Biology Chapter on Membrane Organization

Membrane Organization

• Fluid mosaic model depicts membranes as "protein icebergs in a lipid sea."
• Integral proteins possess lateral mobility and can rearrange based on various interactions including with peripheral proteins and cytoskeletal filaments.
• Membrane asymmetry exists with different chemical compositions on inner and outer bilayer halves.
• Oligosaccharides are found exclusively on the extracellular surface of the plasma membrane.
• Outer bilayer is richer in phosphatidyl choline and sphingomyelin; inner bilayer has more phosphatidyl serine and phosphatidyl ethanolamine.

Membrane Functions

• Cell membranes maintain selective permeability, crucial for preserving homeostasis and preventing the entry of harmful substances.
• Three primary methods enable molecules to traverse membranes:
  • Passive diffusion allows substances to move along concentration gradients without energy expenditure.
  • Facilitated diffusion assists molecules like glucose through specific membrane components without energy use.
  • Active transport requires energy (ATP) for molecules to cross membranes against their concentration gradient; an example is the sodium pump (Na+/K+-ATPase).

Signal Transduction

• Integral receptor proteins on cell membranes are vital for intercellular communication, binding specific signal molecules (ligands) without themselves entering the cell.
• Major receptor classes include:
  • Ligand-gated ion channels allow ion passage upon ligand binding.
  • Enzyme-linked receptors can activate associated enzymes (e.g., protein kinases) upon ligand binding.
  • G protein–coupled receptors (GPCRs) have a structure that passes through the membrane seven times.
  • Steroid hormone receptors play roles in signaling pathways within cells.

Endocytosis

• Cells can internalize extracellular materials through endocytosis, forming membrane-limited vesicles.
• Types of endocytosis include:
  • Phagocytosis (cell eating) for engulfing large substances or bacteria, forming phagosomes.
  • Pinocytosis (cell drinking) for taking in small amounts of fluid and solutes via smaller vesicles.
  • Receptor-mediated endocytosis involves ligand binding to receptors, leading to vesicle formation through clathrin-coated pits.

Cytoplasm

• Cytoplasm is the site of numerous metabolic activities, including glycolysis and cell division.
• Composed of cytosol (liquid with various solutes), organelles, and inclusions; cytosol contains around 70% water.
• The cytoplasmic matrix supports organelles, enabling signaling and metabolic processes within cells.

Cytosol

• Cytosol excludes the nucleus and is rich in dissolved ions and macromolecules; it facilitates signal transduction and maintains cellular functions.
• pH of human cytosol ranges from 7.0 to 7.4, influenced by cellular growth and metabolic activity.
• Ions within cytosol include K+, Na+, Cl-, Mg2+, Ca2+, and bicarbonate.

Intermediate Filaments

• Intermediate filaments are ropelike structures providing tensile strength and durability, formed by protein subunits specific to cell types.
• They stabilize cell structure, particularly in cell types exposed to mechanical stress (e.g., skin, muscle).
• Located throughout the cytoplasm, they form networks around the nucleus and contribute to maintaining cell shape.

Explore the fluid mosaic model that describes biological membranes as integral proteins in a lipid environment. This quiz covers key concepts including membrane asymmetry, protein mobility, and interactions between membrane components and the cytoskeleton.