Key Points

Photosynthesis is a process where green plants use sunlight to synthesize food from carbon dioxide and water. The balanced equation is $6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$.

Photosynthesis takes place in chloroplasts. The light-dependent reactions occur in the thylakoid membranes (grana), while the light-independent (dark) reactions occur in the stroma.

The main pigments are Chlorophyll a (chief pigment), Chlorophyll b, Xanthophylls, and Carotenoids. Accessory pigments (Chlorophyll b and carotenoids) absorb light and transfer energy to Chlorophyll a.

This phase includes light absorption, splitting of water (photolysis), oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.

Photosystems are complexes of pigments and proteins. Photosystem I (PS I) has a reaction center P700 (absorbs at 700 nm), and Photosystem II (PS II) has a reaction center P680 (absorbs at 680 nm).

In non-cyclic photophosphorylation, electrons flow from PS II to PS I and finally to NADP+, forming ATP and NADPH. This is called the Z-scheme due to its characteristic shape on a redox potential scale.

Water is split in association with PS II on the inner side of the thylakoid membrane, producing protons ($H^+$), electrons ($e^-$), and oxygen. The reaction is $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$.

Non-cyclic photophosphorylation involves both PS I and PS II, producing ATP and NADPH. Cyclic photophosphorylation involves only PS I and produces only ATP, not NADPH.

ATP synthesis is linked to a proton gradient across the thylakoid membrane. Protons accumulate in the lumen and their movement back to the stroma through the ATP synthase enzyme drives ATP formation.

This cycle occurs in the stroma and uses ATP and NADPH from the light reaction to fix $CO_2$ and produce sugar. It involves three stages: carboxylation, reduction, and regeneration.

Carboxylation is the first step where $CO_2$ is fixed by the enzyme RuBisCO, which combines $CO_2$ with a 5-carbon sugar, Ribulose-1,5-bisphosphate (RuBP), to form two molecules of 3-PGA.

To synthesize one molecule of glucose ($C_6H_{12}O_6$), six turns of the Calvin cycle are required, consuming 6 $CO_2$, 18 ATP, and 12 NADPH.

In C3 plants, the first product of $CO_2$ fixation is a 3-carbon acid (PGA). In C4 plants, the first product is a 4-carbon acid (oxaloacetic acid, OAA).

C4 plants have a special leaf anatomy called 'Kranz' anatomy, with large bundle sheath cells around the vascular bundles. The Calvin cycle occurs in these bundle sheath cells.

Photorespiration is a wasteful process where RuBisCO binds with $O_2$ instead of $CO_2$, reducing photosynthetic output. It occurs in C3 plants under high light and temperature but is negligible in C4 plants.

Blackman's Law states that the rate of a process is limited by the factor that is nearest to its minimal value. For photosynthesis, key limiting factors are light intensity, $CO_2$ concentration, and temperature.