ATP (Adenosine Triphosphate)
Adenosine triphosphate (ATP) is the primary energy currency of all living cells, consisting of an adenine base, a ribose sugar, and three phosphate groups linked by high-energy phosphoanhydride bonds. When the terminal phosphate group is hydrolysed by ATPase enzymes to yield ADP (adenosine diphosphate) and inorganic phosphate (Pᵢ), approximately 30.5 kJ/mol of free energy is released under standard conditions (and up to ~54 kJ/mol under physiological conditions), which drives endergonic cellular processes including muscle contraction, active transport, biosynthesis, and signal transduction. A typical human cell turns over its own body weight in ATP every day, with mitochondrial oxidative phosphorylation producing the vast majority of this ATP.
Key Formula
ATP + H2O → ADP + Pi; ΔG°' = -30.5 kJ/mol
LaTeX: \text{ATP} + H_2O \xrightarrow{\text{ATPase}} \text{ADP} + P_i \quad \Delta G^{\circ\prime} = -30.5\text{ kJ/mol}
| Symbol | Meaning | Unit |
|---|---|---|
| ATP | Adenosine triphosphate | mol |
| ADP | Adenosine diphosphate (product) | mol |
| Pᵢ | Inorganic phosphate (product) | mol |
| ΔG°' | Standard free energy change at pH 7 | kJ/mol |
| H₂O | Water (reactant in hydrolysis) | mol |
Worked Example
Problem
A myosin motor protein hydrolyses 200 ATP molecules per second during muscle contraction. If ΔG = -54 kJ/mol under physiological conditions, how much free energy (in kJ) is released per second? (Avogadro's number = 6.022 × 10²³)
Solution
Step 1: Convert molecules/s to mol/s: 200 ÷ (6.022 × 10²³) = 3.32 × 10⁻²² mol/s. Step 2: Energy released = 3.32 × 10⁻²² mol/s × 54 kJ/mol. Step 3: Energy = 1.79 × 10⁻²⁰ kJ/s per myosin head. Step 4: Alternative approach — energy per ATP molecule = 54 × 10³ J ÷ (6.022 × 10²³) = 8.97 × 10⁻²⁰ J; for 200 ATP: 200 × 8.97 × 10⁻²⁰ = 1.79 × 10⁻¹⁷ J/s = 1.79 × 10⁻²⁰ kJ/s.
Answer
Approximately 1.79 × 10⁻²⁰ kJ of free energy is released per second per myosin head.
ATP Sources and Their Contribution to Total ATP Yield per Glucose
| Pathway | ATP Produced Directly | Electron Carriers | ATP via ETC | Total ATP |
|---|---|---|---|---|
| Glycolysis | 2 (substrate-level) | 2 NADH (cytoplasmic) | 3–5 | 4–6 |
| Pyruvate oxidation | 0 | 2 NADH (mitochondrial) | 5 | 5 |
| Krebs cycle (×2) | 2 (substrate-level) | 6 NADH + 2 FADH₂ | 22 | 24 |
| Total (aerobic) | 4 | 10 NADH + 2 FADH₂ | 28 | 30–32 |
| Anaerobic only | 2 | Recycled to NAD⁺ | 0 | 2 |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Cellular Respiration
Cellular respiration is the set of metabolic reactions by which cells break down organic molecules — primarily glucose — in the presence or absence of oxygen to generate ATP, the universal energy currency of life. The complete aerobic pathway yields approximately 30–32 ATP molecules per glucose molecule through three sequential stages: glycolysis (cytoplasm), the citric acid (Krebs) cycle (mitochondrial matrix), and oxidative phosphorylation via the electron transport chain (inner mitochondrial membrane). Understanding cellular respiration is fundamental to nutrition science, exercise physiology, and the treatment of metabolic diseases.
Aerobic Respiration
Aerobic respiration is the form of cellular respiration that requires molecular oxygen (O₂) as the final electron acceptor in the electron transport chain, enabling the complete oxidation of glucose to carbon dioxide and water with maximum ATP yield (~30–32 ATP per glucose). It proceeds through glycolysis, the link reaction (pyruvate decarboxylation), the citric acid cycle, and oxidative phosphorylation, all of which are tightly coupled within the mitochondrion. Aerobic respiration is the predominant energy-yielding pathway in all eukaryotes and many prokaryotes under oxygen-sufficient conditions, underpinning sustained muscular activity, brain function, and virtually every energy-demanding cellular process.
Photosynthesis
Photosynthesis is the biochemical process by which chlorophyll-containing organisms — plants, algae, and cyanobacteria — convert light energy into chemical energy stored as glucose, using carbon dioxide and water as raw materials and releasing oxygen as a by-product. The process occurs in two stages: the light-dependent reactions in the thylakoid membranes (which generate ATP and NADPH by splitting water) and the light-independent Calvin cycle in the stroma (which fixes CO₂ into three-carbon sugars). Photosynthesis underpins nearly all food chains on Earth and is responsible for maintaining atmospheric oxygen levels.
The full name "adenosine triphosphate" reflects its components: "adenosine" (adenine + ribose) from the Greek "aden" (ἀδήν) meaning "gland" (as adenine was first isolated from pancreatic tissue) + "triphosphate" from Greek "tri" (three) + Latin "phosphorus" (phosphorus). ATP was first isolated and characterised independently by Karl Lohmann and by Cyrus Fiske and Yellapragada SubbaRow in 1929, with its central role in energy transfer established by Fritz Lipmann in 1941.