Lipids, Membranes and Cell Topology

Question 1

Lipid bilayer

Answer 1

arrangement of phospholipids where the polar heads are exposed to water and fatty acid tails are sandwiched in the middle, this arrangement causes membrane to be selectively permeable and assemble spontaneously

Question 2

Phospholipid composition

Answer 2

head group and phosphate group attached to 3 carbon glycerol backbone with two fatty acid tails attached to 2 carbons of the glycerol backbone

Question 3

head group

Answer 3

polar- hydrophilic

Question 4

unsaturated fatty acid tail

Answer 4

hydrophobic, with one or more double bonds (causes kinks in the molecule)

Question 5

saturated fatty acid tail

Answer 5

hydrophobic, no double bonds so no kinks

Question 6

cholesterol

Answer 6

another component of the lipid bilayer, stabilizes phospholipids by filling in gaps, making it rigid and less fluid, also amphipathic, acts as a membrane fluidity “buffer” so although when there is already more molecular space the addition of cholesterol makes it less fluid, when the membrane starts out more packed together the addition of cholesterol makes it more fluid.

Question 7

amphipathic molecule

Answer 7

molecule that has both hydrophobic and hydrophilic parts

Question 8

selective permeability

Answer 8

some molecules can freely diffuse across membranes and other molecules cannot, allows for cellular compartmentalization

Question 9

freely permeable

Answer 9

hydrophobic molecules (O2, CO2, N2)

Question 10

slightly permeable

Answer 10

small, uncharged polar molecules (H2O, glycerol)

Question 11

impermeable

Answer 11

large, uncharged polar molecules (glucose, sucrose) (due to hydrophobic center), ions (H+, Na+)

Question 12

spontaneous assembly

Answer 12

how phospholipids assemble into lipid bilayer formation using no energy, does not defy the 2nd law of thermodynamics

Question 13

second law of thermodynamics

Answer 13

entropy can only increase so in spontaneous assembly, even though the phospholipids become more ordered, the water molecules become more disordered so the net entropy is actually increases

Question 14

hydrophobic effect

Answer 14

liquid water alone is highly disordered, but water forms cages around hydrophobic molecules (ex. tails of phospholipids) which is unfavorable for H2O entropy, this causes the phospholipids to orient themselves with their hydrophobic tails facing each so the H2O molecules are shielded from the tails and so the H2O can return to its naturally disordered state

Question 15

non-covalent bonds and strengths

Answer 15

ionic (electrostatic)>hydrogen bonds>Van der Waals (transient, fluctuating charges)

Question 16

“fluid mosaic” model for cellular membranes

Answer 16

the fluid refers to how cell membranes are 2-d fluids (phospholipids have movement- lateral diffusion, flexion and rotation) and mosaic refers to how membranes are made up of lots of different molecules

Question 17

fluorescence recovery after photobleaching (FRAP)

Answer 17

refers to bleaching fluorescent phospholipids or other structures, if the dark spot caused by bleaching becomes fluorescent again, it indicates fluidity

Question 18

regulation of membrane fluidity

Answer 18

less molecular space (longer straight phospholipid tails) makes membranes less fluid while more molecular space (shorter tails with kinks) means more fluid; in the example, bacterium adjusts lipids in its membrane to create more fluidity in the new temperature

Question 19

cell topology

Answer 19

the way in which the parts of a cell are arranged relative to one another, as organized by membranes

Question 20

topology of the eukaryotic cell

Answer 20

the cytosol and nucleus are “topologically equivalent” because the nuclear pore complex creates an open channel between the two, however, the ER is “topologically distinct” from the nucleus and cytosol because it is separated by two membranes from those areas

Question 21

why would the pH of the cytosol and nucleus always be the same?

Answer 21

The pH of the cytosol and nucleus must be the same because protons (H+) would be able to diffuse freely through the aqueous pores that connect these topologically equivalent compartments.