Photosynthesis

Light, water, and CO₂ become sugar and oxygen.

The idea

In the light-dependent reactions, chlorophyll in the thylakoid membranes absorbs light; photosystem II splits water (photolysis), releasing O₂ as a by-product and passing electrons down an electron transport chain that pumps H⁺ into the thylakoid space so ATP synthase can make ATP, while photosystem I hands the electrons on to reduce NADP⁺ to NADPH. In the light-independent reactions of the Calvin cycle, in the stroma, the enzyme RuBisCO fixes CO₂ onto the five-carbon sugar RuBP to give two molecules of three-carbon 3-phosphoglycerate, which ATP and NADPH reduce to triose phosphate (G3P); most G3P is recycled to regenerate RuBP and the rest builds glucose, sucrose, starch, and cellulose. Because these are enzyme-driven steps, the overall rate is set by whichever factor is in shortest supply — light intensity, CO₂ concentration, or temperature — the law of limiting factors. Only about 1–2% of the sunlight hitting a leaf ends up stored in sugar.

Work through an example

Making one glucose (six carbons) takes six turns of the Calvin cycle: 6 CO₂ are fixed, 18 ATP and 12 NADPH are consumed, and 12 G3P are produced, of which 2 leave to form glucose and 10 are rearranged back into 6 RuBP. Those 12 NADPH require 24 electrons, which come from splitting 12 H₂O — releasing exactly the 6 O₂ in the equation. Van Helmont’s 17th-century willow makes the scale vivid: it gained about 74 kg in five years while its pot of soil lost only 57 g, because nearly all of the new mass came from CO₂ in the air.

What to watch for

The oxygen a plant releases comes from water, not from carbon dioxide — Ruben and Kamen showed this in 1941 by giving algae water labeled with heavy oxygen (¹⁸O) and finding the label in the O₂ gas. Also, plants do not "photosynthesize by day and respire by night": every plant cell respires around the clock, and in daylight photosynthesis simply outpaces respiration, so the leaf is a net oxygen producer.