Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

The metabolic rate measures how much energy an organism expends over a unit of time. Its breakdown for the human body in terms of its functions is well documented : so much for the heart, for the brain, etc.

In West et al, 2002 I have found an estimate for the metabolic rate of a single cell. But how does this breakdown in terms of elementary functions in the cell?

E.g., how much of the energy made available from nutrients is used in the process of replicating genome, of expressing proteins, of trafficking...?

Following Jeremy Kemball's link suggestions, I find in this paper the fraction of ATP consumption for the following processes:

Protein synthesis    0.34
Na+/K+ ATPase        0.16
Ca2+ ATPase          0.17
RNA/DNA synthesis    0.25
Unidentified         0.09

(which adds up to 1.01 rather than 1 for spurious reasons). Their measurement is indirect, based on $O_2$ consumption for an assumed steady-state regeneration of ATP. I wonder how GTP-based processes are accounted for? Or are they negligible compared to ATP ones?

This is for a specific cell type, rat thymocyte, is there a reason to expect that this will be hugely different for say a fibroblast? Also, they stimulate their thymocyte with concanavalin A. I understood they are not getting RNA/DNA consumption signal without Con-A, but didn't get why it would be so.

Finally, what would be in the remaining 9% ? One candidate for ATP consumption suggested by Jeremy Kemball is actin turnover, I guess it fits only in the "unidentified" line. Tubulin turnover is a GTP process, not sure about intermediate filaments (do they turnover?). All ATP-molecular motors (myosin, kinesin, dynein at least?) have to be there too. What else?

I am particularly interested in the total amount of ATP energy that goes to myosin.

share|improve this question
1  
@DevashishDas: Maybe my question was not clear enough: I'm not looking for a description of ATP production from glucose (and other related processes making energy usable), but of its usage. Some of it actually has to go back to energy production, but some also goes to protein synthesis, etc. Besides, I am looking for numbers (e.g., DNA replication would consume so much over 1 cycle of 24 hrs, which makes so much % of cell metabolic rate) –  Joce Jul 25 at 13:29
    
My instinct is that these figures are going to be hard to find. PET scans and things using fluoridated glucose can pretty easily ballpark metabolic demand for individual organs, but tracking where and when ATP is consumed is going to be much more difficult. A fun complication: ATP consumption and production rates change wildly across an individual cell during normal cell replication, even if the environment is constant. I wish you luck. –  Jeremy Kemball Jul 28 at 15:08
    
@JeremyKemball: thanks for your message. Already knowing that these figures may be completely unavailable is helpful...! I would anyhow be interested in any estimate available: there may be some from a mixture of in-vitro and in-vivo approaches, e.g. a lower estimate for DNA replication might be around, as we know how many b.p. we need to replicate and probably how much energy it takes to replicate one. –  Joce Jul 28 at 20:18
    
@JeremyKemball: Do you have a reference for the fact that "ATP consumption and production rates change wildly across an individual cell during normal cell replication"? Cheers. –  Joce Jul 28 at 20:25
    
I need to get in the habit of finding sources for things I say, even in comments where they won't fit. Just because I say things that seem plausible but turn out to be wrong when you actually do the math, uh, fairly often. :/ –  Jeremy Kemball Jul 28 at 21:59

2 Answers 2

up vote 6 down vote accepted
+50

I was musing on this and did some strange googling, and have some ballpark figures for a bunch of different organisms. It's far from a complete answer but it's at least a start, and all this won't fit in a comment.

DNA replication, I assumed, was a huge metabolic drain on the cell. Turns out that is far from the case. Many helicases are passive, requiring no ATP, and the amount of ATP-equivalent triphosphates to synthesize the entire genome is pretty small compared to the amount that gets used and recycled every day.

According to these guys humans go through their bodyweight in ATP every day, about 50% of which is actin turnover, and 30% is synthesis (60% or more in rapid bacteria). Proteins cost about 4-5 ATP per AA to break down and rebuild.

You're not going to get a really good general breakdown, I don't think, but in crop plants or E. coli there are numbers for this, sort of. A lot of them are based on indirect measurements of protein uptake/turnover and ATP cost per AA or BP. They're fascinating but insane and often contradictory.

share|improve this answer
    
The 50% going into actin turn-over is for brain tissue, probably over-estimated for other tissue. However your link about synthesis lead me to this paper where there are numbers for thymocytes: 1/3rd of ATP consumption for protein synthesis, 1/3rd for calcium+sodium cycling, 1/4th for DNA/RNA synthesis and 1/10th for "all the rest". Firstly, how does this fit with the ATP consumption of actin treadmilling that has to be expected? (it has to fit in the 10% for thymocytes?) What else can one expect to find in these 10%? –  Joce Jul 30 at 12:39
1  
Actin treadmilling for neurons I expect to be a pretty big part of energy expenditure. They recycle neurotransmitters and rearrange their dendrites and things often, and all that has a cost. Thymocytes are just blobs, so they don't have to spend as much energy on the cytoskeleton. genome.jp/dbget-bin/www_bget?cpd:C00002 is a list of some of the things ATP does in the cell. Where and how the ATP budget is spent is going to vary a lot from cell type to cell type. Muscles and nerves do a lot of ion pumps, glands do synthesis, etc. –  Jeremy Kemball Jul 30 at 15:24

A significant amount of heat generated by the cell does not come from the hydrolysis of an NTP. ATP is generated by a H+ gradient in the mitochondria, and this gradient is created by mechanisms which rely only in part by ATP. Most of the energy stores in our bodies are not in the NTP pool. This is why CO2 and urine are used to measure energy expenditure during physical exercise. On the scale of individual cells, it is a challenge to conceive a protocol which would account for all confounding variables. However, on the scale of a human, confounding variables in energy use are easily controlled. If you are interested in the energy expenditure of a single biological process, this would seem a better project rather than quantifying all processes simultaneously.

share|improve this answer
    
Thanks for your answer. In the paper I mention above indeed, there's a lot of cell-scale respiration which is not accounted by ATP processes but attributed to "proton leakage", and I understand that this is linked with heat production. Anyway, what I'm interested in is really the cell-scale energy expenditure, within which I'd like to particularize ATP-based energy. Then, specifically, I am interested in energy consumption by myosin (in nonmuscle cells). –  Joce Aug 2 at 7:18

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.