in Transport

  • Differential equations relate membrane potential and changes in concentration gradients; parts such as hydropathy index used for predicting protein behavior in membranes.

Conclusion

  • The intricate structure and composition of biological membranes facilitate essential functions crucial for cellular integrity and communication. Understanding these mechanisms is fundamental to biochemistry and cellular biology.