Energy

ATP (adenosine triphosphate) is a phosphorylated nucleotide made of three covalently-linked components:

1. Adenine — a nitrogenous base of the purine family. Attached to the 1' carbon of the ribose.
2. Ribose — a five-carbon (pentose) sugar with hydroxyl groups on its 2' and 3' carbons.
3. Three phosphate groups — joined in a linear chain to the 5' carbon of the ribose by phosphoanhydride bonds (sometimes loosely called 'high-energy bonds').

Important name distinctions:

• Adenine = base alone.
• Adenosine = adenine + ribose (a nucleoside).
• AMP = adenosine + 1 phosphate; ADP = adenosine + 2 phosphates; ATP = adenosine + 3 phosphates.

Each phosphate carries a negative charge at cytoplasmic pH, so ATP is highly polar, highly soluble in water, and is repelled from non-polar lipid bilayers — which is why ATP synthase and ATPases are required for transport / hydrolysis at membranes.

ATP is small (mass ~507 g mol⁻¹), stable in the cytoplasm but labile at the active sites of ATPases, and universal: every type of cell from a bacterium to a human neuron uses ATP as its immediate energy carrier.

ATP is synthesised by phosphorylation — the addition of an inorganic phosphate (Pi) to ADP. The reaction is a condensation (water is removed):

$\text{ADP} + \text{P}_\text{i} \rightarrow \text{ATP} + \text{H}_2\text{O}$

This is an endergonic reaction — it requires an input of energy (about 30.6 kJ mol⁻¹ under standard conditions). The energy comes from:

• Respiration — the oxidation of glucose / lipids / amino acids in cells. The bulk of ATP is made by oxidative phosphorylation at the inner mitochondrial membrane during aerobic respiration.
• Photosynthesis — light-dependent reactions in the thylakoid membranes of chloroplasts (photophosphorylation).

The enzyme that catalyses both is ATP synthase, embedded in the mitochondrial inner membrane and the chloroplast thylakoid membrane. It uses the energy of a proton gradient (chemiosmosis) to drive the rotation of part of the enzyme, which couples phosphate to ADP.

A small amount of ATP is also made in the cytoplasm during substrate-level phosphorylation (glycolysis, Krebs cycle), where a phosphate group is transferred directly from a substrate intermediate to ADP.

The cellular pool of ATP is small — only about 5 g at any moment in an adult human — but the turnover is enormous: an adult uses (and resynthesises) approximately 50 kg of ATP per day during normal activity, and many times this during strenuous exercise.

When energy is needed, ATP is hydrolysed by ATPase (ATP hydrolase):

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_\text{i}$

The terminal phosphoanhydride bond is broken (water is added across it) and approximately 30.6 kJ mol⁻¹ of energy is released. The reaction is exergonic because the products (ADP and Pi) are more stable than ATP: the high negative-charge repulsion between adjacent phosphates in ATP is reduced when one phosphate leaves.

A second hydrolysis (ADP → AMP + Pi) is also possible and releases a similar quantity of energy.

The energy is not 'in the bond' as a stored quantity — it is released because of the difference in stability between ATP and ADP + Pi. Modern mark schemes therefore prefer language such as "the energy released when ATP is hydrolysed" rather than "the energy stored in the bond".

Coupling. ATPase is rarely a free-floating enzyme; instead, it is part of the protein that does the work. For example, the Na⁺/K⁺ pump is itself an ATPase — it hydrolyses ATP as part of the same conformational change that moves the ions across the membrane. Coupling like this prevents the energy being released as heat and instead transfers it directly into useful work.

Because the cellular pool of ATP is tiny and the demand is enormous, ATP is continuously recycled between its phosphorylated and dephosphorylated forms. The cycle has two halves:

Right side — synthesis (in mitochondria / chloroplasts / cytoplasm). Energy from respiration (or photosynthesis) is used by ATP synthase to phosphorylate ADP:

$\text{ADP} + \text{P}_\text{i} \xrightarrow{\text{ATP synthase, energy in}} \text{ATP}$

Left side — hydrolysis (anywhere energy is needed). ATP is hydrolysed by an ATPase to release energy and regenerate ADP + Pi:

$\text{ATP} \xrightarrow{\text{ATPase, energy out}} \text{ADP} + \text{P}_\text{i}$

The ADP and Pi released by hydrolysis diffuse back to the mitochondrion (or chloroplast) where ATP synthase rebuilds ATP. The cycle is therefore a conduit that couples energy-releasing (catabolic) reactions to energy-requiring (anabolic) reactions, without the energy having to be released as heat.

A single ADP molecule may be phosphorylated and hydrolysed many thousands of times per day. This is why the body needs only ~5 g of ATP in its pool at any one time, even though it uses ~50 kg per day.

ATP supplies energy for every energy-requiring process in the cell. The Cambridge 9700 specification expects you to know the following examples:

1. Active transport. Carrier proteins such as the Na⁺/K⁺ pump (animal cells) and proton pumps (plant root cells, mitochondrial membranes) are ATPases. ATP binds, is hydrolysed, and the phosphate transferred to the carrier changes its conformation, moving ions against their concentration gradient. The phosphate is then released and the carrier returns to its original shape. Without ATP, no concentration gradients could be maintained — no nerve impulses, no nutrient uptake.

2. Biosynthesis (anabolism). Building macromolecules consumes ATP:

• Protein synthesis — amino acids are activated by attachment to tRNA in an ATP-dependent reaction (one ATP per amino acid); the ribosome uses GTP (similar to ATP) for translocation along the mRNA.
• DNA / RNA synthesis — nucleotide triphosphates (ATP, GTP, CTP, UTP / dATP, dGTP, dCTP, dTTP) are the precursors; the energy of phosphate bond hydrolysis drives polymerisation.
• Glycogen / starch synthesis — glucose is first phosphorylated by ATP to glucose-6-phosphate before being added to the polysaccharide chain.

3. Mechanical work / muscle contraction. In striated muscle, the myosin head binds ATP, hydrolyses it, and undergoes a power stroke that slides the actin filament past the myosin filament (the sliding filament model). Without ATP the myosin head cannot detach from actin — this is the biochemical basis of rigor mortis after death.

4. Cilia and flagella. The dynein motor proteins of microtubules also hydrolyse ATP to produce the bending of cilia and flagella (e.g. the flagellum of a sperm cell, the cilia lining the airways).

5. Nerve impulses. The Na⁺/K⁺ pump in axon membranes uses ATP to re-establish the resting potential after each action potential.

6. Signal transduction. Many cell-signalling pathways use kinases — enzymes that phosphorylate target proteins by transferring the terminal phosphate of ATP. Phosphorylation activates / deactivates the target, transmitting a signal.

7. Endocytosis and exocytosis. Vesicle formation and movement along the cytoskeleton (by motor proteins kinesin and dynein) require ATP.

Glucose stores far more chemical energy per molecule (~2870 kJ mol⁻¹) than ATP releases per hydrolysis (~30.6 kJ mol⁻¹). Why, then, do cells not use glucose directly?

Property	ATP	Glucose
Energy per hydrolysis / oxidation	~30.6 kJ mol⁻¹	~2870 kJ mol⁻¹
Number of steps to release energy	1 (hydrolysis)	~30 (respiration)
Speed of release	Instant	Slow
Universal in all cells	Yes	Glucose is one of several substrates
Solubility / transportability	High	High
Recyclable	Yes (ADP → ATP)	No (once oxidised, gone)
Role	Immediate currency	Long-term storage

The key idea: a cell needs small, manageable packets of energy for individual reactions, not enormous bursts that would be released as heat. Glucose is the bank account (long-term store); ATP is the cash in hand (immediate, divisible currency).

This is also why glucose is stored as glycogen (animals) or starch (plants) — large insoluble polymers that release glucose slowly when needed, which is then converted to ATP for use.