Breaking a bond always costs energy. ATP pays out because its products are more stable, and every payout leaks heat.
A cell is a pocket of order in a universe that drifts toward disorder. It stays ordered only by taking in energy nonstop, spending it through ATP, and losing some as heat at every step.
Key terms
- First law of thermodynamics — energy is never created or destroyed, only converted. Respiration turns the energy in glucose into ATP and heat.
- Second law — every energy conversion increases the total entropy (disorder) of a system and its surroundings. Some energy always escapes as heat.
- Free energy — energy available to do work. Exergonic reactions release it (ΔG negative); endergonic ones need it put in (ΔG positive).
- ATP — the cell's energy currency. Hydrolysis splits off its last phosphate, giving ADP + phosphate and releasing free energy.
- Energy coupling — using an exergonic reaction, usually ATP hydrolysis, to drive an endergonic one. The combined ΔG must be negative.
How it works
- Energy enters as light, captured by photosynthesis, or as food. Respiration uses the energy in food to rebuild ATP from ADP and phosphate.
- ATP's three phosphates carry negative charges packed close together, so they repel. Split off the last one and the products are more stable, so free energy is released.
- The cell couples that release to jobs that cost energy. Usually ATP hands its phosphate to another molecule, making a reactant more reactive or changing a protein's shape.
- A working cell rebuilds ATP about as fast as it spends it. Tissues that spend fastest, like heart muscle, pack in the most mitochondria.
- Every conversion along the way loses some energy as heat. That heat raises the entropy of the surroundings by more than the cell's order lowers its own.
Why it matters
This topic is the rulebook for the rest of Unit 3. Respiration doesn't burn glucose in one blast. It releases the energy in many small enzyme steps, so more of it is caught as ATP instead of lost as heat. On data questions, you can read an animal's oxygen use as how fast it spends free energy.
Exam traps
| Trap | Actually |
|---|
| Breaking ATP's phosphate bond releases energy stored in that bond. | Breaking any bond takes energy in. Hydrolysis releases energy overall because ADP and phosphate are more stable than ATP. |
| Building an ordered body breaks the second law. | Organisms are open systems. The heat they give off raises the entropy of their surroundings by more than their own order lowers it. |
| Cells store ATP in large reserves, like fat. | ATP is spent and rebuilt constantly. A working muscle cell recycles its whole ATP supply in under a minute. Fat is the long-term store. |
| A hibernating animal stops using energy until spring. | Its energy use drops far but never to zero. Ion pumps and repairs keep running, and a cell that stops paying for order dies. |
Worked example
Joining molecules X and Y into XY has ΔG = +20 kJ/mol. A cell can couple this reaction to the hydrolysis of ATP (ΔG = −31 kJ/mol) or of compound Q (ΔG = −13 kJ/mol). Which pairing can make XY, and what is the overall ΔG?
- Coupled reactions add their ΔG values. The pair can proceed only if the total is negative.
- With ATP: +20 + (−31) = −11 kJ/mol. Negative, so the pair releases free energy overall and XY forms.
- With Q: +20 + (−13) = +7 kJ/mol. Still positive, so this pair can't make XY.
Only ATP works. Coupled to ATP hydrolysis, the reaction releases 11 kJ/mol overall. Q falls 7 kJ/mol short.
Coupling doesn't create energy. It lets a large release pay for a smaller cost, so the donor's release must be the bigger number. The near-miss is Q: its hydrolysis is exergonic, so it looks helpful, but it releases less than the job costs.