Lecture 3 – MEMBRANES, IONS, WATER AND PROTEINS Flashcards
Polarity
- Basic meaning: ends/ sides are different
- Epithelial cell: structural polarity
- Magnet: magnetic polarity
- Membrane: electrical polarity
Water
- All life exists in an aqueous environment
- Molecular properties of water influence the structure and behaviour of cells
- Neurones are directly linked to water
Hydrogen bonds
- Electronegativity allow ‘hydrogen bonds’ to occur between adjacent/ neighbouring water molecules allowing it to form a lattice when in ice form
- Oxygen ‘sucks’ electrons away
- Hydrogen atom is shared between two electronegative atoms
- Relatively weak
- Highly directional
Water is unique
- Hydrogen bonds allow water to be a liquid at room temperature
- Compounds with similar, or greater RMM than water are gases
- In liquid state, it’s a loose lattice
- In a solid state, the arrangement of H-bonds are much more ordered
- Ice is less dense than water
The Ions
- An ion is any atom or molecule that has gained or lost one or more electrons and are defined by a charge
- Carry signals around the body in the form of action potentials
- Act as an energy store which is secondary active transport
- Interact biochemically with proteins and other molecules:
1. Ca2+/ troponin C in muscle contraction
2. Mg2+/ ATP
Biologically Important Ions
- Physiologically useful ions act as charge carriers and energy stores
- Biochemically useful ones take part in enzyme reaction or form part of proteins
Ions in an aqueous solution
- Can form electrostatic interactions with water
- Water molecules align themselves to maximise these interactions so the negative oxygens can interact with the ion
- The layer of water molecules immediately in contact with the ion is termed primary hydration shell/ layer
- Negatively charged ions, the hydrogen interacts with the ion and forms the opposite way around
- Looser shell is the secondary hydration shell/layer
Ionic ‘size’ = Ionic Radius?
- The size of the hydration shell depends on the charge density of the ion
- Smaller the ion, the higher the charge density and the larger the shell
- Smaller ions have lower mobility in solution than larger metal ions with the same charge
- Hydration shell affects interactions/ mobility with proteins
Membranes
- polar heads of phospholipids interact with water which drives the formation of the bilayer
- charged substances such as ions, and even moderately sized uncharged polar molecules like glucose, find it almost impossible to pass through bilayers
- very small molecules like ethanol, water and gases find it easier to pass through
- membranes are essentially impermeable to ions
Membrane Proteins
- allow cells to establish ion gradients and use them – impermeable to ions
- form pathways for ions
Pumps/ ATPase
- concentration of ions against gradients REQUIRES energy
- The Sodium Pump
- Basic features of pumps:
Ion Gradients as ‘batteries’
- Gradients represent a source of energy
- Can be used to power cellular processes like the transport of other ions via carriers (2nd class)
Can be used to transmit information by signalling via ion channels
Secondary Active Transport Carriers
Carrier refers to a general class of proteins that bind a substrate on one side of the membrane to undergo a conformational change which leads to the protein moving and releasing the substrate on the other side (inside)
Carriers can be very effective
- Couple downhill flow of ions to uphill flow of a different ion
- Don’t use energy from ATP directly – energy comes from gradient of downhill ions
- Antiporters
- Symporters
- Not all involved in active transport – many are passive and serve to transport polar substrates down their concentration gradient
Ion Channels
ROLE – mediate ion flux through membranes
DISTRIBUTION – everywhere from bacteria to humans
EVOLVED FOR ALL MAJOR PHYSIOLOGICAL IONS – Na+/ K+/ Ca2+/ Cl- channels
Basic properties of ion channels
- Transmembrane proteins
- Passive transport: ‘protein lined hole’ – provide a hydrophilic pathway for ions to travel through hydrophobic regions
- Selectivity: fussy regarding ions – a filter
- Gating: opening and closing is controlled
Voltage-gated channels
- Pores which let ions through
- Open in response to changes in membrane potential
- Have a voltage sensor which tells channel to open
- Coupling mechanism which couples channel opening to voltage sensing
- Inactivation mechanisms which closes the channel
- Tetramer of four equivalent subunits
Ligand-gated ion channels
- Open in response to binding of an activating ligand (agonist) – acetylcholine
- Have pores to let ions through
- Ligand binding site which tells channels to open in response to a ligand binding
- Coupling mechanism which couples channel opening to ligand binding
- Desensitization mechanisms which closes the channels if ligand binds for too long
- Pentamer of five similar subunits