Fats more efficient energy storage than carbohydrates

[dmission]

Anyone have any intuition as to why this might be? Would it have something to do with triglycerides having double bonds, which maybe give more energy when oxidized, than say, glycogen/glucose?

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[Doctor246853]

Anyone have any intuition as to why this might be? Would it have something to do with triglycerides having double bonds, which maybe give more energy when oxidized, than say, glycogen/glucose?

Eh. Makes sense. We went over this in a medical physiology class this week but I forgot exactly why. When I go to class on Tuesday, I will post the answer if you still need it/ hasnt been posted by then.

 

[salim271]

My biochem teacher explained this. Not only do fats give more energy per gram, they also dont increase blood pressure when they're stored as triacylglycerides, sterols and phospholipids. Sounds weird, but it really makes sense. Glycogen is a polymer of glucose that forms 1,4 and 1,6 alpha linkages by dehydration. Every time you form a bond, H2O drops out. If glycogen was stored as the major energy source, water levels in the plasma would skyrocket and probably throw off the osmotic balances of just about everything.

 

[dmission]

Eh. Makes sense. We went over this in a medical physiology class this week but I forgot exactly why. When I go to class on Tuesday, I will post the answer if you still need it/ hasnt been posted by then.

Appreciate it, thanks.

@salim271 Why do they give more energy per gram, though?

 

[salim271]

Appreciate it, thanks.

@salim271 Why do they give more energy per gram, though?

Well you have to look at the process of breakdown. Basically when you start with glucose, you start at glycolysis, and as you oxidize glucose you're gaining energy in the form of ATP, but you're losing energy at the same time to make that ATP. There is still a net gain of ATP however.

With fats, the breakdown is different, basically you take the long fatty acid chain and you cut 2 carbons off at a time and turn it into a bunch of AcCoA molecules and send them to the citric acid cycle, skipping glycolysis completely. There are four steps to get from FA to AcCoA + FA(-2C), and none of them require a direct input of ATP even though the energy powering the process is no doubt coming from elsewhere. The net gain from the breakdown of fats is about double that of glucose, I specifically remember glucose = 2840 kJ/mol and palmitate (a 16C FA) = 9770 kJ/mol.

Just finished biochem lol, I guess i did learn something from that class even if I only got an A-... damn 6 week course.

 

[Axon hillock]

I also took biochem. There are two main reasons for why fats are more effecient. The first is because they are highly reduced molecules thus they release more energy as the guy above me has already mentioned. Also, since fats are highly insoluble in water, there is little or no water of hydration associated with stored fat, unlike the case for glycogen.

Edit: I think as a rule of thumb, fats give 6 times the amount of energy as carbohydrates.

 

[salim271]

I think one of the coolest things I learned in biochem was that your body can make fat from glucose alone, since the precursor for fatty acid synthesis is just AcCoA.

 

[loveoforganic]

I also took biochem. There are two main reasons for why fats are more effecient. The first is because they are highly reduced molecules thus they release more energy as the guy above me has already mentioned. Also, since fats are highly insoluble in water, there is little or no water of hydration associated with stored fat, unlike the case for glycogen.

Edit: I think as a rule of thumb, fats give 6 times the amount of energy as carbohydrates.

 

[dmission]

I also took biochem. There are two main reasons for why fats are more effecient. The first is because they are highly reduced molecules thus they release more energy as the guy above me has already mentioned. Also, since fats are highly insoluble in water, there is little or no water of hydration associated with stored fat, unlike the case for glycogen.

Edit: I think as a rule of thumb, fats give 6 times the amount of energy as carbohydrates.

I've never been totally clear on what being highly reduced actually means. Is that the long carbon-tail chains that are reduced that provide the extra energy somehow?

 

[salim271]

I've never been totally clear on what being highly reduced actually means. Is that the long carbon-tail chains that are reduced that provide the extra energy somehow?

Fully and partially reduced refers to the hydrogens. On fatty acids, each carbon is either attached to a hydrogen or a carbon, barring the carboxyl group at the very beginning. In glucose, every carbon is attached to one hydrogen (C6 is attached to 2,) an oxygen (C1 is a carbonyl, C2-6 have hydroxyls), and a carbons in a chain.

If you look at oxidation numbers (which basically tell how reduced a carbon is), for example at C3, in glucose the carbon C3 has one hydrogen, one OH, and 2 carbons attached. Carbons contribute oxidation numbers of 0 charge, an H is +1, and an OH is -1 (-2 for the O, +1 for the H). This means to balance out charges and have a neutral molecule, C3 must have an oxidation number of 0.

In comparison, C3 in a fatty acid has two hydrogens, and two carbons attached to it. The hydrogens each contribute +1, so C3 must have an oxidation number of -2 to cancel that out and form a neutral molecule. The more negative oxidation number, the more reduced the carbon in question is. Hence CH4 is more reduced than O=CH2, etc.

A long winded explanation that basically sums up to this: FAs have more hydrogens. Apparently I do remember what oxidation numbers are. Huzzah.

 

[dmission]

Fully and partially reduced refers to the hydrogens. On fatty acids, each carbon is either attached to a hydrogen or a carbon, barring the carboxyl group at the very beginning. In glucose, every carbon is attached to one hydrogen (C6 is attached to 2,) an oxygen (C1 is a carbonyl, C2-6 have hydroxyls), and a carbons in a chain.

If you look at oxidation numbers (which basically tell how reduced a carbon is), for example at C3, in glucose the carbon C3 has one hydrogen, one OH, and 2 carbons attached. Carbons contribute oxidation numbers of 0 charge, an H is +1, and an OH is -1 (-2 for the O, +1 for the H). This means to balance out charges and have a neutral molecule, C3 must have an oxidation number of 0.

In comparison, C3 in a fatty acid has two hydrogens, and two carbons attached to it. The hydrogens each contribute +1, so C3 must have an oxidation number of -2 to cancel that out and form a neutral molecule. The more negative oxidation number, the more reduced the carbon in question is. Hence CH4 is more reduced than O=CH2, etc.

A long winded explanation that basically sums up to this: FAs have more hydrogens. Apparently I do remember what oxidation numbers are. Huzzah.

Thanks for the detailed reply. So the number of hydrogens is proportional to the stored energy you'd say?

 

[salim271]

Thanks for the detailed reply. So the number of hydrogens is proportional to the stored energy you'd say?

Yup, think of it this way, glucose and fats are both oxidized, meaning those hydrogens are being removed... since you cannot have oxidation without reduction, what's being reduced? The electron carriers, NADH and FADH2. More hydrogens ---> more electron carriers can be made... basically. Doesnt exactly work like that, but for the MCAT details on FA oxidation and even glucose oxidation isnt needed... i just know because of biochem. That class can be a great supplement for the MCAT because it connects organic and biology, which is something the MCAT does in the BS section.

 

[BilalL]

Thanks for the detailed reply. So the number of hydrogens is proportional to the stored energy you'd say?

Those hydrogens are the ones used in the TCA cycle and than in the ETC. As Acetyl CoA is getting oxidized (less reduced), other molecules are getting reduced (oxidation reduction). These molecules (through more redox) provide for the proton gradient seen across the inner membrane of the mitochondria. This proton gradient powers the synthesis of ATP( More energy!).

The actual process of B-oxidation of fatty acids is not the main source of energy from fatty acids. The process yields the acetyl CoA which than can go on and yield ATP through TCA/ETC.

Each turn of the TCA cycle yields around 12 ATP (3 NADH +1 FADH2 +1 GTP=12 ATP). Now acetyl CoA will have 2 carbons from the FA.

So 16 Carbon fatty acid needs 7 cycles to get 8 acetyl CoA. Each of these cycles produces 5 ATP roughly. So that gives us 35 ATP from just the oxidation of the FA to acetyl CoA.

Now we have 8 acetyl CoAs and can do 8 cycles of the TCA. Each cycle gives 12 ATP. This gives 96 ATP.

96 ATP + 34 ATP (1 ATP needed to transport free FA into Mitochondria) = 130 ATP

So the TCA cycle itself produced roughly 3 times the ATP from the FA oxidation. But the 34 ATP still constitutes a good chunk of ATP so FA proves to be a better storage of energy than carbohydrates.

Unsaturated FAs skip a step in the oxidation of the FA and thus generate a bit less total energy since they cant yield as many protons for ETC.

 

[Integrated MCAT Course]

Think about the 'personality' of oxygen. With electronegativity of 3.5 oxygen is the most electronegative element you will see in biochemistry, fluorine being rare.

Imagine pulling apart a nutrient molecule into single atoms. You are pulling out of the potential energy wells of mutual attraction. The electrons in the molecular bonding orbitals of the reagent had been holding the atoms together so you had to put energy in. Imagine the work.

Now imagine as the atoms fall together to form the product. The electrons would be going into new bonding orbitals. The system would be pulling to the new molecular form through the strength of attraction of the electrons in the new sigma and pi bonds for the atomic nuclei.

When oxygen forms a chemical bond with a less electronegative element, there is not only the standard internal energy decrease of falling into a molecular form, such as H2 bond formation, there is also a lot of internal energy decrease to oxygen's electronegative, electron greedy nucleus taking electron control in redox terms and pulling the electrons towards itself. When a strong positive, like the bare unshielded oxygen nucleus, draws in a negative it's a big internal energy decrease in electrostatic potential energy.

Thermochemistry teaches you that if you add the positive and negatives leading to changes in state functions you have a valid imaginary path. Think of this path to compare it where the nutrient is glucose versus triglyceride. Glucose is a compromise evolution made between solubility and energy density, letting some oxygen in to the molecule to let it dissolve. Think of how the liver can just let glucose flow out or flow into the whole physiology.

Fats are dense energy storage only possible in aqueous solution in micron type, or nonhomogeneous suspension.

Basically because oxygen has already eaten its way into the easy electrons in glucose, there aren't going to be as many deep potential energy wells of new bonds where oxygen, which had been in a tug-of-war with itself - why are we fighting? in the product is bonded to carbon and hydrogen.

Redox is an accounting system to give you a way of keeping track of how much electron control is transitioning. When oxygen gets electron control, it's almost always a potential energy decrease for the system. There are fewer electrons held by the not-so-electron-greedy atoms carbon and hydrogen in glucose from oxygen's perspective. When oxygen gets electron control, the internal energy decrease can become heat flowing into the environment, such as in combustion, or it can be coupled with ATP synthesis in metabolism. The spontaneity is driven by the relationship between the energy lost to the total system as heat flow into the surroundings and the entropy-of-the-universe function, whether an abiotic system like a fire, or an open, dynamic, living system, like the processes of respiration.

 

[vayntraubinator]

Anyone have any intuition as to why this might be? Would it have something to do with triglycerides having double bonds, which maybe give more energy when oxidized, than say, glycogen/glucose?

http://www.bio.miami.edu/~cmallery/255/255atp/255makeatp.htm