Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen. ATPase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATPase enzyme that catalyses the formation of ATP. Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO₂, and synthesis of sugars.

There is a clear division of labour in chloroplasts. The membrane system is responsible for Light reactions. Light energy is trapped by the membrane system and synthesis of ATP and NADPH takes place over there. The stroma utilises CO₂ to synthesize sugar; ATP and NADPH from light reaction are also utilized by stroma.

In C₃ plants some O₂ does bind to RuBisCO, and hence CO₂ fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O₂ to form one molecule and phosphoglycolate in a pathway called photorespiration. In the photo respiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO₂ with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH. Therefore, photorespiration is a wasteful process.

The splitting of water is associated with the PS II; water is split into H+, [0) and electrons. This creates oxygen, one of the net products of photosynthesis. The electrons needed to replace those removed from photo system I are provided by photo system II.
2H₂O ⟶ 4H⁺ + O₂ + 4e^-
2H₂O ⟶ 4H⁺ + O2 + 4e^-

Light reactions or the 'Photochemical' phase include following steps:
(i) light absorption,
(ii) water splitting,
(iii) oxygen release, and
(iv) the formation of high-energy chemical intermediates, ATP and NADPH.
Several complexes are involved in the process. The pigments are organised into two discrete photochemical light harvesting complexes (LHC)within the Photosystem I (PS I) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and also the electrons needed to convert carbon dioxide into carbohydrate, which is a reduction reaction. In general outline, photosynthesis is the opposite of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy.

The particularly large cells around the vascular bundles of the C₄ pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have 'Kranz' anatomy. 'Kranz' means 'wreath' and is a reflection of the arrangement of cells. The bundle sheath cells may form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.

C₄-plants are better adapted to dry tropical regions. They have the following features.
i) Can tolerate high temperatures
ii) Can perform photosynthesis in high light intensities.
iii) Have overcome the problem of photorespiration and have a greater productivity of biomass.

Group A will show more photosynthesis.
(a) Chlorophyll absorbs maximum light in the blue region of the spectrum, i.e., 400-450 nm and hence photosynthetic rate will also be high.
Group B will show negligible amount of photosynthesis or no photosynthesis.
(b) Chlorophyll does not absorb any light in the green region, i.e., 500-550 om, but reflects green.

Carboxylation is the most crucial step of the Calvin cycle.where CO₂ is utilized for the carboxylation of RuBP.This reaction is catalyzed by the enzyme RuBP carboxylase, which results in the formation of two molecules of 3-PGA. Since this enzyme also has an oxygenation activity, it would be more correct to call it RuBP carboxylase oxygenase or RuBisCO.

Photosynthetic organisms at different depths in the ocean do not receive the same light qualitatively and quantitatively. The plants growing thee banks of water bodies get goof quality o light. Algae which can tolerate more dilutions of light penetrate deeper inside water. Green algae remain in the shore line, brown algae descend to intermediate depths, while red algae form the limit below where no other autotrophic plants can grow.

The absorption spectrum is the graph plotted, with the amount of light absorbed as a function of wavelength.The action spectrum is the graph plotted with the rate of photosynthesis as a function of wavelength. Since, the amount of light absorbed and the rate of photosynthesis have a direct relationship, the two curves overlap.They show peaks around wavelengths 450 nm(blue region) and 650-680 nm (red region).

C₄- plants chemically fix carbon dioxide in the cells of the mesophyll by adding it to the 3 molecule Phosphoenol Pyruvate (PEP), a reaction catalysted by an enzyme called PEP carboxylase. It creates the 4 carbon organic acid, oxaloacetic acid. Oxaloacetic acid synthesised by this process is then translocated to specialised bundle sheath cells where translocated to specialised bundle sheath cells where the enzyme, RuBisCO and other Calvin cycle enzymes are located and where CO₂ released by decarboxylation of the 4 carbon acids is then fixed by RuBisCO enzyme to form 3-phosphoglyceric acids (3C).
The physical separation of RuBisCO from the oxygen driven light reactions inhibits photorespiration and increases CO₂-fixation. This way photosynthetic capacity of the C₄-plants increases many folds than C₃ plants.

Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATP synthase enzyme. Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons whithin the thylakoid lumen. ATPsynthase enzyme catalyses the formation of ATP, along with the NADPH  produced by the movement of electrons. The energy carried by ATP molecules will be used immediately in the biosynthetic reaction taking place in the stroma for fixing CO₂ and synthesis of sugars.

(a) In the C₃ pathway the first product of carbon fixation is a 3 carbon atom compound 3-phosphoglyceric acid and in C₄ pathway the first product of carbon fixation is oxaloacetic acid a 4 carbon atom compound.
(b)  Comparison between the cyclic and non-cyclic photophosphorylation

(c)  C₃ leaves have mesophyll, while C₄ leaves have bundle sheath as well.