ch5 membranes

• Membrane structure:

  • Phospholipids arranged in a bilayer
  • Globular proteins inserted in the lipid bilayer
  • Described by the fluid mosaic model, where a mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond
• Cellular membranes have four components:

  1. Phospholipid bilayer: a flexible matrix and a barrier to permeability

  2. Transmembrane proteins: integral membrane proteins

  3. Interior protein network: peripheral or intracellular membrane proteins

  4. Cell surface markers: glycoproteins and glycolipids
• Phospholipids:

  • Structure consists of glycerol, two fatty acids, a phosphate group
  • Spontaneously forms a bilayer with hydrocarbon tails on the inside and polar head groups on the outside
• Phospholipid bilayer:

  • Bilayers are fluid due to weak interactions between phospholipids
  • Influences on fluidity include the types of fatty acids and temperature
  • Phospholipid composition affects membrane structure, with cholesterol playing a role in stability and fluidity
• Membrane proteins:

  • Functions include transporters, enzymes, cell-surface receptors, identity markers, cell-to-cell adhesion proteins, and attachments to the cytoskeleton
  • Structure relates to function, with diverse functions arising from diverse structures
• Passive transport:

  • Movement of molecules through the membrane without requiring energy
  • Occurs in response to an energy gradient, often a concentration gradient
  • Diffusion is a form of passive transport where molecules move from high to low concentration until equilibrium is reached
• In multicellular organisms, the distance for substances to enter cells is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen
• Facilitated diffusion allows molecules that cannot easily cross the membrane to move through proteins
• Facilitated diffusion moves molecules from higher to lower concentration
• Facilitated diffusion can be performed by channel proteins and carrier proteins
• Channel proteins are transmembrane proteins that allow passive transport through hydrophilic channels when open
• Carrier proteins are transmembrane proteins that assist in passive or active transport by binding specifically to molecules
• Ion channels allow the passage of ions through the nonpolar interior of the plasma membrane
• Gated channels open or close in response to stimuli like chemical or electrical signals
• Carrier proteins can transport ions and other solutes via diffusion, requiring a concentration difference across the membrane
• Carrier proteins bind to the molecule they transport, causing a conformational change in the protein
• Osmosis is the net diffusion of water across a membrane toward a higher solute concentration
• Aquaporins are specialized channels for water in the cell membrane that facilitate osmosis
• Water diffuses out of a cell in a hypertonic solution, causing the cell to shrink until osmotic concentrations are equal
• Osmotic pressure is the force needed to stop osmotic flow, affecting cell size and shape
• Some organisms use mechanisms like extrusion, isosmotic regulation, and turgor pressure to maintain osmotic balance
• Active transport requires energy, moving substances from low to high energy/concentration against the gradient
• Carrier proteins used in active transport include uniporters, symporters, and antiporters
• In animal cells not actively dividing, the sodium-potassium (Na+/K+) pump is used for active transport of sodium (Na+) and potassium (K+) ions
• Most animal cells have a low internal concentration of Na+ and a high internal concentration of K+, which is maintained by actively pumping Na+ out of the cell and K+ in
• The sodium-potassium (Na+/K+) pump:

  • Directly uses ATP for active transport
  • Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell
  • Operates against the concentration gradient
  • Uses ATP to change the conformation of the carrier protein, allowing ions to be carried across the membrane
• Symporters:

  • Involve coupled transport that uses ATP indirectly
  • Use the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule
  • Capture the energy released as one molecule moves down its concentration gradient to move a different molecule against its gradient
• Coupled transport via membrane proteins:

  • Transports Na+ into the cell down its concentration gradient while transporting a glucose molecule into the cell against its gradient
  • The Na+ gradient driving the entry allows sugar molecules to be transported against their concentration gradient
  • The Na+ gradient is maintained by the Na+/K+ pump
• Cholesterol is an important component of cell membranes that helps maintain fluidity at low temperatures.