cell membranes

Question 1

Structural components of phospholipids

Answer 1

2 hydrophobic fatty acid tails - hydrophobic
charged phosphate head - hydrophilic - contain anionic and cationic groups - net anionic or neutral
amphiphilic
most based on glycerol
fatty acids are esterified

Question 2

Formation of phospholipid bilayers

Answer 2

Hydrophilic heads interact with the aqueous environment - outside of cell and in cytoplasm
hydrophobic tails form hydrophobic core - between hydrophilic heads - no water, sheltered from aqueous environment
form micelles/droplets
bilayers - layer 2 molecules thick - liposomes

Question 3

Permeability

Answer 3

small non-charged molecules eg water, oxygen and carbon dioxide- permeable through hydrophobic core
charged/polar molecules (cations K+ Na+ Ca2+, anions Cl- HCO3-) impermeable through hydrophobic core - have to pass through channel and carrier proteins
impermeable to macromolecules and biochemical intermediates
permeable to nutrients and waste products
impermeable to hydrophilic molecules like glucose

Question 4

Effect of unsaturated hydrocarbon chains

Answer 4

sparsely packed

Question 5

effect of saturated straight hydrocarbon chains

Answer 5

densely packed

lipid rafts

Question 6

Dimensions of cell membrane

Answer 6

3-7nm

thickness 5nm

Question 7

properties

Answer 7

selective permeability

signal transduction

Question 8

Ceramide structure

Answer 8

not glycerol

it is an amine

Question 9

Structure of cholesterol

Answer 9

ring and tail hydrophobic

Question 10

function of cholesterol

Answer 10

decrease permeability
modulate membrane stiffness
affects interactions with cytoskeleton
fills gaps between phospholipids

Question 11

membrane asymmetry

Answer 11

glycolipids on outside leaflet - neutral on outside

negative on inside - phosphatidylserine

Question 12

How dynamic is a lipid bilayer

Answer 12

phospholipids really switch leaflets - hard move philic through phobic core
they can diffuse laterally easily - no barriers
more dynamic than models would predict - proteins break up order so make it more dynamic - 1970s realised ruffling couldn’t be describes by current models

Question 13

describe the fluid mosaic model

Answer 13

proteins float in a “sea” of lipids in either leaflet of the bilayer, or span both leaflets of lipids that form the bilayer (Singer and Nicolson, 1972).

Question 14

Describe integral proteins

Answer 14

hydrophobic outside
interact with hydrophobic chain
a helical conformation

Question 15

how does the cytoskeleton interact with the membrane

Answer 15

separates it into regions

Question 16

function of membrane proteins

Answer 16

transport impermeable substances across the cell membrane
provide hydrophilic channel or actively transport them across
increase membrane fluidity
transmission of signals

Question 17

Simple diffusion

Answer 17

movement of substance from a high concentration to a low concentration down a concentration gradient
no proteins involved
oxygen

Question 18

Facilitated diffusion

Answer 18

ated diffusion 
Movement of molecules from an area of high concentration to low concentration 
passive 
use channel proteins 
ions

Question 19

Active transport

Answer 19

movement of molecules from low concentration to high concentration
up concentration gradient
requires energy
sodium-potassium pump

Question 20

Sodium-potassium pump

Answer 20

3 Na out of cell
2 K into cell
when Na bind - induce conformational change so K can bind and then both are released in their directions.

Question 21

How does protein composition vary

Answer 21

between cell types
between inner and outer leaflet of bilayer
between organelles

Question 22

Protein pores and transport

Answer 22

route for substance to move down conc gradient

eg glucose transporters - extracellular glucose higher than intracellular GLUT1-10

Question 23

What are the types of coupled transporters

Answer 23

symporters - move substances in same direction, sugar and AA bought in with Na+
antiporters - move in opposite direction to Na+ eg H+ for pH regulation

Question 24

Chloride ions and membrane potential

Answer 24

move into cell - down conc grad through chloride channels
excess -ve charge inside cell push chloride ions back out
steady state - ratio depends on existing membrane potential

Question 25

Purpose of Na-K pumps

Answer 25

prevent dissipation of membrane potential

Question 26

Describe the sodium-potassium pump

Answer 26

pump 3 Na out pump 2 K in high K conc inside - balances fixed ions (proteins and lipids) cell use concentration gradients resting potential -70mv 2 polypeptide chains - a and B a chain spreades membrane 10 times - hydrophilic pore which cations can move through B chain - controller electrogenic - creates -ve intracellular potential mediated by conformational transitions of pump driven by phosphorylation of aspartyl residue using ATP followed by hydrolysis of aspartyl phosph`ate

Question 27

Edit Delete | How does the sodium gradient drive transport of sugars

Answer 27

``` Na+ and ATP bind protein - ADP protein conformational change glucose into cell ATP regenerated cell back to normal shape ```

Question 28

consequence of Na-K

Answer 28

ionic gradients | concentration gradient

Question 29

different method for K to enter cell

Answer 29

through K+ leak channels

Question 30

Describe action potentials

Answer 30

membrane disrupted by brief pulse of current open V gated Na+ channels depolarisation +50mv V gated Na+ channel inactivated by v reversal V gated K+ - repolarisation

Question 31

describe the K channel

Answer 31

4 subunits | pore in middle

Question 32

Specific pumps

Answer 32

Na+, Ca2+ and H+, which use ATP hydrolysis to provide the energy. There are also Cl- pumps in some cells. Some pumps can work in reverse and generate ATP from an ion gradient, e.g. the F1-ATPase in the mitochondria using H+ gradient. Other mechanisms exist for other substances that need to cross the membrane.

Question 33

glucose transport

Answer 33

kidney PCT membrane impermeant reabsorbed from PCT to cell up conc grad- bloodstream glucose binds to specific transporter - flip-flop mechanism

Question 34

Pinocytosis

Answer 34

engulfment by the membrane of extracellular solute and small molecules which end up in small intracellular membrane-bound vesicles.

Question 35

Phagocytosis

Answer 35

engulfment by the membrane of extracellular objects such as bacteria, cell debris, other cells, specifically bound to the cell membrane by receptors. Signalling triggers actin cytoskeleton rearrangement. Again these end up in intracellular membrane-bound vesicles.

Question 36

exocytosis

Answer 36

Movement of proteins (e.g. hormones, blood clotting factors) and other molecules from intracellular vesicles into the extracellular space by fusion with the cell membrane.

Question 37

cell signalling

Answer 37

Some signals are lipid-soluble molecules that cross membranes e.g. steroid hormones, prostaglandins, NO (also drugs) Many impermeable signals rely on trans-membrane receptors intracellular second messengers - change shape of protein and are activated - cascade