C. Metabolic Processes · Metabolic ProcessesATP: The Cell's Energy Currency and Energy Coupling
C3.3 C3.1 C2.1 A1.12About 65 minutes
Learning goals- I can describe the structure of ATP and explain why its hydrolysis releases free energy.
- I can explain the ATP cycle and why cells regenerate ATP constantly.
- I can explain energy coupling and calculate the overall ΔG of a coupled reaction.
- I can tell substrate-level phosphorylation apart from oxidative phosphorylation and photophosphorylation.
Words to know
ATP (adenosine triphosphate)
A nucleotide made of adenine, ribose and three phosphate groups; the main short-term energy carrier in cells.
ADP (adenosine diphosphate)
ATP after it has lost one phosphate group.
inorganic phosphate (Pᵢ)
A free phosphate ion (HPO₄²⁻), released when ATP is hydrolysed.
hydrolysis
Breaking a bond by adding water. ATP + H₂O → ADP + Pᵢ.
phosphorylation
Adding a phosphate group to a molecule.
energy coupling
Using an exergonic reaction (usually ATP hydrolysis) to drive an endergonic one, so that the overall ΔG is negative.
energy carrier
A molecule that picks up energy in one reaction and delivers it to another: ATP carries energy in phosphate bonds; NADH, FADH₂ and NADPH carry energy as high-energy electrons.
substrate-level phosphorylation
Making ATP by transferring a phosphate group directly from a phosphorylated substrate molecule to ADP, using an enzyme.
oxidative phosphorylation
Making ATP using the energy of electrons passed along the electron transport chain to oxygen, through chemiosmosis and ATP synthase.
photophosphorylation
Making ATP in chloroplasts using light energy, through chemiosmosis and ATP synthase.
Why not just burn the glucose?
One mole of glucose holds 2870 kJ of free energy. If a cell released all of it in one step, it would be like lighting a campfire to toast one marshmallow: most of the energy would be lost as heat, and the heat could damage the cell. Instead, cells release energy in small steps and store it in small, spendable packets. Those packets are molecules of ATP.
Think of glucose as a $100 bill and ATP as a pocketful of loonies. You can't put a $100 bill into a parking meter, but you can spend loonies anywhere. Almost every energy-requiring job in a cell is paid for with ATP.
Remember from Biochemistry
In the Biochemistry unit you met the four main types of biochemical reactions: redox, hydrolysis, condensation and neutralization, and you saw that nucleotides are built from a base, a sugar and phosphate. ATP is a nucleotide. Making ATP from ADP is a condensation (water is removed); breaking it is hydrolysis (water is added).
The structure of ATP
ATP (adenosine triphosphate) has three parts:
- Adenine, a nitrogenous base (the same A found in DNA and RNA).
- Ribose, a five-carbon sugar. Adenine + ribose together are called adenosine.
- A chain of three phosphate groups.
Each phosphate group carries negative charges, and they sit close together. Like crowding magnets with their north poles together, the chain is strained and unstable. When the last phosphate is removed by hydrolysis, the charges are farther apart and the products (ADP and Pᵢ) are much more stable, especially because they interact well with water. That is why ATP hydrolysis releases free energy:
ATP + H₂O → ADP + Pᵢ ΔG = −30.5 kJ/mol (standard conditions)
In a living cell, where ATP is kept much more concentrated than ADP, ΔG is even more negative, often around −50 kJ/mol.
A common misconception: people sometimes say ATP has "high-energy bonds" that release energy when they break. Remember from Lesson 1 that breaking any bond needs energy. The free energy comes from the whole reaction: the products are much more stable than the reactants.
The ATP cycle
A cell holds only a small supply of ATP at any moment, so it must keep rebuilding it. A busy muscle cell can turn over its whole ATP supply in about a minute.
- Spending: ATP is hydrolysed to ADP + Pᵢ, releasing energy for work.
- Recharging: ADP is phosphorylated back to ATP: ADP + Pᵢ → ATP + H₂O, ΔG = +30.5 kJ/mol. This is endergonic, so it needs energy from catabolism (cellular respiration) or, in chloroplasts, from light.
Roughly speaking, an adult regenerates about their own body mass of ATP in a day, while only a few grams exist at any one moment. ATP is therefore a carrier of energy, not a long-term store. For long-term storage, cells use glycogen, starch and fats.
Energy coupling: how ATP pays for work
Many reactions a cell needs are endergonic (ΔG > 0), so they won't happen on their own. The cell couples them to ATP hydrolysis. Because free-energy changes add, if the total ΔG is negative, the coupled process goes.
Example: making the amino acid glutamine.
- glutamate + NH₃ → glutamine ΔG = +14.2 kJ/mol (won't go alone)
- ATP + H₂O → ADP + Pᵢ ΔG = −30.5 kJ/mol
- Overall: glutamate + NH₃ + ATP → glutamine + ADP + Pᵢ ΔG = 14.2 + (−30.5) = −16.3 kJ/mol ✔
Coupling only works if the two reactions are physically linked. Usually ATP transfers its phosphate to one of the reactants, making a phosphorylated intermediate that is less stable and more reactive. In the glutamine example, glutamate is first phosphorylated by ATP, then the phosphate is swapped for NH₃.
The same idea powers transport and movement. A membrane pump changes shape when ATP phosphorylates it, moving ions against their gradient (active transport, from the Biochemistry unit). A motor protein in muscle changes shape when ATP binds and is hydrolysed, pulling on an actin filament.
Three ways to make ATP
You will meet all three in this unit, so learn the names now.
| Method | How the phosphate gets onto ADP | Where it happens (eukaryotes) |
|---|
| Substrate-level phosphorylation | An enzyme transfers a phosphate directly from a phosphorylated substrate to ADP | Glycolysis (cytosol) and the Krebs cycle (mitochondrial matrix) |
| Oxidative phosphorylation | Energy from electrons moving along the electron transport chain to O₂ pumps H⁺; H⁺ flowing through ATP synthase makes ATP (chemiosmosis) | Inner mitochondrial membrane |
| Photophosphorylation | Light energy drives electrons along a transport chain; H⁺ flowing through ATP synthase makes ATP (chemiosmosis) | Thylakoid membranes of chloroplasts |
Worked exampleLet's test whether coupling works
The reaction glucose + Pᵢ → glucose 6-phosphate + H₂O has ΔG = +13.8 kJ/mol. In the first step of glycolysis, the cell instead uses ATP: glucose + ATP → glucose 6-phosphate + ADP. Find ΔG of the coupled reaction and decide whether it is spontaneous.
- Write the two reactions to add: (1) glucose + Pᵢ → glucose 6-phosphate + H₂O, ΔG = +13.8 kJ/mol; (2) ATP + H₂O → ADP + Pᵢ, ΔG = −30.5 kJ/mol.
- Add them. Pᵢ and H₂O appear on both sides and cancel: glucose + ATP → glucose 6-phosphate + ADP.
- Add the ΔG values: +13.8 + (−30.5) = −16.7 kJ/mol.
- The total is negative, so the coupled reaction is exergonic.
ΔG = −16.7 kJ/mol, so the coupled reaction is spontaneous. The cell "invests" one ATP to make glucose more reactive, which you will see again in glycolysis.
Worked exampleLet's count the ATP needed for a job
Pumping one mole of a substance into a cell against its concentration gradient requires about +20 kJ. Assume ATP hydrolysis supplies 50 kJ/mol under cell conditions, and that one ATP is hydrolysed per pumping cycle. Is one ATP enough per cycle? What happens to the extra energy?
- Compare the energy needed (20 kJ/mol) with the energy supplied (50 kJ/mol).
- Overall ΔG = +20 + (−50) = −30 kJ/mol: negative, so one ATP is more than enough.
- The extra 30 kJ/mol is not wasted in the sense of being destroyed; it is released as heat (second law).
- This is one reason active cells, and active animals, warm up.
Yes, one ATP per cycle is enough (overall ΔG ≈ −30 kJ/mol). The surplus energy is released as heat.
Just for funWhy did the ADP molecule feel so tired?
It was running low on phosphate and needed to get recharged: one more P and it's back to ATP.
Hands-on activityModel the ATP cycle with a spring toy
You need: 3 small sticky notes or paper squares labelled P, a strip of paper labelled adenosine, a spring-loaded clothespin (or a ballpoint pen with a click spring), and a timer.
- Build ATP on the table: the adenosine strip followed by three P squares in a row.
- The clothespin models the energy in ATP. Squeeze it open and hold it: that is ATP, full of potential energy. Let it snap shut: that is hydrolysis (ATP → ADP + Pᵢ). Move one P square away from the chain as you let go.
- Opening the clothespin again (the recharge) takes effort from your fingers, just as phosphorylating ADP takes energy from catabolism.
- Turnover race: set a timer for 30 seconds. Each cycle (squeeze, release, move the P away and back) is one ATP used and rebuilt. Count your cycles.
Think about it:
- Where does your finger energy come from? (Your own ATP!)
- Your hand gets tired after many cycles. What would a cell need to keep the ATP cycle going?
- Why is it useful that ATP holds a medium-sized packet of energy, not a huge one?
What to look for: the student should connect squeezing to the endergonic phosphorylation of ADP (needs energy) and releasing to exergonic hydrolysis (releases energy for work). A cell needs a constant supply of food (or light) and oxygen to keep regenerating ATP. A medium-sized packet (about 30 to 50 kJ/mol) is big enough to drive most endergonic reactions but small enough that little energy is wasted per job.
Your turn
1Name the three parts of an ATP molecule.
Adenine (a nitrogenous base), ribose (a five-carbon sugar) and three phosphate groups.
2Write the equation for ATP hydrolysis with its standard ΔG.
ATP + H₂O → ADP + Pᵢ, ΔG = −30.5 kJ/mol.
3Why is ATP hydrolysis exergonic?
The three phosphate groups carry crowded negative charges that repel each other. The products, ADP and Pᵢ, are more stable (charges are separated and better stabilized by water), so free energy is released.
4Why must cells keep regenerating ATP?
Cells hold only a small amount of ATP at a time and use it very quickly. ATP is an energy carrier, not a long-term store, so ADP must be continually rephosphorylated using energy from respiration (or light).
5A reaction has ΔG = +22 kJ/mol. Can it be driven by coupling it to the hydrolysis of one ATP (−30.5 kJ/mol)? Show the overall ΔG.
Overall ΔG = +22 + (−30.5) = −8.5 kJ/mol. It is negative, so yes, coupling to one ATP drives it.
6What is substrate-level phosphorylation? Name one pathway where it happens.
An enzyme transfers a phosphate group directly from a phosphorylated substrate to ADP to make ATP. It happens in glycolysis (and the Krebs cycle).
7How do cells physically link ATP hydrolysis to an endergonic reaction?
ATP transfers a phosphate to a reactant (or a protein), forming a phosphorylated intermediate that is more reactive (or a protein that changes shape). The next step then releases the phosphate as the product forms.
More practice: warm-up
3Is the phosphorylation of ADP exergonic or endergonic?
Endergonic (ΔG ≈ +30.5 kJ/mol).
4Name the process that makes ATP using light energy.
5Name one long-term energy store in animal cells.
Glycogen or fat (triglycerides).
More practice: on your own
1Two reactions: A → B, ΔG = +40 kJ/mol. ATP hydrolysis in the cell provides −50 kJ/mol per ATP. How many ATP must be hydrolysed (at minimum) to drive A → B, and what is the overall ΔG?
One ATP gives +40 − 50 = −10 kJ/mol, which is negative, so one ATP is enough. (With the standard value −30.5 kJ/mol, one would not be enough: +40 − 30.5 = +9.5; two would be needed: +40 − 61 = −21 kJ/mol.)
2Explain why ATP is described as "energy currency" rather than "energy savings".
ATP is spent quickly and recycled constantly; a cell holds only enough for a short time. Long-term energy is "saved" in glycogen, starch and fat, which are converted to ATP as needed, just as savings are withdrawn as cash.
3A student says, "Energy is released when the bond in ATP breaks." Explain what is wrong and give a better statement.
Breaking a bond always requires energy. Free energy is released by the overall hydrolysis reaction because the products (ADP + Pᵢ) are much more stable than ATP + H₂O; new, more stable interactions form in the products.
4Compare substrate-level phosphorylation and oxidative phosphorylation (where, and how).
Substrate-level: in the cytosol (glycolysis) and the matrix (Krebs); a phosphate is moved directly from a substrate to ADP by an enzyme; makes little ATP. Oxidative: on the inner mitochondrial membrane; electron transport pumps H⁺, and H⁺ flowing through ATP synthase makes ATP; makes most of the cell's ATP.
5Sodium-potassium pumps use about a quarter or more of the ATP made by a resting human. Explain why stopping ATP production (for example, by a poison) quickly affects these pumps.
The pumps move ions against their concentration gradients, which is endergonic and must be coupled to ATP hydrolysis for every cycle. Because cells hold only a small ATP pool, once ATP production stops the pumps run out of energy within seconds to minutes, and ion gradients start to collapse.
Challenge
1The reaction fructose 6-phosphate + Pᵢ → fructose 1,6-bisphosphate + H₂O has ΔG = +16.3 kJ/mol. Write the coupled reaction with ATP and find its ΔG. Then explain why a cell does not simply use Pᵢ directly.
Coupled: fructose 6-phosphate + ATP → fructose 1,6-bisphosphate + ADP. ΔG = +16.3 + (−30.5) = −14.2 kJ/mol. Using Pᵢ directly would be endergonic (+16.3), so the reaction would not proceed; transferring the phosphate from ATP makes the overall process exergonic.
2Use the second law to explain why the energy captured in ATP from glucose can never equal the 2870 kJ/mol released by oxidizing glucose.
Every energy transfer increases entropy and releases some energy as heat. Making ATP from glucose involves many steps, each one releasing some heat, so the ATP formed always holds less than 2870 kJ per mole of glucose. (You will calculate the efficiency in a later lesson; it is roughly a third.)
3A cell has a 1000-fold higher concentration of ATP than ADP. Without calculating, predict whether ΔG for ATP hydrolysis in the cell is more or less negative than −30.5 kJ/mol, and explain.
More negative. A high ratio of reactant (ATP) to products (ADP, Pᵢ) pushes the reaction further from equilibrium, so more free energy is released per mole hydrolysed. Cells keep ATP high and ADP low precisely so that ATP is a powerful energy source (often about −50 kJ/mol in cells).
Quiz
1. Which part of ATP is released when ATP is hydrolysed to ADP?
2. ATP hydrolysis releases free energy mainly because:
3. A reaction with ΔG = +12 kJ/mol is coupled to ATP hydrolysis (−30.5 kJ/mol). The overall ΔG is:
4. In which process is ATP made by an enzyme transferring a phosphate group directly from a substrate to ADP?
5. Why is ATP not used for long-term energy storage?
6. Where does the energy to rebuild ATP from ADP and Pᵢ come from in a muscle cell?
Big idea: ATP is the link between energy-releasing (catabolic) and energy-using (anabolic) reactions. Coupling works because ΔG values add (C3.3, C2.1).
How to help: The money analogy works well: glucose is a large bill, ATP is pocket change. Ask your teen to add ΔG values for a coupled reaction and say whether the result is spontaneous. The clothespin activity is quick; any spring toy will do.
Common difficulties:
- Saying that "breaking the high-energy bond releases energy". The energy comes from the overall reaction.
- Forgetting that rebuilding ATP is endergonic and needs energy from respiration or light.
- Mixing up the three ways of making ATP. A useful check: only substrate-level phosphorylation happens without ATP synthase.