AP Bio Unit 3: cellular energetics.
AP Bio Unit 3 is Cellular Energetics: five College Board topics on how cells capture energy and put it to work, from enzymes and the conditions that change them to photosynthesis and cellular respiration. College Board gives it 12–16% of the multiple-choice section and no unit weight on the free response. For the other seven units and a plan for the whole exam, see our guide on how to study for AP Biology.
AP Bio Unit 3 in five points
- Five topics, 3.1 to 3.5, over about 12–14 class periods; its 12–16% multiple-choice weight is level with Unit 6.
- Enzymes lower activation energy; heat, pH and inhibitors change their rate.
- Photosynthesis and respiration both use a proton gradient to make ATP.
- Pathway steps and enzyme names are off the exam, except ATP synthase.
- College Board’s three misconceptions: only animals respire, only plants photosynthesize and photosynthesis creates oxygen.
AP Bio Unit 3 at a glance: the five topics
Unit 3 belongs to Big Idea 2, Energetics. It builds on Unit 2, where you learned how cells are put together, and asks a simpler-sounding question: where does a cell get its energy, and what does it do with it? College Board splits AP Biology Unit 3 into five topics and suggests a science practice skill for each one, two in the case of Topic 3.1. The table lists them, with a line on what each topic expects you to know.
| Topic | What you need to know | Skill College Board pairs with it |
|---|---|---|
| 3.1 Enzymes | Enzymes as protein catalysts that lower activation energy; a substrate has to match the active site in both shape and charge. | 1.B Concept Explanation and 3.C Questions and Methods |
| 3.2 Environmental Impacts on Enzyme Function | How temperature, pH, inhibitors and the balance of substrate and product change an enzyme’s rate; denaturation. | 6.E Argumentation |
| 3.3 Cellular Energy | Why life needs a constant energy input; coupled reactions; pathways that run in steps; pathways shared by all three domains. | 6.C Argumentation |
| 3.4 Photosynthesis | Chloroplast structure, the light reactions and their proton gradient, and how ATP and NADPH drive the Calvin cycle. | 6.B Argumentation |
| 3.5 Cellular Respiration | Glycolysis, the Krebs cycle, the electron transport chain and chemiosmosis in mitochondria; fermentation without oxygen. | 4.A Representing and Describing Data |
Two threads run through every topic. One is structure and function: an enzyme works because of its shape, and a membrane can store energy because it keeps protons on one side. The other is energy flow through a cell’s metabolism, from light, to sugar, to ATP, to the work a cell does.
College Board also uses this unit to drill argumentation, so most Unit 3 AP Bio questions ask you to back a claim with evidence or to predict what a change will do. This AP Bio Unit 3 review covers the topics in course order (CED pp.64–78). Some older study guides, and some search-engine answers, still describe this unit’s earlier layout, with more topics, different topic numbers and longer pacing; this page follows the current course description, the edition effective fall 2025. The ideas come back later in the course, when Unit 6 looks at how cells spend energy to keep life going.
3.1 Enzymes
Enzymes are proteins, and they are biological catalysts: they make chemical reactions in the cell go faster without being used up. They do it by lowering the activation energy, the energy a reaction has to take in before it can get started. An enzyme does not change how much energy the products hold compared with the reactants, and it cannot make a reaction happen that would not happen anyway. It only lowers the hill, so the reaction runs fast enough to be useful at the temperature of a living cell.
Each enzyme has an active site, a region whose shape and charge suit one particular substrate. When a substrate with the right shape and charge meets it, the two bind and form an enzyme-substrate complex. The reaction takes place, the products leave and the enzyme comes away unchanged, ready for the next substrate. That fit is why enzymes are specific, usually working on one substrate or a few very similar ones.
Textbooks often describe the binding with the induced-fit model, in which the active site shifts a little as the substrate settles in. College Board’s name for the idea is the enzyme-substrate complex model, so that is the term to use in an answer.
Enzymes are also how a cell runs its chemistry on a schedule. By making more or less of an enzyme, or by switching one on and off, a cell decides which reactions run and how fast. That is why the course ties enzyme structure and function to the regulation of biological processes, and why a damaged enzyme can upset a whole pathway.
The course uses Topic 3.1 to practice experimental design (skill 3.C). In an enzyme experiment you should be able to name the independent variable (the condition the experimenter changes, such as temperature) and the dependent variable (the rate they measure, such as product formed per minute). Then pick a control and say why it is there. A tube with no enzyme, or with an enzyme that has been boiled, shows how fast the reaction goes with no catalyst at all.
3.2 Environmental impacts on enzyme function
An enzyme works only while it keeps its shape, and its shape depends on its surroundings. Topic 3.2 covers the conditions that change enzyme activity: temperature, pH, inhibitors and the amounts of substrate and product.
Warming a solution makes its molecules move faster, so enzymes and substrates bump into each other more often and the rate climbs. That only holds up to the enzyme’s optimum temperature. Past it, the extra heat breaks the hydrogen bonds that hold the enzyme in shape, the active site deforms and the rate drops quickly. That loss of shape is denaturation.
pH acts in a similar way. Each enzyme has an optimal pH range: a digestive enzyme in the stomach does best in strong acid, while most enzymes in the cytoplasm prefer a pH near neutral. Outside that range, the bonds holding the enzyme’s structure are disrupted and it catalyzes less efficiently. A large enough change in temperature, pH or the chemical surroundings, such as salt concentration, denatures it completely.
Denaturation is not always permanent. College Board says that in some cases it can be reversed, and the enzyme regains its activity when conditions return to normal. An answer claiming a denatured enzyme can never work again goes further than the course does.
The relative amounts of substrate and product matter too. More substrate means more collisions and a faster rate, until every active site is busy and adding substrate changes nothing: the enzyme is saturated. Product that builds up slows the reaction, and in many pathways the final product loops back to slow an enzyme near the start.
Inhibitors come in two kinds, and the exam keeps them apart. A competitive inhibitor looks enough like the substrate to bind the active site reversibly and keep the substrate out. Adding plenty of substrate can win that competition. A noncompetitive inhibitor binds elsewhere on the enzyme, at an allosteric site. That changes the enzyme’s shape so the active site works less well, and extra substrate does not undo it.
| Factor | What changes | What the rate does |
|---|---|---|
| Temperature below the optimum | Molecules move faster and collide more often | Rises as the temperature rises |
| Temperature above the optimum | Hydrogen bonds break and the active site loses its shape | Falls steeply as the enzyme denatures |
| pH away from the optimum | The bonds that hold the enzyme’s structure are disrupted | Falls on either side of the optimum |
| More substrate | More substrate molecules meet the enzyme | Rises, then levels off once every active site is busy |
| Competitive inhibitor | It occupies the active site | Falls; more substrate can bring it back up |
| Noncompetitive inhibitor | It binds an allosteric site and changes the enzyme’s shape | Falls; more substrate does not bring it back |
Most of these effects reach you as graphs. A curve of rate against temperature rises gently to a peak at the optimum and falls more steeply after it, and a curve of rate against substrate concentration rises and then flattens. Read the axes before the biology: the rise on the first graph is about collisions, the fall is about shape, and the flat part of the second is about every active site being full.
Working out a rate, with units
Rate questions hand you a table and ask how fast something changed. Say an enzyme that breaks down hydrogen peroxide releases 6.0 mL of oxygen in the first 2 minutes and 13.5 mL by the end of 5 minutes. The rate over the whole run is the change in volume divided by the change in time: 13.5 mL ÷ 5 min = 2.7 mL/min. Over the last three minutes it is (13.5 − 6.0) mL ÷ (5 − 2) min = 2.5 mL/min, slower than the 3.0 mL/min of the first two minutes, which fits a substrate that is being used up.
On a graph, the same number is the slope of the line between two points. Keep the units on every line of your work, not just the answer.
3.3 Cellular energy
Living systems need a steady input of energy. A cell is a highly ordered system, and staying ordered costs energy all the time: the energy coming in has to be more than the energy lost, or the order falls apart. College Board puts it bluntly. When order or energy flow is lost on a large scale, the organism dies.
None of this breaks the laws of thermodynamics. A cell does not create energy. It captures energy from light or from food and converts it from one form to another, which is the first law. Every conversion also loses some energy as heat, so the disorder (entropy) of the universe goes up even while a cell builds order inside itself, which is the second law. Living things stay organized by taking in energy and giving off heat to their surroundings.
Cells pay for reactions that need energy with reactions that release it. This is energy coupling: energy-releasing (exergonic) reactions driving energy-requiring (endergonic) ones. ATP is the usual go-between. It is built from the base adenine, the sugar ribose and a chain of three phosphate groups, and the bond to the last phosphate is the one the cell breaks. Hydrolyzing ATP to ADP and inorganic phosphate releases energy, and the cell couples that release to work that needs it, such as building a large molecule or pumping an ion against its gradient. Hydrolysis uses a water molecule to break a covalent bond in ATP, a detail the 2024 free-response question asked for directly.
Energy pathways are sequential: they run in steps. In a metabolic pathway, the product of one reaction is usually what the next reaction uses, and each step has its own enzyme. Releasing energy a little at a time lets the cell capture more of it and control the flow, instead of losing it all at once as heat. It also sets up a common question: block one enzyme and the molecule just before that step piles up, while everything after it runs short.
Topic 3.3 ends with an evolution point. Glycolysis and oxidative phosphorylation, the core of how cells release energy, are conserved in organisms from all three domains: Archaea, Bacteria and Eukarya. Pathways shared that widely most likely trace back to a common ancestor, so College Board counts them as evidence of common ancestry.
3.4 Photosynthesis
Photosynthesis turns light energy into chemical energy, using it to build carbohydrates from carbon dioxide and water and giving off oxygen. The sugars it makes are used right away or stored, and they feed nearly everything else in an ecosystem.
It did not start in plants. Photosynthesis first evolved in prokaryotes, and the evidence points to cyanobacteria as the organisms whose photosynthesis filled Earth’s atmosphere with oxygen. The pathways eukaryotes use were built on those prokaryotic ones. That history is why College Board counts the belief that only plants photosynthesize as a misconception: algae and cyanobacteria do it too.
Inside a chloroplast, the stroma is the fluid that fills the space inside the inner membrane and around the thylakoids. Thylakoids are flattened membrane sacs, and they are stacked into grana. Their membranes hold chlorophyll and other pigments, organized into two photosystems, along with the proteins of an electron transport chain. The light reactions (the light-dependent reactions) happen there, in the grana. Carbon fixation, the Calvin cycle, runs in the stroma.
The light reactions start when chlorophyll absorbs light and its electrons are boosted to a higher energy level, in both photosystem II and photosystem I. Photosystem II loses electrons this way, and splitting water replaces them. That split is where the oxygen comes from. The oxygen photosynthesis releases was part of a water molecule a moment earlier, so it is rearranged, not created, which is the answer to the second misconception College Board lists.
From photosystem II the electrons travel along the electron transport chain to photosystem I, passed from molecule to molecule in a series of oxidation-reduction (redox) reactions. In photosystem I they are boosted again and finally handed to NADP⁺, which is reduced to NADPH.
As the electrons move down the chain, the energy they give up moves protons (hydrogen ions) into the thylakoid space. Protons pile up inside the thylakoid and stay scarce in the stroma, so an electrochemical gradient forms across the membrane. The only way back out is through ATP synthase, and that flow drives it to make ATP from ADP and inorganic phosphate. Making ATP this way, by chemiosmosis powered by light, is called photophosphorylation.
ATP and NADPH carry the captured energy to the Calvin cycle in the stroma, which uses it to build carbohydrates from carbon dioxide. ADP, phosphate and NADP⁺ go back to the thylakoids to be recharged. The Calvin cycle does not use light itself (its reactions are often called light-independent), but it stops in the dark because its supply of ATP and NADPH runs out.
One more detail turned up on a released question. Electrons normally take the noncyclic route described above, from water all the way to NADPH. They can also cycle from photosystem I back into the electron transport chain, a route called cyclic electron flow that moves more protons and makes ATP but no NADPH. When NADP⁺ runs short, electrons are pushed onto that cyclic route, because NADP⁺ is what accepts them at the end of the noncyclic one. That reasoning is what most students missed on the 2023 short question about the two routes.
3.5 Cellular respiration
Cellular respiration releases the energy stored in biological macromolecules (most often glucose) and captures it as ATP. Respiration and fermentation happen in every form of life, which is the point behind the first misconception College Board lists: plants, fungi and bacteria respire as well as animals. A leaf cell has mitochondria working alongside its chloroplasts, day and night.
In eukaryotes, aerobic respiration is a series of enzyme-catalyzed stages, each in its own place. Glycolysis runs in the cytosol and needs no oxygen. It splits glucose into two molecules of pyruvate and yields a little ATP and some NADH, which is NAD⁺ loaded with electrons.
Pyruvate is then carried into the mitochondrion and oxidized there. That oxidation and the Krebs cycle, also called the citric acid cycle, run in the mitochondrial matrix. Between them they release the carbon from the original glucose as carbon dioxide, make a small amount of ATP and load electrons onto the coenzymes NAD⁺ and FAD, reducing them to NADH and FADH₂.
NADH and FADH₂ deliver those electrons to the electron transport chain in the inner mitochondrial membrane. As the electrons pass from carrier to carrier in redox reactions, the energy released moves protons out of the matrix into the intermembrane space. At the end of the chain, oxygen is the terminal electron acceptor: it takes the electrons, picks up protons and becomes water. With no oxygen to accept them, electrons back up and the chain stops.
The result is a proton gradient across the inner membrane, high in the intermembrane space and low in the matrix. Fewer protons means a higher pH, so the matrix has a higher pH than the intermembrane space, a comparison College Board spells out. Protons return to the matrix by way of ATP synthase, which uses their movement to make ATP from ADP and inorganic phosphate. In aerobic respiration this is oxidative phosphorylation, and it makes most of the ATP that respiration produces.
Inside the smooth outer membrane, the inner membrane is folded into cristae, and the folds add surface area for more transport chains and more ATP synthase, so more ATP can be made.
Cells can also let protons leak back without making ATP. Uncoupling electron transport from ATP synthesis this way turns the gradient’s energy into heat, and endotherms use that heat to hold their body temperature, as in the brown fat of hibernating mammals and newborn babies.
Prokaryotes have no mitochondria, so they run their electron transport chains in the plasma membrane and build the gradient across it. Aerobic prokaryotes use oxygen as the terminal electron acceptor, while anaerobic ones pass their electrons to other molecules.
When no oxygen is available, many cells fall back on fermentation, which keeps glycolysis going without oxygen. It converts pyruvate into an organic product, alcohol (ethanol, with carbon dioxide) in yeast or lactic acid in muscle cells, and the point of that step is to regenerate the NAD⁺ glycolysis needs. All the ATP still comes from glycolysis, so a fermenting cell makes far less ATP per glucose than a cell that has oxygen. Our concept page goes through the stages of cellular respiration in more depth, along with how the topic is tested on its own.
Photosynthesis vs cellular respiration
Photosynthesis and cellular respiration are often taught as opposites, and their overall equations do run in reverse. The machinery inside the two organelles is what they share. Each passes electrons down a transport chain in a membrane, uses the energy to move protons across it and lets the protons flow back through ATP synthase. College Board wants you to know the differences, to see how the two work together in an ecosystem and to predict what happens when either one is disrupted.
Photosynthesis 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Cellular respiration C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (captured as ATP)
College Board describes both in words rather than as balanced equations, so know what goes in and what comes out; the numbers only balance the atoms.
| Feature | Photosynthesis | Cellular respiration |
|---|---|---|
| Where it happens (eukaryotes) | Chloroplasts: thylakoid membranes, then the stroma | The cytosol, then the mitochondrial matrix and inner membrane |
| What goes in | Carbon dioxide, water and light energy | Glucose (or other organic molecules) and oxygen |
| What comes out | Carbohydrates and oxygen | Carbon dioxide, water and ATP |
| Electron carrier made | NADPH | NADH and FADH₂ |
| Path of the electrons | From water to NADP⁺ | From glucose to oxygen |
| Side of the membrane rich in protons | Inside the thylakoid | The intermembrane space |
| How the ATP is made | Photophosphorylation, through ATP synthase | Mostly oxidative phosphorylation through ATP synthase, plus a little made directly in glycolysis and the Krebs cycle (substrate-level phosphorylation) |
| Which organisms do it | Plants, algae and photosynthetic prokaryotes | Every form of life, plants included |
In an ecosystem the two processes form a loop. Photosynthesis pulls carbon out of the air and fixes it into sugar, and respiration in plants, animals, fungi and microbes returns it as carbon dioxide. Disrupt either side and the effects spread outward: less photosynthesis means less sugar, less new plant tissue (biomass) and less food for everything that eats plants. The same chain of cause and effect, from a molecule to a cell to an organism to an ecosystem, is what the exam expects you to follow.
How Unit 3 is tested on the AP Biology exam
Unit 3 counts for 12–16% of the multiple-choice section (CED pp.17 and 198). The exam has 60 multiple-choice questions, so expect roughly 7 to 10 of them to come from this unit. College Board does not publish unit weightings for the free-response section.
There are six free-response questions. The two long ones are worth 9 points apiece and the four short ones 4 points apiece. Each short question is drawn from a different unit, so Unit 3 is never promised a slot, but its content has turned up in long questions as well as short ones. For a method that works on any of them, see our AP Bio FRQ tips, which take each question type part by part.
Skills weigh as much as content. College Board pairs the topics in this unit with argumentation (6.B, 6.C and 6.E), graphing (4.A), explaining concepts (1.B) and experimental design (3.C). Argumentation questions make up 20–26% of the whole multiple-choice section, so expect to support claims and predict effects far more often than to recite steps. The course description adds a warning written for this unit: you may be asked to graph data and calculate a reaction rate, and you should show the calculation with units on the final answer.
Five free-response questions released from 2023 to 2026 were built on Unit 3 content. Only one is tied to the unit by College Board: the 2023 Chief Reader Report files question 4 under Unit 3 as Topic 3.5, a number from the course’s earlier layout (photosynthesis is Topic 3.4 now). The rest are our reading of each question’s subject.
| Exam | Question type | What it asked about | Average score |
|---|---|---|---|
| 2023, question 2 | Long, with a graph | Whether more carbon dioxide raises the number of mitochondria in plant cells, as it does chloroplasts; graphing the data and the inner membrane’s role. | 4.10 of 9 |
| 2023, question 4 | Short, conceptual analysis | The two electron routes in the light reactions, and what happens to NADPH, NADP⁺ and plant growth when photosystem I slows or NADPH piles up. | 1.17 of 4 |
| 2024, question 2 | Long, with a graph | How temperature changes oxygen use and ATP synthesis in toad liver cells: ATP hydrolysis, a rate calculation with units and a compound that blocks ATP synthase’s proton channel. | 3.65 of 9 |
| 2025, question 5 | Short, analyzing a model | A multistep enzyme pathway that turns one amino acid into another: the active site, inhibition by the end product and what a pH change does to one enzyme. | 2.00 of 4 |
| 2026, question 3 | Short, scientific investigation | A poison that slows an enzyme of the mitochondrial electron transport chain: why aerobic respiration beats glycolysis alone, choosing controls and predicting lactic acid output. | report not out yet (4 points) |
The averages show where points go missing. On the 2023 electron-flow question, students averaged 1.17 out of 4. Many saw that NADPH would build up, but few explained that a shortage of NADP⁺ is what pushes electrons onto the cyclic route. On the 2024 toad question, many answers described what the blocking compound does to ATP synthase instead of predicting what happens to the proton gradient. The gradient grows, because the chain keeps pumping protons out while none can flow back in.
The reports also flag graphing slips that cost easy points: a missing unit on an axis, error bars plotted unevenly and a line graph drawn for categories that needed bars. The habits that fix them are simple. Label both axes with units and plot the error bars. Write every calculation with its units. When a question asks for a prediction, give the prediction and the reason in the same answer, because a prediction with no justification earns little.
The labs behind Unit 3 questions
The course description points teachers to three AP lab investigations for these topics, the Enzyme Lab, the Photosynthesis Lab and the Cellular Respiration Lab (CED pp.69, 73 and 76), and it suggests a yeast fermentation lab that measures carbon dioxide. Released questions keep coming back to the same designs. Know what each one changes, what it measures and what a fair control looks like.
| Lab | What changes (independent variable) | What is measured (dependent variable) | A sensible control |
|---|---|---|---|
| Enzyme activity (3.2) | Temperature, pH or substrate concentration | How fast product forms, such as oxygen released from hydrogen peroxide each minute | A tube with boiled enzyme, or with no enzyme |
| Photosynthesis (3.4) | Light intensity, light color or the carbon dioxide supply | How fast oxygen is made, often timed as leaf disks that float once oxygen builds up inside them | Leaf disks kept in the dark |
| Cellular respiration (3.5) | Temperature, or germinating seeds against dry ones | Oxygen used each minute, read from a respirometer | Glass beads in place of seeds, to catch changes in temperature or pressure |
| Fermentation (3.5) | Sugar concentration or the type of sugar | Carbon dioxide produced by yeast | Yeast with no sugar added |
Two released questions were built on these designs: the 2024 long question measured oxygen use in toad liver cells at different temperatures, and the 2023 long question counted mitochondria under normal and raised carbon dioxide. When a question describes a setup, name the variables first, then make the prediction.
Mistakes College Board names for Unit 3
College Board’s course description flags several Unit 3 misunderstandings (CED p.65), and the Chief Reader Reports for the released questions above add more. Every entry pairs the mistake with the correction.
In the course description
- Confusing metabolic pathways with other processes. Know each pathway’s inputs and outputs, predict what a change in a reactant does to it and explain how that change reaches the organism and the ecosystem.
- Thinking only animals carry out cellular respiration. Plants, fungi, protists and prokaryotes respire too.
- Saying photosynthesis creates oxygen. The oxygen it releases comes from water split at photosystem II. The atoms are rearranged, not made.
- Thinking only plants photosynthesize. Algae and cyanobacteria do it as well, and photosynthesis began in prokaryotes long before there were plants.
- Memorizing molecules instead of following the logic. The course separates reciting structures from explaining how events at the molecular level connect to what the organism does and to the flow of carbon through an ecosystem.
- Leaving the units off a calculation. When you work out a rate, show each step and put the units on the final answer.
In the Chief Reader Reports
- Putting every stage of respiration in one place. Glycolysis is in the cytosol, the Krebs cycle in the matrix and the electron transport chain in the inner mitochondrial membrane.
- Saying chlorophyll’s job is to make plants green. Its job is to absorb light energy. The green we see is the light it does not absorb.
- Forgetting what NADP⁺ does. It is the final electron acceptor of the noncyclic route, so when it runs short, electrons shift to the cyclic route.
- Reading a ratio one way only. In a fixed pool, when the ratio of NADPH to NADP⁺ rises, NADPH goes up and NADP⁺ goes down at the same time.
- Saying hydrolysis breaks hydrogen bonds. Hydrolysis of ATP uses water to break a covalent bond, leaving ADP and inorganic phosphate.
- Describing the blocked enzyme instead of the gradient. If protons cannot flow back through ATP synthase, the gradient across the inner membrane grows, because the transport chain keeps pumping.
- Treating enzymes as receptors and substrates as ligands. An enzyme changes its substrate into a product, while a receptor binds a signal molecule and passes the message on.
- Mixing up the two kinds of inhibitor. A competitive inhibitor takes the active site; a noncompetitive one binds an allosteric site. The two sites are not the same place.
- Misreading a pathway diagram. An arrow shows a step or a push forward, and a line ending in a short bar (a T-bar) shows inhibition.
What the AP exam leaves out of Unit 3
Five exclusion statements in the course description take a large part of a textbook’s energy chapter off the exam. They tell you what you can stop memorizing, and they explain why some study guides spend pages on things you will not be asked to recall. Each one still leaves something you do need.
| Topic | Left out of the exam | What you still need |
|---|---|---|
| 3.3 Cellular energy | The equation for Gibbs free energy | The idea of energy itself: input, loss as heat, coupling and the two laws of thermodynamics |
| 3.4 Photosynthesis | The steps of the Calvin cycle, the structures of its molecules (G3P among them) and its enzyme names (RuBisCO among them); ATP synthase is the exception | That ATP and NADPH from the light reactions power the Calvin cycle in the stroma |
| 3.4 and 3.5 | The full names of the electron carriers in each transport chain | What the chain does: redox reactions that build a proton gradient |
| 3.4 and 3.5 | The specific steps, enzyme names and intermediates of each pathway | Inputs, outputs, locations and what happens when one step is blocked |
| 3.5 Cellular respiration | Glycolysis and Krebs-cycle steps, with their structures and enzyme names (acetyl-CoA and citrate synthase among them) | Where each stage happens, what goes in and what comes out |
ATP totals fall in the same bucket. Textbooks quote different ATP yields per glucose, anywhere from about thirty to thirty-eight, and the course description asks for none of them. What you do need is the comparison: aerobic respiration yields far more ATP from each glucose than glycolysis alone, which is what the 2026 short question asked you to explain.
AP Bio Unit 3 key terms
The unit’s core vocabulary in plain definitions, using College Board’s own term wherever the course description has one.
- Activation energy
- The energy a reaction needs to get started; enzymes lower it.
- Active site
- The part of an enzyme where the substrate binds.
- Allosteric site
- A site away from the active site; binding there changes the enzyme’s shape.
- ATP
- The cell’s energy carrier; hydrolysis to ADP and phosphate releases energy.
- ATP synthase
- The membrane enzyme that makes ATP as protons flow through it.
- Calvin cycle
- Stroma reactions that use ATP and NADPH to build sugar from carbon dioxide.
- Cellular respiration
- Releasing the energy in food molecules to make ATP.
- Chemiosmosis
- Making ATP from protons flowing down their gradient through ATP synthase.
- Chlorophyll
- The thylakoid pigment that absorbs light energy.
- Competitive inhibitor
- A molecule that binds the active site and blocks the substrate.
- Coupled reactions
- An energy-releasing reaction driving one that needs energy.
- Denaturation
- Loss of an enzyme’s shape and function; sometimes reversible.
- Electron transport chain (ETC)
- Membrane proteins that pass electrons along and pump protons.
- Enzyme
- A protein that speeds a reaction by lowering its activation energy.
- Enzyme-substrate complex
- An enzyme and its substrate bound together at the active site.
- Fermentation
- A pathway that keeps glycolysis going without oxygen.
- Glycolysis
- Splitting glucose into pyruvate in the cytosol, with a little ATP made.
- Grana
- Stacks of thylakoids, where the light reactions happen.
- Krebs cycle
- The matrix cycle that releases carbon dioxide and reduces NAD⁺ and FAD.
- Light reactions
- Thylakoid reactions that turn light energy into ATP and NADPH.
- NADH and FADH₂
- Carriers that bring electrons from glycolysis and the Krebs cycle to the ETC.
- NADPH
- The carrier made at photosystem I and spent in the Calvin cycle.
- Noncompetitive inhibitor
- A molecule that binds an allosteric site; extra substrate does not undo it.
- Oxidative phosphorylation
- ATP made by chemiosmosis in mitochondria.
- Photophosphorylation
- ATP made by chemiosmosis in chloroplasts, powered by light.
- Photosystems I and II
- Chlorophyll-protein clusters in the thylakoid membrane, linked by an ETC.
- Proton gradient
- A difference in proton concentration across a membrane that stores energy.
- Stroma
- The fluid around the thylakoids, where the Calvin cycle runs.
- Substrate
- The molecule an enzyme acts on.
- Thylakoid
- A flattened chloroplast membrane sac that holds the photosystems.
How to study AP Bio Unit 3
Unit 3 AP Biology questions reward understanding more than memory, which is good news, because the pathway diagrams in most textbooks are far more detailed than the exam needs. This order builds each idea on the one before it:
- Start with enzymes. Be able to sketch a curve of rate against temperature and explain both sides of the peak.
- Learn the energy rules of Topic 3.3 next: why a cell needs a constant input, what coupling means and why pathways run in steps.
- Study photosynthesis and respiration side by side, using the comparison table on this page. For each, know where it happens, what goes in, what comes out and where the proton gradient forms.
- Draw the chloroplast and mitochondrion diagrams from memory until you can place every stage, carrier and gradient without looking.
- Practice disruption questions. Pick one step, block it and trace what builds up and what runs out, all the way to the organism.
- Work one rate calculation with units and one graph with labeled axes and error bars, since both have cost students points on released questions.
- Read the exclusion list once and cross its pathway steps and enzyme names off your notes.
If you are using this page as an AP Bio Unit 3 study guide the night before a test, the checklist below is the short version.
Before the test, can you…
- explain how an enzyme speeds up a reaction and sketch the energy curve that shows it
- predict how temperature, pH, substrate and each kind of inhibitor change an enzyme’s rate
- say why living things need a constant energy input without breaking the laws of thermodynamics
- trace electrons from water to NADPH, and from glucose to oxygen
- name the side of each membrane that holds more protons and say how ATP synthase uses it
- predict what happens to the gradient, ATP and NADPH when one step is blocked
- explain why fermentation keeps glycolysis running without oxygen
- calculate a reaction rate from a table, with units
Where to go next
- Our walkthrough of the AP Bio Unit 3 MCQ on AP Classroom goes through each kind of question in the unit’s Progress Check (about 19 multiple-choice and two free-response questions) and the reasoning it rewards.
- After a full practice exam, check your raw points against the AP Biology score curve to see the AP score they would likely earn.
Frequently asked questions
Quick answers, each checked against the course description and College Board’s exam reports.
What is AP Bio Unit 3 about?
Cellular energetics: how cells capture energy and use it. You study enzymes and what affects them, why living things need a constant supply of energy and the two processes at the center of the unit, photosynthesis and cellular respiration.
How many topics does AP Bio Unit 3 have?
Five, numbered 3.1 to 3.5: enzymes, the environment’s effect on enzymes, cellular energy, photosynthesis and cellular respiration. College Board plans them over about 12–14 class periods, the same pacing as Unit 4.
How much of the AP Bio exam is Unit 3?
12–16%, counted on the multiple-choice section only. That works out to roughly 7–10 questions out of 60. The free-response section has no published unit weights, though Unit 3 content has appeared there often, including the long graphing questions on the 2023 and 2024 exams.
Is Unit 3 the hardest AP Bio unit?
There is no official difficulty ranking, and at about 12–14 class periods it is mid-length; Units 2, 6, 7 and 8 take longer. Its reputation comes from the free response, where the one short question College Board tied to it averaged 1.17 of 4 points. Our page on how hard is AP Biology looks at the course as a whole.
Is the Gibbs free energy equation on the AP Bio exam?
No. The course description puts the formula outside the exam. You still need the idea behind it: reactions that release energy can drive reactions that need it, and a cell has to keep taking in energy to stay ordered.
Do you need to memorize the Krebs cycle for AP Bio?
No. Its steps, the structures of its molecules and its enzyme names are outside the exam, and the same goes for glycolysis. Know that it runs in the mitochondrial matrix, gives off carbon dioxide and passes electrons to NADH and FADH₂ for the transport chain.
What is the difference between photosynthesis and cellular respiration?
Photosynthesis stores energy: chloroplasts use light to build sugar from carbon dioxide and water, releasing oxygen. Respiration releases it: cells break sugar down, usually with oxygen, to make ATP and give off carbon dioxide and water. Both build a proton gradient with an electron transport chain. Plants do both, and animals rely on respiration alone.
Is fermentation part of AP Bio Unit 3?
Yes, in Topic 3.5. Fermentation regenerates NAD⁺ so glycolysis can continue when there is no oxygen, producing alcohol or lactic acid. The 2026 exam asked a related question: why a cell gets more ATP from each glucose by aerobic respiration than by glycolysis alone.
Sources: every College Board fact here was checked against the AP Biology Course and Exam Description, edition effective fall 2025, with Unit 3 on pp.64–78 and the unit weightings on pp.17 and 198. The free-response evidence comes from College Board’s past free-response questions page: the questions and scoring guidelines for 2023–2026 and the Chief Reader Reports for 2023–2025. The exam date is from AP Students, College Board’s AP Biology exam page. Last checked October 2026.