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IOE > Cell membranes > Flashcards

Cell membranes Flashcards

1
Q

define cell membranes

A

boundaries of cells

act as barriers defining the inside and outside of the cell

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2
Q

cell membranes prevent

A

imp molecules leaking out

unwanted molecules diffusing in

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3
Q

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

A

transport systems

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4
Q

transport systems confer on membranes

A

selective permeability (imp membrane property)

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5
Q

describe cell membranes

A
  • 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
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6
Q

what is the relevance of fatty acids

A
  • hydrocarbon chains
  • hydrophobic properties
  • can be saturated or unsaturated
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7
Q

give an example of

a) saturated fatty acid
b) unsaturated fatty acid

A

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

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

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8
Q

what are phospholipids

A
  • 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
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9
Q

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

A

phosphoglycerides

phosphatidate

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10
Q

major phosphoglycerides are derived from

A

phosphatidates

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

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11
Q

list some common alcohol moieties of phosphoglycerides

A
  • amino acid serine
  • ethanolamine
  • choline
  • glycerol
  • inositol
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12
Q

what is sphingomyelin

A
  • phospholipid in membranes
  • not derived from glycerol BUT…
  • has sphingosine backbone (amino alcohol w long unsaturated hydrocarbon chain)
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13
Q

glycolipids are

A
  • sugar containing lipids
  • derived from sphingosine in animal cells
  • orientated asymmetrically with sugar residues on extracellular side of membrane
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14
Q

cholesterol is

A

lipid steroid (membrane lipid based on a steroid nucleus)

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15
Q

what is the structure of cholesterol

A
  • 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
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16
Q

where is cholesterol present / absent

A
  • 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
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17
Q

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

A

AMPHIPATHIC

contain both a hydrophilic and hydrophobic moiety

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18
Q

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

A

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
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19
Q

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

A

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

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20
Q

what 2 categories can membrane proteins be split into

A

1) integral membrane proteins (intrinsic)

2) peripheral (extrinsic)

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21
Q

describe integral membrane proteins

A
  • 1+ segments embedded in the bilayer (transverse it - transmembrane proteins)
  • interact with bilayers hydrocarbon region
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22
Q

describe peripheral proteins

A
  • 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
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23
Q

what can proteins span the membrane with

A

alpha helices

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

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24
Q

what is bacteriorhodopsin

A
  • consists largely of membrane spanning alpha helices embedded in membrane
  • amino acids in these = non-polar
25
Q

what is bacterial porin

A
  • 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
26
Q

what does the amino acid sequence of a porin demonstrate

A
  • 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
27
Q

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

A
  • 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
28
Q

what is converted to prostaglandin H2 and how

A

arachidonic acid

by prostaglandin H2 synthase-1

29
Q

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

A

ONLY by the action of detergents

30
Q

how can we predict transmembrane helices

A

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

31
Q

when is free energy change estimated

A

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

32
Q

which membrane part is mostly non-polar residues

A

residues in alpha helices which span hydrophobic part of the membrane

33
Q

what is glycophorin

A

membrane spanning helix

34
Q

how can glycophorin be located

A

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
35
Q

what is the essence of the fluid mosaic model

A
  • allows lateral movement but not rotation through the membrane
  • membranes are 2D solutions of oriented lipids and globular proteins
36
Q

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

A

DUAL

its both

a) solvent for integral membrane proteins
b) permeability barrier

37
Q

which membrane processes

a) can be rapid
b) are very slow

A

a) lateral diffusion of membrane components

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

38
Q

what is membrane fluidity controlled by

A
  • fatty acid composition

- cholesterol content (key regulator in animals)

39
Q

how does cholesterol control membrane fluidity

A
  • 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)
40
Q

which fatty acid properties influence fluidity

A
  • number of double bonds

- length

41
Q

what do cis double bonds do and give an example

A
  • 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
42
Q

how are prokaryote cell membranes different

A

many bacteria have 2 membranes (ie e.coli)

others have 1

43
Q

how can we classify bacteria based on cell membrane number

A

gram stain

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

44
Q

how do animal cells take up cholesterol

A

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

45
Q

what are the stages of receptor mediated endocytosis

A

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

46
Q

where is the reverse process of receptor mediated endocytosis key

A

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

47
Q

what is cholesterol taken up by receptor mediated endocytosis used for

A

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

48
Q

what are some active and passive transport methods

A
  • 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
49
Q

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

A

Na+/K+ ATPase pump

hydrolyses ATP providing energy needed for active transport

  • 3 Na+ OUT
  • 2 K+ IN
  • generates a gradient
50
Q

what are the 6 stages of P-type ATPase action

A

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

51
Q

what do P-type ATPases have the ability to do

A

transport lipid molecules across membrane

by flippase enzymes - maintain membrane asymmetry by ‘flipping’ phospholipids from outer to inner layer of membrane

52
Q

what is MDR and what is it due to

A

multidrug resistance protein

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

53
Q

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

A
  • each comprises 2 membrane binding domains AND 2 ATP binding domains (ATP-binding cassettes)
  • MDR, CFTR + ABC transporters are single polypeptide chains
54
Q

what is the action of histidine permease

A

hydrolyses ATP which drives transport of histidine into the cell

55
Q

what do secondary transporters do, give 2 examples

A

use one concentration gradient to power the formation of another

1) antiporters
2) symporters

56
Q

what happens in antiporters and give an example of one

A

2 species flow in opposite directions

ie Na/Ca exchanger uses electrochemical gradient of Na to pump Ca out

57
Q

what happens in symporters and give an example of one

A

2 species move in same direction

ie glucose symporter uses Na entry to power glucose entry to cells

58
Q

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

A
  • 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)

IOE (22 decks)

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