Pathways and Regulation Flashcards
(13 cards)
chemiosmotic circuits underlie many functions
-initially idea was cells were bags of enzymes and they were doing everything
-how did cells make ATP? they knew in these membranes there were proteins arranged in the electron transport chain (ETC)- extract electrons from NADH and pass it along and it goes downhill in terms of energetics and its final recipient is hydroxyl –> uses oxygen and generates water
-they also knew that protons moved across the membrane and membranes had ATP synthase, which took ADP+Pi and made ATP but rxn uses a lot of energy and coupled to ETC
-some high energy intermediate that ran from ETC to ATP synthase and gave it energy –> energy intermediate was proton that transported across the membrane- protons pumped across the ETC then electrochemical gradient was itself a form of intermediate energy
-if protons run back downhill through ATP synthase, they could somehow drive formation of ATP
what types of work do chemiosmotic circuits do?
- chemical work
- mechanical work- Ex. bacteria have flagella and when they rotate their flagella, they tumble through media- proton gradient goes through set of proteins and causes flagella to rotate
- osmotic work- it can come downhill through transporter and drag solute either in the same or opposite direction
variations of chemiosmotic theme
-anaerobic bacteria do not have O2 to drive ETC but can do glycolysis- takes in glucose to break it down and ferment it to lactic acid and make ATPs that way –> uses ATP to drive ATP synthase backwards- enzyme is breaking down ATP and making proton gradient to drive the other functions
-vibrio lives in alkaline water and cannot maintain proton gradients- replaced proton by Na so the ETC can move Na ions and protons and Na ions can come through ATP synthase and make ATP
-another bacterium uses a completely different exothermic rxn of decarboxylation and decarboxylation is linked to pushing Na ions across the membrane- Na ions then drive formation of ATP
-another bacterium is Ex. of virtual proton pump- bacteria exchanges oxylate for formate- oxylate has 3 negative charges and formate has 1, so doing this exchange creates difference of ions across the membrane
pumps initiate chemiosmotic circuits
-pumps use form of energy like ATP, light and they move ions across the membrane to create gradient that can move back across membrane through carriers and they can run downhill through ion channels
-pumps, carriers, and channels constitute chemiosmotic circuits across every biological membrane whether it’s an organelle or cell membrane they do work of all kinds
circuits build on protons are widespread
-proton pumps use ATP to drive protons across membrane and proton drives uptake of sugar or amino acid
-chloroplast in plants harness light to make ATP and then there’s another organelle with its own pumps, carriers, and ion channels
movement of ions across membrane
-gradient of K and Cl facing outwards
-measure charge across membrane –> for every K ion there’s also a Cl ion inside and outside is the same
-everything inside is electrically balanced and neutral- if you could measure charge across membrane, it would be zero but there’s a [] gradient
-introduce an ion channel that is selective for K ions- allows K ions to cross membrane but not Cl ions
-when the channel first opens, K ions will go out and in –> flux of K ions going out is greater than the fux going in (unequal movement) and K ions carry charge so outside becomes positive and inside is negative b/c of the Cl ions left behind
-next instance of time: K is still moving out b/c it’s a gradient and K is still moving in, however, the movement of K ions outside is slower b/c it’s repelled by positive charges outside so flux of K ions going out is less and conversely K ions going in will be faster because there’s an attractive negative force inside pulling it in and charge across continues to get negative inside but not that fast
-eventually rate of K ions going out will be equal and opposite to K going in driven by electrical gradient –> electrochemical equilibrium where these 2 rates are equal and balanced
nernst potential
[] gradient and electrical gradient are equal and opposite and balanced –> no net change in voltage or flux and the current = zero
what happened to the [] gradient of Ek…did it change?
-NO the [] gradient changes before it gets to Ek and everything stops –> doesn’t change at all
-charge moving across membrane is proportional to voltage gradient
-capacitance determines membrane potential for charge that’s moved
-biological membranes are made of lipid bilayer with no conductivity –> no water, nothing hydrophilic so it has a low capacitance so very few charges need to move across to have equivalent membrane potential
rule-of-thumb
- number of ions moved to generate change in voltage is small
- [] gradients remain largely unchanged
- electrical potentials change a lot faster than [] gradients
- must shunt the potential to generate a [] gradient
acidification of lysosome
-proton pump hydrolyzes ATP and pumps protons one at a time inside the cell
-if no other ion is moving only protons, very quickly you build up a positive charge and you can’t move any more protons
-reach equilibrium where you have a charge difference but no pH difference
-cell could fix this either by if the vesicle had a lot of K and some sort of ion channel for K, the K can go out so every proton that comes in, a K ion goes out so more protons come in to make more acidic inside
-you could also have a Cl channel to allow negative ion to come in, so for every positive charge that comes in so does a negative one –> no electrical component anymore
ion gradients in mammalian cells
-a lot of K inside typical cell compared to outside and converse is true for Na
-more Cl outside
-huge Ca gradient with small amount inside and large amount outside
-if K channel opened, membrane would quickly become negative inside- -90 mV
-if K channel closed and Na opens, Na rushes in and cell becomes positive and inside becomes +69 mV
if one set of channels open, voltage moves to Eion
-when K channel opens, goes down to Ek and leaves negative charges behind
-when K channel closes but Na channel opens, goes up to Ena
-if Cl channels are open, negative inside
-if Ca channels are open, positive inside
-signals drive APs that get sent to muscles
what happens if Na and K channels are open at the same time?
-gradients would run down and burn out
-ion channels regulate how they open and shut b/c if they didn’t the gradients would cancel each other
