Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- All living systems need a continuous energy input, from light (photosynthesis) or from food molecules (respiration).
- Life is highly ordered. Keeping that order costs energy all the time.
- First law: energy is converted, never created or destroyed. Second law: every conversion releases some heat, so total disorder (entropy) of organism plus surroundings rises.
- Organisms are open systems: they gain order by releasing heat, CO₂ and H₂O, which disorder the surroundings more.
- Energy coupling: energy-releasing (exergonic) processes drive energy-requiring (endergonic) ones, usually through ATP + H₂O → ADP + Pᵢ.
- Pathways are sequential: the product of one step is the reactant of the next, so energy comes out in small, controllable amounts.
- Glycolysis and ATP synthesis by a membrane ATP synthase (as in oxidative phosphorylation) are found in Bacteria, Archaea and Eukarya: evidence of common ancestry.
- The Gibbs free energy equation is not needed.
Key relationships
| Idea | What to write |
|---|---|
| Staying alive | energy input > energy loss; a large, lasting deficit → loss of order → death |
| Coupling test | overall change = energy required (+) + energy released (−); it must come out negative |
| ATP hydrolysis | about −30 kJ mol⁻¹ in standard lab conditions (more in cells) |
| Number of ATP | smallest whole number whose total release exceeds the energy required |
| Energy budget | intake = losses (faeces, heat) + energy stored |
| Blocked pathway step | substrate of that step builds up; later products fall |
Assumptions behind the numbers
- Energy values are per mole of reaction and use the rounded 30 kJ mol⁻¹ for ATP unless a question gives another value.
- An energy budget assumes every route of energy loss has been measured.
Mistakes to avoid
- Saying organisms “make” or “create” energy.
- Saying living things break the second law.
- Saying energy is released when a bond in ATP breaks; the whole hydrolysis reaction releases it.
- Using a fraction of an ATP when counting how many are needed.
- Saying stepwise pathways release more energy than burning; the total is the same.
- Explaining shared pathways only by “all cells need energy” instead of common ancestry.
Quick self-check
- A reaction requires +12 kJ mol⁻¹. What is the overall change when coupled to one ATP? (+12 − 30 = −18 kJ mol⁻¹, so it can proceed.)
- A reaction requires +75 kJ mol⁻¹. How many ATP, at minimum? (Three: 2 × 30 = 60 is too little; 3 × 30 = 90 gives an overall −15 kJ mol⁻¹.)
- How can a seedling become more ordered without breaking the second law? (It releases heat and simple molecules that increase the disorder of its surroundings by more than its own order increases.)
Next: practice questions.