= Photosynthesis
{wiki}
<Photosynthesis> captures <light> <energy> and transforms it into chemical forms that a <biological cell> can use. In <plants>, <algae> and <cyanobacteria>, this supports <carbon fixation>: incorporation of <carbon> from <carbon dioxide> into <chemical compounds> used to build living material. Capturing <light> and incorporating <carbon> are distinct processes coupled through <metabolism>. See https://openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis[OpenStax Biology 2e, 8.1: overview of photosynthesis].
For <oxygenic photosynthesis>, a simplified net equation writes a six-<carbon> <sugar>, <glucose>, as the representative product:
$$
6\mathrm{CO}_2+6\mathrm{H}_2\mathrm{O}+\text{light}\longrightarrow\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6+6\mathrm{O}_2.
$$
This is net <stoichiometry>, not a one-step mechanism. A worked bookkeeping check counts six <carbon> <chemical atoms>, twelve <hydrogen> <chemical atoms>, and eighteen <oxygen> <chemical atoms> on each side: the product side has six <oxygen> <chemical atoms> in the <sugar> and twelve in <molecular oxygen>. The <light> label supplies <energy>, not additional <chemical atoms>.
Crucially, the released <molecular oxygen> comes from <water>, not <carbon dioxide>. Cancelling <water> made and consumed in different steps hides this distinction in the net equation. The <Calvin cycle> supplies a net three-<carbon> <triose phosphate>, such as <glyceraldehyde 3-phosphate>, for subsequent <sugar> synthesis; it does not directly export <glucose>. See https://openstax.org/books/biology-2e/pages/8-2-the-light-dependent-reactions-of-photosynthesis[OpenStax Biology 2e, 8.2: light-dependent reactions] and https://openstax.org/books/biology-2e/pages/8-3-using-light-energy-to-make-organic-molecules[OpenStax Biology 2e, 8.3: using light energy to make organic molecules].
\Image[/photosynthesis-energy-and-matter.png]
{title=<Photosynthesis>: <water> supplies released <molecular oxygen>; <carbon dioxide> supplies <sugar> <carbon>}
{description=Original schematic of <oxygenic photosynthesis>. Solid blue arrows trace selected material transfers; dashed orange arrows show <energy> transfer and the coupling through <ATP> and <NADPH>. These carriers are also <molecules>, not pure <energy>. Arrows omit intermediate <chemical reactions> and do not specify rates or yields.}
The useful achievement is connecting incoming <light> to both usable <energy> and new biological material. <ATP> is an immediate <energy> currency; <NADPH> supplies <electrons> for synthesis; <carbon dioxide> supplies the <carbon> incorporated into <carbohydrates>. These roles are complementary. <Conservation of energy> still holds: <photosynthesis> transforms <energy>, never creates it, and some absorbed <energy> is dissipated as <heat>. A <plant> also carries out <cellular respiration>, including in <biological cells> that perform <photosynthesis>. Gas exchange reflects both processes. <Oxygenic photosynthesis> remains central to interpreting terrestrial <molecular oxygen> as an <atmospheric biosignature gas>. See https://openstax.org/books/biology-2e/pages/8-3-using-light-energy-to-make-organic-molecules[OpenStax Biology 2e, 8.3: photosynthesis and respiration].
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