Photosynthesis is a vital physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the primary source of all food on Earth and is responsible for releasing oxygen into the atmosphere.
The overall balanced chemical equation is:
$$6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$$
Photosynthesis occurs in the green leaves and other green parts of plants, specifically within the mesophyll cells which contain a large number of chloroplasts. Within the chloroplast, there is a clear division of labor:
Chromatographic separation reveals that leaf color is due to four distinct pigments:
Accessory pigments (Chl b, xanthophylls, carotenoids) absorb light at different wavelengths, transfer energy to Chl a, and protect it from photo-oxidation.
The light reactions include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates (ATP and NADPH). Pigments are organized into two discrete photochemical light harvesting complexes (LHC) within Photosystem I (PS I) (reaction center $P_{700}$) and Photosystem II (PS II) (reaction center $P_{680}$).
When PS II absorbs red light at 680 nm, electrons become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor and passed through an electron transport system consisting of cytochromes to PS I.
Simultaneously, electrons in the reaction center of PS I are excited by red light of 700 nm and transferred to another acceptor with greater redox potential. These electrons are then moved downhill to reduce $NADP^+$ to $NADPH + H^+$. This continuous pathway is called the Z-scheme due to its characteristic shape on a redox potential scale.
To supply electrons continuously to PS II, water is split on the inner side of the thylakoid membrane:
$$2H_2O \rightarrow 4H^+ + O_2 + 4e^-$$
This process releases oxygen as a byproduct and accumulates protons ($H^+$) inside the thylakoid lumen.
ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane. Protons accumulate in the lumen due to water splitting, proton translocation via plastoquinone, and the consumption of stroma protons during $NADP^+$ reduction.
This gradient is broken down as protons diffuse back to the stroma through the transmembrane channel of ATP synthase ($CF_0$ and $CF_1$). This movement releases enough energy to catalyze the synthesis of ATP from ADP and inorganic phosphate.
The products of the light reaction (ATP and NADPH) are used to drive the synthesis of sugars from $CO_2$ and $H_2O$. The primary $CO_2$ acceptor is a 5-carbon ketose sugar, ribulose-1,5-bisphosphate (RuBP), and the first stable product is a 3-carbon acid, 3-phosphoglyceric acid (PGA). The cycle proceeds in three stages:
To synthesize one molecule of glucose, 6 turns of the cycle are required:
Plants adapted to dry tropical regions use the $C_4$ pathway. They exhibit Kranz anatomy, characterized by large bundle sheath cells around vascular bundles that contain many chloroplasts and lack intercellular spaces.
A wasteful process in $C_3$ plants where RuBisCO binds with $O_2$ instead of $CO_2$ under high oxygen concentrations. RuBP combines with $O_2$ to form one molecule of PGA and one molecule of phosphoglycolate ($C_2$ compound). This pathway does not synthesize sugar, ATP, or NADPH, but instead consumes ATP and releases $CO_2$.
$C_4$ plants lack photorespiration because they have a mechanism that increases the concentration of $CO_2$ at the RuBisCO enzyme site, ensuring high productivity and tolerance to high temperatures.
Formulated in 1905, this law states: "If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value."