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BIOL 130 > Unit 4 > Flashcards

Unit 4 Flashcards

1
Q

Membrane functions

A
  • Compartmentalization (euks): create separate environments for different activities
  • Selectively permeable barrier: prevent unrestricted molecule exchange
  • Transport solutes: molecule exchange across membrane
  • Energy transduction: convert from one form to another
  • Respond to external signals: signals travelling from distance/nearby cells
  • Scaffold for biochemical activities
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2
Q

What are membrane phospholipids composed of?

A

Polar head group, phosphate, glycerol, 2 fatty acid chains

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3
Q

Major polar head groups of phospholipids

A

Used for naming
- Phosphatidyl… choline (PC), serine (PS), ethanolamine (PE), inositol (PI)

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4
Q

Amphipathic

A

Have both hydrophilic and hydrophobic parts
e.g. phospholipid, cholesterol (sterol), glycolipid

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5
Q

Formation of lipid bilayers

A
  • Hydrophobic molecules exclude water, clustering together to minimize energy cost of organizing water molecules
  • Amphipathic molecules –> conflicting forces –> solved by formation of bilayer (energetically most favourable, spontaneous)
  • Lipid bilayers are: closed, self-sealing
  • Sealed compartment formed by phospholipid bilayer –> energetically favourable vs planar phospholipid bilayers (edges exposed to water)
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6
Q

Movement of phospholipids within membrane

A
  • Phospholipids –> constantly moving within leaflet: lateral diffusion (swapping places within leaflet), twisting, turning, rotation
  • Intentional movement: flipping to opposite leaflet during membrane synthesis (rarely flip back)

Allows membrane fluidity

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7
Q

Factors that affect membrane fluidity

A
  • Temperature
  • Change in lipid composition that affect alignment of phospholipid tails
    Tightly packed tails –> more viscous, less fluid
    Freely moving tails –> higher fluidity
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8
Q

Interaction between temperature + lipid composition

A

Membrane fluidity will change if:
- temp changes, lipid composition stays constant
- lipid composition changes, temp stays constant

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9
Q

Transition temperature (Tm)

A

Temp at which a membrane transitions b/w fluid phase + gel phase

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10
Q

If temp above Tm

A

Membrane ‘melts’ –> lipids move more freely, rotationally, laterally within leaflets

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11
Q

If temp below Tm

A

Hydrophobic tails pack together (as if cold, huddling together) –> membrane gels –> incompatible with life

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12
Q

Tm + membrane fluidity

A

Cells must maintain fluidity within a relatively narrow range even w/ changes in environmental temp
Usually deal with too cold temps

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13
Q

Factors that affect Tm (fatty acids)

A
  1. Altering length of fatty acid chains
    - Longer chains –> more interactions b/w fatty acid tails –> tighter packing –> less fluid at a given temp, higher Tm, higher temp to melt
  2. Altering saturation of fatty acids –> # of db
    - More db (more unsat) –> less packing –> more fluid at given temp, lower Tm, lower temp to melt
    - Membrane phospholipids typically have 1 sat fatty acid and 1 w/ 1+ db (unsat)
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14
Q

Factor that affects Tm (sterols)

A
  1. Altering amount of sterol (e.g. cholestrol) - (can be up to 50% of membrane lipid in animal cells)
    - Cholesterol acts as “buffer”, inhibiting phase transitions when temp changes
    - Higher cholesterol at cool temps –> more fluid
    - Higher cholesterol at warm temps –> less fluid
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15
Q

Examples: variability of membrane fluidity between organisms (within life compatible range)

A
  • Organisms from warm climates –> membranes near melt
  • Organisms from cold climates –> membranes near gel
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16
Q

Mechanisms of membrane fluidity regulation in living cells

A

Homeo viscous adaptation (whole organism level): maintaining fluidity at temps low enough for potential gelling by altering membrane composition

Dealing with low temps:
- Shorter fatty acid chain length (e.g. enzymes cut C18 –> C16)
- Increase # db (= decrease sat): (e.g. desaturase enzymes triggered by low temps)

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17
Q

Membranes of archaea vs eubacteria + eukaryotic

A

Both: glycerol, phosphate, 2 HC chains

Archaea: branched ISOPRENE chains (instead of fatty acids) ETHER-linked to L-glycerol
- Allow extremophile archaebacteria to not suffer from membrane breakdown

Eubacteria/Euk: fatty acid chains ESTER-linked to D-glycerol

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18
Q

Ways proteins can associate w/ membranes

A

Integral:
- Transmembrane (across entire membrane, both leaflets, external parts outside membrane: alpha-helix, beta sheet
- Monolayer-associated: 1 leaflet
- Lipid-linked: linked to membrane by a lipid

Peripheral:
- Protein-linked: fully outside membrane, linked by integral protein

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19
Q

Integral protein association w/ membrane

A
  • Hydrophobic R groups allow protein to be located in hydrophobic environment (hold orientation in membrane) since the backbone of protein is always hydrophilic (polar)
  • Polypeptide chains usually cross membrane as alpha-helices
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20
Q

Formation of hydrophilic channels

A
  • From several alpha-helices
  • Hydrophobic parts hold protein in membrane
  • Hydrophobic parts (very polar) form pore that water and small soluble molecules can flow through
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21
Q

Formation of Porins

A
  • From beta pleated sheets
  • Common in outer membranes of gram-negative bacteria + endosymbiont derived organelles
  • Not common in animals
  • Very common in bacteria
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22
Q

Movement restriction of membrane proteins

A

Restriction of movement by:
- Cytosolic protein
- Extracellular protein
- Identical/non-identical proteins of separate cells interact
- Large membrane protein prevents flow of membrane proteins (e.g. tight junction)

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23
Q

Membrane protein distribution in epithelium

A
  • Tight junctions create 2 different domains in the membrane, prevent protein movement
  • Epithelial cells: line tubular + spherical organs (can bring things in/move things out)
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24
Q

Apical domain

A

Domain on top section of cell

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25
Q

Basalateral domain

A

Domain on bottom and side (separated from apical domain)

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26
Q

Membrane glycoproteins

A
  • Membrane proteins are coated with sugars on the extracellular side of the membrane
  • “Glyco calyx” –> sugar coat(ed)
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27
Q

Preservation of membrane symmetry during transport process (sugars)

A
  • Sugar added to protein in Golgi always on non-cytosolic side
  • First in the Golgi lumen, then inside the vesicle, then the extracellular side of membrane
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28
Q

What is the most extensive membrane compartment?

A

The ER, especially rough ER

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29
Q

Where is new membrane added in a cell?

A

The ER

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30
Q

Role of ER in membrane assembly

A

Free fatty acids in the cytosol –> catalyzed by enzymes bound to cytosolic side of ER –> new phospholipids added to cytosolic side –> scramblases transfer random phospholipids to other leaflet –> membranes w/ “scrambled” phospholipids emerge from ER

31
Q

Role of Golgi in membrane assembly

A

Membranes with evenly distributed phospholipids arrive from ER –> flippases selectively move PS and PE to cytosolic leaflet (phospholipids no longer symmetrically distributed) –> membrane asymmetry maintained from this point on

32
Q

Asymmetrical composition of membrane leaflets

A
  • Cytosolic: PS, PE, little bit of PI,
  • Non- cytosolic: PC, SM, sugars
  • Both: cholesterol equal
33
Q

What does the appearance of PS in outer leaflet indicate?

A

The cell is going to die

34
Q

Human RBC as model organisms for plasma membrane

A
  • Best understood plasma membrane
  • Cells are inexpensive, available in large numbers
  • Already present in single cell suspension (don’t have to break it down out of tissues)
  • Simple: no nucleus, ER, mitochondria, lysosomes (very pure reps of plasma membranes)
  • Purified intact plasma membranes –> prepared by producing RBC “ghosts”
35
Q

Membrane transport overview

A
  • Allow passage of certain substances in/out (gases, ions, nutrients, waste)
  • Lipid bilayers tend to block passage of polar (water-soluble) molecules
  • Can enter by: passing directly, transported by membrane proteins (carriers, channels), engulfed by cell (avoiding passage through membrane)
36
Q

Diffusion

A
  • Dissolved solutes in constant random motion
  • Movement high to low concentration
  • Solutes spontaneously spread out (increase entropy) until concentrations = in all regions –> no NET flux
37
Q

Semi-permeable membrane

A

Lets some solutes through and blocks others

38
Q

Osmosis

A
  • Diffusion of water (across semi-permeable membrane) from low solute conc. to high to equalize conc. on both sides
  • Outcomes: conc. of water + total solute = on both sides as long as water is allowed to cross –> no NET movement of water
  • Solute that CANNOT cross the membrane draws the water across
39
Q

Osmotic pressure

A

Force opposing the force of water moving from high to low conc. of solute

40
Q

Osmosis + cells

A
  • Water constantly moving through cell membrane in both directions
  • If “osmotic tone” (conc. of osmotically active particles) = inside + outside cell –> fluids are isotonic
  • If total solute conc. changes on either side (usually outside), net movement of water towards fluid w/ higher solute conc.
41
Q

Cell in hypotonic solution

A

Net water gain –> cell swells (can lead to hypotonic lysis (pop))

42
Q

Cell in hypertonic solution

A

Net water loss –> cell shrinks

43
Q

Cell wall in hypotonic solution

A
  • Net water gain
  • Turgid (normal) –> rigid cell wall limits increase in size
44
Q

Cell wall in isotonic solution

A
  • No NET water flux, less water coming in than normal
  • Flaccid –> wilted, shape changes
45
Q

Cell wall in hypertonic solution

A
  • Net water loss –> plasmolyzed
  • Rips cell away from cell wall
46
Q

Turgor pressure

A

Pressure of cell contents against cell wall in plant + bacterial cells

47
Q

Strategies for maintaining osmotic balance

A

Osmoconformers: adjust internal salt concentrations to match environment (marine organisms)

Osmoregulators:
- contractile vacuoles –> periodically pump out water (single celled euks –> lack cell walls)
- Terrestrial –> circulate fluid isotonic to cystoplasm throughout body (humans –> blood plasma + extracellular fluid)

Turgor: most plants –> hypotonic to environment –> water pulled into cell –> presses membrane out to cell wall

48
Q

What molecules can pass directly through membranes?

A
  • Depends on size, polarity, charge
  • Small nonpolar molecules (conc. gradients): O2, CO2, N2, steroid hormones
  • Small uncharged polar molecules (H2O, ethanol, glycerol)
  • Larger uncharged polar molecules (AAs, glucose, nucleosides) (negligible ability)
49
Q

What molecules cannot pass directly through membranes?

A
  • Larger uncharged polar molecules (AAs, glucose, nucleosides)
  • Ions (H+, Na+, K+, Ca2+, Cl-, Mg2+, HCO3-)
50
Q

Membrane transporter proteins

A

Carrier protein (shuttle) and channel protein (tunnel) –> both passageways for particular classes of molecules, most are multipass

51
Q

Carrier proteins (shuttle)

A
  • Changes shape to allow transport
  • Molecules must fit particular binding site
  • 1 molecule at a time
  • e.g. Revolving door
52
Q

Channel protein (tunnel)

A
  • Ion channels
  • Detects size + charge, as long as channel open –> anything w/ that size/charge can pass
    e.g. Open double doors
53
Q

Ion channels

A

Function: when open, allow movement (‘conductance’) of Na+, K+, Ca2+, Cl- down their gradients

Critical in many cell activities:
- cell volume regulation
- formation + propagation of nerve impulses
- secretion of substances into extracellular space
- muscle contraction

Features:
- Discriminate charge + size
- Highly selective
- Much faster than carriers (1000x)
- Bidirectional

Ion flux det. by electrical + concentration gradients (electrochemical)

54
Q

Electrochemical gradients driving ion movements

A
  • Voltage + conc. gradients work in same direction –> large flux to lower conc. area
  • Voltage + conc. gradients work in opposite directions: net flux changes in different circumstances (e.g. K+)
55
Q

Chemical gradient

A

Conc. inside vs outside

56
Q

Electrical gradient

A

Whether it is being attracted across membrane (by oppositely charged molecules) or repelled (by “like” charges)

57
Q

Ion selectivety

A
  • One side determines what moves through (if ion fits), acts as selectively filter
  • Other side can be open (e.g. bacterial K+ channel) or closed
58
Q

What determines whether ion channels are open or closed?

A
  • Voltage-gated: charge (reversal) effects protein shape
  • Ligand-gated (extra/intracellular ligand): binding of signal changes protein shape
  • Mechanically-gated: something like pressure opens the channel
59
Q

How do carrier proteins mediate facilitated diffusion?

A
  • Binding of solute at specific site temp. changes shape of protein
  • Solute moves down conc. gradient so carrier protein facilitates passive diffusion
  • Many carriers work in both directions
60
Q

Example of carrier protein

A
  • GLUT1 (glucose transporter on mammalian cells)
  • Will move glucose but not fructose, D-glucose but not L-glucose
  • High specificity
61
Q

Features of membrane carrier proteins

A

Similar to enzymes:
- Specificity, saturable (revolving door, can only transport amount that fits)
- Can be inhibited/blocked by substances resembling normal cargo (substrate) (like comp. inhibition of an enzyme)

  • Passive (facilitates diffusion)
62
Q

Passive vs active transports

A
  • Passive: down conc. gradient (simple diffusion, channel-mediated, carrier-mediated)
  • Active: against conc. gradient, requires energy
63
Q

Active transport

A
  • Using energy (directly/indirectly) to move ions against their gradient
  • Transport closely coupled to energy release –> hydrolysis of ATP, absorption of light, movement of electrons
64
Q

Sodium-potassium ATPase

A
  • 1st pump discovered (nerve cells of crab)
  • ONLY in animal cells
  • Moves Na+ out, K+ in
  • Coupled to hydrolysis of ATP –> shape change caused by addition of phosphate group
  • 3 Na+ out for every 2K+ in –> electrogenic (contributes to electrical diff, inside cell slightly negative)
  • AGAINST GRADIENTS of both
65
Q

Action of Na+/K+ ATPase

A
  1. Na+ binds
  2. Pump phosphorylates itself, hydrolyzing ATP
  3. Phosphorylation triggers conformational change + Na+ is ejected
  4. K+ binds
  5. Pump dephosphorylates itself
  6. Pump returns to original conformation + K+ is ejected
66
Q

Significance of Na+/K+ ATPase

A
  • Both membrane protein + enzyme
  • Present in all animal cells: running pump consumes 1/3 energy produced, major contributor to basal metabolic rate, target of many drugs
  • Helps maintain a Na+ gradient (high outside, low inside), used to co-transport other molecules (glucose, AAs)
67
Q

Other examples of pumps

A

Muscle cells: Ca2+-ATPase
Stomach lining: H+/K+-ATPase

68
Q

Function of ion pumps

A

Allow cells to concentrate certain substances/set up gradients that can be used to drive other processes

69
Q

Carrier proteins + coupled transport

A
  • Symporter: coupled transport in same direction (e.g. Na+ and glucose/AAs)
  • Antiporter: coupled transport in opposite directions (H+ exchanged for Na+/K+ or Na+ exchanged for Ca2+)
70
Q

Glucose-Na+ symport in intestinal epithelium

A
  • Using massive Na+ gradient to cytosol (inward) to transport glucose against its gradient
  1. Occluded empty
  2. Outward open: Na+ binds to active site and waits for glucose to bind
  3. Glucose binds –> occluded occupied
  4. Flips open randomly, if inward: Na+ and glucose brought in
  5. Occluded empty again
71
Q

Glucose transport across gut lining

A
  1. Na-glucose symporter
  2. Glut2 facilitated diffusion (uniport, down conc. gradient)
  3. Na+/K+-ATPase –> primary active transport (uses ATP)
72
Q

General features of coupled transport

A
  • Membrane carrier protein used driving force of ion (Na+) moving DOWN its gradient to move a solute (small molecule, ion) across the membrane even AGAINST its gradient
  • Gradient for ion created by active transport (primary, using ATP)
  • Coupled mediated transport (also known as secondary active transport)
73
Q

Pumps in plant cells

A
  • No Na+/K+-ATPase (animal only)
  • H+ pumps instead –> protein driven coupled symport
74
Q

Membrane proteins + gene expression

A
  • Each membrane has own characteristic set of channels and carriers (plasma, lysosomal, mitochondrial)
  • Transporters are proteins encoded by genes
  • Array of transporters present depends on: genes present in organism, whether or not they are expressed (in that cell/patch of membrane at that time)
  • Mutation in widely-expressed membrane transporter can have devastating consequences (Cl- –> cystic fibrosis, K+ –> long QT syndrome, Ca2+ –> malignant hyperthermia)

BIOL 130 (6 decks)

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