Every process in the last two lessons ended with a bill in ATP, and the bill can only be understood by asking what makes any chemical change happen at all.
This lesson is the thermodynamics the rest of the course spends. It assumes nothing beyond the idea that energy is conserved, and it derives what it needs. Readers who have met free energy elsewhere can skim the first section and start at the one about the phosphate bond, which is where the biology begins and where the standard story is wrong.
What decides whether a reaction goes
Two things change when a reaction happens: the energy stored in bonds, and the number of ways the system and its surroundings can be arranged. The first is the enthalpy change , negative when heat is released. The second is the entropy change , positive when the arrangement becomes more disordered. Neither alone predicts direction. Ammonium nitrate dissolves in water spontaneously while absorbing heat, so a reaction can be uphill in enthalpy and still happen, driven by entropy.
The combination that does predict direction, at constant temperature and pressure, is the Gibbs free energy
A process runs forward when is negative, runs backward when it is positive, and is at equilibrium when it is zero. This is the second law in the form a chemist can use: negative for the system is exactly the condition for the total entropy of system plus surroundings to increase.
The essential further point is that depends on concentrations, not only on the identity of the reactants. For a reaction with reaction quotient , the ratio of product to reactant activities as they actually are,
where is the value under a defined standard state: one molar for everything, pH 7, 25 degrees Celsius. Nothing in a cell is at one molar and pH 7 is only approximately right, so tabulated values are a reference point rather than a description. Mistaking one for the other is the commonest error in cellular energetics, and the next section is built on the difference.
One thing never tells you is speed. Glucose in air has a of oxidation near kJ/mol and sits in a sugar bowl indefinitely, because the path to the products runs over an activation barrier. Enzymes lower that barrier and change nothing about the equilibrium. Thermodynamics says what is allowed and catalysis says what happens this afternoon, and a cell needs both.
The bond that is not special
ATP is adenosine with three phosphates in a chain. Hydrolysing the terminal one gives ADP and inorganic phosphate:
with kJ/mol.
Textbooks have called this a "high energy phosphate bond" for eighty years and drawn it with a squiggle, and the phrase is actively misleading. Breaking a bond always costs energy. There is no bond anywhere that releases energy by breaking. What releases energy here is the whole reaction, products against reactants, and three separate features of the products are responsible.
First, electrostatics. At pH 7 the triphosphate chain carries about four negative charges within a few tenths of a nanometre of each other, and they repel. Splitting the chain relieves that strain, and it is the reactant being unstable rather than the products being unusually stable.
Second, resonance. Free inorganic phosphate spreads its negative charge over four equivalent oxygens, an arrangement unavailable to the same phosphate while it is tied into a chain. That delocalisation is worth real stabilisation energy.
Third, solvation. Two separate ions are hydrated better than one larger one, so water is happier with the products.
None of this makes ATP special among phosphate compounds. Phosphoenolpyruvate hydrolyses at kJ/mol, twice ATP's figure, and creatine phosphate at kJ/mol. ATP sits deliberately in the middle of the range, which is what allows it to be a currency: compounds above it can phosphorylate ADP to make ATP, and ATP can phosphorylate compounds below it. A currency has to be worth less than what you sell and more than what you buy.
Mass action does most of the work
Now put the cell's real concentrations into the equation, which almost no textbook diagram does. A typical cytosol holds roughly 3 mM ATP, 0.3 mM ADP and 5 mM inorganic phosphate. The reaction quotient, in molar units, is
and at 310 K, where kJ/mol,
The real figure is about kJ/mol, sixty per cent larger in magnitude than the tabulated one, and the whole of that extra came from concentrations. This is the number the previous lesson used to show that one ATP is enough to run one cycle of the sodium potassium pump, and with the tabulated 30.5 it would not have been.
The distance from equilibrium can be quantified. The equilibrium constant for ATP hydrolysis is , while the cell's actual mass action ratio is . The cell holds ATP about times away from where the reaction wants to be. At true equilibrium essentially every ATP molecule in you would be ADP.
That is the real nature of ATP. It is not a battery with energy stored in a bond. It is a molecule kept enormously far from its own equilibrium by continuous resupply, and its useful free energy is the size of that displacement. Stop the resupply and the value of the currency collapses within seconds, which is precisely what happens when a tissue loses its blood supply.
Example. A student proposes that a cell could get more energy per ATP by evolving a version whose hydrolysis has a of kJ/mol instead of . What would go wrong?
Two things, and they are both about ATP's job rather than its energy content. First, the compound has to be makeable. ATP is regenerated by transferring phosphate from donors such as phosphoenolpyruvate at kJ/mol, and no ordinary metabolite could phosphorylate a molecule whose own hydrolysis is worth , so the cell would have to find a much stronger donor and would face the same problem one step further back. Second, the compound has to be stable enough to keep. A larger driving force for hydrolysis means a greater tendency to react with water, and ATP's non-enzymatic half-life in neutral solution is already only hours to days. A molecule that hydrolysed spontaneously in minutes would be useless as a store, because a cell recycles its entire ATP pool in about a minute but individual molecules must survive the trip. The general principle is that a currency is defined by sitting between what earns it and what spends it, and moving it to one extreme of the range breaks half of its transactions.
Now you. Sprinting muscle exhausts its ATP in roughly two seconds, yet a sprinter runs for ten. Muscle also holds creatine phosphate at about five times the concentration of ATP, and its hydrolysis is worth kJ/mol. What is creatine phosphate for, and why is it not simply used as the currency instead?
Answer
It is a buffer, not a currency. Because its phosphate transfer potential is higher than ATP's, the enzyme creatine kinase can regenerate ATP from ADP directly and almost instantly, with no metabolism involved at all, so the ATP concentration in a working muscle barely falls even while the demand is enormous. Five times the pool at a comparable energy per molecule extends the supply from about two seconds to about ten, which is the duration of a sprint, and it explains why creatine supplementation has measurable effects on short maximal efforts and none on endurance. It is not used as the currency because a currency must be recognised by thousands of different enzymes: ATP is a substrate for kinases, polymerases, motors, pumps and synthetases, and its adenine and ribose give it a large distinctive surface for those enzymes to bind. Creatine phosphate is small and featureless, good for one fast transfer to one partner. The division of labour is between a universally recognised token and a local reserve that can be converted into it.
A currency, not a store
The arithmetic of turnover makes the point better than any argument. Intracellular water in an adult is roughly 28 litres, and at 3 mM the total ATP in a human body is mol, which at 507 g/mol is about 43 g. Call it fifty grams.
Now the daily consumption. A 2000 kcal diet is 8368 kJ. If something like half of that passes through ATP, at 50 kJ per mole, the body makes and spends mol per day, which is 42 kg of ATP. The efficiency assumption is soft and the honest range is roughly 30 to 60 kg, but every version of the calculation lands at the same qualitative result: an adult turns over something close to their own body mass in ATP each day while never holding more than about fifty grams of it.
Divide the two figures and each ATP molecule is recycled about a thousand times a day, once every minute and a half. There is no reservoir. The system is a pipeline running at full rate, which is why cardiac arrest damages the brain in minutes rather than hours, and why every remaining lesson on metabolism is about the rate at which ATP can be regenerated rather than about how much energy a fuel contains.
Coupling, and why it is mechanical rather than clerical
The most common misconception in bioenergetics is that an unfavourable reaction can be driven by a favourable one simply because the sum of the two free energies is negative. It cannot. Two reactions in the same beaker with no connection between them each go their own way, and the exergonic one merely warms the solution.
Coupling requires a shared intermediate: the two reactions must be steps of a single mechanism, so that the unfavourable one physically cannot happen except as part of the favourable one.
Hexokinase, the first enzyme of glycolysis, is the clean example. Attaching a phosphate to glucose,
has kJ/mol, so at standard concentrations the equilibrium ratio of product to reactant is , and less than half a per cent of the glucose would be phosphorylated. The cell instead runs
with kJ/mol and an equilibrium ratio of . The shift is a factor of , which is exactly the equilibrium constant of ATP hydrolysis, as it has to be.
The crucial detail is that free inorganic phosphate never appears. Hexokinase binds glucose and ATP side by side and transfers the terminal phosphate directly from one to the other in a single step. There is no moment at which ATP has been hydrolysed and the glucose has not yet been phosphorylated, which is what makes the coupling mechanical rather than a piece of accounting. The enzyme also closes around the glucose as it binds, excluding water from the active site, because water is a small and abundant alternative acceptor and an enzyme that let it in would be an ATPase rather than a kinase.
Example. Glutamine synthetase attaches ammonia to glutamate, a reaction with kJ/mol, and it uses one ATP. The enzyme does not transfer the phosphate to ammonia. Where must it go, and why does the answer follow from the requirement for a shared intermediate?
It must go onto the glutamate, and it does: the enzyme first makes gamma-glutamyl phosphate, then ammonia attacks that. The reasoning is that the shared intermediate has to lie on the path to the product, so the phosphate must activate the partner that is going to be attacked, not the attacking group. Phosphorylating ammonia would create a dead end, since the cell would then have to get the phosphate off again and nothing would have been gained. The general pattern is worth carrying forward, because it recurs throughout metabolism: ATP is used to convert a poor leaving group, in this case the hydroxyl of a carboxylic acid, into a good one, in this case phosphate. Aminoacyl-tRNA synthetases do the same thing to attach an amino acid to its tRNA, and fatty acid activation does it with coenzyme A. When you see ATP consumed in a biosynthesis, the useful question is not "where did the energy go" but "which group was made into a better leaving group".
Now you. Some biosynthetic reactions consume ATP by splitting it into AMP and pyrophosphate rather than ADP and phosphate, and the cell then immediately hydrolyses the pyrophosphate to two phosphates with a separate enzyme. Why bother with the second step?
Answer
Because it makes the first step effectively irreversible. Splitting ATP to AMP and pyrophosphate is worth about the same as splitting it to ADP and phosphate, roughly kJ/mol under standard conditions, so on its own it buys nothing extra. Destroying the pyrophosphate afterwards, worth a further kJ/mol, removes one of the products of the first reaction, and by mass action a reaction whose product is continuously removed cannot run backwards. The cell is spending a second phosphoanhydride bond purely to buy directionality. This matters most where a mistake would be expensive: DNA and RNA polymerases, aminoacyl-tRNA synthetases and fatty acid activation all use the AMP route, and all of them are committing steps in the assembly of something large. Pyrophosphatase is one of the busiest enzymes in the cell for this reason, and its job is not metabolism but the enforcement of one-way traffic.
The other currencies
ATP is the main one but not the only one, and the others are worth naming because later lessons use them without ceremony.
GTP is chemically almost identical to ATP and is used where a distinct signal is wanted: protein synthesis, the G proteins of signalling, and one step of the citric acid cycle. Interconversion with ATP is fast, so it is not a separate energy pool so much as a separate label.
NADH and NADPH carry electrons rather than phosphate, delivering a hydride ion to whatever needs reducing. They differ by a single phosphate on the ribose, a group that carries no energy and exists purely so that enzymes can tell them apart, and the cell keeps them in opposite states: NADH is held at roughly one part in a thousand relative to NAD⁺, an oxidising pool suited to stripping electrons off fuel, while NADPH is held at around a hundred to one over NADP⁺, a reducing pool suited to building things. One extra phosphate group lets a cell run oxidation and reduction at full tilt in the same compartment at the same time.
Acetyl-CoA carries an activated two-carbon unit as a thioester, whose hydrolysis is worth about kJ/mol, comparable to ATP. And the proton gradient across a membrane, which the previous lesson treated as an expense, turns out in mitochondria to be the largest energy currency of all.
What a cell spends it on
Rough figures make the priorities visible. Making one peptide bond costs four high-energy phosphate bonds, so an average protein of 300 residues costs about 1200 ATP, roughly 60 kJ per mole of protein, before the amino acids themselves are made. A fast-growing bacterium spends the majority of its total energy budget on protein synthesis, with most estimates falling between sixty and seventy-five per cent. Maintaining ion gradients, as the previous lesson noted, takes twenty to thirty per cent of the energy of a resting animal cell and more in neurons. Everything else, including DNA replication, is small by comparison.
So a cell is an object that spends nearly all of its income on making proteins and on keeping sodium outside. Both are continuous, neither can be paused, and the supply has to run at the rate of the demand.
Example. The human brain is about 1.4 kg, two per cent of a 70 kg body, and consumes roughly twenty per cent of resting energy. Work out its energy use per gram against the body average, and say what the difference is spent on.
Twenty per cent of 8368 kJ is 1674 kJ a day, or about 400 kilocalories, which per gram of tissue is kJ. The whole-body average is kJ per gram. The brain runs at ten times the average rate of the tissue around it. At 50 kJ per mole of ATP that is roughly 17 moles of ATP a day for the organ alone. What it is spent on is mostly the previous lesson's subject rather than this one's: estimates attribute the majority of the brain's energy budget to restoring the sodium and potassium gradients discharged by synaptic and action potential activity, with a further large share to packaging and recycling neurotransmitter. Thinking is expensive because it is electrical, and electrical signalling in a cell means moving ions across a membrane that a pump then has to move back.
Now you. Given that figure, why does concentrating hard on a difficult problem not measurably increase how much you eat?
Answer
Because almost all of the brain's consumption is a standing cost rather than a task cost. The gradients have to be maintained whether or not anything interesting is happening, and the resting firing rates of neurons are already high, so the marginal energy of a demanding task is small against that baseline. Imaging studies that measure regional blood flow and metabolism find task-related increases of a few per cent in the activated regions, which is a large signal locally and a negligible one against total body expenditure of 2000 kilocalories: a few per cent of 400 kilocalories is under 20, which is a biscuit and lies well inside the day-to-day noise in what anyone eats. The wider lesson is one worth carrying into any energy argument about cells. A large total is not the same as a large variable component, and metabolic regulation acts on the part that varies.
Which raises the question this lesson has deferred throughout. If ATP is held times from equilibrium by continuous resupply, what does the resupplying? The next lesson takes the oldest and most widespread answer, a ten-step pathway that every domain of life still runs, and follows the accounting to an uncomfortable conclusion: it captures almost none of the energy available in the fuel it consumes.