Water, membranes and ions

• STRUCTURE OF WATER

  • Water is a polar molecule:

  1. oxygen is electronegative and has a tendency to pull electrons away from the Hs to which it is bonded
  2. this allows for hydrogen bonding

  • ice is less dense than liquid so ice floats
• IONS

  • ions are charged molecules/ atoms that have lost or gained an electron
  • ions carry signals in the body (action potentials); acts as an energy store (secondary active transport) and interact biochemically with proteins and other molecules (Ca2+/ troponin C in muscle contraction and Mg2+/ATP)
  • Physiologically useful ions act as charge carriers (Na+, K+, Cl-)
  • Biochemically useful ions take part in enzymic reactions/ form part of proteins (Mg2+, Fe3+, Zn2+)

  1. Ca2+ is both physiologically and and biochemically useful
• IONS cont.

  • ion can form electrostatic interactions with water:

  1. water molecules align themselves to maximise these interactions

  • the smaller the ion; the higher the charge density and so the larger the shell

  1. therefore small ions have lower mobility in solution
  2. hydration shell affects mobility in solution and interactions with proteins
• MEMBRANES AND ION GRADIENTS

  • all biological membranes are lipid bilayers formed from phospholipids

  1. phospholipids have hydrophilic polar heads and hydrophobic tails which forms the bilayer

  • impermeable to charged substances such as ions and moderately sized uncharged polar molecules like glucose
  • permeable to small molecules such as ethanol, water and gases
  • the permeability of membranes allows concentration gradients to form
• PUMPS AND TRANSPORTERS

  • membrane proteins form pathways for ions across the membrane

  1. this allows cells to establish and to use gradients

  • pumps use ATP to move ions against a concentration gradient- called primary active transport

  1. cells get this energy from the hydrolysis of ATP

  • Basic feature of pumps:

  1. live in membranes
  2. move ions 'uphill'
  3. couple to ATP
  4. fairly slow
  5. nearly always move cations

  • the sodium-potassium ATPase (sodium pump) generates a Na+ and K+ gradient (Electrogenic- 2K+ in, 3Na+ out)
  • ion gradients can be used to transmit info and power cellular processes
• SODIUM-POTASSIUM EXCHANGER: antiporter

  • Na+, Ca2+ exchanger extrudes 2000 Ca2+/ sec
  • Ca2+ pump 30 Ca2+/ sec
• ION CHANNELS

  • Carier/ transporter proteins are active or passive with a maximum rate ~ 10,000/s (e.g. Na+/K+ ATPase)
  • Ion Chanels use passive transport- maximum rate ~ 1,000,000/s

  1. transmembrane proteins
  2. selectively permeable
• LIGAND GATED ION CHANNELS

  • Cos loop receptors: nicotinic AChR, GABAa, 5HT3 receptor and inhibitory glycine receptor
  • Ionotropic glutamate receptors
  • open in response to binding of an activating ligand (agonist-acetylcholine)
  • all ligand gated channels have:

  1. PORE- lets ions through
  2. LIGAND BINDING SITE- signals the channel to open
  3. COUPLING MECHANISM- couples channel opening to Logan binding
  4. DESENSITIZATION MECHANISM- closes channel if ligand binds for too long
• STRUCTURE OF VOLTAGE-GATED ION CHANNELS

  • EXAMPLES: calcium channels (Cav); sodium channels (Nav) and potassium channels (Kv)
  • Kv channels appear early in evolution (present in prokaryotes)
  • Human genome has 40kv channel genes (subdivided into 12 families)