Core Course: Block 1, Lecuture 10

Front 1

Biomembranes (functions)

Back 1

separate the cell from the outside world

separate the organelles from the inside of the cell (compartmentalize important processes and activities)

diverse function depending on location

Front 2

"unit" membrane structure

Back 2

at the electron microscope level, membranes share common structure

resemble railroad track with 2 densely stained lines, separated by a relatively clear space

on the cell surface, many proteins are glycosylated

Front 3

What are the major constituents of the cell membrane

Back 3

phospholipids

Front 4

Phospholipid structure

Back 4

polar head group

phosphate

glycerol backbone

2 FA tails

Front 5

What does the carboxyl group of the FA do?

Back 5

if free, it would be ionized

usually, it is linked to other groups to form esthers of amides

Front 6

sphingolipids make up what % of membrane phospholipids?

Back 6

15-20%

Front 7

Cholesterol: classification and function in membranes

Back 7

a sterol

a membrane plasticiser: inserts in membrane to alter packing of phospholipids

Front 8

Lipid composition of membranes

Back 8

different membranes have very different lipid composition depending on function

Front 9

why do membranes spontaneously form?

Back 9

hydrophobic molecules aggregate to minimize SA contact with water.

At the air-water interface, hydrophobic FAs tails of a lipid monolayer will stick up to avoid association with water

Front 10

What determines lipid packing (and whether a micelle or bilayer will form)

Back 10

the shape of the lipid

lysosome lipids: lipase removed one acyl chain, more conical in shape

phospholipids are cylindrical

Front 11

multilamellar phospholipids

Back 11

if you just shake up phospholipids in water, an onionlike structure results

to make vesicles you must sonicate or mix with detergent and slowly dialize away detergent.

Front 12

How do we know that membranes are bilayers?

Back 12

Indirectly: differential scanning calorimetry allows us to see that lipid melting occurs in much the same way each time. (for a single sample)

X-ray diffraction data was the definitive evidence for bilayer arrangement

Front 13

Phospholipids most common in the outer hemileaflet:

Back 13

sphingomyelin, phosphatadylcholine

Front 14

Phospholipids most common in the inner hemileaflet

Back 14

Phophatadylthanolamine, phosphatadylserine

Front 15

Lipid bilayers are:

Back 15

dynamic, noncovalent structures

Front 16

FRAP

Back 16

Fluorescence recovery after photobleaching:

attach fluorophors to phospholipid heads, bleach with laser beam, the observe diffusion

Front 17

lipid molecule diffusion (lateral and transverse)

Back 17

lateral: 1 micon/sec

transverse: much slower, need flipases

Front 18

Transport definition

Back 18

the movement of molecules across membranes

Front 19

Why do phospholipids form bilayers?

Back 19

The molecule is cyllindrical in shape, which allows sheet formation

Planar sheets are thermodynamically unstable because the hydrophobic edge of the plane is exposed to water, therefore they tend to fold in to make spheres.

Front 20

Leakage (diffusion) plot

Back 20

plot of uptake (Yaxis) v. [S] (x axis) is linear

V=k[S]

Front 21

protein mediated transport plot

Back 21

plot of uptake (y axis) v. [S] (x axis) shows michaelis menton kinetics

V=(Vmax[S])/(Km + [S])

Front 22

sum of both leakage and protein mediated transport plot

Back 22

plot of uptake (y axis) v. [S] (x axis) shows a mix between linear and michaelis-menton kinetics

V=k[S] + (Vmax[S])/(Km + [S])

Front 23

is protein mediated or diffusion more common in cells

Back 23

sum of both is most common

inhibit the protein-mediated process to obtain leakage only rate, then determine protein-mediated rate from that

Front 24

Passive transport

Back 24

The distribution of solute across the membrane is identical to the distribution that would be achieved without a transport protein (ie simple diffusion)

Front 25

Active transport

Back 25

A distribution of solute across a membrane that is not that produced by simple diffusion

Front 26

Primary active transporters

Back 26

Exploit high-energy intermediates (atp hydrolysis) to catalyze rapid net solute movement against a concentration gradient

Front 27

Secondary active transporters

Back 27

Exploit Na+, K+ or H+ gradients to drive a molecule against an electrochemical gradient

Front 28

2 factors govern the rate of diffusion

Back 28

1) the membrane solubility of a specific molecule

2) the size of the molecule that diffuses across the cell membrane

Front 29

Permiability depends on 3 factors:

Back 29

1) partitioning into the membrane
2) mobility within the membrane
3) thickness of the membrane

Front 30

Equation for permiability

Back 30

P=(K x Dm)/λ

where:

P = the permiability coefficient

K= partitioning coefficient

Dm= diffusion coefficient (related to mobility within the membrane)

λ= thickness of the membrane

Front 31

Why do larger molecules have lower Dm?

Back 31

The lipid bilayer contains “void space”
o The mobility within the membrane depends on 2 factors:
• 1) frequency of appearance of void spaces
• 2) the average void space volume in the membrane

o so even if a molecule can partition into the bilayer, it may be diffusionally restricted by the amount of void volume in the membrane
• increasing temperature can increase void space volume, thus increasing permeability of some molecules