The reaction
Inputs
Light energy, water, and carbon dioxide
Outputs
Sugar and oxygen released to the air
Formula
Light plus 6 H2O plus 6 CO2 becomes C6H12O6 plus 6 O2
Use
Sugar builds tissue and powers metabolism
Where it happens
Chloroplast
An inner organelle that turns photons into the electrical charge used to make sugar
Old partner
Chloroplasts likely began as cyanobacteria that took up residence in plant cells
Own genes
They still carry DNA for about 200 proteins. The nucleus supplies the rest of the roughly 1,000 parts
Rooms
An outer leaky membrane, a tight inner membrane, stroma, and thylakoid stacks around the lumen
Light reactions
Two systems
Photosystem II runs first, then Photosystem I, named by discovery order, not sequence
Water split
The oxygen-evolving center pulls electrons from water and leaves protons and oxygen
Speed
One Photosystem II complex can free as many as 50 oxygen molecules per second
Hand-off
The electron is boosted again in Photosystem I and used to make ATP
Dark reactions and storage
Not night-only
The carbon-fixing steps use the energy the light steps just made. They run in the stroma
Rubisco
This enzyme locks CO2 onto a sugar backbone and builds carbohydrates and amino acids
Starch
Extra sugar is stored as glucose polymers for later growth
Garden meaning
Light, water, and CO2 are not optional extras. They are the recipe