AP Biology Unit 3: Cellular Energetics
Study photosynthesis, cellular respiration, ATP, metabolic pathways with exam-format practice questions and rubric-based scoring.
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Inside This Unit: The Full Breakdown
Cellular Energetics covers how cells capture, store, and use energy. Photosynthesis converts light energy into chemical energy in glucose, while cellular respiration breaks glucose down to produce ATP — the universal energy currency of life.
Why it matters
Photosynthesis and cellular respiration are among the most heavily tested topics on the AP Bio exam. You must understand the inputs, outputs, and locations of each stage, as well as how the two processes are interconnected.
Key concepts
- Photosynthesis occurs in chloroplasts: light reactions in the thylakoid membranes produce ATP and NADPH; the Calvin cycle in the stroma fixes CO₂ into G3P.
- Cellular respiration occurs in mitochondria: glycolysis (cytoplasm), the Krebs cycle (matrix), and oxidative phosphorylation (inner membrane) progressively extract energy from glucose.
- Chemiosmosis couples electron transport to ATP synthesis via a proton gradient across a membrane in both photosynthesis and respiration.
- Fermentation allows glycolysis to continue when oxygen is absent by regenerating NAD⁺, but yields far less ATP than aerobic respiration.
Photosynthesis
Photosynthesis takes place in chloroplasts and has two main stages. The light-dependent reactions occur in the thylakoid membranes, where chlorophyll absorbs light energy to split water molecules, releasing oxygen as a byproduct. This energy drives an electron transport chain that pumps H⁺ ions into the thylakoid space, creating a proton gradient used by ATP synthase to produce ATP. NADP⁺ is reduced to NADPH. In the Calvin cycle, which occurs in the stroma, the enzyme RuBisCO fixes CO₂ into a three-carbon molecule (G3P) using the ATP and NADPH from the light reactions. Three turns of the Calvin cycle produce one G3P molecule that can be used to build glucose and other organic molecules.
Cellular Respiration
Cellular respiration extracts energy from glucose in three stages. Glycolysis occurs in the cytoplasm and splits one glucose molecule into two pyruvate molecules, producing a small amount of ATP and NADH. If oxygen is available, pyruvate enters the mitochondria and is converted to acetyl-CoA, which feeds into the Krebs cycle in the mitochondrial matrix. The Krebs cycle completes the oxidation of glucose, releasing CO₂ and generating NADH and FADH₂. These electron carriers deliver electrons to the electron transport chain on the inner mitochondrial membrane, where oxidative phosphorylation produces the majority of ATP — about 30-32 molecules per glucose. Oxygen serves as the final electron acceptor, forming water.
Chemiosmosis and Fermentation
Chemiosmosis is the mechanism that links electron transport to ATP production in both photosynthesis and respiration. In both cases, an electron transport chain pumps protons across a membrane, building a concentration gradient. Protons then flow back through ATP synthase, which uses the energy of this flow to phosphorylate ADP into ATP. This coupling of a proton gradient to ATP synthesis is one of the most important unifying concepts in biology. When oxygen is unavailable, cells use fermentation to regenerate NAD⁺ so glycolysis can continue. Alcoholic fermentation (in yeast) produces ethanol and CO₂, while lactic acid fermentation (in muscle cells) produces lactate. Fermentation yields only 2 ATP per glucose compared to about 30-32 from aerobic respiration.
AP exam tip
For AP questions on energetics, always identify WHERE each process occurs (thylakoid vs. stroma, cytoplasm vs. mitochondrial matrix vs. inner membrane) and WHAT goes in and comes out. Location-based questions are extremely common.
Connections to other units
- Unit 1 (Chemistry of Life): Enzyme function and macromolecule structure underpin every metabolic reaction.
- Unit 2 (Cell Structure): Organelle structure (cristae, thylakoid stacks) directly enables chemiosmosis.
- Unit 8 (Ecology): Energy flow through ecosystems depends on producers capturing energy via photosynthesis.
