ACT Science: Compare Photosynthesis and Respiration to Understand Energy Flow in Organisms

Photosynthesis: Uses light energy to convert CO2 and H2O into glucose and O2. Occurs in chloroplasts. Equation: 6CO2+6H2O+light energy→C6H12O6+6O2. Builds energy-rich molecules. Respiration: Uses glucose and O2 to produce CO2, H2O, and ATP (energy). Occurs in mitochondria. Equation: C6H12O6+6O2→6CO2+6H2O+ATP (energy). Breaks down energy-rich molecules. Photosynthesis stores energy from the Sun into chemical bonds. Respiration releases that energy from chemical bonds to power cells. They are complementary: the products of one are the reactants of the other.

Why it matters: Photosynthesis is how plants (and some bacteria) capture sunlight. Respiration is how all organisms (plants and animals) release energy for life processes. Understanding both is essential for ecology (food chains, energy flow), cell biology (ATP production), and biogeochemistry (carbon cycle).

Difference 1: Light requirement. Photosynthesis requires light (occurs in daytime, in the light-dependent reactions). Respiration does not require light; it happens constantly, day and night. Difference 2: Energy direction. Photosynthesis stores energy (endergonic, requires input). Respiration releases energy (exergonic, releases output). Difference 3: Location. Photosynthesis occurs in chloroplasts (in plants, algae). Respiration occurs in mitochondria (in all living cells). Difference 4: Products. Photosynthesis produces glucose (food) and oxygen. Respiration produces ATP (energy), CO2, and water. A common mistake is thinking plants only photosynthesize; they also respire. All living cells respire to release energy from glucose.

On the ACT, questions might ask: "Where does this process occur?" or "What is the energy source?" or "What are the products?" Use the differences above to identify which process is being described and predict its characteristics.

Characteristic 1: "Requires sunlight." Answer: Photosynthesis (specifically the light-dependent reactions). Characteristic 2: "Occurs in mitochondria." Answer: Respiration (all types, in all cells). Characteristic 3: "Produces ATP." Answer: Respiration (ATP is the cell's energy currency). Characteristic 4: "Produces oxygen as a byproduct." Answer: Photosynthesis (from splitting water in the light reactions). Characteristic 5: "Uses CO2 from the air." Answer: Photosynthesis (the Calvin cycle uses CO2 to make glucose). Characteristic 6: "Happens in plants 24/7." Answer: Respiration (photosynthesis is only in light, but respiration is constant). For each characteristic, determine which process it describes by checking the location, light requirement, energy direction, and products.

On the next ACT Science practice test, find questions about photosynthesis or respiration. For each, ask: What location is mentioned? What is the energy source? What are the products? Use these clues to match the process correctly and predict what should happen under different conditions (different light, different temperatures, etc.).

Photosynthesis and respiration questions appear 2-4 times per ACT Science section and connect to ecology (food webs, energy pyramids), cell biology (ATP), and biochemistry (metabolic pathways). Understanding both processes is foundational for predicting what happens when conditions change (increased light, lower temperature, lack of oxygen). Once you internalize photosynthesis and respiration as complementary processes, you answer questions about energy flow in ecosystems with confidence grounded in understanding, not memorization.

Spend 20 minutes this week learning the inputs and outputs of both photosynthesis and respiration, and the locations where each occurs. Draw side-by-side diagrams or equations. Then answer 10 questions about energy transformation in organisms, using your understanding to predict outcomes. By test day, you will answer these questions with the confidence that comes from grasping how organisms capture and use energy.