Organisms can be divided into autotrophs, which can synthesize their own food using inorganic minerals, and heterotrophs which require organic food.  Plants produce organic compounds from simple compounds but cannot produce so-called “organic food” from “organic products”. The heterotrophs can do it and most plants are not heterotrophs. Plants are autotrophs. Hence all labels of “Organic food “ are misnomer untils and unless someone explains what is meant by “organic food”  and “Organic farming”.

Here the basic process of production of food by plants is  presented in a simplified way. 

Autotrophs fix atmospheric carbon dioxide 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.  The general equation for photosynthesis is therefore:

2n CO₂ + 2n H₂A + photons → 2(CH₂O)n + n O₂ + 2n A

Carbon dioxide + electron donor + light energy → carbohydrate + oxygen + oxidized electron donor

Since water is used as the electron donor in oxygen photosynthesis, the equation for this process is:

2n CO₂ + 2n H₂O + photons → 2(CH₂O)n + 2n O₂

Carbon dioxide + water + light energy → carbohydrate +oxygen

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH+H^+. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide. However many of such “light independent reactions” have recently  been proved to be “light dependent directly or indirectly”  as activation of certain enzymes of "light independent reactions" is found to be " light dependent".

Most organisms that utilize photosynthesis to produce oxygen use visible light.

Photosynthetic Membranes and organelles:

In plants and algae, photosynthesis takes place in organelles called chloroplasts. A typical plant cell contains about 15 to 50 chloroplasts or even more. The chloroplast is enclosed by a membrane which contains aqueous fluid called the stroma. The stroma contains stacks (grana) of thylakoids,which are the site of photosynthesis.

The thylakoids are flattened disks  with a lumen or thylakoid space within it. The site of photosynthesis is the thylakoid membrane which contains chlorophyll a, chlorophyll b , carotene, and xanthophyll , arranged in photosynthetic units commonly known as PS I and PS II.  The pigments  of the photosystems absorb light energy and transfer it to antenna pigment.

 LIGHT REACTION

Light reactions or the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH + H⁺.  Products of light reaction are ATP, NADPH and O₂.

Splitting of Water

The splitting of water is associated with the PS II; water is split into H⁺, O₂ and electrons.

2H₂O → 4H⁺ + O₂ + 4e^-

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

When the two photosystems work in a series, first PS II and then the PS I, a process called non-cyclic photo-phosphorylation occurs.

The Calvin Cycle

The special lab of the late Nobel Laureate Professor Melvin Calvin at the University of California, Berkley is in an oval building with a tomb-like structure to accommodate very large, up to 15 feet long and 1 to 2 inch in diameter, tubes in which he used Chlorella as organism, radio labeled CO2 coming from sodium bicarbonate once acid is poured on it and hot and steaming alcohol  is used to stop the reactions within a few seconds. I had a chance to witness  this original building  and had the privilege to meet Professor Calvin, who took pains to show me his special but simple  devices which won him the Nobel Prize.  It  the was discovery of radio isotopes and technique of chromatography during early 40s which was  made use of by Melvin Calvin and his team of workers to elucidate the path of carbon in photosynthesis. Not all those working in his team got the Nobel, hence some people prefer to call it the Photosynthetic Carbon reduction (PCR) cycle and Photosynthetic Carbon  Oxidation (PCO) cycle. RuBisCO, that is the most abundant enzyme in the world, is characterized by the fact that its active site can bind to both CO2 and O2. In C3 plants some O2 does bind to RuBisCO, and hence CO2 fixation is decreased. In C4 plants photorespiration does not occur. 

 That is how the initial enzyme is now named as Ribulose bis phosphate carboxylase oxygenase ( RuBISCO). 

The Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.

1. Carboxylation – Carboxylation is the fixation of CO2 intoa stable organic intermediate. This reaction is catalysed by the enzyme RuBPcarboxylase-oxygenase (RuBISCO) which results in the formation of two moleculesof 3-PGA.

2. Reduction – 3-PGA undergoes reduction  to the formation of glucose. The steps involveutilisation of 2 molecules of ATP for phosphorylation and two of NADPH

3. Regeneration –The regeneration steps require one ATP forphosphorylation to form RuBP.

Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. 

 THE C4 PATHWAY

I fondly remember a comment by Professor Melvin Calvin when someone pointed out to him that now there are two cycles of carbon fixation viz C3 and C4. (The C4 pathway has been named as  the Hatch and Slack Pathway). The basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C3 and C4 plants. Professor Melvin Calvin replied there is only one cycle which fixes CO2 to its end product and that is C3 cycle only.The C4 cycle is merely a CO2 enriching cycle to cope up extra demand of CO2 in tropical plants exposed to higher energy levels and high temperature.  (Exceptions to this is anaplerotic CO2 fixation).  The CO2 fixed in cytoplasm of mesophyll cells is decarboxylated in bundle sheath cells where it is fixed in Calvin cycle. The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle.

The objective of this article was to provide basic information about photosynthesis to a wider audience.