Module 3b- Membrane function
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- is the difference in electrical charge across a membrane
- created when there's a difference in ion concentrations across a membrane
resting membrane potential
is an equilibrium condition where there is no net flow occurring across the membrane
how do cells establish a membrane potential?
what is the significance of membrane transport? (2)
- important in setting up different environments inside versus outside the cell or within organism
- optimizes the environment for function and sets up membrane potentials that store energy
what is the difference between solute and solvent?
- solvent is a fluid in which another substance, the solute, can dissolve
- water is an excellent solvent
why is water able to dissolve a large variety of substances?
hydrogen has a positive electric charge and oxygen has a negative electric charge which allows is to attract different kinds of molecules
Describe the key feature that makes water a universal solvent.
- polar: has a partial positive charge and a complementary negative charge
- most of the molecules in cells are also polar and can form hydrogen or ionic bonds with water
what type of chemical bonds are found in these molecules?
- have an affinity for water & dissolve in it easily
- in general, polar molecules or ions
- small molecules, such as sugars, organic acids, & some amino acids
- polar covalent or ionic bonds
what type of chemical bonds are found in these molecules?
- not easily soluble in water
- In general, large and non-polar
- lipids and proteins in membranes
- nonpolar covalent bonds
what type of chemical bond exists between a cation and an anion? Is this a strong or weak interaction?
- ionic bonds
- strong interaction
what needs to happen in order for a hydrophilic solute (NaCl) to dissolve in a liquid?
to dissolve in a liquid, the attraction between anions and cations in the salt must be overcome
what do polar water molecules do to ions?
polar molecules form spheres of hydration around ions
how does water interact with proteins?
ionic and polar regions on the protein's surface attract water molecules
- release of ordered water is necessary for the formation of an enzyme-substrate complex
- ordered water interacts through hydrogen bonding
how is enzyme-substrate interaction stabilized in desolvation?
stabilized by hydrogen bonding, ionic and hydrophobic interactions
Fatty acids hydrophilic or hydrophobic?
- tend to be hydrophobic
- some fatty acids with no net charge are polar and thus hydrophilic if they have a region that is positively charge and another that is negatively charged
how is the movement of a molecule that has no net charged determined?
determined by its concentration gradient
difference between simple or facilitated diffusion and active transport.
- simple or facilitated diffusion involves exergonic movement "down" the concentration gradient
- active transport involves endergonic movement "up" the concentration gradient
what is ion movement influenced by?
influenced by the combined effect of its concentration gradient and it's electrochemical potential (which is impacted by the charge gradient across the membrane)
what does active transport of ions across a membrane create?
what is solute movement affected by?
- a charge gradient or membrane potential across the membrane
- affected by electrochemical potential
The 3 permeability guidelines for a typical lipid bilayer are?
- 1. small nonpolar molecule readily dissolve in lipid bilayer and therefore diffuse across the membrane
- 2. uncharged polar molecules may also diffuse rapidly depending on size (only small, water etc.)
- 3. Ions and charged molecules do not cross without assistance (ions, sugars, amino acids, nucleotides, and wastes)
The three methods through which solutes cross the membrane are:
- 1.simple diffusion (osmosis) [passive transport]
- 2. facilitated diffusion [passive transport]
- 3. active transport (direct, indirect)
simple diffusion (passive transport)
- movement down the concentration gradient
- direct unaided movement is dictated by difference in the concentration of the solute on the two sides of the membrane (higher to lower concentration)
- ex. gases, nonpolar molecules, and small polar molecules such as water, glycerol, ethanol
facilitated diffusion (passive transport)
- movement down the gradient that requires transport proteins
- transport proteins(integral membrane proteins) assist most solute across membranes
- some proteins move solutes to regions of lower concentration, this does not use energy
- transport proteins move solutes against the concentration gradient
- requires energy like the hydrolysis of ATP or by the simultaneous transport of another solute down an energy gradient
since diffusion always moves solutes toward equilibrium:(2)
- free energy is minimized, as molecules move down their gradient
- solutes always move toward regions of lower concentration until concentrations are equal
describe the three factors affecting diffusion.
- size: smaller molecules are able to diffuse across the membrane
- solute polarity: more permeable to nonpolar substances than polar ones; polar molecules will repel the fatty acids and NOT cross membrane; small polar molecule will diffuse very slowly
- solute charge: for charged solutes to move into a membrane, the water molecules (that form a shell of hydration around polar substance) must be removed; requires energy
- measure of solute polarity
- measured by the ratio of its solubility in an organic solvent to its solubility in water
- more nonpolar (hydrophobic) a substance is, the higher the partition coefficient (solute can pass through membrane)
rate of simple diffusion
- rate of simple diffusion is directly proportional to the concentration gradient (steepness of the concentration gradient)
- is an exergonic process
permeability coefficient and diffusion through membrane.
- larger permeability coefficient, diffusion through a membrane is easier
- and vice versa
- membrane characteristics also influence permeability
what is the name of the interaction between water molecules? Are these weak or strong interactions?
- hydrogen bonds
- weak initially but will become a force to contend with when together
what happens to water molecules when solute molecules are dissolved in water?
- solute molecule that are dissolved in water disrupt the interactions that normally occur between water molecules
- decreases free energy of the solution, allows water to move from regions of low to high solute 
- for most cells water tends to move inward
- diffusion of water across a selectively permeable membrane
- water will move toward region of higher solute concentration
- the pressure required to stop osmosis
- solutes exert an osmotic pressure
- measure of the tendency of a solution to take in water by osmosis
- isotonic solution: no net movement of water
- hypotonic solution: water moves into the cell and may burst
- hypertonic solution: water leaves the cell and shrivels
how do cells use osmotic pressure to regulate water balance? (Na, Cl, and K)
- the  of Na+ and Cl- are higher outside the cell than inside the cell, whereas the  K+ is higher inside than out
- the low  of Na+ and Cl- inside the cell balance the high intracellular  of organic compounds. this equalizes the osmotic pressure and prevents the net influx of water
- Animal cells can regulate water using aquaporins
- transporter specific to water molecules
- single file movement through channels
plant cells prefer a __________ internal environment (ex. central vacuole)
- cell wall has poor osmotic pressure which moves water through the cell wall from an area of low  to are of high  which is the central vacuole
- guard cells in plant tissue regulate water loss by controlling stoma opening and closing
- guard cells take up K+ by active transport; causes water to enter the cell my osmosis
- stoma opening: guard cell walls are unevenly thickened and have radially oriented cellulose microfibrils; causes cells to bow as they become turgid
- stoma closing: when K+ ions are pumped out of the cell, water follows my osmosis and they stomata closes
water regulation in protozoans
some protozoans use contractile vacuole to remove water and prevent osmotic lysis
what are the transport proteins in facilitated diffusion? describe the 2 types.
- large, integral membrane proteins with multiple transmembrane segments
- carrier proteins: bind solute molecules on one side of the membrane, undergo a conformation change, and release the solute of the other side of the membrane; high specificity, not energizing
- channel proteins: form hydrophilic transmembrane channels through the membrane to provide a passage route for solutes; creates pore in membrane, very specific
what are the 3 types of channels?
- ion channels
what are the characteristics?
- tiny pores lined with hydrophilic atoms
- passive transport occurs based on electrochemical gradients
- -selective: very specific ions
- -gated: closed when not in use
- -Rapid Transporters: move millions of ions/ sec across membrane
Selectivity of the K+ channel (check diagram)
- pore is just wide enough to allow passage of dehydrated K+ from which all associated water molecules have been displaced as a result of interactions between K+ and carbonyl oxygen
- Na+ is too small to interact with the carbonyl oxygen of the selectivity filter, so it remains bound to water in a complex that is too large to pass through the channel pore
what does it mean that channels are gated?
- means that they open and close in response to stimulus
- closed conformation: flow of ions blocked by gate
- Open conformation: allows ions to flow rapidly through the channel
describe the 3 types of gate channels.
- voltage-gated channels: open and close in response to changes in membrane potential (depends on resting membrane potential)
- ligand-gated channels: are triggered when specific substances bind to the channel protein
- Mechanosensitive Channels: respond to mechanical forces acting on the membrane
- (ex. pressure on hand)
importance of Gated Channels
- because there is a  difference across a membrane, a transient opening of one of these channels is able to locally alter the membrane potential
- this can lead to electrical signaling
Functions of ion channels?
- establish a resting membrane potential
- role in cellular communication (muscle contraction & electrical signaling in nerve cells)
- maintaining salt balance in cells and airways linking the lungs
what are porins?
what is the function of porins?
- multipass transmembrane proteins that form membrane pores
- allow the rapid but non-specific passage of solutes
describe the structure of porins and how the embed in the plasma membrane.
- Beta barrel is the tertiary structure of porin
- 3 Beta barrel in a ring is quaternary structure or porin
- Beta barrel has hydrophilic and hydrophobic residue. The hydrophobic residue, facing towards outside of barrel, attract lipids in plasma membrane which allows it to be embedded
aquaporin structure (3)
- all are tetrameric integral proteins
- 4 identical monomers associate with their 24 transmembrane segments in an orientation that forms 4 central channels
- channels lined with hydrophilic side chains that are just large enough to let water pass through single file
function of aquaporin.
- found in animal cells
- allow the rapid passage of water by interrupting a minimum number of hydrogen bonds
describe structure of gap junction and function.
- 6 transmembrane proteins called connexins come together and create a channel in the center called the gap junction
- resting potential change is what allows cells to communicate
describe what the alternating conformation model state about carrier proteins.
- states that a carrier protein is an allosteric protein and alternates between two conformational proteins
- in one state, the solute binding site of the protein is accessible on one side of the membrane
- the protein shifts to its alternate conformation, with the solute binding site on the other side of the membrane, triggering the solute's release
how are carrier proteins analogous to enzymes?
how are carrier proteins regulated?
- facilitated diffusion involves binding the substrate on a specific solute binding site
- carrier protein and solute form an intermediate after conformational change, the "product" is released (the transported solute)
regulated by external environments
carrier protein specificity
are often highly specific for a single compound or a small group of closely related compounds
when does competitive inhibition of carrier proteins occur?
can occur in the presence of molecules or ions that are structurally related to the correct substrate
Active transport performs 3 important cellular functions:
- uptake of essential nutrients
- removal of wastes
- maintenance of non-equilibrium concentrations of certain ions
how does active transport differ from diffusion in regards to the direction of transport .
- diffusion is non-directional with respect to the membrane (direction of movement driven by concentration gradient)
- active transport has an intrinsic directionality and is therefore unidirectional
how is the energy for direct active transport provided?
provided through ATP-hydrolysis
how is indirect active transport driven?
- driven by ion gradients
- ATP does not fuel pump directly (takes energy from direct active transport pump)
what are the 2 types of direct active transport?
- ATP-driven pump
- light-driven pump
compare uniport and coupled transport.
- uniport: when a carrier protein (uniporter) transports a single solute across a membrane
- coupled transport: when two solutes are transported simultaneously and their transport is coupled
what does coupled transport depend on?
- In couple transport, the transport of one solute depends on the transport of another solute
- energy stored in the electrochemical gradient of one solute is used to move the other
compare symport (cotransport) and antiport (countertransport).
- symport: if the 2 solutes are moved across a membrane in the same direction
- antiport: if the 2 solutes are moved in opposite directions
proton pump (ex. interior of lysosome within animal cells)
- the transmembrane protein (acts as the proton pump) binds to the proton in the cytoplasm while in conformation A
- the energy from ATP then drives the transport of the proton to the interior of the lysosome through the formation of conformation B
- release of the proton results in the protein reassuming conformation A
describe the structure and function of Ca2+ pump. (generally in cell)
- transmembrane protein that has 9 alpha helixes passing through
- takes Ca2+ from cytoplasm and transports it to ER membrane which stores it until needed
- ATP hydrolyzes to ADP and collects energy to move Ca2+
- Ca2+ a second messenger, helps neurotransmitters and enzyme function
- moves from high to low  because charged
what is the glucose transporter and describe the structure?
- a uniport carrier that is uniquely active
- erythrocyte capable of glucose uptake by facilitated diffusion because level of blood glucose is much higher outside than the inside of the cell
- glucose transported inward by GLUT1
- GLUT1 is integral membrane protein (single polypeptide with 12 passes)
- doesn't use ATP
describe the 4 steps in the transport of glucose by GLUT1.
- 1. Glucose binds to 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 the next cycle
what is the benefit of phosphorylation of glucose upon entry into a cell?
- the immediate phosphorylation of glucose upon entry into the cell keeps the concentration of glucose low
- once phosphorylated, glucose cannot bind the carrier protein any longer and is stuck in the cell
- because highly polar (negative) can't diffuse back
what is the Na+/K+ pump?
- maintains electrochemical ion gradients
- typical animal cell, K+ inside cell and Na+ outside cell
- electrochemical potentials for sodium and potassium are essential as a driving force for coupled transport and for transmission of nerve impulses
what's the energy requirement for Na+/K+ pump?
- the pump uses the exergonic hydrolysis of ATP to drive the transport of both ions
- it's responsible for the asymmetric distribution of ions across the membrane of animal cells
structure of Na+/K+ ATPase
- 4 polypeptides transmembrane protein
- 2 alpha subunits that connect to the ion
- 2 Beta subunits, function unclear, for structure?
the Na+/K+ pump conformational states.
- Na+/K+ pump is an allosteric protein
- 2 alternative conformational states:
- E1 conformation: open to he inside of the cell and has high affinity for Na+ ions
- E2 Conformation: open to the outside of the cell and has a high affinity for K+ ions
what are the six steps in the Na+/K+ pump mechanism?
- 1. on the inside of the cell, three Na+ ions bind to the E1 conformation
- 2. this triggers phosphorylation of the alpha subunit by ATP
- 3. the pump undergoes a shift to the E2 conformation, which causes the release of the Na+ ions on the outside of the cell
- 4. two K+ ions bind to the E2 conformation on the outside of the cell
- 5. this triggers dephosphorylation of the alpha subunits by ATP and a return to the E1 conformation
- 6. In the conformational change, K+ ions are carried to the inside of the cell and released
what are the 3 functions of the Na+/K+ pump?
- 1. set up the membrane potential
- 2. maintain high [Na] outside the cell and high [K] inside the cell
- 3. maintain the osmotic balance of the animal cell
what does direct transport depend on?
what are they?
- direct transport depends on four types of transport ATPases
- ATPases are a class of enzymes that hydrolyzes ATP to ADP and a free phosphate ion. This releases energy that can be used to drive other chemical reactions
what are transmembrane ATPases? what are the types?
- integral membrane proteins that use ATP energy to move solutes across the membrane against their concentration gradient
- 4 types are:
what are P-type ATPases? (3)
- members of a large family that are reversibly phosphorylated by ATP on a specific aspartic acid residue
- 8-10 transmembrane segments
- sensitive to inhibition by vanadate
what are the 5 subfamilies of p-type ATPases?
- P1-ATPases: Transport heavy metals, like copper, zinc
- P2-ATPases: maintain gradients of ions
- P3-ATPases: pump protons out across the plasma membrane in plants & fungi, which acidifies the external medium
- P4-ATPases: pump hydrophobic molecules such as cholesterol and fatty acids & act as flippases
- P5-ATPases: some are known to transport cations in ER, vacuoles or lysosomes
what is the V-type ATPases?
what is the structure?
what are the 3 functions?
- It pumps protons into organelles such as vacuoles, vesicle, lysosomes, endosomes, and Golgi complex
- have 2 multi-subunit components
- -V0 is the integral component embedded in the membrane
- -V1 is a peripheral component that has the ATPase activity (made of 7 subunits)
- Three functions are:
- generate a proton gradient
- act as a pH sensor
- energize the membrane
what is the F-type ATPases?
what is the structure?
what are the 3 functions?
- transport protons in bacteria, mitochondria and chloroplasts
- 2 multi-subunit components:
- -F0 a transmembrane pore
- -F1 a peripheral membrane component that contains the ATP binding site
- function by hydrolyzing ATP to obtain the energy needed to pump protons against the  gradient
what are the ABC-Type ATPases?
- transport amino acids, sugars, ions, polypeptides, polysaccharides or proteins
- in eukaryotes, they only export. in prokaryotes they import and export
- typically have 4 protein domains:
- -2 domains are highly hydrophobic and are embedded in the membrane
- -2 domains are peripheral and are associated with the cytoplasmic side of the membrane. bind ATP and couple its hydrolysis to transport molecules
what are multi-drug resistance proteins? (MDR Proteins)
- are ABC-type ATPases
- pump hydrophobic drugs out of cells
- reduces the cytoplasmic concentration and hence their effectiveness
- can be overcome by inhbitors
- small integral membrane protein in the plasma membrane of archaea
- uses energy from photons of light to drive the active transport of protons out of the cells
- creates an electrochemical proton gradient that powers synthesis of ATP by and ATP synthase
what is the light-absorbing pigment in bacteriorhodospim?
- light-absorbing pigment is retinal, which is related to vitamin A
- embedded in plasma membrane
- linear in resting state, when it's energized by light it gets bent or cised
function of bacteriorhodopsin
- retinal chromophore in bacteriorhodopsin present in all-trans conformation
- when the retinal absorbs a photon, one of its double bonds isomerases to a cis form
- photo activated molecule than transfers protons to the outside of the cell
- proton gradient created by pumping protons out of the cell is used to produce ATP
- protons flow back into the cell down the concentration gradient and causes ATP synthases to produce ATP
some cells use Na+/glucose symporter called sodium-dependent glucose transporters or SGLT proteins to take up glucose. what are the six steps in this mechanism?
- 1. 2 external Na+ ions bind their sites on the symporter, which is open to the exterior
- 2. this allows one molecule of glucose to bind
- 3. a conformational change in their protein exposes the glucose and Na+ inside the cell
- 4. the 2 Na+ ions dissociate in response to the low internal Na+ 
- 5. this locks the symporter in its inward-facing conformation until the glucose dissociates
- 6. the loss of glucose frees the symporter to return to the outward-facing conformation
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