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Amphipathic proteins

• Proteins can be amphipathic
- the polar and charged amino acids are hydrophilic
- the non polar amino acids are hydrophobic
• Amphipathic proteins can integrate into lipid bilayers
- alpha helix can insert itself into a lipid bi layer because its side groups are hydrophobic but the end it is hydrophilic and able to react with water
- proteins don't move up or down in lipid bilayer, but they do move sideways (due to hydrophobic amino acids)- this gives rise to fluid mosaic


Fluid Mosaic

• Integral (transmembrane proteins)- inside the lipid bi layer
• Peripheral proteins (outside of lipid bilayer)- pointing in and out of the cell- do most of the work in and out of the cell


Function of Membranes

• barrier, transport
a) Planar bilayers: Artificial membranes
- things can go through lipid bilayer with out regulation
b) Artificial-membrane experiments
- how rapidly can different solutes cross the membrane (if at all) when different types of phospholipids are used to make the protein
- find out that membranes are selectively permeable


Selectively permeable

• O2, CO2, N2 and other small non polar molecules are able to diffuse through
• H20, glycerol and other small, uncharged polar molecules are somewhat able to go through
• Glucose, sucrose are too large, uncharged polar molecules so they cant get through
• Ions are also not bake to get through


Passive Transport; Diffusion

The passive mixing of substances resulting in
a net transport along a concentration gradient
- need few molecules on one side and more on the other
at t=0 you have a high ∆G


Why do molecules diffuse?

• Brownian motion
- due to thermal motions and collisions


Diffusion rates

determined by distance,
temperature, size of molecule, and
steepness of concentration gradient
- very steep means very negative ∆G
- large molecules dont diffuse because they dont move around a lot


Diffusion across a lipid bilayer

1. Separation of solutes by lipid bilayers
2. Diffusion
3. Equilibrium (at equilibrium there is still movement back and forth but there is no net movement)



• diffusion of water across a
selectively permeable membrane
• water will flow from high water concentration to low water concentration


Hypertonic solution

• The concentration of solute outside of the cell is higher than inside the cell
- water will flow out of the cell and shrink
- reason why you shouldn't drink 200g sodium chloride solution, although you dont die when you drink 200g of glucose because the molecules are larger so you need less molecules to get 200g


Hypotonic solution

• The concentration inside the cell is higher than outside of the cell
- net flow into the cell, and the cell can expand and even burst (happens if you drink a lot of water- simply too much water for your kidneys to filter, especially during marathons- this is why you're supposed to drink isotonic solutions)


Isotonic Solution

• The concentration inside and outside of the cell are equal
- therefore, no net movement in one direction or the other


Passive Transport: Facilitated Diffusion

• Facilitated diffusion depends on two types of membrane proteins: channel proteins and carrier proteins.
- uses concentration gradients that are already around


Electrochemical gradients

- composed of ions
- gives rise to a very -∆G so it will want to cross the cell
- high concentration of Na+ on outside (so net + charge
- net negative charge on inside due to Cl- ions and the low concentration of Na+


Potassium channels

- allow only potassium ions to pass through
- electrical charge outside the membrane triggers shape change that allows ions to pass through
- open for a short amount of tim


Saturation point

- When all binding sites are occupied, the carrier is
saturated; therefore the rate of diffusion levels off
- channel proteins also can be saturated if concentration is too high due to the increase of molecules going through the channels (less molecules go through carrier molecules)


Active Transport

• Requires the expenditure of energy (directly or indirectly).
• Substances are moved across the membrane against the concentration gradient.


How the Sodium Potassium Pump works

• Pump found in animal cells
- transports 3 sodium ions out and then 2 potassium ions in (net transport of one ion out)- helps regulate osmolarity
- generation of membrane potential (electric charge due to concentration gradient that allows other transport processes to occur)
1. Unbound protein
2. Sodium binds
3. Shape changes
4. The sodium is released


Secondary Active Transport

- at end lots of Na ions outside and K inside, and they want to move back- drive secondary active transport
- systems use established gradients to move substances.
- have glucose cotransporter (transports glucose against concentration gradient), also can enter by facilitated transport
- Very +∆G by coupling it with the inflow of Na ions that have a very -∆G


Na+ driven glucose cotransport

- found in gut
- food flows through intestinal lumen, wanting the nutrients to be extracted
- one side of the lumen you ave a cotransporter (moves against concentration gradient)
- on the other side, you have a passive glucose carrier (with blood flowing by)
- the nutrients (glucose and Na+) are extracted then sent into the extracellular fluid where they are sent into the blood.