ATP is the molecule cells reach for whenever they need energy now. It is hydrolysed in a single step to release a controlled pulse of usable energy, then regenerated within seconds from its products.
ATP is built from adenine, ribose, and three phosphate groups.
ATP is a nucleotide derivative; it shares its family of components with the nucleotides that build RNA. Three components account for its full structure. The structure determines the chemistry, and the chemistry determines the function.
Three components in series. Adenine: a nitrogenous base, the same adenine found in DNA and RNA nucleotides. Ribose: a five-carbon sugar, the same ribose that builds RNA nucleotides. Three phosphate groups: attached in series to the ribose, one after another. All three must be named for the AQA structure mark.
Name the sugar
ribose, notpentose. AQA's specification names ribose specifically for this topic, andpentoseis explicitly rejected on ATP questions.
Don't name the bonds inside ATP.
Glycosidic bondsandhydrogen bondsare both rejected on this topic; describe the inter-phosphate bonds without a formal name.
ATP releases energy when hydrolysed to ADP and inorganic phosphate.
Cells need energy continuously, and ATP supplies it through a single enzyme-catalysed hydrolysis reaction. Two features make this practically useful: the energy arrives in a single step, and the quantity released is sized for one cellular reaction at a time.
Equation: ATP + H₂O → ADP + Pi, catalysed by ATP hydrolase; the reaction releases energy. The bond broken is the one between the second and third phosphate groups. The products are ADP (adenosine diphosphate) and inorganic phosphate (Pi), and the Pi can be transferred onto another molecule afterwards.
ATP releases energy when hydrolysed; produces, makes, creates, and generates are not credited by AQA.
Phosphorylationis the covalent attachment of Pi to a target molecule, raising its reactivity. It is not the formation of anenzyme-substrate complex; don't carry active-site vocabulary into phosphorylation answers.
ATP is regenerated by condensation, catalysed by ATP synthase.
ATP is not stockpiled. It is regenerated from ADP and Pi within seconds of hydrolysis, by the reverse reaction. The energy that drives the condensation comes from aerobic respiration in all living cells, and from photosynthesis in photosynthetic cells such as plant mesophyll cells and algae.
Equation: ADP + Pi → ATP + H₂O, catalysed by ATP synthase; the reaction requires energy input. The reaction type is condensation: water is released as the new bond between the second and third phosphate groups forms. The energy is supplied by aerobic respiration and, in photosynthetic cells, by photosynthesis.
Hydrolysis and synthesis are the same reaction in opposite directions.
| Direction | Enzyme | Reaction type | Position of water |
|---|---|---|---|
| Hydrolysis | ATP hydrolase | Hydrolysis | Reactant (left) |
| Synthesis | ATP synthase | Condensation | Product (right) |
The enzyme name tells you the direction.
Synthasemeans synthesis, with water on the right.Hydrolasemeans hydrolysis, with water on the left. Direction errors score zero.
Pitfall — Two-step ATP questions need both halves
Two-step ATP questions need both halves.
Phosphorylation and ATP-function questions are marked as paired mark points: a source step (energy released from ATP hydrolysis) and a consequence step (the energy drives a named process such as the active transport of ions). Stating only one half caps the answer at one mark, no matter how detailed the named half is.
ATP powers active transport, muscle contraction, secretion, and biosynthesis.
ATP is the energy currency for a defined set of cellular processes. The 3.1.6 expectation is to name these processes and link them to ATP hydrolysis as the driving step. Detailed mechanism for each process belongs to later topics.
Active transport: ATP hydrolysis drives transport proteins that move ions against a concentration gradient. Muscle contraction: ATP is required across the filament cycle (mechanism at 3.6.3). Secretion: vesicle packaging, transport, and fusion with the plasma membrane. Anabolic biosynthesis: protein, polysaccharide, and nucleic acid synthesis. Activation of molecules: Pi attaches covalently to a substrate in phosphorylation, raising its reactivity.
In ion-pump questions, the substance transported is
ions(often Na⁺ or K⁺), not glucose. Substituting glucose for ions is a recurring reject in active-transport mark schemes.
Key terms
- Pi
- ribose
- ADP
- hydrolysis
- adenine
- enzyme
- ATP synthase
- ATP hydrolase
- inorganic phosphate
- condensation