Energy is necessary for organisms to live. All organisms use ATP (adenosine triphosphate) as a common energy currency.
Basic Knowledge
ATP is a compound consisting of adenine (a base)+ribose (a sugar)+3 phosphate groups. When the bonds between phosphates (high-energy phosphate bonds) are hydrolyzed, about 30.5 kJ/mol of energy is released.
ATP ⇄ ADP + phosphate + energy
(ATP = adenosine triphosphate, ADP = adenosine diphosphate)
This reaction is reversible. ATP is synthesized using energy obtained from catabolic reactions that release energy (such as respiration), and ATP is broken down in anabolic reactions that use energy (such as muscle contraction・active transport・and substance synthesis).
Organisms consume and regenerate large amounts of ATP every second. In an adult human, it is estimated that an amount of ATP nearly equal to body weight (40〜70 kg) cycles through the body each day. This is a dynamic equilibrium in which new ATP is constantly produced and consumed.
ATP (Adenosine triphosphate. Common energy currency)
ADP (Adenosine diphosphate. The form produced when one phosphate is removed from ATP)
High-energy phosphate bond (A bond between phosphates. Releases a large amount of energy upon hydrolysis)
Metabolism (The general term for chemical reactions occurring in living organisms)
Anabolism (Synthesis of complex substances from simple substances・an energy-absorbing reaction)
Catabolism (Breakdown of complex substances・an energy-releasing reaction)
Enzyme (A biological catalyst. A protein that promotes reactions by lowering activation energy)
In Depth
Why is ATP an energy currency? Chemically, ATP's three phosphate groups carry many negative charges and strongly repel one another. When one phosphate group is removed by hydrolysis, this repulsion is relieved and energy is released as the system becomes more stable. In addition, the hydrolysis products (ADP・phosphate) gain resonance stabilization, so the reaction's Gibbs free-energy change ΔG becomes greatly negative (about -30.5 kJ/mol).
Another important factor is the enzyme. Reactions in living organisms have activation energies that are too high to proceed at room temperature on their own, so enzymes change the reaction pathway and lower the energy barrier. Enzymes are proteins (some are RNA) with an active site that binds specifically to a particular substrate. Activity depends on temperature and pH; human enzymes generally show optimum activity around body temperature (37℃) and near-neutral pH.
Anabolism and catabolism work as a set. Organic substances made by photosynthesis (anabolism) are broken down by respiration (catabolism) to obtain ATP, and ATP is used to make proteins and nucleic acids again (anabolism). This is the metabolic cycle.
- ATP = adenine+ribose+3 phosphates
- Reversible reaction: ATP⇄ADP+phosphate+energy
- Hydrolysis releases about 30.5 kJ/mol of energy
- Anabolism = energy absorption・substance synthesis, catabolism = energy release・substance breakdown
- Daily ATP turnover in humans = equivalent to body weight (several dozen kilograms)
- Enzymes = proteins that lower activation energy
- Enzymes have substrate specificity and temperature/pH dependence
Points to Note
① Distinguish ATP (three phosphates) from ADP (two phosphates). When one phosphate is removed from ATP, it becomes ADP. ② Do not reverse anabolism (substance synthesis・energy absorption) and catabolism (substance breakdown・energy release). Associate “synthesis” with “building up in the same direction.” ③ Enzymes are proteins, but not all proteins are enzymes. ④ “High-energy phosphate bond” does not mean that a large amount of energy is stored in the bond itself; it means that hydrolysis releases a large amount of energy.
Practice
- State the base・sugar・and number of phosphates that make up ATP.
- What is the metabolic pathway that breaks down substances to obtain energy?
- Explain how enzymes promote reactions using the term “activation energy.”