Light-independent reaction

The Calvin Cycle

Light-independent reactions take place within the chloroplasts. During this reaction, sugars are made using carbon dioxide and the products of the light-dependent reactions (ATP and NADPH) and various other chemicals found in the plant during the Calvin cycle. Therefore, the light-independent reaction cannot happen without the light-dependent reaction. Sugars made in the light-independent reactions are moved around the plant by translocation.

The Calvin Cycle

1. A sugar (Ribulose Biphosphate or RuBP) made of 5 carbon atoms combines with carbon dioxide to form a 6-carbon sugar (phosphoglycerate). An enzyme called RuBisCO speeds this reaction up
2. Phosphoglycerate is reduced with hydrogen atoms from the light-dependent to form two molecules of triose phosphate (each have 3 carbon atoms). ATP is needed for this to happen
3. Some triose phosphate is converted (using ATP) back into Ribulose Biphosphate (which is why this is called a cycle)
4. The rest left over is used to produce glucose.

Complete process RuBP combine with CO2 with the help of enzyme known as Rubisco ( this known as carbon fixation or phase 1).As RuBP undergoes phase 1,this change from 5 carbon to 6 carbon(unstable intermediate). So this 6 carbon catalyst(split) into (2) of 3 carbon which is known as 3phosphoglycerate(PGA). At this process ATP reduce to ADP + P.Thus NADPH reduced to NADP+. This reduction of ATP and NADPH are in phase 2. Now the 3phosphoglycerate are known as glycerate-3-phosphate. So,as complete of 3 cycle of RuBP and form 6 of glycerate-3-phosphate (G3P), 1 of the G3P release to form glucose. Another 5 of G3P are now undergoes carbon fixation and this is in phase 3. The carbon fixation rearrange the 5 of G3P and form 3 RuBP. At this phase ATP from outside are reduce to ADP. This is because the phosphate group from the reduction process are needed for carbon fixation. The process are repeats.

overall (for three RuBP): 3x2PGA > 3x2ATP and 2x3NADH are reduce > 3x2(G3P) > 2x3ATP > 3x3 RuBP

Other websites

• The Biochemistry of the Calvin Cycle at Rensselaer Polytechnic Institute
• The Calvin Cycle and the Pentose Phosphate Pathway from Biochemistry, Fifth Edition by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer. Published by W. H. Freeman and Company (2002).