ATP

In a nutshell

ATP (adenosine triphosphate) is the molecule every cell uses as its immediate energy currency: it holds energy in a form that can be released in a single, quick step exactly where and when a reaction needs it.

This subtopic is about what ATP is made of, how its hydrolysis releases usable energy, how that energy is coupled to reactions in the cell, and how ATP is rebuilt from ADP.

Assumed knowledge: Nucleic acids.

Core content

The structure of ATP

ATP is a nucleotide derivative: it has the same three parts as a nucleotide, but with three phosphate groups instead of one.

A single molecule of ATP is made of:

• adenine, a nitrogenous base
• ribose, a pentose (5-carbon) sugar
• three phosphate groups

The adenine and ribose together are called adenosine, which is why the molecule is adenosine triphosphate. Removing phosphates gives ADP (adenosine diphosphate, two phosphates) and AMP (adenosine monophosphate, one phosphate).

Because it is built from ribose and a base like an RNA nucleotide, ATP is described as a nucleotide derivative:

Hydrolysis of ATP releases energy

ATP releases its energy by hydrolysis: a water molecule is added to break the bond to the terminal (third) phosphate.

ATP + H₂O → ADP + P_i  (energy released)

• The products are ADP (adenosine diphosphate) and an inorganic phosphate group, written P_i.
• The reaction is catalysed by the enzyme ATP hydrolase.
• Breaking that one bond releases a small, usable amount of energy. The energy is released, not made: it was already stored in the molecule.

Still don't get it? · how ATP "releases" energy to drive a reaction

Think of ATP as a loaded mousetrap and ADP as the sprung, empty one. Setting the trap took effort and stored energy in the spring; when the trap snaps shut, that stored energy is let go. Snapping the trap does not create energy, it releases what was put in.

Now the cell's version, one step at a time. A reaction the cell needs (say, moving an ion against a gradient) does not happen on its own because it needs an energy input. So the cell runs ATP hydrolysis at the same time and place: ATP + water breaks to ADP + phosphate and lets go of energy, and that energy is used to make the difficult reaction go. Two reactions run together like this are said to be coupled.

Back to the exam wording: the hydrolysis of ATP is coupled to energy-requiring reactions in the cell, and energy is released (never "produced" or "made"). Say "released", and say the two reactions are coupled.

Coupling and phosphorylation: two ways the energy is used

The energy from ATP hydrolysis is used inside the cell in two linked ways.

• Energy coupling. The hydrolysis of ATP is coupled to energy-requiring reactions within cells, so the energy released drives reactions that would not happen on their own (for example active transport, muscle contraction and synthesising large molecules).
• Phosphorylation. The inorganic phosphate (P_i) released can be transferred onto another compound. Adding a phosphate group is called phosphorylation, and it often makes that compound more reactive.

Still don't get it? · why adding a phosphate makes a molecule "more reactive"

Imagine a heavy, still boulder. Hard to get moving. Now wedge a compressed spring against it: the boulder is now primed, and a small nudge sends it off. You have not changed what the boulder is, you have loaded it so it reacts more easily.

Step by step in the cell: an unreactive molecule sits there and will not change. ATP hands it a phosphate group (this is phosphorylation). Carrying that phosphate, the molecule is now less stable and easier to react, so the next reaction happens more readily.

In mark-scheme words: the inorganic phosphate released from ATP can phosphorylate other compounds, making them more reactive. Note it is the phosphate group that is transferred, so never write "phosphorus" or "P" on its own.

Resynthesis of ATP

ATP is not stored. It is used and remade continuously, so cells constantly rebuild it from ADP and phosphate.

ADP + P_i → ATP + H₂O

• This is a condensation reaction: ADP and P_i join and a molecule of water is released.
• It is catalysed by the enzyme ATP synthase.
• It happens during respiration and during photosynthesis.

Hydrolysis and resynthesis together form the ATP/ADP cycle: the same pool of adenosine is recharged and reused over and over.

Why ATP is a good immediate energy source

ATP suits its job as the cell's energy currency because:

• it releases energy in small, manageable amounts, so little is wasted as heat
• its hydrolysis is a single, one-step reaction, so energy is available rapidly / instantaneously
• it can be rapidly resynthesised from ADP and phosphate
• it is soluble and cannot leave the cell, so each cell makes and uses its own

Worked examples

Model 4-mark answer, "Explain why ATP is a suitable immediate source of energy in cells."

Each numbered point is one distinct, creditable idea. A 4-mark "explain" needs four such points.

1. ATP is hydrolysed in a single-step reaction, so energy is released quickly / immediately when it is needed.
2. Hydrolysis releases a small, manageable amount of energy, so little energy is wasted (lost as heat).
3. The reaction is reversible: ATP can be rapidly resynthesised from ADP and P_i, so it is continuously available.
4. The phosphate released can phosphorylate other compounds, making them more reactive, and the released energy can be coupled to energy-requiring reactions.

Notice what earns the marks: precise words like "small amounts", "single/one-step", "rapidly resynthesised" and "phosphorylate ... more reactive", not vague phrases like "ATP gives lots of energy quickly".

Common exam mistakes

• Writing that ATP or a reaction "produces" or "makes" energy. Energy is released (in hydrolysis) or transferred, never created. "Energy produced" is rejected and can cancel an otherwise correct point.
• Naming the products of hydrolysis as "adenine diphosphate". It is adenosine diphosphate (ADP). Adenine is only the base.
• Writing "P" or "phosphorus" instead of phosphate (or P_i). Phosphorus is the element P; the group transferred is a phosphate group. AQA rejects a bare P (it must be shown as P_i or "P in a circle").
• Calling the reaction ATP → ADP + P_i a "condensation" or "phosphorylation". Breaking ATP down with water is hydrolysis.
• Forgetting the water: hydrolysis of ATP needs water added; resynthesis of ATP is a condensation that releases water. Do not put water on the reactant side of the resynthesis.
• Being vague about why ATP is useful ("it is quick and easy"). The marks need the specific ideas: small/manageable amounts, single reaction, rapidly resynthesised, cannot leave the cell.
• Confusing the two enzymes: ATP hydrolase breaks ATP down; ATP synthase builds it up.

Key definitions

• ATP (adenosine triphosphate): a nucleotide derivative formed from ribose, adenine and three phosphate groups; the immediate source of energy in cells.
• Hydrolysis of ATP: the breaking of ATP into ADP and an inorganic phosphate (P_i) by the addition of water, catalysed by ATP hydrolase, releasing energy.
• Condensation (resynthesis) of ATP: the joining of ADP and an inorganic phosphate to form ATP with the removal of water, catalysed by ATP synthase.
• Phosphorylation: the addition of a phosphate group to a compound, often making it more reactive.
• ATP hydrolase: the enzyme that catalyses the hydrolysis of ATP to ADP and P_i.
• ATP synthase: the enzyme that catalyses the resynthesis of ATP from ADP and P_i during respiration or photosynthesis.

Specification

• I can state that a molecule of ATP is a nucleotide derivative, formed from ribose, adenine and three phosphate groups.
• I can state that the hydrolysis of ATP to ADP and an inorganic phosphate (P_i) is catalysed by ATP hydrolase.
• I can explain that the hydrolysis of ATP can be coupled to energy-requiring reactions within cells.
• I can explain that the inorganic phosphate released during ATP hydrolysis can be used to phosphorylate other compounds, often making them more reactive.
• I can state that ATP is resynthesised by the condensation of ADP and P_i, catalysed by ATP synthase during photosynthesis or during respiration.

Related notes

• Nucleic acids
• Photosynthesis
• Respiration