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Foundations of Medical Science > Cell Biology and Signalling > Flashcards

Cell Biology and Signalling Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

what is a cell?

A

a semi-independent living unit within the body, sites the mechanisms for metabolism, growth and replication

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2
Q

what is an organelle?

A

subunit within a cell with a defined structure and performing specific, integrated activities. different functions can operate under different conditions

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3
Q

what is a tissue

A

organised assembly of cells which carry out coordinated activities within the body

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4
Q

what is an organ?

A

assembly of tissues coordinated to perform specific functions within the body

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5
Q

what is a system?

A

assembly of organs with specific activities sharing regulatory infuences

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6
Q

what is a prokaryote?

A

single celled organism, chromosome circular and free, no membranous organelles

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7
Q

what is a eukaryote?

A

chromosomes enclosed in a nucleus, linear DNA, membrane bound organelles, all complex organisms

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8
Q

what is a virus?

A

an assemblage of nucleic acid (DNA or RNA) and proteins. invade cells, subvert their protein synthesis machinery to make more viruses

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9
Q

genetic material (prokaryote vs eukaryote)

A 
P:
chromosomes - single circular
location - nuclear region
nucleolus - absent
histones - absent
extrachromosomal DNA - in plasmids
ribosomes - 70S
cell division - binary fission
E:
chromosomes - paired linear
location - membrane-bound nucleus
nucleolus - present
histones - present
extrachromosomal DNA - in mitochondria
ribosomes - 80S cytoplasmic / 70S mitchondrial
cell division - mitosis or meiosis
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10
Q

intracellular structure (prokaryote vs eukaryote)

A 
P:
mitotic spindle - absent
sterols in plasma membrane - absent
internal membranes - only for photosynthetic organisms
endoplasmic reticulum - absent
mitochondria - absent
lysosomes - absent
Golgi - absent
peroxisomes - absent
cytoskeleton - absent
cell wall - present
E:
mitotic spindle - present
sterols in plasma membrane - present
internal membranes - numerous membrane bound organelles
endoplasmic reticulum - present
mitochondria - present
lysosomes - present
Golgi - present
peroxisomes - present
cytoskeleton - present
cell wall - absent (apart from some fungi)
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11
Q

microscopes (SEM vs TEM)

A 
SEM:
cell surface shown
electrons scattered off cell surface by heavy metal
TEM:
looks inside the cell

BOTH:
elaborate prep involved
can only use dead cells

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12
Q

what limits max size of the cell?

A

diffusion at distance less than 50um is efficient, needs efficient SA:V (bigger cells less efficient)

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13
Q

how do specialised cells overcome the max size limitation?

A

thin processes - directed transport of substances around cell via cytoskeleton

“giant” multinucleate cells - gene expression can occur in more than one place

gap junctions - channels between cells allow movt of substances between cells

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14
Q

nucleus

A

largest organelle (3-10um)

only organelle clearly visible by light microscopy

contains genetic material:

  • DNA organised as chromosomes; chromatin = complex of DNA/histone and non-histone proteins
  • DNA winds round histones into nucleosomes
  • unless cell is dividing chromatin is decondensed

nucleolus - where rDNA is transcribed and ribosome subunits assembled
nuclear envelope - surrounded by two layers of membrane
nuclear pores - allows transport in and out

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15
Q

SER and RER

A

SER:
biosynthesis of membrane lipids and steroids, starts of N-linked glycosylation, detoxification of xenobiotics

RER:
coated with ribosomes (translation, proteins for secretion or insertion into cell membrane), proteins are folded (cya-cys bridges form), vesicles budded from RER and transported to the Golgi

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16
Q

Golgi complex / body / apparatus

A
  • 4-8 closely stacked membrane bound channels (cisterna)
  • modifies proteins delivered from RER via vesicles (modifying N-linked carbohydrates, glycosylation of O-linked carbs and lipids)
  • synthesise/package materials to be secreted
  • direct new proteins in vesicles to their correct compartments
    transport membrane lipids around cell
  • creates lysosomes
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17
Q

secretory vesicles

A
  • vesicles bud off from the Golgi

- vesicles fuse with the inner surface of the plasma membrane and release their contents (exocytosis)

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18
Q

peroxisomes

A

Contain enzymes for breaking down toxic materials, also involved in phospholipid synthesis, oxidation of very long chain fatty acids

  • enzymes which generate H202
  • Zellweger syndrome
  • adrenoleukodystrophy (ALD)
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19
Q

lysosomes

A
  • electron dense spheres in EM
  • membrane-bound
  • 50 different hydrolytic enzymes
  • all require low pH
  • involved in organelle turnover/replacement (autophagy)
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20
Q

mitochondria

A
  • 2 layers of membrane
  • number per cell reflects metabolic activity
  • contain DNA (encode some of their proteins - own genome)
  • sugars oxidised (generate ATP krebs)
  • inner membrane in folds (cristae inc SA)
  • Krebs cycle enzymes/electron transport chain are located in diff parts of structure
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21
Q

peptide bond between AA’s

A 
formed by enzyme reaction
strong
carboxyl and amino group
hydrolysis (h20 removed) to give CN link
only happens under digestion and lysosome
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22
Q

peptide bond features (like double bond)

A 
C-N bond short
no rotation
-ve charge on O
\+ve charge on N
peptides can form H-bonds with other polar groups in polypeptide chain
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23
Q

direction of polypeptides

A

first AA has NH3+ group

last AA has COO- group

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24
Q

other covalent linkages (apart from peptide bonds)

A

disulphide S-S bridges between two cys
(intrachain and interchain)

glycosylation:
O-linked -OH of thr and ser
N-linked -NH2 of asn

modifying structure changes function:

phosphorylation (+/- phosphate group):

eg. cell signal transduction
eg. change in activity of enzyme

methylation (+/- methyl groups) via -NH2 groups of lys and arg:
eg. histones affect gene expression

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25
Q

how is the alpha helix formed?

A

formed by H-bonds in same polypeptide chain

particularly H-bonds between peptide bond carbonyl-O and H of N-H every 4th peptide

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26
Q

alpha helix features

A

3.6 AA residues

R groups on outside

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27
Q

what bonds are in beta pleated sheets

A

linear peptide chains

H-bonding between peptide chains

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28
Q

collagen triple helix features:

  • where are H-bonds
  • how many residues
  • common repeating primary sequence
A
  • H-bonds between chains
  • 3 residues

Gly-X-Y-Gly-X-Y

X=mainly proline
Y=mainly hydroxy-proline

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29
Q

define tertiary structure

A

how the whole polypeptide is folded in 3D, will consist of a number of diff super secondary structures (domains)

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30
Q

define quaternary structure

A

how the whole functional protein is formed in 3D, may consist of a number of subunits (eg. haemoglobin)

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31
Q

forces that stabilise protein structure (2)

A

covalent:
- disulphide bridges

non-covalent:

  • H bonds
  • electrostatic interactions
  • VdeW forces
  • hydrophobic effect
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32
Q

electrostatic interactions and 2 examples

A

between charged side chains

Asp and Glu carboxyl groups are ionised
-COO-

Lys and Arg amino groups are ionised
-NH3+

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33
Q

define van de waal forces

A

sum of the attractive or repulsive forces between molecules
(excluding those due to covalent, hydrogen, electrostatic)

dependent on dipole affect caused by unequal distributions of electrons

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34
Q

hydrophobic regions are _____ to form hydrogen bonds

A

unable

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35
Q

proteins are sensitive to denaturation by

A
  • pH
  • temp
  • ionic strength
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36
Q

Creutzfeldt-Jakob disease symptoms

aggregation of misfolded proteins

A

neurological symptoms:

  • difficulties with walking
  • slurred speech
  • numbness
  • dizziness

psychological symptoms:

  • severe depression
  • withdrawal
  • anxiety
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37
Q

3 key features of enzymes

A
  • speed (rate enhancement by 10^6-10^14)
  • selectivity (Some will only act on one type of substrate)
  • specificity (eg. Will only add glucose onto 2 position of another glucose not 3,4 or 6 positions
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38
Q

classification of enzymes (6)*

A
  • oxidoreductases
    Lactate Dehydrogenase
    (add O2 or remove 2H)
  • transferases
    Alanine aminotransferase
    (catalyse transfer of functional groups from donors to acceptors)
  • hydrolases
    Trypsin
    (catalyse cleavage of bonds by addition of water, hydrolysis)
  • lyases
    ATP-citrate lyase
    (catalyse cleavage of C-C, C-O or C-N, form double bonds by removal of groups)
  • isomerases
    Phosphoglucose isomerase
    (catalyse the transfer of functional groups within the same molecule, isomerisation reactions)
  • ligases
    DNA ligase
    (use ATP to catalyse the formation of new covalent bonds)

classes are divided in subgroups according to their substrate and source

eg. alcohol dehydrogenase

IUB name, alcohol, substrate, reaction type followed by ace, IUB number, E.C.1.1.1.1

6 classes further divided into subgroups according to substrate or source, each enzyme is identified by its own 4 digit number
(eg. catalase is E.C.1.11.1.6)

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39
Q

what does the induced fit theory imply?

A

enzymes undergo conformational changes upon substage binding

these changes ca affect residues in AS as well as repositioning entire domains

it serves to bring specific functional group within enzyme in the proper position to catalyse reaction

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40
Q

catalysis of peptide bond hydrolysis by chymotrypsin

A
  • polypeptide substrate binds to hydrophobic pocket
  • H+ is transferred from Ser to His, substrate forms tetrahedral transition state with enzyme
  • H+ transferred to C-terminal fragment, which is released by cleavage of the C-N bond. The N-terminal peptide is bound through acyl linkage to Ser
  • water molecule binds to enzyme in place of departed polypeptide
  • water molecule transfers its proton to His and its -OH to the remaining substrate fragment. tetrahedral transition state formed
  • the second peptide fragment is released: acyl bond cleaved, proton transferred from His back to Ser, enzyme returns to initial state
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41
Q

enzyme with low substrate specificity

A

Papain

a cysteine protease from papaya
used as a meat tenderiser

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42
Q

other ways of plotting enzyme rate vs [S]

A

Lineweaver-Burk (double reciprocal) plot
1/v against 1/[S]
Intercepts: 1/Vmax and -1/Km

Eadie-Hofstee plot
v/[S] against v
intercepts: Vmax and Vmax/Km

Hanes plot
[S]/v against [S]
Intercepts: Km/Vmax and -Km

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43
Q

relationship between Km and E-S affinity

A

lower the value of Km the higher the affinity of a particular substrate for the enzyme that catalyses it

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44
Q

Types of E-S inhibition

A

Competitive:
Bind directly to the AS of an enzyme, competing with substrate
Increases Km but does not affect Vmax

Non-competitive:
Binds to enzyme away from AS, alters shape of enzyme so even if substrate can bind, the AS functions less effectively
Reduces Vmax but does not affect Km

Uncompetitive:
Only bind once the E-S complex has formed. The E-S-I complex does not produce any product
Reduces both Km and Vmax

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45
Q

Example of competitive inhibition

A

Succinate dehydrogenase:

  • oxidation of succinate to fumarate
  • inhibited reversibly by malonate (resembles substrate, can’t be oxidised)

Increases Km
Doesn’t affect Vmax

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46
Q

Example of non-competitive inhibition

A

Fluoride inhibition of enolase

  • key enzyme of glycolysis
  • forms PEP
    F- is a non-competitive inhibitor
    Fluoride ions replace oxygen s of carboxylate of PEP

Reduces Vmax
Doesn’t affect Km

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47
Q

Example of uncompetitive inhibition

A

Examples involved with certain types of cancer

A number of genes that code for TSAPs are inhibited uncompetitively by amino acids such as leucine and phenylalanine

Reduces both Km and Vmax

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48
Q

What is allosteric regulation

A

A form of regulation where the regulatory molecule (an activator or inhibitor) binds to an enzyme someplace other than the AS

All cases of non-competitive inhibition are forms of allosteric regulation

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49
Q

Features of allosteric enzymes

A

Multiple activate sites located on different protein subunits

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50
Q

Allosteric inhibitor
Allosteric activators
Cooperatively

A

Allosteric inhibitors:
Bind to enzyme away from AS, all AS’s on protein subunits are changed so they work less well
T-state

Allosteric activators:
Bind to locations on an enzyme other than AS causing inc in function of the active site
R-state

Cooperativity:
Substrate itself serves as an allosteric activator: binds to one AS, the activity of the other AS’s goes up

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51
Q

Feedback inhibition (cycle)

A

In metabolic pathways an end product of a chain of enzymatic reactions can act as an allosteric inhibitor using a feedback mechanism

  • substrate binds to enzyme
  • intermediate substrate
  • this binds to another enzyme, producing another intermediate
  • finally producing end product
  • if want to inhibit the formation of this final product, the product can bind away from active site on the first enzyme
  • this depresses the formation of the end product
52
Q

What does an allosteric graph look like?

Reaction velocity against substrate conc

A

Sigmoidal

Can be considered to be a result of combing two Michaelis-Menten enzymes
One with high Km value (T-state)
One with low Km value (R-state)

The sigmoidal response is retained but shifted by regulators

53
Q

Example of allosteric enzyme

A

ATCase
Aspartate transcarbamylase

Catalyses the condensation of aspartate and carbamoyl phosphate to form N-carbamoylaspartate, in pyrimidine synthesis
(CTP end product)

Enzymes exists in T state (favoured by CTP binding) and R state (favoured by substrate binding)

In absense of substrate/regulators ATCase exists in equilibrium between 2 states, where T state is favoured

54
Q

amino acid structure

A

central carbon with amino group, carboxylic group, and a R group attached

20 universal amino acids

55
Q

essential amino acids

A

de novo
cannot be synthesised by humans
need to be obtained nutritionally

56
Q

the 20 coded amino acids

and their side chain charges

A 
aspartic acid - negative (carboxylic)
glutamic acid - negative (carboxylic)
arginine - positive (amino)
lysine - positive (amino)
histidine - positive (complex ring amino)
asparagine - uncharged polar (amide)
glutamine - uncharged polar (amide)
serine - uncharged polar (hydroxyl)
threonine - uncharged polar (hydroxyl)
tyrosine - uncharged polar (benzene uncharged)
alanine - non polar (aliphatic)
glycine - non polar (none)
valine - non polar (aliphatic)
leucine - non polar (aliphatic)
isoleucine - non polar (aliphatic)
proline - non polar (ring structure including alpha amino)
phenylalanine - non polar (benzene ring)
methionine - non polar (sulphur)
tryptophan - non polar (ring structure)
cysteine - non polar (thiol)
57
Q

not obvious 3 letter codes (3)

A

Gln - glutamine
Asn - asparagine
Ile - isoleucine

58
Q

not obvious single letter amino acid codes

A 
W - tryptophan
E - glutamate
D - aspartate
K - lysine
Q - glutamine
N - asparagine
59
Q

other roles of amino acids

A

taste enhancement
- MSG (monosodium glutamate)

neurotransmitters
- glutamate

synthesis of neurotransmitters

  • eg. tyrosine to dopamine
  • eg. tryptophan to serotonin

synthesis of hormones
- eg. tyrosine to melanin

60
Q

what does acidity depend on?

A

depends only on free hydrogen ions

not those still bound to anions

61
Q

where do acids in the body come from?

A
  • breakdown of proteins
  • incomplete oxidation of fats or glucose
  • loading and transport of carbon dioxide in the blood
62
Q

how is acid-base balance regulated in the body?

A
  • lungs
  • kidneys
  • chemical buffers
63
Q

what is blood pH

A

7.4

64
Q

what is pKa?

A

the pH at which the acid is half dissociated

the lower the pKa, the stronger the acid

65
Q

what are buffers mixtures of?

A

weak acids and their conjugate bases

66
Q

why is the pKa the best buffering point

A

if H+ is added they can be picked up by the conjugate base

if OH- is added the acid can donate a proton to form H2O

67
Q

examples of:

metabolic acidosis

respiratory acidosis

A

conjugate base (bicarbonate) is low, probs diabetic

the acid (carbonic acid) is high, carbon dioxide partial pressure is proportional to carbonic acid conc so can monitor

68
Q

why is glycine not a good physiological buffer?

A

it has pKa points at 2.3 and 9.6 which means it buffers best at non-useful pH’s

the alpha carboxyl and alpha amino groups are involved in the peptide bond so wouldn’t be able to dissociate anyway

69
Q

best amino acid as a physiological buffer

A

histidine
pKa of 6

most amino acids do not buffer in the physiological range

70
Q

what makes haemoglobin a good blood buffer

A

has a large number of histidine residues

in haemoglobin, the pKa of histidine is different from that of free histidine (which is 6), neighbouring groups affect the pH

oxyhaemoglobin pKa = 6.8
deoxyhaemoglobin pKa = 7.8

71
Q

What are the 3 key residues in the catalytic triad

A

Ser
His
Asp

72
Q

What is albinism caused by

A

Defective tyrosinase

Affects the synthesis of melanin from Tyrosine

73
Q

What is a cytoskeleton (give 3 components)

A

Protein filaments

Actin = thinnest (muscle)
Microtubules = thickest (pull daughter cells apart)
Intermediate filaments = mechanical strength of cell

74
Q

What’s another name for lipids

A

Triacylglycerols

75
Q

Two types of lipids in membranes and their structures

A

Phospholipids

  • glycerol & 2 FA’s & phosphate containing group
  • composed of a polar head group attached to glycerol backbone through a phosphate group (hydrophilic)
  • FA’s linked time glycerol via ester bonds (hydrophobic)

Glycolipids
- glycerol & 2 FA’s & sugars

76
Q

Define amphipathic

A

Polar head group and non-polar FA tail (polar intact with aqueous environment)

77
Q

Common head groups found in phospholipids (4)

Learn their struct?

A
  • choline (involved in signalling)
  • serine
  • ethanolamine
  • inositol (involved in signalling)
78
Q

What is ceramide composed of?

A

Sphingosine and fatty acyl chain together

79
Q

Variation in composition of cellular membranes

Plasma membrane

Outer mitochondrial membrane

Inner mitochondrial membrane

Nuclear membrane

A

Only plasma membrane contains carbohydrates

Higher amount of protein in the inner mitochondria membrane and nuclear membrane %

All are different in lipid composition %

Plasma membrane has far more cholesterol %

80
Q

Composition of the 2 halves of the bilayer

A

External side:

  • glycolipid
  • PC and SPH

Internal side:
- PS and PE

81
Q

Regulation of fluidity of the bilayer

A

Increase fluidity:

  • inc in short chain fatty acids reduce VdeW interactions between so inc fluidity
  • kinks in unsaturated fatty acids reduce VdeW with other lipids so inc fluidity

Decrease fluidity:
- high cholesterol content restricts random movt of polar heads

82
Q

Composition of lipid rafts

A

Specialised membrane

Less fluid

Inc level of cholesterol and sphingomyelin

83
Q

Membrane protein classes:

Integral (intrinsic) proteins
Anchored proteins
Peripheral (extrinsic) proteins

A

Integral
- embedded in lipid bilayer

Anchored

  • have covalent bonds with fatty acids from phospholipids
  • example is RAS a signalling G-protein

Peripheral

  • attach to membrane surface by ionic interactions with integral proteins or phospholipid heads
  • example is Spectrin (structural protein on erythrocytes that interact with other proteins such as ankyrin)
84
Q

Cation vs anion

A

Cations carry one or more positive charges.

Anions carry one or more negative charges.

85
Q

pKa values for amino acids

A

Alpha carboxyl group:
~2.5

Alpha amino group:
~9.6

86
Q

Equivalence point

A

When an equivalent number of MOLES of base has been added to the weak acid

87
Q

The Bohr effect

A

An increase in pH (and decrease in CO2 conc) increases Hb’s affinity for O2

88
Q

BPG effect

A

Whole blood must contain something that lowers its affinity for O2 compared to pure Hb

89
Q

Globin chains in adult Hb and fetal Hb

A

HbA:
Alpha 2 beta 2

HbF:
Alpha 2 gamma 2
(Practically none left after 3months)

90
Q

Ferrous vs ferric iron

A 
Ferrous = Fe2+
Ferric = Fe3+ (rust/cannot react with O2)
91
Q

Deoxygenated structure vs oxygenated Hb

A

When no oxygen:
Iron centre is shifted 0.4armstrong from the plane of the Haem

When oxygen binds, iron centre moves into plane of the Haem (due to partial change in electron distribution, Fe2+ temp goes Fe3+)

Allosteric transition from a low O2 affinity state (T) to a high O2 affinity state (R)

Distal histidine stabilises other side of oxygen

92
Q

Molecular equations in respiring tissues and in the lungs

A

In respiring tissues:

CO2 enters erythrocyte
CO2 + H2O -> HCO3- + H+ + Cl-
Bicarbonate dissolves in blood plasma whilst Cl- enters erythrocyte
H+ produced decreases pH so less affinity for O2

In lungs = opposite equation and CO2 leaves erythrocyte and is exhaled

93
Q

Molecular cause of sickle cell anaemia

A

Genetic disease resulting from a mutation that converts Glu (acidic & hydrophilic) in the beta chains to Val (non-polar & hydrophobic)

Oxygenated molecules are soluble but upon deoxygenation, conformation of HbS differs from HbA and it aggregates into insoluble fibers -> sickle shaped cell

94
Q

Protein signalling at ER (example)

A

Newly synthesised protein at the ribosome

Signal sequence on growing polypeptide chain is recognised by SRP (signal recognising protein)

SRP binds to receptor in the ER membrane

SRP displaced (& recycled), newly synthesised polypeptide guided through translocation channel on membrane and into ER lumen

95
Q

When does the protein remain in the ER membrane and not fully through?

A

When the protein has an additional stop sequence it stops the process so protein is embedded within membrane

96
Q

Newly translated protein targeted to the mitochondria

A

Protein completed and released by ribosome into cytoplasm

Chaperone (HSP70) takes protein to mitochondria

Signal sequence on protein, binds to receptor on mitochondrial membrane, protein guided through translocator contact site and into mitochondrial matrix

Signal sequence cleaved off protein

Protein is folded into mature protein

97
Q

Targeting proteins to nucleus

A

Newly translated protein is fully folded and released into cytoplasm

Nuclear proteins contain NLS (nuclear localisation signal) which has lots of basic AA’s

NLS binds to importin and this complex is transported through nuclear pore

(Exportin performs reverse function)

98
Q

Targeting proteins to lysosome

A

Lysosomal protein tagged with sugar in Golgi
(Mannose-6-phosphate)

Sugar receptor in golgi directs proteins into transport vesicles

Vesicle becomes the lysosome

99
Q

Composition of cytoskeleton

Smallest to largest

A

Actin microfilaments:

  • monomer is globular protein G-actin
  • dynamic
  • 2 tightly wound chains (polymerised)
  • eg. In microvilli = mechanical support

Intermediate filaments:

  • different IF proteins in diff cell types
  • epithelia = keratin
  • axons = neurofilamin
  • universal (nucleus) = lamin A, B, C
  • structure process: monomer-dimers-tetramers-link up end to end

Microtubules (polymers of tubulin):

  • tubulin monomer is heterodimer: alpha&beta
  • 13 parallel protofilaments arranged in hollow tube
  • dynamic scaffold (chromatid separation)
  • movt of cargo within cells
100
Q

Lamellapodia

A

Extensions of cells containing actin network
(Generated by rapid actin growth in cell membrane)

Contraction involving myosin allows cell movt

101
Q

Spindle formation

A

Spindle formation initiated from centrosome
(type of microtubule organising centre)

Centrosomes contain centrioles
(a form of stable microtubules)

Centrosomes form 2 poles of cells

Kinetochore MT attach to centromere of chromatid

Aster MT attach centromere to cell membrane

102
Q

Movement of cargo within cells

A

2 motor proteins asso with microtubules

ATP hydrolysed to move cargo along microtubule

Kinesin moves to + end (cell periphery)

Dynein moves to - end (near nucleus)

103
Q

Cilia

A

Microtubules central support

MTOC called Basal body close to membrane

MT’s move components up and down cilia

104
Q

Primary vs motile cilium

A

In cross section

Primary have no central doublet (9+0)

Motile have a central doublet
(9+2)
Contains additional dyenin component that provides the ATP synthesis for movement of cilia

105
Q

Selective permeability

A

Block almost all hydrophilic molecules
(Charged polar molecules needed specialist proteins)

Small, uncharged or hydrophobic can freely cross

106
Q

Passive transport

A

Rate of diffusion depends on partition coefficient of solute

Solutes that are more hydrophobic have higher partition coefficient and equilibrate more quickly

107
Q

Facilitate diffusion of glucose

A

GLUT1

  • most cells
  • high affinity

GLUT2

  • liver, pancreatic cells
  • low affinity
  • never fully saturated so will work at all glucose concs

GLUT3

  • neurones
  • low Km

GLUT4

  • muscle, adipocytes
  • regulated by insulin
  • (muscle glucose to glycogen) (adipocyte glucose to fatty acids)
108
Q

Insulin stimulated uptake of glucose

A

I stimulates uptake of glucose in muscle and adipose

I inc the amount of GLUT4 in plasma membrane (via vesicles)

Inc uptake of glucose into cell

(When decrease of glucose, transporters move into intra-cellular storage pool - endosome)

109
Q

Gated ion channels

A

Open or close in response to stimulus
Either:

Ligand-gated:
(Acetylcholine on Na+/K+ channel on post synaptic membranes)

Voltage-gated:
(Na+ and K+ channels in axons)

110
Q

Na+/K+ pump

Primary active transport

A

Na+/K+ pump consists of tetramer

Na+ enters open cytoplasmic access

Phosphorylation from ATP at cytoplasmic site causes conformational change (closes cytoplasmic access)

Conformational change means pump binds K+ and releases Na+ outside cell

Hydrolysis of phosphate group closes external access, opens cytoplasmic, releases K+

111
Q

Secondary active transport

A

Pre-establishes gradient is used to drive transport of solute across membrane against gradient

ATP hydrolysis used to establish primary gradient

Example:
Na+-glucose cotransporter
Symport
(Na+/K+ used before to set up gradient)

112
Q

Cholera treatment

A

Cholera toxin stimulates inc in cAMP level that activates CFTR and secretion of chloride ions out of cell

Na+ and water follow via osmosis

Oral rehydration therapy includes high glucose conc which drives Na+ (and therefore Cl- and water) uptake into the cells via SGLUT

113
Q

Secretion of insulin by beta-cells

A

Glucose transported into beta cells by facilitated diffusion by GLUT2

Glucose is metabolised increasing ATP level

ATP/ADP ratio closes ATP-sensitive K+ channels, leading to cell membrane depolarisation

Voltage-gated Ca2+ channels open, intracellular Ca2+ increases

Inc in Ca2+ triggers executors is of insulin in vesicles

114
Q

Different ways for cells to signal to each other

A

Endocrine:
Signal produced by cells in one part of body travels in blood to target cells elsewhere

Autocrine:
Signal acts on same cell that produced it

Paracrine:
Signal produced by cell and act on other cells that are close

Contact dependent:
Signal is integral part of one cell and interacts directly with another cell

Neuronal:
Electrical signal transmitted down cell and message to another via synapse

115
Q

Location of receptor

In terms of hormones

A

Cell surface receptor
- hormone is hydrophilic (adrenaline) so doesn’t enter

Intracellular receptor

  • hormone is hydrophobic
  • binds to receptor in cytosol
116
Q

2nd messengers generated by enzymes

A

G protein coupled receptors
(GPCR)
(Transmembrane alpha helix)

  • activation of adenylyl cyclase (forms cAMP)
  • activation of phospholipase C
    (forms IP3, DAG)
117
Q

G-proteins

A

Heterotrimeric complex
(Alpha, beta, gamma)

Dissociates when GTP binds

Free active G-alpha activates effector enzymes (leads to production of secondary messenger)

Complex re-associates when GTP hydrolysed to GDP

118
Q

Signal amplification via kinase cascade

A

Glucagon -> glucagon receptor

ATP -> cAMP

inactive PKA -> active PKA

inactive phosphorylase kinase -> active phosphorylase kinase

Inactive phosphorylase -> active phosphorylase

Glycogen -> glucose-1-phosphate

119
Q

cAMP dependent protein kinase A (PKA)

A

Tetrameric enzyme, 2 regulatory (R) and 2 catalytic (C) subunits

cAMP binds to R subunit and tetramer dissociates

C monomers now active enzymes (PKA)

120
Q

IP3 / DAG

Secondary messengers

A

Some GPCR contain G(alpha)q subunit

Dissociated Gq activates phospholipase C

Phospholipase C cleaves inositol phospholipids in membrane (-> DAG & IP3)

IP3 = activates Ca2+ channel in ER (inc conc in cytosol)
DAG = together with Ca2+ activates protein kinase C
121
Q

Types of signalling

After binding of signal to receptor

A

Depolarisation of membrane due to flow of ions
(Acetylcholine)

Direction activation of transcription factor
(Steroid)
(Steroid into cell, binds to receptor, complex binds to specific DNA)

Generation of secondary message inside cell
(Glucagon - cAMP)

Direct activation of enzymatic kinase cascade
(EGF - MAP kinase pathway)

122
Q

Hyperpolarisation vs depolarisation

A

Hyperpolarisation:
Membrane potential becomes more negative

Depolarisation:
Membrane potential becomes less negative (more positive)
(Opening of a channel that lets positive ions flow into the cell cause depolarisation)

123
Q

Direct activation of transcription factors

A

Binding of steroid hormones to receptor (in cytoplasm) induces conformational change that allows DNA binding and activation of transcription of target genes

Steroids are ligand-dependent transcription factors

124
Q

GCPR signalling to effector enzymes

Glycogen breakdown

A

Signal binds to receptor

GTP/GDP exchange on G protein (GTP bound)

GTPalpha activates effector enzyme (cyclase)

Cyclase produces cAMP (2nd messenger)

cAMP binds to R subunit
(on R2C2, PKA enzyme)

C monomers now active enzymes

(Refer to kinase cascade for the rest of breakdown)

125
Q

cAMP on gene transcription

A

cAMP activates PKA

PKA phosphorylates CREB
(cAMP response element binding protein)

CREB binds to specific sequences in target genes & stimulates transcription

126
Q

EGF (no secondary messenger)

A

Binding of EGF triggers autophosphorylation of tyrosine residues -> receptor tyrosine kinase (RTK)

Adaptor proteins contain phosphotyrosine binding domains (SH2, PTB)

Adaptor proteins Grb2 and Sos bind to receptor

This activates the exchange:
GDP-Ras -> GTP-Ras (Ras is a G protein)

GTP-Ras triggers kinase cascade:
(Ras-MAPkinase pathway)
MAPKKK activates MAPKK activates MAPK activates transcription factor

Foundations of Medical Science (2 decks)

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