Chemistry/Human Biology Help!?

Question

My chemistry teacher tells me that to break chemical bonds it requires energy and when bonds are formed energy is released, however my human biology teacher says that in cells ATP releases an inorganic phosphate and the bond that holds it on is broken to release energy?

Can someone clear this up for me please?

Thanks

Answer 1

Breaking bonds does NOT release energy.  Breaking bonds is uphill, and requires an input of energy, ALWAYS.  The apparent contradiction only arises because biologists aren't very good at balancing chemical reactions.

That's a bit facetious. But biology books, particularly introductory ones, tend to focus just on the molecules they care about, and downplay or ignore what else is going on. As a result, they give the impression that what you're doing in a typical catabolic reaction, or in ATP hydrolysis, is only breaking bonds, when what really happens is that you break some (that are relatively weak) and then you also form some (that are relatively strong) and the net result is an exothermic (downhill) reaction, because you put in less to break that you get out to make. But if you just focus on the organic molecules that do the breaking part and ignore the rest, it seems counter-intuitive.

example: ATP hydrolysis is often written as ATP => ADP + Pi. You broke one phosphate bond, energy comes out, right?  Buzz. The reaction isn't balanced. It's really ATP + HOH ==> ADP + Pi. You break a P-O and an O-H, and you also form a new P-O and an O-H in the phosphate.  In terms of bond enthalpies, this one is almost thermoneutral.  Most of the driving force has more to do with increased entropy and solvation enthalpy of the free phosphate than the bonds you form.

example: reaction of α-ketoglutarate in the citric acid cycle is often written as breaking a C-C bond: –[O2C(CH2)3C(O)CO2]– (αKG) ==> –[O2C(CH2)3C(O)SCoA] + CO2 (succinylCoA + carbon dioxide). Break a bond, energy comes out.

But you formed a C-S bond from the CoA as well. And the C-O bonds in CO2 are far far stronger than the C-O bonds in a carboxylate group. And you lost two electrons, which form a new C-H bond in NADH. The whole balanced reaction is:

–[O2C(CH2)3C(O)CO2]– + –[SCoA] + H+ + [NAD]+ ==>

–[O2C(CH2)3C(O)SCoA] + NADH + CO2.

break: C-C and a C-O (from αKG carboxylate)

make: C-S (in succinylCoA), C=O (in CO2), C-H (in NADH).

Even though it's catabolism, breaking the organic species into smaller bits via bond breaking, you form more new bonds that you lost, you form stronger bonds than you broke (and you're making CO2 as an added entropic bonus), and that's the reason that the reaction releases energy.

Your chem teacher is right: breaking bonds is always uphill and requires energy, forming bonds is always downhill and releases energy. Your biology teacher is also right, if you promise to remember that the molecules that are breaking apart are not the only molecules involved in the reaction, that you're also forming new bonds in the products, and that the reason the reaction releases energy overall is that you put in less energy to break weak bonds in the reactants so that you can release more energy when you form the stronger bonds of the products.

Answer 2

Think of chemical bonds as cookies on the top shelf of your cabinet. They store energy (since the contain sugar). In order for you to get that energy, you need to walk all the way to the cabinate, reach for the cookie, eat it and digest it. All these processes require energy. But on the contrary, by completely digesting the cookie you get energy since the cookie stored energy originally.

Similarily, bonds story energy. To break them, energy is needed similar to how you need energy to get and breakdown the cookie. But bonds also release energy once broken.

ATP requires very little energy to be broken down into ADP and Phosphate but release a lot of energy, similar to how a cookie requires little energy to get but releases a lot of energy after digestion. However, to convert ADP and phosphate into ATP requires a lot of energy.

Not all compounds release a lot of energy when broken. Metal oxides (like Magnesium Oxide) release a lot of energy when formed, require a lot of energy to be broken and release little energy after being broken.

Answer 3

bonds store energy. it takes energy to create bonds. when a bond is broken it releases its stored energy. it takes energy to break bonds but more energy is released from the bonds