Where this fits in Biology 20

In Alberta Biology 20 Unit A, photosynthesis is central to understanding energy flow and matter cycling in ecosystems. Plants, algae, and some bacteria capture light energy and convert it into chemical energy stored in glucose. That glucose then supports growth, cellular respiration, and food webs. When students understand photosynthesis clearly, many other topics become easier, including productivity, trophic levels, and carbon cycling.

Photosynthesis has two linked stages. In the light-dependent reactions (in the thylakoid membranes), chlorophyll absorbs light, water is split, oxygen is released, and ATP plus NADPH are produced. In the light-independent reactions (Calvin cycle, in the stroma), ATP and NADPH are used to fix carbon dioxide into organic molecules that can be built into glucose. A common misconception is that the Calvin cycle only happens at night; in reality, it can run whenever ATP and NADPH from light reactions are available.

From a matter perspective, atoms are rearranged rather than created. Carbon atoms in glucose come from atmospheric CO2, and the oxygen gas released comes from water molecules split during light reactions. This is why isotope tracing evidence is so important in biology: it shows exactly where atoms move. If you can explain where carbon, hydrogen, and oxygen come from in glucose, you are demonstrating high-level understanding expected in high school biology.

Environmental factors affect photosynthesis rate. Increasing light intensity, carbon dioxide concentration, or temperature can increase rate up to a limit, after which another factor becomes limiting. In labs, students often graph rate versus one variable and identify the plateau. On tests, be ready to interpret these graphs and justify which variable is limiting under specific conditions, such as greenhouse settings or drought stress.