Chloroplast

Photosynthesis is the only process on Earth by which solar energy is trapped by green plants and converted into food. In precise terms, it is the synthesis of the carbohydrate glucose from the inorganic raw materials carbon dioxide and water, powered by solar energy that chlorophyll pigments capture, and it can be summarised in one overall equation:

6CO₂ + 12H₂O → (Light; Chloroplast) C₆H₁₂O₆ + 6O₂ + 6H₂O

This process is unique to green plants, and it is the final step by which light energy enters the living world -- every food chain on Earth ultimately depends on it, which is why it is sometimes described as turning all life into 'bottled solar energy'. It is also one of the largest chemical processes happening on the planet: the atmosphere contains only about 0.03% carbon dioxide by volume, but that thin fraction still represents roughly 2200 billion tonnes of CO2, and the oceans hold more than 50 times as much again, dissolved as gas or locked up as carbonate. Between these two reservoirs, green plants fix around 70 billion tonnes of carbon every single year through photosynthesis.

Photosynthesis itself happens inside chloroplasts, organelles found mainly in the mesophyll cells of leaves, where CO2 diffuses in through the stomata and water arrives via the leaf's veins. In higher plants, chloroplasts are typically discoid or lens-shaped, and each one is enclosed by a double membrane (an outer and an inner membrane, with a narrow intermembrane space between them). Inside this envelope lies a ground substance called the stroma, and suspended within the stroma is an internal membrane system: chlorophyll-bearing double-membrane sacs, or lamellae, stacked on top of one another into cylindrical piles called grana (singular: granum). Each individual disc-shaped sac within a granum is a thylakoid, and separate grana are interconnected by unstacked membrane bridges called stroma lamellae.

All of the light-absorbing pigments -- the chlorophylls, carotenes and xanthophylls -- sit in the thylakoid membranes, embedded there because they are fat-soluble and belong in the membrane's lipid layer. Each pigment absorbs light of particular wavelengths in the visible region, and with the help of specific enzymes they convert that absorbed solar energy into ATP and NADPH2; separate enzymes located in the stroma then use this ATP and NADPH2 to build carbohydrates. …