How do cells obtain, store and use energy, and what makes a reaction favorable?
Topic 3.4 Cellular Energy: explain how cells use free energy, ATP and coupled reactions to drive endergonic processes, and how energy flows into and out of biological systems.
A focused answer to AP Biology Topic 3.4, covering free energy, exergonic and endergonic reactions, ATP as the energy currency, energy coupling, and why living systems require a constant input of free energy.
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What this topic is asking
The College Board (Topic 3.4) wants you to explain how cells handle energy: the meaning of free energy, the difference between exergonic and endergonic reactions, the role of ATP as the cell's energy currency, and how cells use energy coupling to drive unfavorable reactions. The big idea is energetics: living systems require a constant input of free energy to maintain their order.
Free energy, exergonic and endergonic
Whether a reaction is exergonic or endergonic is a separate question from how fast it goes. An enzyme changes the rate (it lowers activation energy) but does not change whether a reaction is exergonic or endergonic.
Why life needs constant energy input
ATP, the energy currency
ATP is ideal as an energy currency because the amount of energy released by its hydrolysis is enough to power many cellular jobs but not so large as to be wasteful, and because it is quickly made and broken down.
Energy coupling
Cells run endergonic reactions by coupling them to exergonic ones. The most common partner is ATP hydrolysis.
When ATP is hydrolyzed next to (or as part of) an endergonic reaction, the free energy released, often by transferring a phosphate group to a reactant (phosphorylation), is enough that the overall coupled process is exergonic and therefore proceeds. Coupling is how cells build large molecules, do mechanical work (such as muscle contraction), and move substances against gradients (active transport).
Try this
Q1. Identify whether the hydrolysis of ATP is exergonic or endergonic, and state what the cell uses the released energy for. [2 points]
- Cue. Exergonic; the released free energy is coupled to endergonic processes such as building macromolecules, active transport and mechanical work.
Q2. Explain why organisms need a continuous input of free energy. [2 points]
- Cue. Living systems are highly ordered, and maintaining order is not spontaneous; without a constant input of free energy, organization breaks down.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (short FRQ). A cell builds a large molecule from smaller subunits, a process that is energetically unfavorable on its own. (a) Identify the molecule the cell uses to drive this process. (b) Explain how an unfavorable (endergonic) reaction can be made to proceed in the cell.Show worked answer →
A 4-point concept-explanation FRQ on energy coupling.
(a) Identify (1 point): ATP (adenosine triphosphate).
(b) Explain (3 points): (1 point) the hydrolysis of ATP to ADP and inorganic phosphate is exergonic and releases free energy; (1 point) the cell couples this exergonic reaction to the endergonic building reaction; (1 point) the energy released by ATP hydrolysis (often by transferring a phosphate to a reactant) makes the overall coupled process exergonic, so it proceeds.
Markers reward naming ATP and explaining that coupling an exergonic reaction to an endergonic one makes the overall process favorable.
AP 2021 (style)1 marksSection I (multiple choice). Which statement correctly compares exergonic and endergonic reactions? (A) Exergonic reactions absorb free energy; endergonic release it. (B) Exergonic reactions release free energy; endergonic require an input of free energy. (C) Both release free energy. (D) Neither involves a change in free energy.Show worked answer →
The correct answer is (B).
Exergonic reactions release free energy (they are spontaneous), while endergonic reactions require an input of free energy to proceed. Cells run endergonic reactions by coupling them to exergonic ones, most often ATP hydrolysis. (A) reverses the definitions; (C) and (D) are incorrect.
Related dot points
- Topic 3.2 Enzyme Catalysis: explain how enzymes lower activation energy and how substrate concentration, enzyme concentration and inhibitors affect the rate of an enzyme-catalyzed reaction.
A focused answer to AP Biology Topic 3.2, covering activation energy, the transition state, saturation, the effect of substrate and enzyme concentration, and competitive versus noncompetitive inhibition, with a worked rate calculation.
- Topic 3.5 Photosynthesis: explain how the light-dependent reactions and the Calvin cycle capture light energy and use it to fix carbon dioxide into sugar.
A focused answer to AP Biology Topic 3.5, covering the light-dependent reactions, the electron transport chain, chemiosmosis, the Calvin cycle, and how light energy is converted to the chemical energy of sugars.
- Topic 3.6 Cellular Respiration: explain how glycolysis, the Krebs cycle and oxidative phosphorylation release energy from glucose to make ATP, and how fermentation allows ATP production without oxygen.
A focused answer to AP Biology Topic 3.6, covering glycolysis, the Krebs cycle, oxidative phosphorylation, chemiosmosis, the role of oxygen, and fermentation, with the link back to photosynthesis.
- Topic 2.9 Mechanisms of Transport: explain how active transport and bulk transport move ions and large molecules across membranes and establish electrochemical gradients.
A focused answer to AP Biology Topic 2.9, covering active transport, the sodium-potassium pump, electrochemical gradients, secondary active transport, and bulk transport by endocytosis and exocytosis.
- Topic 1.5 Structure and Function of Biological Macromolecules: explain how a change in the subunit composition or sequence of a polymer may affect its structure and function.
A focused answer to AP Biology Topic 1.5, covering how the sequence and composition of monomers determine the structure and function of macromolecules, illustrated with proteins, sickle-cell haemoglobin, and the directionality of polymers.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)