Biology: Osmosis and Ionic Compounds

Water and Salts

• Water dissolves salts like NaCl by hydrating and stabilizing Na+ and Cl- ions, weakening electrostatic interactions between them.
• Amphipathic compounds have a hydrophilic or polar head group and a hydrophobic tail, usually a hydrocarbon.

Osmosis

• Osmosis is the movement of water across a semi-permeable membrane driven by differences in osmotic pressure.
• Types of osmotic systems:
  • Isotonic: balanced, no net water movement
  • Hypotonic: diluted solution, water moves into the cell (RBC cell wall may burst)
  • Hypertonic: concentrated solution, water moves out of cells (shrinking)

Buffers, Weak Acids, and Bases

• Buffers: aqueous systems that resist a change in pH upon the addition of an acid or base, consisting of a weak acid (proton donor) and its conjugated base (proton acceptor).
• Ionization of water:
  • Water molecules undergo reversible ionization to yield a hydrogen ion (a proton) and a hydroxide ion.
  • Hydrogen ions formed in water are immediately hydrated to form hydronium ions (H3O+).
• Example: calculating the concentration of H+ in water using Keq.

Acids and Bases

• Acids: proton donors, strong acids disassociate fully in aqueous solutions to give H+ ions.
• Bases: proton acceptors, strong bases disassociate fully in aqueous solutions to give OH- ions.
• Acid-base pairs: a proton donor and corresponding proton acceptor make up a conjugate acid-base pair.

Biological Buffers

• Examples of biological buffers:
  • Hemoglobin
  • Proteins (histidine)
  • Carbonate/bicarbonate system: H2CO3 = H+ + HCO3-
  • Phosphate buffer system: H2PO4- = H+ + HPO42-

Carbohydrates

• Carbohydrates are polyhydroxy aldehydes, ketones, or substances that yield such compounds.
• Functions of carbohydrates:
  • Energy yielding (by oxidation of carbohydrates)
  • Insoluble carbohydrates serve as structural and protective elements
• Types of carbohydrates:
  • Glycoconjugates: carbohydrate joined to protein or lipid
  • Peptidoglycan: linear (acetylglucosamine + acetylmuramic acid), hetero, cross links, forms strong sheath, present in bacterial cell wall
  • Proteoglycans: macromolecules in ECM or cell surface, in connective cartilage tissues, do non-covalent interactions (strength) between cells and ECM
  • Glycoproteins: carbohydrate + protein, in out face of plasma membrane, ECM, and blood
  • Glycolipids: membrane proteins (hydrophilic part is an oligosaccharide), intercellular communication

Proteins

• Primary structure: sequence of amino acid residues
• Secondary structure: residues giving rise to recurring structural proteins, e.g., a-helix and B-conformations
  • a-helix: hydrogen bonding stabilizes the structure
  • B-helix: backbone is extended as a zig-zag rather than helical in the form of sheets known as pleats
• Tertiary structure: 3D folding of polypeptide
• Quaternary structure: 2 or more polypeptides
• Interactions that stabilize the tertiary structure:
  • Disulfide bonds: covalent linkage formed from SH of 2 cysteines forming cystine
  • Heat shock proteins: hsp70 binds and hydrolyses ATP
  • Chaperonins: facilitate folding
• Diseases related to protein misfolding:
  • Alzheimer's: Amyloid B protein (formation of brain plaques)
  • Parkinson's: a-synuclein

Separation and Purification of Proteins

• Methods:
  • Column chromatography:
    • Ion exchange chromatography: based on charge
    • Size inclusion chromatography/gel filtration: based on size
    • Affinity chromatography: based on binding specificities