Photosynthesis inplants can fall into three types C3 , C 4 and CAM.

C3 photosynthesis:

Photosynthesis is conversion of light energy into chemical energy utilizing chlorophyll (  LHC II.PS II and LHC I  PS I ) along with water and generating ATP, reducing NADP+ to NADPH + H+  and photolysis of water with release of Oxygen.  

Earlier Photosynthesis was divided into two type of reactions: Light reactions and dark reactions. 

It was assumed that Carbon fixation does not require light and the reaction is light independent. Here I would like to put together recent results from various research  groups  from USA, U.K.  Germany  France, and  Japan indicating that dark reaction also takes place in light only. It may not be possible to cite those who have contributed to this understanding but I acknowledge the work of all persons working in the field. 

Dark reaction requires presence of light and takes place at the same time as Light reaction: 

 In a way dark reaction is actually also light dependent Carbon fixation. Although  Calvin Cycle’ or ‘Carbon Reactions Pathway  do not Require Light energy to occur but they do require energy captured by light reactions and they occur at same time as light reactions.

This reaction takes place in light and is termed as “Light reaction”. C3 reduction  or Calvin cycle CO₂→ C3 → C6  (Melvin Calvin 1950s,  Nobel prize in 1961) was termed “dark reaction”.

 Enzymes&intermediates of the Calvin Cycle are located in the chloroplast stroma, a compartment somewhat analogous to the mitochondrial matrix. The Calvin Cycle, earlier designated  as the photosynthetic"dark reactions,"  is now called the carbon reactions pathway Carbon fixation. I propose to call it “light dependent carbon fixation” as this takes place during light only.      

 Light-activated e^- transfer is linked to pumping of H⁺ into thylakoid disks. This leads to increase of  pH in the stroma to about 8. This pH range ( 7.8 to pH 8.0 )is  required for activity of RuBisco to take place effectively.  Alkaline pH activates stromal Calvin Cycle enzymes RuBP Carboxylase,Fructose-1,6-Bisphosphatase&Sedoheptulose Bisphosphatase. 

The light-activated H⁺ shift is countered by Mg⁺⁺release from thylakoids to stroma. RuBP Carboxylase (in stroma) requires Mg⁺⁺ binding to carbamate at the active site.  Some plants synthesize atransition-state inhibitor, carboxyarabinitol-1-phosphate (CA1P), in thedark. RuBP Carboxylase Activase facilitates release of CA1P from RuBP Carboxylase, when it is activated under conditions of light by thioredoxin.  The activase is a member of the AAA family of ATPases, many of which have chaperone-like roles. 

Rubisco activity is dependent on light activation and dark inactivation: 

Regulation prevents the Calvin Cycle from being active in the dark, when itmight function in a futile cycle with Glycolysis & Pentose Phosphate Pathway, wasting ATP & NADPH. Light activates, or dark inhibits, the Calvin Cycle (previously called the “dark reaction”) in several ways.  Since photosynthetic light reactions produce ATP, the ATP dependence of RuBisCO activation provides a mechanism for light-dependent activation of the enzyme. 

Activation of RuBisco activase and coversion of inactive form of RuBisco  require electron transport and it can only take place in presence of light. Thioredoxin is a small protein with a disulfide that is reduced in chloroplasts via light-activated electron transfer. During illumination, the thioredoxin disulfide is reduced  to a dithiol by ferredoxin, a constituent of the photosynthetic light reaction pathway, via an enzyme Ferredoxin-Thioredoxin Reductase. Reduced thioredoxin activates several Calvin Cycle enzymes, including Fructose-1,6-bisphosphatase,Sedoheptulose-1,7-bisphosphatase, and RuBP Carboxylase Activase, by reducing disulfides in those enzymes to thiols.  Thus most of the important enzymes of Carbon dioxide fixation require light activation and hence Calvin cycle is Light dependent carbon fixation? 

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