Question

i just want to confirm...if in a reaction ATP are used...then that reaction necessarily has to be irreversible?? or no??

Answer 1

for eg the reaction of conversion of glucose to glucose-6-po4 uses ATP(external energy) and this reaction is irreversible...@chmvijay

Answer 2

with my limited chemistry knowledge - I don't think why this should be necessary

Answer 3

The breakdown of ATP to ADP and inorganic phosphate is highly exergonic. This means that thermodynamics is driving the given reaction in that direction. The reverse wouldn't happen spontaneously.

Many biochemical reactions are impossible from a thermodynamical point of view - that is - they are endergonic - they need to absorb energy in order to occur. However, organisms are smart - they bind the two reactions together - the breakdown of ATP with a huge amount of energy released, with another reaction, say, phosphorilation of glucose, which needs energy in order to be initiated. By summing the two reactions up, you still have some of the released energy left, so thermodynamics say it can occur spontaneously.

Let me give an example to make it more clear. First, I'll note that exergonic reactions have -ΔG, energy is released and they occur spontaneously according to the laws of thermodynamics. Conversely, endergonic reactions have +ΔG, they need to absorb energy from the surroundings in order to occur - the substances wouldn't react unless energy is added.

ATP -> ADP + P -ΔG1
Glu + P -> Glu-P +ΔG2

However, as we said, -ΔG1 is a hugely negative number and +ΔG2 is a small positive number. That's why -ΔG1 + ΔG2 is still less than zero and the phosphorilation reaction will occur in the presence of ATP.

Answer 4

thanks @suzanka
i totally understood d concept of 1 exergonic reacn clubbed in wid 1 endergonic....
but then when this clubbing occurs then the endergonic reacn has 2 b irreversible right? i kind of deduced it from ur explanation...

Answer 5

@suzanka says right ?...:) i guess
But once energy is released by ATP to ADP :) where does that ADP goes :) does it makes bond with other phosphate to have ATP again :)
ADP will go back to ATP by oxidative phosphorylation as for my knowledge of biology is concerned

Answer 6

We can't have an irreversible endergonic reaction simply because the reactants would never interact with each other. So, we have the reversible conversion of Glu to Glu-P. One of the reagents here is P which is taken from the breakdown of ATP. That's why we can bind the two reactions together.

The reverse reaction Glu-P -> Glu + P does not occur when ATP is around as it can't bind with the breakdown of ATP.

Answer 7

so u r saying that due to unavailability of ADP the backward reaction does not occur because there is no molecule to accept the Pi..???

Answer 8

ATP are like biological red-ox couples like in chemistry gain and lose electrons like heree ATP converts to ADP and then back to ATP again :) it goes on like this

Answer 9

@chmvijay yes, you're right that ADP has to be converted to ATP in the process of oxidative phosphorylation, however, that's a process that takes in energy and occurs in the mitochondria. It doesn't happen spontaneously, say, in the cytosol.

Answer 10

@chmvijay this reaction occurs in the cytoplasm and ADP to ATP reaction occurs in mitochondria...so we cannot club those reaction due to physical barrier

Answer 11

@suzanka is persuing honors in BSc final she may give u good idea regarding this :) i have lost the bio completely :P :)

Answer 12

see this ques arose..because i was writing all d reactions in glycolysis..and it just struck me that whenever there was usage of ATP the reaction became irreversible...and the source is lehninger so yes the arrows are correct...:P
dis made me think weder it is true for all such reactions

Answer 13

@shrutipande9, yes, it is the binded reactions that become irreversible. ΔG is less than zero.

Answer 14

@chmvijay It is wrong that ATP/ADP is a red-ox pair. Oxidation states don't change.

Answer 15

yayy..ok..thanks both of u...:)
i really appreciate it...^_^

Answer 16

sorry its not exactly redox couples :) i mean lose and gain of 3 rd phosphate group :)

Answer 17

i dint understand d last point though...redox couple???
sry i m nt dat gud in chem

Answer 18

haaa haaaa :) ok leave it :)

Answer 19

@shrutipande9 ignore the red-ox thing, I was replying to @chmvijay. But seems everything's clear now :)

Answer 20

Crystal clear @suzanka :)

Answer 21

yes yes...my ques is done...thnk u soo much..is it ok if i tag u in such ques? @suzanka
i m kind of havin difficulty wid thermodynamics n bioenergetics?

Answer 22

and obv @chmvijay tula tar me nehmich tag karat rahnar in chem...:P

Answer 23

haaa haaa kara tele kaya problem nahi :) @shrutipande9

Answer 24

Okay, I don't mind. I'll do my best to help :)

Answer 25

thank u guys...☺

Answer 26

You're welcome :)