The unit where cells get to work. Enzymes that speed up chemistry, ATP that powers everything, and the two great metabolic pathways: photosynthesis (light energy → glucose) and cellular respiration (glucose → ATP). This is one of the heaviest-weighted units on the AP Bio exam.
Seven topics from the College Board CED, in order.
Topic 3.1
Enzyme Structure
Active site, substrate specificity, induced fit, and how protein shape determines enzyme function.
Topic 3.2
Enzyme Catalysis
How enzymes lower activation energy and speed up reactions without being consumed.
Topic 3.3
Environmental Impacts on Enzyme Function
Temperature, pH, salinity, and inhibitors (competitive vs. noncompetitive) — and how each changes enzyme activity.
Topic 3.4
Cellular Energy
ATP, coupled reactions, endergonic vs. exergonic — and how cells move energy around.
Topic 3.5
Photosynthesis
Light reactions in the thylakoid, Calvin cycle in the stroma, and the inputs/outputs of each.
Topic 3.6
Cellular Respiration
Glycolysis, pyruvate oxidation, Krebs cycle, the electron transport chain, and chemiosmosis.
Topic 3.7
Fitness
Fermentation (lactic acid and alcoholic) — how cells make ATP without oxygen.
About Unit 3
Unit 3 is the metabolism unit. You'll learn how enzymes catalyze the thousands of reactions that keep cells alive — how they work, how to regulate them, and how environmental factors change their activity. You'll learn the structure and function of ATP, the universal energy currency. And then you'll meet the two great metabolic pathways: photosynthesis (capturing light energy in glucose) and cellular respiration (extracting that energy as ATP).
At 12–16% of the AP Bio exam, this is one of the heaviest units — tied with Unit 6 (Gene Expression) for second-highest weight. The unit takes about 14–16 class periods. Expect questions on enzyme kinetics, the inputs and outputs of photosynthesis and respiration, where each step happens in the chloroplast or mitochondrion, and how cells produce ATP without oxygen (fermentation).
The College Board ties Unit 3 to three of the four Big Ideas:
Big Idea 1
Evolution — fermentation evolved before aerobic respiration