Lecture 7: Membranes

Question 1

Amphipathic proteins

Answer 1

• 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

Question 2

Fluid Mosaic

Answer 2

• 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

Question 3

Function of Membranes

Answer 3

• 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

Question 4

Selectively permeable

Answer 4

• 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

Question 5

Passive Transport; Diffusion

Answer 5

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

Question 6

Why do molecules diffuse?

Answer 6

• Brownian motion

- due to thermal motions and collisions

Question 7

Diffusion rates

Answer 7

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

Question 8

Diffusion across a lipid bilayer

Answer 8

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)

Question 9

Osmosis

Answer 9

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

Question 10

Hypertonic solution

Answer 10

• 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

Question 11

Hypotonic solution

Answer 11

• 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)

Question 12

Isotonic Solution

Answer 12

• The concentration inside and outside of the cell are equal

- therefore, no net movement in one direction or the other

Question 13

Passive Transport: Facilitated Diffusion

Answer 13

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

Question 14

Electrochemical gradients

Answer 14

• 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+

Question 15

Potassium channels

Answer 15

• 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

Question 16

Saturation point

Answer 16

• 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)

Question 17

Active Transport

Answer 17

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

Question 18

How the Sodium Potassium Pump works

Answer 18

• 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

Question 19

Secondary Active Transport

Answer 19

• 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

Question 20

Na+ driven glucose cotransport

Answer 20

• 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.