nonpolar molecules > small uncharged polar molecules > large polar molcules & ions >macromolecules
1.Selective Barrier
a) Surround cells to hold enzymes and metabolites inside
b) surround organelles inside cells
2.Contain Enzyme Systems--energy metabolism (oxidative phosphorylation, photosynthesis etc.)
3.Contain Transport Systems--bring food molecules inside and maintain ion concentrations
4.Contain Specific Recognition Sites--for hormones etc.
1. Lipids--mostly phospholipids but also glycolipids (contain carbohydrate) and steroids (cholesterol). Lipid bilayers form a barrier to diffusion of hydrophilic molecules.
2. Proteins
a) peripheral membrane proteins bound to bilayer surface
b) integral membrane proteins are intrinsic structural parts and have hydrophobic and hydrophilic domains ===> amphipathic
c) glycoproteins (integral) contain covalently bound CHO which may be very complex ===> cell surface receptors? e.g. glycophorin--MW 30,000 with 130 aa's and 60% CHO
3. Composition of membranes varies greatly:
Membrane Lipid Protein Myelin Sheath 80% 20% Plasma Membrane 50% 50% Mitochondrial Inner Membrane
25% 75%
1. Lipids and Proteins are free to move about (diffuse) in the plane of the membrane and to rotate about an axis perpendicular to the membrane. The membrane is like a two-dimensional solution of proteins in a phospholipid solvent (Phospholipid Bilayer) ===> Fluid.
2. Proteins and phospholipids cannot flip from one side of the membrane to the other however.
3. Fluidity of lipids depends on hydrocarbon tails of fatty acids: fatty acids with cis double bonds don't condense easily into solid state ==> bilayer is more fluid.
4. Steroid lipids (Cholesterol) make bilayers less fluid but also inhibits transformation to solid state at low temperature.
5. Membrane proteins are classified as either Integral or Peripheral
1.. Lipid Bilayer -- primary barrier selective based upon size and polarity of molecules
a) Nonpolar molecules (O2, hydrocarbons, fatty acids)-- bilayers are most permeable to small nonpolar molecules
b) Small uncharged polar molecules (H2O, CO2) -- bilayers are somewhat permeable to small uncharged polar molecules
c) Large polar molecules and ions -- bilayers are relatively impermeable to large polar molecules and to ions
d) Macromolecules (proteins, nucleic acids, polysaccharides) -- cannot pass unless a special mechanism exists (signal hypothesis).
2. Integral Membrane Proteins -- modify permeability selectively
a) Facilitated Diffusion -- help molecules move to lower free energy state
- down concentration gradient to lower concentration
- for ions - to region of opposite charge
b) Active Transport -- use a source of energy to transport molecules to region of higher energy
c) Selective -- can recognize specific molecules (just like an enzyme).
1. Free Energy -- why do molecules want to diffuse?
a) Think of diffusion as a chemical reactionb) What is DG for this reaction? DG = D G° + RT ln [A]out/[A]in but DG° = 0 ===> D G = RT ln [A]out/[A]in . Thus, if [A]out < [A]in then DG < 0; this is because having a higher [A] in one space is lower in Entropy (higher order) than having the molecules spread out evenly
c) Rate of Diffusion -- is proportional to the concentration gradient (difference in concentration)
2. Membrane Potentials -- difference in charge on 2 sides of the membrane
a) Affects the free energy of ions
- cations -- have +charge and are attracted to -side
- anions -- have -charge and are attracted to +side
b) Extra term in free energy equation: DG = RT ln [A]out/[A]in + zFDY
where DY is the membrane potential in volts, z is the charge on the ion and F is a conversion constant.c) Membrane Potential can create a concentration difference by attracting ions to a region of opposite charge. The final concentration gradient will be a balance between free energy due to concentration difference and free energy from membrane potential
3. Gradients of Concentration -- a way of storing free energy -- Active Transport
a) use chemical energy (ATP) to create concentration gradients
b) use energy of one gradient to create another gradient
c) use a gradient to create chemical energy -- synthesis of ATP in mitochondria and chloroplasts
4. The Sodium Potassium Pump is found in the plasmamembes of most eukayotic cells. Using the energy from ATP hydrolysis, it pumps three Na+ out of the cell and two K+ into the cell. This creates a high [Na+] concentration outside the cell and a high [K+] inside the cell.
1. Solute Concentration -- determines water balance ...Figure 5.8
a) Osmosis -- passive transport of water across a semipermeable membrane
- the driving force is free energy
- H2O tends to move across the membrane until the solute concentration on both sides are the same
b) Osmotic Pressure -- the tendency for H2O to diffuse to higher solute concentration can be measured as a pressure
Two chambers containing different solute concentrations are separated by a semipermeable membrane (a). In (b) water diffuses across the membrane until the difference in height, h, equals the osmotic pressure. In (c) a piston pushes on the liquid in the righthand chamber; the amount of pressure required to keep water from diffusing into this chamber is equal to the osmotic pressure
2. Osmotic Pressure -- affects cells
a) Hypertonic Solution -- the solution around the cell has a higher [solute] (called osmolarity) than the cytoplasm ==>
- water diffuses out of the cell and the volume of the cytoplasm decreases
- affects both cells with and without cell walls.
b) Isotonic Solution -- solution around the cell has the same osmolarity as the cytoplasm ==> no net movement of water
c) Hypotonic Solution -- the solution around the cell has a lower osmolarity than the cytoplasm ==> water diffuses into the cell
- cells without cell walls (animal cells) increase in volume, and if the osmotic pressure is great enough the plasma membrane may break. This method is sometimes intentionally used by scientists to break open cells or vesicles.
- cells with cell walls (plant cells and bacteria) increase in volume until the inward pressure exerted by the cell wall equals the osmotic pressure and the cell becomes more rigid or turgid. Turgor gives structural support to plants.
