Light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on light. Assimilatory powers (ATP and NADPH2) are mainly produced in this light reaction.
Robin Hill (1939) first of all showed that if chloroplasts extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptors, e.g., Potassium ferroxalate (Some plants require only this chemical) and potassium ferricyanide, oxygen is released due to photochemical splitting of water. Under these conditions, no CO2 was consumed and no carbohydrate was produced, but light-driven reduction of the electron acceptors was accompained, by O2 evolution.
The splitting of water during photosynthesis is called photolysis. This reaction on the name of its discoverer is known as Hill reaction.

# Hill reaction proves that
(i) In photosynthesis oxygen is released from water.
(ii) Electrons for the reduction of CO2 are obtained from water [i.e., a reduced substance (hydrogen donor) is produced which later reduces CO2].
- Dichlorophenol indophenol is the dye used by Hill for his famous Hill reaction.

According to Arnon (1961), in this process light energy is converted to chemical energy. This energy is stored in ATP (this process of ATP formation in chloroplasts is known as photophosphorylation) and from electron acceptor NADP+, a substance which found in all living beings NADP*H is formed as hydrogen donor. Formation of hydrogen donor NADPH from electron acceptor NADP+ is known as photoreduction or production of reducing power NADPH.

# Light phase can be explained under the following headings :
(i) Transfer of energy
(ii) Quantum yield
(iii) Emerson effect
(iv) Two pigment systems
(v) Z-scheme
(vi) Cyclic and non-cyclic photophosphorylation

When photon of light energy falls on chlorophyll molecule, one of the electrons pair from ground or singlet state passes into higher energy level called excited singlet state. It comes back to hole of chlorophyll molecule within 10–9 seconds.
This light energy absorbed by chlorophyll molecule before coming back to ground state appears as radiation energy, while that coming back from excited singlet state is called fluorescence and is temperature independent. Sometimes the electron at excited singlet state gets its spin reversed because two electrons at the same energy level cannot stay; for some time it fails to return to its partner electron. As a result it gets trapped at a high energy level. Due to little loss of energy, it stays at comparatively lower energy level (Triplet state) from excited singlet state. Now at this moment, it can change its spin and from this triplet state, it comes back to ground state again losing excess of energy in the form of radiation. This type of loss of energy is called as phosphorescence.
When electron is raised to higher energy level, it is called at second singlet state. It can lose its energy in the form of heat also. Migration of electron from excited singlet state to ground state along with the release of excess energy into radiation energy is of no importance to this process. Somehow when this excess energy is converted to chemical energy, it plays a definite constructive role in the process.

 Rate or yield of photosynthesis is measured in terms of quantum yield or O2 evolution, which may be defined as, "Number of O2 mols evolved per quantum of light absorbed in photosynthesis."
On the other hand quantum requirement is defined as, "Number of quanta of light required for evolution of one mol of O2 in photosynthesis."
 Quantum requirement in photosynthesis = 8, i.e., 8 quanta of light are required to evolve one mol. of O2.
 Hence quantum yield = 1 / 8 = 0.125 (i.e., a fraction of 1) as 12%.

R. Emerson and C.M. Lewis (1943) observed that the quantum yield of photosynthesis decreased towards the far red end of the spectrum (680nm or longer). Quantum yield is the number of oxygen molecules evolved per light quantum absorbed. Since this decrease in quantum yield is observed at the far region or beyond red region of spectrum is called red drop.
Emerson et al. (1957) further observed that photosynthetic efficiency of light of 680nm or longer is increased if light of shorter wavelengths (Less than 680nm) is supplied simultaneously. When both short and long wavelengths were given together the quantum-yield of photosynthesis was greater than the total effect when both the wavelengths were given separately. This increase in photosynthetic efficiency (or quantum yield) is known as Emerson effect or Emerson enhancement effect.

The discovery of Emerson effect has clearly shown the existence of two distinct photochemical processes, which are believed to be associated with two different specific group of pigments. One group of pigments absorbs light of both shorter and longer wavelengths (More than 680nm) and another group of pigments absorbs light of only shorter wavelengths (Less than 680nm). These two groups of pigments are known as pigment systems or photosystems.
Pigment system I or Photosystem I : The important pigments of this system are chlorophyll a 670, chlorophyll a 683, chlorophyll a 695, P700. Some physiologist also include carotenes and chlorophyll b in pigment system I. P700 acts as the reaction centre. Thus, this system absorbs both wavelengths shorter and longer than 680nm.
Pigment system II or photosystem II : The main pigments of this system are chlorophyll a 673, P680, chlorophyll b and phycobilins. This pigment system absorbs wavelengths shorter than 680nm only. P680 acts as the reaction centre.
Pigment systems I and II are involved in non-cyclic electron transport, while pigment system I is involved only in cyclic electron transport. Photosystem I generates strong reductant NADPH. Photosystem II produces a strong oxidant that forms oxygen from water.

When sunlight strikes the thylakoid membrane, the energy is absorbed simultaneously by the antenna pigments of both PS I and PS II and passed on to the reaction centers of both photosystems. Electrons of both reaction center pigments are boosted to an outer orbital and each photoexcited electron is transferred to a primary electron acceptor. The transfer of electrons out of the photosystems leaves the two reaction center pigments missing an electron and thus, positively charged. After losing their electrons, the reaction centers of PS I and PS II can be denoted as P700+ and P680+ respectively. Positively charged reaction centers act as attractants for electrons, which sets the stage for the flow of electrons between carriers.
In oxygenic photosynthesis, in which two photosystems act in series, electron flow occurs along three legs-between water and PS II, between PS II and PS I and between PS I and NADP+ an arrangement which is described as the Z scheme. The Z scheme as originally proposed by Hill and Bendall, 1960.

Light phase includes the interaction of two pigment systems. PS I and PS II constitute various type of pigments. Arnon showed that during light reaction not only reduced NADP is formed and oxygen is evolved but ATP is also formed. This formation of high energy phosphates (ATP) is dependent on light hence called photophosphorylation.

@@@ Detailed study in ARTICLE - PHOSPHORYLATION

Dark phase : The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO2 into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR) cycle or dark reactions. The dark reactions are sensitive to temperature changes, but are independent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e. NADP .2H and ATP. The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation of CO2 and synthesis of sugar. The techniques used for studying different steps were Radioactive tracer technique using 14C (Half life – 5720 years), Chromatography and Autoradiography and the material used was Chlorella (Cloacal alga) and Scenedesmus (these are microscopic, unicellular algae and can be easily maintained in laboratory).
The assimilation and reduction of CO2 takes place in this reaction by which carbohydrate is synthesized through following three pathways :
(i) Calvin cycle (C3) (ii) Hatch and Slack cycle (C4) (iii) Crassulacean acid metabolism (CAM plants)

Calvin and Benson discovered the path of carbon in this process. This is known as C3 cycle because CO2 reduction is cyclic process and first stable product in this cycle is a 3-C compound (i.e., 3-Phosphoglyceric acid or 3-PGA).
Calvin cycle is divided into three distinct phases : Carboxylation, Glycolytic reversal, Regeneration of RuBP.

Kortschak and Hart supplied CO2 to the leaves of sugarcane, they found that the first stable product is a four carbon (C4) compound oxalo acetic acid instead of 3-carbon atom compound. The detailed study of this cycle has introduced by M.D. Hatch and C.R. Slack (1966). So it is called as "Hatch and Slack cycle". The stable product in C4 plant is dicarboxylic group. Hence it is called dicarboxylic acid cycle or DCA-cycle. C4 plants are true xerophytic plants.

The plants that perform C4 cycle are found in tropical (Dry and hot regions) and sub-tropical regions. There are more than 900 known species in which C4 cycle occurs. Among them, more than 300 species belong to dicots and the rest belong to monocots. The important among them are sugarcane, maize, Sorghum, Cyperus rotundus, Digitaria brownii, Amaranthus, etc. These plants have "Kranz" (German term meaning halo or wreath) type of leaf anatomy. The vascular bundles, in C4 leaves are surrounded by a layer of bundle sheath cells that contain large number of chloroplasts. The chloroplasts in C4 leaves are dimorphic (Two morphologically distinct types). The chloroplasts of bundle sheath cells are larger in size and arranged centripetally. They contain starch grains but lack grana. The mesophyll cells, on the other hand, contain normal types of chloroplasts. The mesophyll cells perform C4 cycle and the cells of bundle sheath perform C3 cycle.

CO2 taken from the atmosphere is accepted by phosphoenolpyruvic acid (PEP) present in the chloroplasts of mesophyll cells of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA). This acid is converted to another 4-C acid, the malic acid which enters into the chloroplasts of bundle sheath cells and there undergoes oxidative decarboxylation yielding pyruvic acid (a 3-C compound) and CO2. CO2 released in bundle sheath cells reacts with Ribulose-1,5-biphosphate (RuBP) already present in the chloroplasts of bundle sheath cells and thus Calvin cycle starts from here. Pyruvic acid re-enters mesophyll cells and regenerates phosphoenol pyruvic acid. CO2 after reacting with RuBP gives rise to sugars and other carbohydrates. Mesophyll cells have PEP carboxylase and pyruvate orthophosphate dikinase enzyme while the bundle sheath cells have decarboxylase and complete enzymes of Calvin cycle. In C4 plants, there are 2 carboxylation reactions, first in mesophyll chloroplast and second in bundle sheath chloroplast.
C4 plants are better photosynthesizers. There is no photorespiration in these plants. In C4 plants, for formation of one mole of hexose (glucose) 30 ATP and 12 NADPH2 are required. There is difference in different C4 plants in mechenism of C4 mode of photosynthesis. The main difference is in the way the 4C dicarboxylic acid is decarboxylated in the bundle sheath cells.

# The three categories of C4 pathways in C4 plants are recognised such as :
(a) Some C4 plants e.g., Zea mays, Saccharum officinarum, Sorghum utilise NADP+ specific malic enzyme for decarboxylation. This mechanism of C4 pathway in these C4 plants is said to be of NADP+ –Me Type.
(b) Some C4 plants e.g., Atriplex, Portulaca, Amaranthus utilise NAD+ specific malic enzyme for decarboxylation. This mechanism of C4 pathways in these C4 plants is said to be of NAD+ –Me Type.
(c) Some C4 plants e.g., Panicum, Chloris utilise PEP-carboxykinase enzyme. The mechanism of C4 pathway in these plants is called as PCK-me-Type.

# Characteristics of C4 cycle
(1) C4 species have greater rate of CO2 assimilation than C3 species. This is on account of the fact that
(a) PEP carboxylase has great affinity for CO2.
(b) C4 plants show little photorespiration as compared to C3 plants, resulting in higher production of dry matter.
(2) C4 plants are more adapted to environmental stresses than C3 plants.
(3) CO2 fixation by C4 plants require more ATP than that by C3 plants. This additional ATP is needed for conversion of pyruvic acid to phosphoenol pyruvic acid and its transport.
(4) CO2 acceptor molecule in C4 plants is PEP. Further, PEP-carboxylase (PEPCO) is the key enzyme (RuBP-carboxylase enzyme is negligible or absent in mesophyll chloroplast, but is present in bundle sheath chloroplast).

This dark CO2 fixation pathway proposed by Ting (1971). It operates in succulent or fleshy plants e.g. Cactus, Sedum, Kalanchose, Opuntia, Agave, orchid, pine apple and Bryophyllum helping them to continue photosynthesis under extremely dry condition.
The stomata of succulent plants remain closed during day and open during night to avoid water loss (Scotactive stomata). They store CO2 during night in the form of malic acid in presence of enzyme PEP carboxylase. The CO2 stored during night is used in Calvin cycle during day time. Succulents refix CO2 released during respiration and use it during photosynthesis.
This diurnal change in acidity was first discovered in crassulacean plants e.g. Bryophyllum. So it is called as crassulacean acid metabolism. The metabolic pathways are –

# Acidification : In dark, stored carbohydrates are converted to phosphoenol pyruvic acid (PEP) by the process of glycolysis. The opening of stomata in CAM plants in dark causes entry of CO2 in leaf. So, phosphoenol pyruvic acid in presence of PEP carboxylase is converted to oxaloacetic acid (OAA). OAA is then reduced to malic acid in presence of enzyme malic dehydrogenase with the help of NADH2. This malic acid (Produced by acidification) is stored in vacuole.

# Deacidification : In light the malic acid is decarboxylated to produce pyruvic acid and evolve CO2. This process is called deacidification.
The malate may be decarboxylated in two ways –
(a) In some CAM plants the malate is directly decarboxylated in the presence of NADP+ malic enzyme into CO2 and pyruvate (ME-CAM plants).
(b) In other CAM plants, the malate is first oxidised to oxaloacetic acid by enzyme malate dehydrogenase which is then converted into CO2 and phosphoenol pyruvate with the utilization of ATP by enzyme PEP carboxykinase (PEPCK-CAM plants).
The CO2 produced by any above process is then consumed in normal photosynthetic process to produce carbohydrate.

# Characteristics of CAM pathway
(1) CO2 assimilation and malic acid assimilation take place during the night.
(2) There is decrease in pH during the night and increase in pH during the day.
(3) Malic acid is stored in the vacuoles during the night which is decarboxylated to release CO2 during the day.
(4) CAM plants have enzymes of both C3 and C4 cycle in mesophyll cells. This metabolism enable CAM plants to survive under xeric habitats. These plants have also the capability of fixing the CO2 lost in respiration.