Lecture 7 - Membrane Transport Flashcards
(57 cards)
1
Q
Selective permeability barrier
A
- allows regulated exchange of substances between compartments
2
Q
solute transport
A
- membrane proteins facilitate the movement of substances between compartments
3
Q
What is the importance of the transport of solutes
A
- needed to maintain an internal environment (dynamic steady state) that is very different from the extracellular environment
4
Q
what is passive transport
A
- solutes move down a concentration gradient
- no energy is needed
- transport proteins may or may not be needed
5
Q
what is active transport
A
- solutes move against a concentration gradient
- requires input of energy (often but not always ATP)
- transport proteins “pumps” are required
6
Q
carriers
A
- transporters that alternate between two conformations
- used for transporting glucose
7
Q
channels
A
- water filled pore through which specific ions or small molecules can diffuse
8
Q
what does simple diffusion require
A
- membrane permeability
- favourable gradient conditions
9
Q
What determines permeability
A
- molecular size
- partition coefficient (measure of polarity)
- charge
10
Q
what is passive/simple diffusion
A
- unassisted movement down a concentration gradient at a rate proportional to the gradient
- only possible for gases, nonpolar molecules, small polar molecules (water, glycerol, or ethanol)
- diffusion always moves solutes toward equilibrium
11
Q
Example of simple diffusion
A
- oxygen inhaled by the lungs and taken up by erythrocytes
- in the capillaries of body tissues low o2 and high co2 - > o2 is released from hemoglobin and diffuses outwards to meet tissue
- in capillaries of lungs high o2 and low co2 -> o2 diffuses inward and binds to hemoglobin
12
Q
osmosis
A
- diffusion of water through a semi-permeable membrane where water moves from an are of lower solute concentration to an area of higher solute concentration
13
Q
Hypotonic solution
A
- net water gain
- cell swells
- high solute concentration inside the cell
14
Q
hypertonic solution
A
- net water loss
- cell shrinks
- high solute concentration outside the cell
15
Q
isotonic solution
A
- no net loss or gain
- same concentration
16
Q
What is the difference between simple diffusion and osmosis
A
simple diffusion - membrane permeable to solute, impermeable to water
- concentration of solute moves to be the same on both sides
osmosis - membrane impermeable to solute, permeable to water
- water moves from low conc. to high conc. to dilute solute -> so solute conc. will be equal
17
Q
affects of osmosis on a plant cell
A
- plants are usually hypotonic compared to their fluid environment
- tendency for water to enter the cell causing turgor pressure that pushes against its surrounding wall
- in hypertonic solutions the plant cell undergoes plasmolysis and the plant loses it support and wilts
18
Q
What is facilitated diffusion and the 2 main classes of facilitated diffusion transports
A
- passive transport requiring a protein
- channels and carrier proteins
19
Q
What are channel proteins
A
- integral membrane proteins
- form hydrophilic channels through the membrane that allow passage of solutes without a major conformational change
20
Q
what are carrier proteins
A
- integral membrane proteins
- bind one or more solute molecules, undergoes a conformational change that transfers the solutes to the other side
21
Q
what are the 3 kinds of transmembrane channel proteins
A
- ion channels -> highly specific channel that can conduct almost a million ions per second
- porins -> passage of a variety of hydrophilic solutes determined by pore size, some antibiotic resistance has been linked to mutations in certain bacterial porins
- aquaporins -> water flows through at a rate of several billion per second. amino acid residues discriminate against other ions of smaller size
22
Q
what is special about frog oocytes
A
- most cells swell when placed in a hypotonic solution including erythrocytes
- frog oocytes do not
- expression of an erythrocytes aquaporin in frog oocytes causes then to swell in burst when placed in hypotonic solution
23
Q
what is the structure of an aquaporin? how is this channel selective for water?
A
- tetramer
- several conserved hydrophilic amino acids whose side chains and carbonyl groups extend into the middle of the channel
- forms hydrogen bonds with transported water molecules
- the arrangement of the H bonds and narrow pore diameter prevent passage of protons or other ions
24
Q
what are porins
A
- allow for rapid passage of various solutes (low specificity)
- found on outer membranes of mitochondria, chloroplasts, and gram-negative bacteria
- close cylindrical beta-barrel with a water filled pore at its center
- polar side chains line the inside
- nonpolar side chains point into the membrane
- upper size limit of the solute molecules is determined by the pore size
- mutations in bacterial porins have been associated with antibiotic resistance
25
what is a concentration gradient
- molecules want to move down a concentration gradient
- entropy increases
26
what is an electric potential gradient
- charged molecules want to move towards the compartment with the net opposite charge
27
what is the thermodynamically favourable transport direction for molecules with no net charge
- determined by concentration gradient
28
what is the thermodynamically favourable transport direction for charged molecules
- determined by electrochemical gradient
29
what is the electrochemical gradient
- the combined effect of the concentration gradient and the electrical potential gradient across the membrane
30
what is the electrical potential difference across almost all plasma membranes
- (-70mV)
- cytosolic side of the membrane is negative with respect to the outside of the cell
31
what are the typical intracellular and extracellular ion concentrations for K+, Na+, and Ca2+
- Na+ higher outside the cell -> more positive
- K+ higher inside the cell -> more negative
- Ca2+ in cytoplasm are kept very low (10^-7 M)
- would see more Na+ going in then K+ going out because of membrane potential
32
what are ion channels
- allow for rapid passage of very specific ions
- these channels are bidirectional with flow determined by electrochemical gradient
- different channels transport different ions
- possess tiny pores which are lined with hydrophilic amino acids
- when open 10 million ions can pass per second
- more than 200 ion channels
33
what are the different types of ion gated channels
- voltage gated -> open and close in response to changes in membrane potential
- ligand gated -> triggered by the binding of specific substances to the channel protein
- mechanosensitive gated -> respond to mechanical forces that act on the membrane
- most ion channels are selective for a specific ion
34
how does the structure of bacterial K+ channel show how an ion channel works and is selective
- K⁺ channel has a central pore and a selectivity filter
- Filter fits K⁺ perfectly, using backbone oxygen atoms
- gated by pH
- change in pH result in conformation change of the position of M2 helix
35
why does K+ work for the channel but not Na+
- when ions are in the vestibule they are hydrated with water
- in the selectivity filter they lose their hydration with water and carbonyl oxygens are placed precisely to accommodate a dehydrated K+ ion
- the dehydration of the K+ ion requires energy which is balanced precisely with the energy regained by interaction with the carbonyl oxygens
- Na+ is too small to interact with the carbonyl oxygens and requires a great expense of energy to enter the selectivity filter
36
what is a eukaryotic voltage gated K+ channel
- pore domain similar to the bacterial K+ channel (helices S5 and S6)
- a voltage sensing domain consisting of helices S1-S4 - > S4 = voltage sensing helix
- when closed, resting potential more (-) inside, S4 helix very (+) charged
- when open, depolarized, more (+) charged on inside
37
what is the glucose transporter GLUT1
- facilitated diffusion of glucose by a uniport carrier protein -> carrying 1 molecule
- found on all mammalian plasms membranes
- human genome encodes 14 glucose transporters, only GLUT1 is ubiquitous
- process is reversible depending on concentration gradient
- glucose is rapidly phosphorylated inside the cell -> keeps the intracellular concentration of glucose low and thus maintains a concentration gradient
38
what are the steps of glucose binding to GLUT1
1. glucose binds to a GLUT1 transporter protein that has its binding site open to the outside of the cell (T1 conformation)
2. glucose binding causes the GLUT1 transporter to shift to its T2 conformation with the binding site open to the inside of the cell
3. glucose is released to the interior of the cell, initiating a second conformational change in GLUT1
4. loss of bound glucose causes GLUT1 to return to its original (T1) conformation ready for a further transport cycle
39
what similarities does facilitated diffusion share with kinetics
1. like enzymes, carrier proteins are very specific for their target molecule
2. carriers proteins can have their activity regulated
3. carrier proteins exhibit saturation kinetics
- carrier proteins can move 100s-1000s of molecules per second -> much slower than channels
40
what is active transport
- used to move solutes up a concentration gradient away from equilibrium
- energy is required to move substances against their concentration gradient
- couples endergonic transport to an exergonic process
- unidirectional
41
what are the 3 important cellular functions of active transport
1. uptake of essential nutrients
2. removal of wastes
3. creation of gradients and maintenance of non-equilibrium concentrations of ions
42
what are primary active transport-ATPases
- harness the energy of ATP hydrolysis to move ions or small molecules against a concentration gradient/electrical potential
43
what are the 4 classes of ATPases and their characteristics
- P type ATPases
- V type ATPases
- F type ATPases
- ABC-type ATPases
- high selectivity
- differ in structure, mechanism, localization, and physiological roles
- all have one or more ATP binding sites on the cytosolic membrane leaflet
44
what are p-type ATPases
- move all types of ions
- P for phosphorylation
45
what are v-type ATPases
- V for vacuoles
- keeps pH of compartments low which activates hydrolytic enzymes
- pump protons into organelles (components in cells that have pH lower than
- only found in eukaryotes
- have two multi-subunit components and integral component embedded in the membrane and a peripheral component that juts out from the membrane surface
- acidify organelles
46
what are f-type ATPases
- F for factor
- uses H+ gradient to drive ATP synthesis
47
what are ABC-ATPases
- ABC for ATP-binding cassette
- importers and exporters
- comprise a very large family of transport proteins
- first discovered were importers -> involved in uptake of nutrients
- later exporters were identified in both bacteria and humans
- all family members have 2 ATP binding domains and 2 transmembrane domains
48
what is the scheme of the Na+/K+ ATPases transport cycle
- maintains intracellular sodium and potassium concentrations in animal cells which maintains osmotic balance and stabilizes cell volume
- most energy consumed by the brain (2/3) is used to maintain Na+/K+ ATPase which maintains the membrane potential required for the transmission of nerve impulses
49
what is an example of P-class pumps
- Ca2+ ATPase in SR
- pumps Ca2+ from cytosol into lumen of SR
- over 80% of the SR is composed of the muscle Ca2+ ATPase
50
What does the H+/K+ ATPase in the stomach control and what type of pump is it
- controls acid secretion
- acidify lumen of your gut
- p type pump
51
what is multidrug -resistant transporter
- abc transporters are medically important because some of them pump antibiotics or drugs out of cells - > cell becomes resistant to drug
- some human tumors are resistant to drugs that normally inhibit growth of tumors -> resistant cells have high concentrations of an ABC transporter call MDR transport protein
- MDR transport protein pumps hydrophobic drug out of cells reducing the cytoplasmic concentration and hence their effectiveness
- MDR protein transports a wide range of chemically dissimilar drugs
52
what is direct (primary) active transport
- solute accumulation is coupled directly to an exergonic chemical reaction -> ATP hydrolysis
- drives the outward transport of protons thereby establishing an electrochemical potential for protons across the membrane
53
what is indirect (secondary) active transport
- an endergonic is coupled to the exergonic
- symporters and antiporters
- simultaneous transport of two solutes
- exergonic inward movement of H+ provides the energy to move the transported solute against its concentration gradient or electrochemical potential
54
how is indirect active transport powered
- not by ATP hydrolysis
- potential energy stored in ionic gradients is utilized to transport other solutes
- the inward transport of molecules up their electrochemical gradients is coupled to and driven by simultaneous inward movement Na+ or H+ down their gradients
- in co transport neither molecule can move alone
55
examples of indirect active transporters
- Na+/glucose = symporter
- Na+/ Ca2+ = antiporter
- Na+/H+ = antiporter
56
what is the difference between symporters and antiporters
- symporters = both substances move in same direction
- antiporters = substances move in opposite directions
57
is Cl higher inside or outside the cell
higher outside
