BIOL10008 · Foundations Of Biology: Life's Machinery
Metabolism
Metabolism is the whole set of chemical reactions in a cell, and it runs two ways at once. Catabolism breaks big molecules into small ones and releases energy; anabolism builds big molecules from small ones and consumes energy. This chapter shows how the cell links the two with one universal carrier, ATP: catabolism recharges ATP, anabolism spends it. It explains ATP as a rechargeable battery (ATP ⇌ ADP + Pi) whose job is to capture and transfer free energy, and the logic of energy coupling — an endergonic building reaction is run alongside the exergonic hydrolysis of ATP so the combined process becomes favourable. At the molecular level it maps the build/break logic back to the master reaction from the biomolecules chapter: condensation (releases water) is anabolic, hydrolysis (adds water) is catabolic. The test asks you to classify reactions, state ATP's job, and explain why anabolism must be coupled to ATP hydrolysis.
What this chapter covers
- 01Catabolism vs anabolism — direction of energy flow
- 02ATP as the energy currency: capture and transfer free energy
- 03The ATP cycle (ATP ⇌ ADP + Pi) — a rechargeable battery
- 04Energy coupling: how anabolism gets paid for
- 05Condensation (anabolic) vs hydrolysis (catabolic) at the molecular level
- 06Energy flow — the big picture pipeline
Worked example: classify the reaction and explain the coupling
- +1(a) Classify: building a large molecule (protein) from small ones (amino acids) is anabolic, so energy is required (the reaction is endergonic).
- +1(b) Reaction at each bond: each peptide bond is formed by condensation, which releases one water molecule.
- +1(b) Confirm the direction: condensation = build = water out = anabolic; this matches part (a).
- +1(c) The problem: an anabolic reaction needs an energy input it cannot supply itself, so on its own it will not proceed.
- +1(c) The coupling: the cell runs it alongside the exergonic hydrolysis of ATP (ATP → ADP + Pi); the energy released by ATP hydrolysis is handed to the building reaction, making the combined process favourable — catabolism pays for anabolism via ATP.
Key terms
- Catabolism vs anabolism
- Catabolism breaks large molecules into small ones and releases energy (exergonic); it hydrolyses and recharges ATP. Anabolism builds large molecules from small ones and consumes energy (endergonic); it condenses and spends ATP.
- ATP (adenosine triphosphate)
- The cell's universal energy currency. Its job is to capture and transfer free energy: hydrolysing the terminal phosphate (ATP → ADP + Pi) releases energy to do work, and ADP + Pi is recharged to ATP using energy from respiration or photosynthesis.
- Energy coupling
- Running an unfavourable (endergonic) building reaction alongside the favourable (exergonic) hydrolysis of ATP so that the combined process is favourable. This is how the cell makes uphill reactions run.
- Exergonic vs endergonic
- Exergonic reactions release free energy (e.g. catabolism, ATP hydrolysis); endergonic reactions require a free-energy input (e.g. anabolism). Coupling pairs an endergonic reaction with an exergonic one.
- Condensation = anabolic, hydrolysis = catabolic
- At the molecular level the build/break logic of metabolism is the master reaction: condensation joins monomers and releases water (anabolic, energy in); hydrolysis adds water and breaks bonds (catabolic, energy out).
Metabolism FAQ
What is the actual job of ATP?
To capture and transfer free energy within the cell — it is the intermediary, not the ultimate energy source. Energy released by catabolic reactions is used to recharge ATP from ADP + Pi; the cell then spends ATP (ATP → ADP + Pi) to power anabolic building and other work. ATP links the reactions that release energy to the reactions that need it, which is why it is called the energy currency.
Why is ATP described as a rechargeable battery rather than a fuel tank?
Because the cell does not store ATP in bulk like petrol in a tank — it holds only a small amount and cycles it thousands of times a day. ATP → ADP + Pi discharges energy to power a process; ADP + Pi → ATP recharges it using energy from respiration or photosynthesis. The same few 'batteries' are swapped between discharged and charged states constantly.
Why must anabolism be coupled to ATP hydrolysis?
Because anabolic building is endergonic — it needs an energy input it cannot supply itself, so it will not proceed alone. The cell solves this by coupling: it runs the building reaction alongside the exergonic hydrolysis of ATP, so the energy released by ATP → ADP + Pi drives the building forward. In one line: anabolic building requires energy, and ATP hydrolysis is an energy-releasing reaction that supplies it.
How does metabolism connect back to the biomolecules chapter?
Through the master reaction. Whenever you see 'polymer → monomers' (digestion, respiration, recycling proteins), it is hydrolysis = catabolism = energy out. Whenever you see 'monomers → polymer' (making DNA, protein or starch), it is condensation = anabolism = energy in. One pair of reactions explains both halves of metabolism, which is why linking metabolism to condensation/hydrolysis is a favourite integrative exam move.
Exam move
Hold one master grid in your head: catabolism = break = hydrolysis = water in = energy out = ATP made; anabolism = build = condensation = water out = energy in = ATP spent. Then practise classifying any reaction by spotting whether a big molecule is being broken or a big one built. State ATP's job in one line (capture and transfer free energy) and be ready to explain energy coupling: an endergonic build is run alongside exergonic ATP hydrolysis so the combined process becomes favourable. Keep the rechargeable-battery analogy and the condensation/hydrolysis link ready — the integrative question that ties metabolism back to the biomolecules chapter recurs.
