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BIOL 130 Midterm > Unit 5 > Flashcards

Unit 5 Flashcards

1
Q

Biological Membranes

A
  • all cells -> plasma membrane
    encloses contents of entire cell
  • eukaryotic cells -> membrane-bound organelles
    -nuclear ‘envelope’
  • double membranes of mitochondria and chloroplasts

endoplasmic reticulum
Golgi apparatus
lysosomes / vacuoles
temporary transport vesicles
et al.

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2
Q

Membrane Functions pt 1

A
  • compartmentalization (eukaryotes)
  • create separate environments for different activities
  • provide a selectively permeable barrier
  • prevent unrestricted exchange of molecules
  • transport solutes
  • exchange of molecules across the membrane
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3
Q

Membrane Functions pt 2

A
  • energy transduction - conversion of one form of energy into another
  • respond to external signals - signal transduction
  • signals travelling from a distance or from nearby cells
  • scaffold for biochemical activities
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4
Q

Membrane Phospholipids

A

phosphatidyl choline (PC)
phosphatidyl serine (PS)
phosphatidyl ethanolamine (PE)
phosphatidyl inositol (PI)

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5
Q

Movement of Phospholipids within Membrane

A
  • phospholipids are constantly moving
  • spinning in place; travelling laterally within ‘leaflet’
  • phospholipids are occasionally ‘flipped’ to the opposite leaflet during membrane synthesis but they rarely ‘flop’ back
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6
Q

Lipid bilayers form spontaneously.

A
  • hydrophobic molecules would exclude water, clustering together to minimize energy cost of organizing water molecules
    = > energetically favourable
  • form large droplets or surface film
  • are closed – no free edges
  • self-sealing
  • important feature for cell fusion, budding, locomotion
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7
Q

Membrane Fluidity

A

how easily lipid molecules move …
- rotationally
- laterally within a membrane leaflet

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8
Q

Membrane fluidity affected by

A
  • temperature
  • changes in lipid composition that affect alignment of phospholipid tails
  • tightly packed tails -> membrane more viscous, less fluid
  • freely moving tails = higher fluidity
  • temp changes while lipid composition stays constant
    lipid composition changes with constant temp
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9
Q

Transition Temperature (Tm)

A
  • temperature at which a membrane transitions between the fluid phase and gel phase
  • above Tm -> membrane ‘melts’ -> lipids free
  • below Tm -> hydrophobic tails pack together -> membrane gels -> incompatible with life
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10
Q

Transition Temperature (Tm) and Membrane Fluidity

A
  • cells must maintain fluidity within a relatively narrow range even in the face of changes in environmental temperature
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11
Q

Tm (fluid/gel transition temp) affected by: pt 1

A
  1. altering length of fatty acid chains
    - longer chains -> more interactions between fatty acid tails -> tighter packing -> less fluid at a given temp
    - higher Tm, higher temp to ‘melt’
    - range 14-24 carbons in membrane fatty acids
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12
Q

Tm (fluid/gel transition temp) affected by: pt 2

A
  1. altering degree of saturation of fatty acids -> # double bonds
    more double bonds -> less packing > more fluid at a given temp
    - lower Tm, lower temp to ‘melt’
    - membrane phospholipids typically have one saturated fatty acid and one with one or more double bonds
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13
Q

trans un sat

A

double bonds of H, one on each side

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14
Q

cis unsat

A

double bonds of H, both on same side

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15
Q

saturated

A

single bonds, 2 H atoms on each side

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16
Q

Tm (fluid/gel transition temp) in eukaryotic cells also affected by:
pt 3

A
  1. altering amount of sterol (eg cholesterol)
    especially animal cells - can be up to 50% of membrane lipid
    - cholesterol acts as a ‘buffer’, inhibiting phase transitions when temp changes
    - higher cholesterol at cool temps -> membrane more fluid
    - higher cholesterol at warm temps -> membrane less fluid
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17
Q

Regulation of Membrane Fluidity in Living Cells pt 1

A

homeo viscous adaptation
- maintaining membrane fluidity at temps potentially low enough to cause membrane to enter gel phase by altering membrane lipid composition

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18
Q

Regulation of Membrane Fluidity in Living Cells pt 2

A

dealing with low temperatures:
- shorter fatty acid chain length
- eg. enzymes that cut C18  C16
- many bacteria
- pond fish dealing with dramatic day / night temp shifts

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19
Q

Regulation of Membrane Fluidity in Living Cells pt 3

A
  • increase # double bonds (= decrease saturation)
  • eg. desaturase enzymes triggered by low temps
  • bacteria, cold-hardy plants (winter wheat)
  • coldwater fish species
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20
Q

Membrane Lipids in the 3 Domains of Life

A
  • membranes of all cells consist of phospholipids.
  • glycerol-phosphate plus two hydrocarbon chains
  • membrane phospholipids of both eubacterial and eukaryotic cells have fatty acid chains ester-linked to D-glycerol
  • archaea have branched isoprene chains instead of fatty acids
  • L-glycerol instead of D-glycerol
  • ether linkages instead of ester linkages
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21
Q

Polypeptide chains usually cross as

A

α-helices.

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22
Q

Hydrophilic channels can be formed from

A

several α-helices
- hydrophilic side chains form an aqueous pore
- α helix amphipathic
- hydrophobic side chains interact w phospholipid tails

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23
Q

Proteins folded into pleated sheets can form

A

pores.
- common in outer membranes of gram-negative bacteria and endosymbiont-derived organelles
eg sucrose-specific bacterial porin (S. typhimurium)

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24
Q

Cells can restrict the movement of membrane proteins

A

Membrane Anchoring, Membrane Domains and Compartments, Membrane Protein-Protein Interactions, Membrane Protein-Lipid Interactions

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25
Q

Membrane Protein Distribution in an Epithelium

A
  • protein a (apical surface) helps protein b stays in the basolateral side
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26
Q

Eukaryotic cells are coated with

A
  • sugars.
    ‘glyco calyx’
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27
Q

Membrane asymmetry is preserved during transport processes.

A

Sugar is on non cytosolic side — even when it joins the plasma membrane

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28
Q

Secretory Pathway: outward to plasma membrane

A
  • Rough ER: synthesis of proteins for export (secretion), insertion into membranes, lysosomes
  • Golgi apparatus: collection, packaging, and distribution
29
Q

Membrane Assembly: Role of ER

A
  • free fatty acids in cytosol (catalyzed by enzymes bound to cytosolic side of ER)
  • New phospholipids added to cytosolic side (scramblases transfer phospholipids to other leaflet)
  • Membranes with scrambled phospholipids emerge from ER — random distribution
30
Q

Membrane Assembly: Role of Golgi Apparatus

A
  • membranes with evenly distributed phospholipids arrive from ER (due to action of flippases phospholipids no longer symmetrically distributed)
  • Membrane asymmetry maintained from this point on
31
Q

Asymmetrical Composition of Membrane Leaflets

A
  • out: phosphatidyl choline (PC), sphingomyelin
  • in: phosphatidyl inositol (PI) and phosphatidyl serine (PS), phosphatidyl ethanolamine (PE)
32
Q

Asymmetry of Lipid Composition

A

the appearance of PS in outer leaflet of membrane usually indicates that cell is going to die

33
Q

cytosol is enriched for

A

PS and PE

34
Q

Human RBCs as ‘Model Organisms’ for Plasma Membrane

A
  • best understood plasma membrane
  • Cells are inexpensive and available in large numbers
  • Already present in single cell suspension
  • Simple - no nucleus, no ER, no mitochondria, no lysosomes, very pure preps of plasma membranes
35
Q

Membrane Transport: Overview

A
  • need to allow passage of certain substances in/out of cell — gases, ions, nutrients/waste products
  • Lipid bilayers tend to block passage of polar (water-soluble) molecules
  • Substances can enter a cell by — passing directly through lipid bilayer, being transported across bilayer by membrane proteins acting as carriers or channels, being engulfed by cell, avoiding passing through the membrane
36
Q

How do molecules move? -> diffusion

A
  • dissolved solutes (molecules/iosn in solution) are in constant, random motion
  • Solutes will spontaneously ‘spread out’ (increasing entropy) until concentrations in all regions are equal — no net flux
  • Across semi-permeable membrane
37
Q

Water also moves down its concentration gradient

A

Starts with more solute on right side of membrane — solute cannot cross
* Water moves across membrane to attempt to equalize its concentration
Outcomes:
* concentration of water equal on both sides
* concentration of total solute equal on both sides as long as water is allowed to cross
- Water can cross but large solute cannot; next flux of water will be into upper chamber

38
Q

Osmosis

A
  • diffusion of water across semi-permeable membrane down its concentration gradient (toward a higher solute concentration)
  • Once ‘water concentration’ equal on both sides, no net movement of water
  • Water concentration depends on total concentration of osmotically active particles (solutes)
  • All ions, molecules dissolved in fluid
39
Q

Osmosis and Cells

A
  • water is constantly moving through cell membrane in both directions
  • If osmotic tone (concentration of osmotically active particles) is equal inside and outside cell — intracellular and extracellular fluid are isotonic
  • Net movement of water will be toward fluid with higher concentration of solutes
40
Q

Osmotic effects on cells with walls

A
  • hypotonic solution — lysed
  • Isotonic solution — normal
  • Hypertonic solution — shriveled
41
Q

Tonicity (Mammalian Red Blood Cell)

A

hypotonic solution - cell swells
isotonic solution, no net loss or gain
hypertonic, cell shrinks

42
Q

Strategies for Maintaining ‘Osmotic Balance’

A

osmoconformers
osmoregulators
turgor

43
Q

osmoconformers

A

most marine organisms adjust their internal salt concentrations to match seawater

44
Q

osmoregulators

A

some single-celled eukaryotes have contractile vacuoles that periodically pump out water
- terrestrial organisms carefully regulate the osmolarity of a fluid they circulate through their bodies such that it is iso-osmotic with their cytoplasm

45
Q

turgor

A
  • most plants are hyper-osmotic to their environment
  • water pulled into cell, presses membrane out to cell wall
46
Q

What molecules can pass directly through membranes?

A

depends on size, polarity, charge

47
Q

small non polar molecules

A

easily can cross

48
Q

small uncharged polar molecules

A

still able to cross without special mechanisms

49
Q

larger uncharged polar molecules

A

somewhat of an ability to cross - needs help

50
Q

ions

A

tiny, no ability to cross lipid bilayer

51
Q

Membrane Transporter Proteins

A
  • carrier protein (shuttle) — require molecules that fit a particular binding site, one molecule at a time can be transported
  • Channel protein (tunnel) — ions across membrane, channels mostly detect size and charge, as long as channel open, it can pass
52
Q

Ion Channels

A
  • when open, allow movement na, k, ca, cl, down their gradients
  • Critical in many cell activities — regulation of cell volume, formation and propagation of nerve impulses, secretion of substances into extracellular space, muscle contraction
  • Ion flux determined by both electrical and concentration gradients — “electrochemical” gradient
53
Q

features of ion channels

A
  1. Discriminate on both charge and size
  2. Usually highly selective
  3. Much faster than carriers (1000x)
  4. Bidirectional
54
Q

Electrochemical gradients drive ion movements.

A
  • ‘chemical’ gradient: concentration inside versus outside
  • ‘electrical’ gradient: whether it is being attracted across membrane (by oppositely charged molecules) or repelled (by ‘like’ charges)
55
Q

What determines ion selectivity?

A

depends on — gate open — ion has to fit

56
Q

What determines whether channels are open or closed?

A
  • channels are open or closed depending on the slight negative charge on the inside of the membrane
57
Q

Carrier proteins mediate facilitated diffusion.

A
  • binding of solute at specific site temporarily changes shape of carrier protein
  • Solute is moves down its concentration gradient so carrier protein facilitates passive diffusion — many carriers work in both directions
58
Q

GLUT1 (glucose transporter on mammalian cells)

A
  • will move glucose, not fructose
  • D-glucose but not L-glucose
59
Q

Features of Membrane Carrier Proteins

A
  • specificity
  • Passive (facilitates diffusion)
  • Saturable
  • Can be inhibited/blocked by substances resembling normal cargo (‘substrate’)
60
Q

Transporting against a Gradient

A

Active transport:
* using energy (directly or indirectly) to move ions against their gradient
* Transport closely coupled to energy release — hydrolysis of ATP, absorption of light, movement of electrons

61
Q

Animal Cells: Sodium-Potassium ATPase

A
  • only present in animal cells
  • Moves Na out, K in
  • Coupled to hydrolysis of ATP — Change in shape caused by addition of phosphate group — moves 3 Na out for every 2 K in — electrogenic
62
Q

Significance of Na/K-ATPase

A
  • both a membrane protein and an enzyme
  • Present in all animal cells — running this pump consumes — one third of the energy produced by animal cells
  • Major contributor to basal metabolic rate — target of many drugs
  • Helps maintain a Na+ gradient (high outside cell, low inside)
  • Animal cells use this gradient move (co-transport) other molecules — glucose, amino acids
63
Q

Muscle Cells in Animals:

A

Ca2+-ATPase = pump

64
Q

other Membrane Pumps

A
  • H+/K+-ATPase (proton pump) in parietal cells lining stomach
  • Pump K+ into cell (against gradient) in exchange for H+
    Ion pumps in general:
  • allow cells to concentrate certain substances or set up gradients that can be used to drive other processes
65
Q

Carrier Proteins and Coupled Transport

A
  • uniport, symport, antiport — symport and antiport coupled transport
  • e.g. Na+ is often co-transported with glucose or amino acids
  • E.g. antiporters: H+ exchanged for Na+ or K+
66
Q

Glucose-Na+ Symport in Intestinal Epithelium

A
  1. Na-glucose transporter
  2. glut 2 facilitated diff.
67
Q

General Features of Coupled Transport

A
  • membrane carrier protein uses driving force of an ion moving DOWN its gradient to move a solute (small molecule, ion) across the membrane even AGAINST that solutes gradient
  • symport — ion and other solute move in same direction
  • antiport — opposite directions
  • Gradient for ion is created by active transport
  • Coupled-mediated transport is thus also known as … indirect active transport, secondary active transport
68
Q

Pumps in Animal vs Plant Cells

A
  1. Animal cell — Na+-K+ ATPase, Na+ driven symport
  2. Plant cell — H+ ATPase, H+ driven symport
69
Q

Membrane Transporters and Gene Expression

A
  • each membrane has its own characteristic set of channels and carriers — plasma membrane, lysosomal membranes, mitochondrial membranes (inner, outer),..
  • Transporters are proteins, encoded by genes
  • The array of transporters present in a given membrane depends on — genes present in that organism, whether or not they are expressed … in that cell, in that patch of membrane, at that time
  • A mutation in a widely-expressed membrane transporter can have devastating consequences
    e.g. channelopathies (Ion channel diseases)

BIOL 130 Midterm (4 decks)

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