Cell membranes

Question 1

define cell membranes

Answer 1

boundaries of cells

act as barriers defining the inside and outside of the cell

Question 2

cell membranes prevent

Answer 2

imp molecules leaking out

unwanted molecules diffusing in

Question 3

what do cell membranes contain to allow specific molecules to be taken up or removed

Answer 3

transport systems

Question 4

transport systems confer on membranes

Answer 4

selective permeability (imp membrane property)

Question 5

describe cell membranes

Answer 5

• sheet like (2 molecules thick)
• mainly lipids (form permeability barrier) and proteins (acts as a transport system of pumped channels - specific proteins mediate specific membrane functions)
• fluid structures
• lipids + proteins can diffuse rapidly in plane of the membrane

Question 6

what is the relevance of fatty acids

Answer 6

• hydrocarbon chains
• hydrophobic properties
• can be saturated or unsaturated

Question 7

give an example of

a) saturated fatty acid
b) unsaturated fatty acid

Answer 7

a) palmitate
- 16 carbon
- ionised form of palmitic acid

b) oleate
- 18 carbon
- 1 cis double bond between carbons 9 + 10
- cis-Δ 9octadecanoate

Question 8

what are phospholipids

Answer 8

• major class of lipids (common membrane lipid)
• abundant in all biological membranes
• constructed from:
  fatty acids (provide hydrophobic barrier)
  platform fatty acids = attached to
  a phosphate with an alcohol attached
• non fatty acid components have hydrophilic properties

Question 9

what is the name of phospholipids based on / derived from glycerol
name the simplest one

Answer 9

phosphoglycerides

phosphatidate

Question 10

major phosphoglycerides are derived from

Answer 10

phosphatidates

- ester bond forms between phosphate group of phosphatidate and hydroxyl group of an alcohol

Question 11

list some common alcohol moieties of phosphoglycerides

Answer 11

• amino acid serine
• ethanolamine
• choline
• glycerol
• inositol

Question 12

what is sphingomyelin

Answer 12

• phospholipid in membranes
• not derived from glycerol BUT…
• has sphingosine backbone (amino alcohol w long unsaturated hydrocarbon chain)

Question 13

glycolipids are

Answer 13

• sugar containing lipids
• derived from sphingosine in animal cells
• orientated asymmetrically with sugar residues on extracellular side of membrane

Question 14

cholesterol is

Answer 14

lipid steroid (membrane lipid based on a steroid nucleus)

Question 15

what is the structure of cholesterol

Answer 15

• 4 linked hydrocarbon rings linked to a steroid at one end and hydroxyl group at the other
• oriented parallel to fatty acid chains of phospholipids
• hydroxyl group interacts with nearby phospholipid heads

Question 16

where is cholesterol present / absent

Answer 16

• NOT in prokaryotes
• found in varying degrees in most animal membranes
• almost 25% of membrane lipids in some nerve cells
• almost absent from some intracellular membranes

Question 17

what type of molecules are membrane lipids and what does this mean

Answer 17

AMPHIPATHIC

contain both a hydrophilic and hydrophobic moiety

Question 18

what is the favoured structure of phospholipids and what does this enable

Answer 18

bimolecular sheet (formed in an aqueous environment)

• allows membranes to form due to amphipathic nature as polar heads favour water and hydrocarbon tails interact with each other

Question 19

what are the 3 biological consequences of the many reinforcing, non-covalent, hydrophobic interactions holding lipid bilayers together (makes them cooperative structures)

Answer 19

1) bilayer = extensive
2) lipids tend to close in on themselves so there are no edges with exposed hydrocarbon chains + so form compartments
3) lipid bilayers are self-sealing because a hole in the bilayer would be energetically unfavourable

Question 20

what 2 categories can membrane proteins be split into

Answer 20

1) integral membrane proteins (intrinsic)

2) peripheral (extrinsic)

Question 21

describe integral membrane proteins

Answer 21

• 1+ segments embedded in the bilayer (transverse it - transmembrane proteins)
• interact with bilayers hydrocarbon region

Question 22

describe peripheral proteins

Answer 22

• dont interact with hydrophobic core of bilayer
• bound to membrane indirectly through interactions with integral membrane proteins
• bound directly through interactions with polar head groups of integral lipids

Question 23

what can proteins span the membrane with

Answer 23

alpha helices

by integral proteins containing membrane spanning alpha helical domains (these are the most common structure motifs in membrane proteins)

Question 24

what is bacteriorhodopsin

Answer 24

• consists largely of membrane spanning alpha helices embedded in membrane
• amino acids in these = non-polar

Question 25

what is bacterial porin

Answer 25

- porin = membrane protein built entirely of beta strands that have hydrophobic + hydrophilic amino acids in adjacent positions - each beta strand = H bonded to its neighbour in anti-parallel arrangement forming a single beta sheet - beta sheet = curls up to form hollow cylinder that functions as the active unit - SO form pores / channels in the membrane

Question 26

what does the amino acid sequence of a porin demonstrate

Answer 26

- amino acids are adjacent - diagonal lines indicate direction of H bonding along beta sheet - hydrophobic residues (yellow) lie on outside of structure in contact with membranes hydrophobic core

Question 27

what is prostaglandin H2 synthase-1 and where is it held

Answer 27

- membrane bound enzyme - integral membrane protein BUT NOT a membrane spanning protein - in the membrane by a set of alpha helices coated with hydrophobic amino acid side chains that extend from the bottom of the protein into membrane - not largely embedded in the membrane - lies along membranes outer surface + is bound by the set of alpha helices

Question 28

what is converted to prostaglandin H2 and how

Answer 28

arachidonic acid by prostaglandin H2 synthase-1

Question 29

how can prostaglandin H2 synthase-1 be released from the membrane (strong link)

Answer 29

ONLY by the action of detergents

Question 30

how can we predict transmembrane helices

Answer 30

from amino acid sequences and their free energy changes for transfer of individual amino acid residues from hydrophobic to an aqueous environment ie amino acids showing -ve values = more likely located in an aqueous environment

Question 31

when is free energy change estimated

Answer 31

when a helical segment is transferred from the interior of a membrane to water

Question 32

which membrane part is mostly non-polar residues

Answer 32

residues in alpha helices which span hydrophobic part of the membrane

Question 33

what is glycophorin

Answer 33

membrane spanning helix

Question 34

how can glycophorin be located

Answer 34

create hydropathy plot based on free energy values of transferring amino acid residues from hydrophobic environment to water (predicts single transmembrane domain) - hydrocarbon core of a membrane is typically a length that can be transversed by an alpha helix - free energy change measured when hypothetically alpha helix formed of residues transferred from membrane interior to water

Question 35

what is the essence of the fluid mosaic model

Answer 35

- allows lateral movement but not rotation through the membrane - membranes are 2D solutions of oriented lipids and globular proteins

Question 36

what type of role does the lipid bilayer have, explain this

Answer 36

DUAL its both a) solvent for integral membrane proteins b) permeability barrier

Question 37

which membrane processes a) can be rapid b) are very slow

Answer 37

a) lateral diffusion of membrane components | b) transverse diffusion / flipflop - spontaneous rotation of lipids from one face of a membrane to another

Question 38

what is membrane fluidity controlled by

Answer 38

- fatty acid composition | - cholesterol content (key regulator in animals)

Question 39

how does cholesterol control membrane fluidity

Answer 39

- different shape to phospholipid so disrupts regular interactions between fatty acyl chains - forms complexes which concentrate in specific regions in membranes with some phospholipids - result = moderation of membrane fluidity (less fluid + less subject to phase transitions)

Question 40

which fatty acid properties influence fluidity

Answer 40

- number of double bonds | - length

Question 41

what do cis double bonds do and give an example

Answer 41

- disrupt ordered packing of fatty acid chains - saturated fatty acids pack more tightly as do not have a kink in their structure ie - 3 molecules of stearate (C18, saturated) lie flat - 1 oleate (C18, unsat) molecule between 2 stearate disrupts the layers

Question 42

how are prokaryote cell membranes different

Answer 42

many bacteria have 2 membranes (ie e.coli) | others have 1

Question 43

how can we classify bacteria based on cell membrane number

Answer 43

gram stain 1) 2 membranes = -ve result 2) 1 membrane = +ve result

Question 44

how do animal cells take up cholesterol

Answer 44

receptor mediated endocytosis for cholesterol carrying complex LDL (low density lipoprotein - way that cholesterol is transported in the blood as cholesteryl ester)

Question 45

what are the stages of receptor mediated endocytosis

Answer 45

1) ldl binding = ldl binds to specific ldl receptor 2) internalisation = the complex invaginates to form internal vesicle 3) after seperation from receptor, ldl containing vesicle fuses with lysosome 4) lysosomal hydrolysis = leading to degradation of ldl and cholesterol release

Question 46

where is the reverse process of receptor mediated endocytosis key

Answer 46

key step in release of neurotransmitters from a neuron (fusion of vesicle to a membrane)

Question 47

what is cholesterol taken up by receptor mediated endocytosis used for

Answer 47

to make new membrane (this method provides most of the cholesterol needed for this process)

Question 48

what are some active and passive transport methods

Answer 48

- passive diffusion - small molecules dissolve in lipid bilayer - many proteins require protein transporters - active transport - ATP hydrolysis to pump ions across, creates a concentration gradient - energy for transport can be generated by ion gradients

Question 49

give an example of an enzyme which generates a gradient (active transport)

Answer 49

Na+/K+ ATPase pump hydrolyses ATP providing energy needed for active transport - 3 Na+ OUT - 2 K+ IN - generates a gradient

Question 50

what are the 6 stages of P-type ATPase action

Answer 50

1) binding 2) phosphorylation of ATPase 3) leads to eversion of binding sites 4) release of 2Ca2+ to luminal side of membrane 5) hydrolysis of phosphoasparate 6) eversion - resets enzyme to initial state

Question 51

what do P-type ATPases have the ability to do

Answer 51

transport lipid molecules across membrane by flippase enzymes - maintain membrane asymmetry by 'flipping' phospholipids from outer to inner layer of membrane

Question 52

what is MDR and what is it due to

Answer 52

multidrug resistance protein expression and activity of a membrane protein which acts as an ATP dependent pump extruding small molecules from cells expressing it

Question 53

what do amino acid sequences of MDR and related proteins (ie CFTR) reveal

Answer 53

- each comprises 2 membrane binding domains AND 2 ATP binding domains (ATP-binding cassettes) - MDR, CFTR + ABC transporters are single polypeptide chains

Question 54

what is the action of histidine permease

Answer 54

hydrolyses ATP which drives transport of histidine into the cell

Question 55

what do secondary transporters do, give 2 examples

Answer 55

use one concentration gradient to power the formation of another 1) antiporters 2) symporters

Question 56

what happens in antiporters and give an example of one

Answer 56

2 species flow in opposite directions ie Na/Ca exchanger uses electrochemical gradient of Na to pump Ca out

Question 57

what happens in symporters and give an example of one

Answer 57

2 species move in same direction ie glucose symporter uses Na entry to power glucose entry to cells

Question 58

how does the Na+/K+ pump use action of a secondary transporter

Answer 58

- converts free energy of phosphoryl transfer into free energy of Na+ gradient (energy transduction) - Ion gradient used to pump materials into cell through secondary transporter (eg sodium glucose symporter)