Review AP Bio Unit 3 to understand how living cells capture, transfer, and use energy through enzymes, photosynthesis, and cellular respiration. This unit covers the metabolic pathways and structural features that power all life, making it one of the most concept-dense and exam-relevant units in the course.
Use this hub to review all five topics, check key terms, and access topic guides and practice questions for Unit 3.
Unit 3 asks you to trace energy from sunlight to ATP, understanding the molecular machinery that makes each step possible. You will work with enzyme structure and function, the thermodynamic rules that govern all cellular processes, the two-stage process of photosynthesis in the chloroplast, and the three-stage process of aerobic respiration in the mitochondrion.
Cellular energetics is the study of how cells capture energy from the environment, convert it into usable forms like ATP, and use that energy to maintain order and carry out life functions. Enzymes make reactions fast enough to sustain life, photosynthesis stores light energy in sugars, and cellular respiration releases that stored energy as ATP.
Enzymes are proteins that act as biological catalysts. They speed up reactions by lowering the activation energy required, not by changing the overall energy difference between reactants and products. The active site binds a specific substrate to form an enzyme-substrate complex, and the induced-fit model explains how the active site adjusts shape upon binding.
In the thylakoid membranes, light reactions use photosystems I and II to capture light, split water, and produce ATP and NADPH. In the stroma, the Calvin cycle uses that ATP and NADPH to fix CO2 into sugar. The overall inputs are CO2, H2O, and light; the outputs are glucose and O2.
Glycolysis in the cytosol breaks glucose into pyruvate, producing 2 ATP and 2 NADH. Pyruvate enters the mitochondrion, is oxidized to acetyl-CoA, and enters the Krebs cycle in the matrix. NADH and FADH2 carry electrons to the electron transport chain in the inner mitochondrial membrane, where chemiosmosis through ATP synthase produces most of the ATP.
The metabolic pathways in Unit 3, including glycolysis, oxidative phosphorylation, and photosynthesis, are conserved across Archaea, Bacteria, and Eukarya. This conservation is direct evidence for common ancestry. Every process in the unit, from enzyme catalysis to the proton gradient in ATP synthase, reflects the same principle: cells must continuously capture and transform energy to maintain the order that defines life.
Enzymes are protein catalysts that lower activation energy. The active site binds a specific substrate to form an enzyme-substrate complex, and the induced-fit model explains how the active site adjusts to stabilize the transition state and speed up the reaction.
Temperature, pH, substrate concentration, and inhibitors all affect enzyme activity. Conditions outside the optimal range disrupt hydrogen bonds and can denature the enzyme. Competitive inhibitors block the active site; noncompetitive inhibitors bind an allosteric site and change the active site's shape.
All living systems require continuous energy input to maintain order. Cells obey thermodynamic laws by coupling exergonic reactions to endergonic ones through ATP. Metabolic pathways are sequential, and core pathways like glycolysis and oxidative phosphorylation are conserved across all domains of life.
Photosynthesis uses CO2, H2O, and light to produce glucose and O2. Light reactions in the thylakoid membranes produce ATP and NADPH through photosystems I and II and chemiosmosis. The Calvin cycle in the stroma uses those products to fix CO2 into sugar.
Cellular respiration breaks down macromolecules to produce ATP through glycolysis (cytosol), the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane). The electron transport chain builds a proton gradient that drives ATP synthase. Fermentation allows ATP production without oxygen by regenerating NAD+.
AP Bio cellular respiration review: glycolysis, Krebs cycle, electron transport chain, chemiosmosis, ATP synthase, and fermentation, with locations, inputs, and outputs.
AP Bio fitness and natural selection review: how energy efficiency, molecular variation, and adaptations like hemoglobin and chlorophyll drive survival and reproduction.
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Review Cellular Respiration with attention to how the concept appears in AP-style source and evidence questions.
Review Cellular Energy with attention to how the concept appears in AP-style source and evidence questions.
Review Photosynthesis with attention to how the concept appears in AP-style source and evidence questions.
Review Environmental Impacts on Enzyme Function with attention to how the concept appears in AP-style source and evidence questions.
Enzymes are proteins that lower the activation energy of chemical reactions without being consumed. The active site is a three-dimensional pocket whose shape and charge are complementary to a specific substrate. When the substrate binds, an enzyme-substrate complex forms. The induced-fit model describes how the active site flexes slightly to better accommodate the substrate, stabilizing the transition state and accelerating the reaction.
Can you explain why changing one amino acid in the active site could eliminate enzyme function, and how that connects to protein structure from Unit 1?
Enzyme activity depends on temperature, pH, and the concentrations of substrates, products, and inhibitors. Each enzyme has an optimal temperature and pH at which its active site shape is maintained. Outside that range, hydrogen bonds and other weak interactions that hold the enzyme's tertiary structure are disrupted, causing denaturation. In some cases denaturation is reversible; in others it is permanent. Competitive inhibitors bind the active site reversibly, while noncompetitive inhibitors bind an allosteric site and change the active site's shape without competing for it.
Given a graph of enzyme activity vs. temperature, can you identify the optimal temperature, explain what happens above it, and distinguish that from competitive inhibition?
| Feature | Competitive Inhibition | Noncompetitive Inhibition |
|---|---|---|
| Binding site | Active site | Allosteric site |
| Effect on active site shape | Blocks access; shape unchanged | Changes active site shape |
| Reversibility | Reversible | Can be reversible or irreversible |
| Overcome by excess substrate? | Yes | No |
| Example context | Drug mimics substrate | Feedback inhibition in metabolic pathways |
All living systems require a continuous input of energy to maintain order. Life obeys the first law of thermodynamics (energy is conserved) and the second law (entropy increases in isolated systems). Cells counteract entropy by coupling exergonic reactions (like ATP hydrolysis) to endergonic reactions (like biosynthesis). Metabolic pathways are sequential: the product of one reaction is the substrate for the next, allowing controlled, stepwise energy transfer. Core pathways like glycolysis and oxidative phosphorylation are conserved across all three domains of life, which is evidence for common ancestry.
Why must energy input exceed energy loss for a cell to survive, and how does ATP coupling make otherwise unfavorable reactions possible?
Photosynthesis occurs in two stages inside the chloroplast. The light-dependent reactions take place in the thylakoid membranes: photosystem II absorbs light and splits water (releasing O2 and electrons), electrons pass through an electron transport chain to photosystem I, and NADP+ is reduced to NADPH. The proton gradient built across the thylakoid membrane drives ATP synthesis through ATP synthase via chemiosmosis. The Calvin cycle in the stroma uses ATP and NADPH to fix CO2 into G3P, which can be used to build glucose. Photosynthesis first evolved in prokaryotes (cyanobacteria), whose oxygenic photosynthesis produced Earth's oxygenated atmosphere and whose pathways became the foundation of eukaryotic photosynthesis.
Trace an electron from water through photosystem II, the ETC, and photosystem I to NADPH. Then explain how the proton gradient produced along the way drives ATP synthesis.
| Feature | Light Reactions | Calvin Cycle |
|---|---|---|
| Location | Thylakoid membranes | Stroma |
| Inputs | H2O, light, ADP, NADP+ | CO2, ATP, NADPH |
| Outputs | ATP, NADPH, O2 | G3P (sugar precursor), ADP, NADP+ |
| Energy conversion | Light to chemical (ATP, NADPH) | Chemical to stored carbon bonds |
Aerobic cellular respiration extracts energy from glucose and other macromolecules in three connected stages. Glycolysis in the cytosol converts glucose to two pyruvate molecules, yielding 2 ATP and 2 NADH. Pyruvate is transported into the mitochondrial matrix, oxidized to acetyl-CoA (releasing CO2), and fed into the Krebs cycle, which produces NADH, FADH2, ATP, and CO2 per turn. NADH and FADH2 deliver electrons to the electron transport chain in the inner mitochondrial membrane; as electrons pass through protein complexes to the final electron acceptor (O2), protons are pumped into the intermembrane space. This proton gradient drives ATP synthesis through ATP synthase via chemiosmosis, producing the majority of ATP. When oxygen is unavailable, fermentation regenerates NAD+ so glycolysis can continue, producing lactic acid or ethanol as byproducts.
Starting from glucose, identify where each stage of cellular respiration occurs, what electron carriers are produced, and how the ETC converts that electron flow into ATP.
| Stage | Location | Key Inputs | Key Outputs | ATP produced |
|---|---|---|---|---|
| Glycolysis | Cytosol | Glucose, NAD+, ADP | Pyruvate, NADH | 2 net |
| Pyruvate oxidation | Mitochondrial matrix | Pyruvate, NAD+ | Acetyl-CoA, NADH, CO2 | 0 |
| Krebs cycle | Mitochondrial matrix | Acetyl-CoA, NAD+, FAD | NADH, FADH2, CO2 | 2 per glucose |
| Oxidative phosphorylation | Inner mitochondrial membrane | NADH, FADH2, O2 | H2O, ATP | ~28-32 |
Try stimulus-based AP practice questions and written prompts after you review the notes.
A timed, AP-style test built only from unit 3 questions — multiple choice plus free response, scored with AI feedback. Use the standard format or build your own.
| Term | Definition |
|---|---|
| Activation Energy | The minimum energy required to start a chemical reaction; enzymes lower this barrier to speed up biological reactions. |
| Active Site | The specific region on an enzyme where the substrate binds; its shape and charge determine which substrates the enzyme can catalyze. |
| Enzyme-Substrate Complex | The temporary structure formed when a substrate binds to an enzyme's active site, enabling the reaction to proceed. |
| induced fit | A model of enzyme-substrate interaction in which the active site adjusts its shape upon substrate binding to better stabilize the transition state. |
| Denaturation | Loss of a protein's 3D structure due to disruption of hydrogen bonds by extreme temperature, pH, or chemical conditions, eliminating enzyme function. |
| Allosteric Regulation | A molecule binds a site other than the active site, causing a shape change that increases or decreases enzyme activity. |
| coupled reactions | Cellular processes in which energy released by an exergonic reaction (such as ATP hydrolysis) powers an endergonic reaction. |
| ATP | The primary energy currency of cells; stores and transfers chemical energy through its phosphate bonds to power cellular processes. |
| Chemiosmosis | Movement of protons down their electrochemical gradient through ATP synthase to generate ATP; occurs in both mitochondria and chloroplasts. |
| Light-Dependent Reactions | Reactions in the thylakoid membranes that use light energy to split water, produce ATP and NADPH, and release O2. |
| Carbon Fixation | The Calvin cycle process in which CO2 is incorporated into organic molecules using ATP and NADPH from the light reactions. |
| Glycolysis | Cytosolic pathway that converts glucose into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH. |
| Krebs Cycle | Metabolic pathway in the mitochondrial matrix that oxidizes acetyl-CoA, releasing CO2 and producing NADH, FADH2, and ATP. |
| Electron Transport Chain | Protein complexes in the inner mitochondrial membrane (or thylakoid membrane) that transfer electrons through redox reactions, building a proton gradient for ATP synthesis. |
| Fermentation | Anaerobic process that regenerates NAD+ from NADH so glycolysis can continue, producing lactic acid or ethanol as byproducts. |
Denaturation permanently (or in some cases reversibly) destroys the enzyme's 3D structure. Inhibition, whether competitive or noncompetitive, does not destroy the enzyme; it temporarily reduces activity. Do not say an inhibitor denatures the enzyme.
Glycolysis is in the cytosol, not the mitochondrion. The Krebs cycle is in the mitochondrial matrix. The electron transport chain and ATP synthase are in the inner mitochondrial membrane. Getting locations wrong on free-response questions costs points.
Plants carry out both photosynthesis and cellular respiration. Respiration and fermentation are characteristic of all forms of life, including plants. Do not write that plants get energy only from photosynthesis.
In the thylakoid, protons accumulate inside the thylakoid lumen (high concentration) and flow out through ATP synthase. In the mitochondrion, protons accumulate in the intermembrane space and flow into the matrix. Students often flip these directions.
The Calvin cycle does not require light directly, but it depends on ATP and NADPH from the light reactions. Without ongoing light reactions, the Calvin cycle stops because it runs out of ATP and NADPH.
AP Bio frequently presents experimental data on enzyme activity, such as graphs of reaction rate vs. temperature, pH, or inhibitor concentration, and asks you to explain the results using molecular mechanisms. Practice connecting a change in conditions to a specific structural effect on the active site, then to the observed change in reaction rate.
Questions often ask you to identify where a specific molecule is produced or consumed, or to predict what happens to ATP yield when a step is blocked. Be ready to explain how disrupting the ETC, removing oxygen, or inhibiting ATP synthase affects the entire respiration sequence, and how analogous disruptions affect photosynthesis.
The structural features of the chloroplast (thylakoid membranes, stroma) and mitochondrion (inner membrane, cristae, matrix, intermembrane space) are directly tied to where and how each reaction occurs. Exam tasks often ask you to explain why a structural feature, such as the folded cristae increasing surface area, supports a specific function.
Use this list to confirm you can handle every major concept before the exam.
Describe how enzymes lower activation energy, how the active site determines substrate specificity, and how the induced-fit model differs from a rigid lock-and-key view.
Given a change in temperature, pH, substrate concentration, or inhibitor type, explain what happens to active site shape and reaction rate, including whether denaturation is reversible.
Explain how coupled reactions and ATP hydrolysis allow cells to maintain order without violating the laws of thermodynamics, and why sequential metabolic pathways allow controlled energy transfer.
Follow electrons from water through photosystem II, the ETC, and photosystem I to NADPH, and explain how the resulting proton gradient drives ATP synthesis. Identify inputs and outputs of both the light reactions and the Calvin cycle.
Identify the location, inputs, and outputs of glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation. Explain how NADH and FADH2 connect the earlier stages to the ETC and ATP synthase.
Explain why the presence of glycolysis and oxidative phosphorylation across Archaea, Bacteria, and Eukarya is evidence for common ancestry, and note that cyanobacterial photosynthesis produced Earth's oxygenated atmosphere.
Read the topic guides for 3.1 and 3.2. Draw the energy diagram showing how an enzyme lowers activation energy. Practice explaining competitive vs. noncompetitive inhibition and predicting what happens to enzyme activity when temperature or pH shifts away from optimal. Check your understanding with the available practice questions.
Review the topic guide for 3.3. Write out in your own words how coupled reactions use ATP hydrolysis to power endergonic processes. Make a note connecting conserved pathways (glycolysis, oxidative phosphorylation) to the concept of common ancestry across all three domains.
Use the topic guide for 3.4 to map the chloroplast: label the thylakoid membranes, stroma, and grana. Trace electron flow from water through PSII, the ETC, and PSI to NADPH. Then trace how the proton gradient drives ATP synthase. Summarize Calvin cycle inputs and outputs separately from the light reactions.
Work through the topic guides for 3.5 and the cellular respiration review. Build a table with columns for stage, location, inputs, outputs, and ATP yield. Practice explaining how NADH and FADH2 connect glycolysis and the Krebs cycle to the ETC, and why fermentation is necessary when oxygen is absent.
Use the key terms list to self-quiz on all 8-15 terms. Attempt FRQ practice focused on explaining mechanisms, predicting outcomes of experimental changes, and connecting photosynthesis to cellular respiration as complementary pathways. Use the AP score calculator to estimate your current scoring range.
Open the individual guides for Unit 3 when you want a closer review of one topic.
Use AP-style practice after you review the notes so you can check what you understand.
Practice free-response reasoning and compare your answer with scoring guidance.
Watch past review streams filtered to Unit 3 when you want a video walkthrough.
Open the Fiveable one-page unit review, then explore visual cheatsheets for a quick refresher.
Estimate your broader AP score goal after you review the course and exam format.
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get printablesAP Bio Unit 3 covers 5 topics: enzyme structure and function (3.1), environmental impacts on enzyme function (3.2), cellular energy and ATP (3.3), photosynthesis including the light-dependent and Calvin cycle reactions (3.4), and cellular respiration including glycolysis, the Krebs cycle, and oxidative phosphorylation (3.5). These topics connect around one big idea: how living systems capture, store, and use energy to stay organized and alive. If you want a full breakdown, check out AP Bio Unit 3.
AP Bio Unit 3 makes up 12-16% of the AP exam, making it one of the more heavily tested units. That weight comes from photosynthesis, cellular respiration, enzyme function, and ATP production, all of which show up in both multiple-choice and free-response questions. Knowing the inputs, outputs, and regulation of these pathways is key to scoring well.
The AP Bio Unit 3 progress check includes MCQ and FRQ parts that draw from all 5 topics in the unit: enzyme structure and function, how environmental factors like temperature and pH affect enzymes, cellular energy and ATP, photosynthesis, and cellular respiration. MCQ questions often ask you to interpret graphs of enzyme activity or metabolic rates. FRQ prompts typically ask you to predict what happens when a variable changes in photosynthesis or cellular respiration and explain the mechanism behind it. For practice questions matched to each progress check topic, visit AP Bio Unit 3.
AP Bio Unit 3 FRQs most often focus on photosynthesis, cellular respiration, and enzyme function because these topics require you to explain mechanisms, analyze experimental data, and predict outcomes, which is exactly what free-response questions test. A typical prompt might give you a graph of oxygen production in plants and ask you to connect it to the light-dependent reactions or the Calvin cycle. To practice, write out full explanations using the correct vocabulary (ATP, glycolysis, Krebs cycle, substrate concentration) and then check your reasoning against the scoring criteria. You can find FRQ-style practice at AP Bio Unit 3.
The best place to find AP Bio Unit 3 practice questions, including multiple-choice and practice test sets, is AP Bio Unit 3. You'll find MCQs covering photosynthesis, cellular respiration, glycolysis, the Krebs cycle, enzyme function, and ATP production, organized by topic so you can target exactly what you need. Working through topic-specific MCQs before doing a full practice test helps you spot gaps in your understanding of metabolism before they show up on exam day.
Start AP Bio Unit 3 by building a strong foundation in enzyme function (topics 3.1 and 3.2) before moving into photosynthesis and cellular respiration, since enzymes drive both pathways. For photosynthesis, map out the two stages separately: light-dependent reactions and the Calvin cycle, with their inputs and outputs. For cellular respiration, trace glucose through glycolysis, the Krebs cycle, and oxidative phosphorylation, tracking ATP yield at each stage. A few concrete steps that help: - Draw the pathways from memory, then check your diagram against your notes. - Practice interpreting graphs of enzyme activity, oxygen production, and CO2 output. - Write out short explanations of what happens when a variable changes (temperature, pH, light intensity) to prep for FRQs. - Do topic-by-topic MCQs to confirm your understanding before a full practice test. Visit AP Bio Unit 3 for organized practice resources for each topic.