Lecture 21: Structure and Function

Question 1

what makes up the membrane structure?

Answer 1

thin flexible, sturdy barrier that surrounds the cytoplasm of a cell
- sea of lipids in which proteins float like icebergs
- membrane is 50% lipid and 50% protein (held together by hydrogen bonds)
- lipid is barrier to entry or exit of polar substances
- gatekeepers that regulate traffic

Question 2

what makes up the lipid bilayer of the cell membrane?

Answer 2

2 back-to-back layers of 3 types of lipid molecules
- cholesterol, and glycolipids scattered among a double row of phospholipid molecules

phospholipids:
- comprises of 75% lipids
- 2 parallel layers
- each molecule is amphipatic

Question 3

membrane fluidity

Answer 3

lipids moe around within the plane of the membrane leaflet
- they rarely flip flop between leaflets, therefore lipid composition of leaflets can be asynmetric

Question 4

what is membrane fluidity determined by?

Answer 4

tail length (long tail = less fluid)
number of double bonds (more = more)
amount of cholesterol (more = less fluid)

Question 5

what are the two types of membrane proteins?

Answer 5

integral proteins: extend complete across cell membrane (transmembrane) or into it
- amphipatic
- hydrophobic region that spans hydrophobic core of l.be which consists of non polar amino acids coiled into helices

peripheral proteins: attach to inner/outer surface of cell membrane and are easily removed from it.

Question 6

what is the function of membrane proteins?

Answer 6

receptor proteins
cell identity markers
linkers (holds cell into place)
enzymes (takes/break down molecules)
ion channels (water filled pore)
transporter protein (changes shape)

Question 7

selective permeability

Answer 7

allows some substances to cross but excludes others

lipid bilayer is permeable to:
- non polar uncharged molecules (02, n2, benzene)
- lipid soluble molecules (steroids, fatty acids, some vitamins)
- small uncharged polar molecules (water, urea, glycerol, co2)

impermeable to:
- large uncharged polar molecules; glucose (respiration), amino acids (protein), too large can’t react
- impermeable to ions (na+, k+, cl-, ca2+, h+)

**membrane proteins mediate transport of substances across the membrane that cannot permeate

Question 8

diffusion

Answer 8

molecules move until they are equal concentration (equilibrium)

random mixing of particles in a solution as a result of the particle’s kinetic energy
- more molecules move from high conc to low conc
- the greater the difference in conc, the faster the rate of diffusion

Question 9

what is the rate of diffusion affected by?

Answer 9

higher temperatre = fast
larger size of substance = slow
increase in surface area = fast
increase diffusion distance = slow

Question 10

what are the two types of gradients across cell membrane?

Answer 10

1. concentration gradient: non-charged molecules diffuse down conc gradient
2. electrical gradient: ions influenced by membrane potential + conc gradient. movement of ions influenced by electrochemical gradients

**cells mimic capacitors and separate and store charge

Question 11

random facts about gradient

Answer 11

cells use 30% of resting energy to maintain concentration and electrical gradiets
gradients represent stored energy

Question 12

osmosis

Answer 12

net movement of water through a selectively permeable membrane from high water conc to low water conc
only occurs if membrane is permeable to water only

Question 13

membrane permeability to water

Answer 13

pw = pd + pf

pd; through lipid bilayer; small, mercury insensitive, temperature dependent

pf: through water channel; large, mercury sensitive, temperature independent + mediated by aquaporins (9)

cells have diff pw because they express diff aquaporin isoforms)

Question 14

what is osmotic pressure?

Answer 14

the pressure applied to a solution to prevent the inward flow of water across a semipermeable membrane