ATP

AQA spec ref: 3.1.4 - ATP

ATP (adenosine triphosphate) is the universal energy currency of all living cells. Energy released by respiration is not used directly to drive cellular processes - it is first used to synthesise ATP, which then acts as the immediate energy source. This indirect transfer is more controlled and efficient than burning glucose directly.

Structure of ATP

ATP is a nucleotide derivative - it shares the same basic structure as the nucleotides in RNA:

• Adenine - a nitrogenous base (purine)
• Ribose - a pentose sugar (5-carbon)
• Three phosphate groups - linked in series (α, β, γ phosphate)

The three phosphate groups are linked by phosphoanhydride bonds. These bonds are relatively unstable due to the negative charges on adjacent phosphate groups repelling each other. This makes them readily hydrolysed, releasing energy.

Hydrolysis of ATP

When ATP is hydrolysed by the enzyme ATP hydrolase, the terminal (γ) phosphate group is removed, releasing:

• ADP (adenosine diphosphate)
• Inorganic phosphate (Pᵢ)
• Energy - approximately 30.5 kJ mol⁻¹ under standard conditions (though available energy in cells is closer to 50 kJ mol⁻¹ due to actual concentrations)

Hydrolysis of a second phosphate produces AMP (adenosine monophosphate), releasing additional energy. AMP is less commonly used in this way but is important as a signal of low energy status.

The released inorganic phosphate can be transferred to another molecule (phosphorylation), which often activates it - for example, phosphorylating a protein to change its conformation, or phosphorylating glucose in the first step of glycolysis.

Synthesis of ATP

ATP is continuously regenerated from ADP and Pᵢ by condensation - the reverse reaction, catalysed by ATP synthase:

There are two mechanisms for driving this synthesis:

Oxidative phosphorylation - in the inner mitochondrial membrane (and thylakoid membrane in chloroplasts), ATP synthase is driven by the flow of H⁺ ions down an electrochemical gradient (chemiosmosis). This produces the vast majority of ATP in aerobic respiration. See Respiration and Photosynthesis.

Substrate-level phosphorylation - a phosphate group is transferred directly from a phosphorylated intermediate to ADP, without involving the electron transport chain. This occurs in glycolysis (e.g. pyruvate kinase phosphorylates ADP) and in the Krebs cycle. It produces a small yield of ATP regardless of oxygen availability.

Why ATP is the Ideal Energy Currency

AQA sometimes asks why cells use ATP rather than releasing energy directly from glucose. The reasons are:

• Releases energy in small, manageable amounts - the hydrolysis of one phosphate releases exactly the right amount of energy for most cellular reactions. Direct glucose oxidation would release too much energy at once, most of which would be lost as heat.
• Immediate energy source - no intermediate reactions needed. ATP is ready to donate energy directly.
• Couples energy-releasing reactions to energy-requiring reactions - ATP acts as a link between catabolic (energy-releasing) and anabolic (energy-requiring) reactions.
• Cannot leave the cell - ATP is too large and charged to cross the plasma membrane, so energy is kept within the cell that produces it.
• Universal - all organisms use ATP, so the same enzymatic machinery works across all life.

Roles of ATP in Cells

ATP provides energy for virtually every energy-requiring process:

• Active transport - ATP hydrolysis drives conformational changes in carrier proteins (e.g. Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in per ATP)
• Muscle contraction - ATP binds to myosin heads, allowing them to detach from actin and recock (see Skeletal Muscles as Effectors)
• Biosynthesis - forming peptide bonds (protein synthesis), glycosidic bonds (polysaccharide synthesis), and phosphodiester bonds (nucleic acid synthesis)
• Nerve impulse transmission - Na⁺/K⁺ ATPase restores resting potential after an action potential (see Neurons and Synapses)
• Secretion - moving vesicles along cytoskeletal tracks (kinesin uses ATP)

Summary

• ATP=adenine+ribose+3 phosphate groups
• Hydrolysis (ATP hydrolase): ATP → ADP + Pᵢ + energy
• Synthesis (ATP synthase): ADP + Pᵢ + energy → ATP
• Two synthesis routes: oxidative phosphorylation (chemiosmosis) and substrate-level phosphorylation
• ATP is ideal because: releases energy in small amounts; immediate; universal; cannot leave the cell

AQA Exam Tips

• ATP is not the energy source for respiration - glucose is. ATP is the product of respiration and the immediate energy source for cellular work. This distinction comes up in mark schemes.
• Hydrolysis vs hydration: ATP hydrolysis requires water (H₂O splits the phosphoanhydride bond). Make sure you write "hydrolysis" not just "breakdown."
• Phosphorylation activates molecules: when Pᵢ is transferred to glucose (in glycolysis), it destabilises the molecule and makes it more reactive. AQA often asks why glucose is phosphorylated - state: it makes glucose more reactive and prevents it from leaving the cell.
• Substrate-level vs oxidative phosphorylation: substrate-level = direct transfer of phosphate from an intermediate to ADP (no membrane, no H⁺ gradient). Oxidative phosphorylation = ATP synthase driven by chemiosmosis. Know which occurs where.
• ADP + Pᵢ → ATP requires energy input - it is endergonic. The energy comes from the oxidation of reduced coenzymes (NADH, FADH₂) in the electron transport chain.