Glossary
photosynthesis
Photosynthesis is how a plant uses light energy to turn carbon dioxide and water into sugar, releasing oxygen; respiration runs the reaction the other way.
Photosynthesis is how a plant makes food. Carbon dioxide from the air and water from the soil are combined, using energy from light, into sugar; oxygen is given off as a by-product. Energy that started in sunlight is now stored in the sugar's chemical bonds, and every food chain on Earth starts from that store.
This subtest usually asks it as a pair. Photosynthesis and respiration are opposites: photosynthesis takes carbon dioxide out of the air and returns oxygen, while respiration takes in oxygen and returns carbon dioxide and water. Hold that sentence, because the questions are most often about which gas moves which way. Photosynthesis needs light and happens in the green parts of the plant — the leaves — while respiration runs in the plant's cells day and night. At night a plant is net-adding carbon dioxide to the air.
Energy is never recycled this way; only the material is. Sunlight keeps arriving, and that is the input every food chain ultimately spends.
Taught in
Related terms
- mitochondria — Mitochondria are the cell structures that release the energy stored in food, which is why they are called the cell's powerhouses; a hard-working cell like a muscle cell is packed with them.
- food chain — A food chain is the path energy takes through an ecosystem: producers such as plants make food from sunlight, consumers eat them, and each step passes on less energy than the last.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)