The Light-Independent Reaction

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ATP and reduced NADP from the LDR are used to reduce carbon dioxide in the second stage of photosynthesis. This stage may not require light directly, however it requires the products of the LDR, so would rapidly cease in the absence of light.

The LIR takes place in the stroma of the chloroplasts.

The LIR is also known as the Calvin Cycle.

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The Calvin Cycle

> CO2 from the atmosphere diffuses into the leaf through stomata > dissolves in water around the walls of the mesophyll cells > diffuses through the plasma membrane, cytoplasm and chloroplast membranes into > stroma.

> In the stroma the CO2 combines with the 5-Carbon compound (RuBP) using an enzyme.

> This combination produces two molecules of the 3-carbon glycerate 3-phosphate (GP).

> The ATP and reduced NADP from the LDR are used to reduce the activated GP into  triose phosphate (TP).

> The NADP is reformed and goes back to the LDR to be reduced again by accepting more hydrogen.

> Some TP molecules are converted to useful organic substances, such as glucose.

> Most TP molecules are used to regenerate RuBP using ATP from the LDR.

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Site of the Light-Independent Reaction

The LIR of photosynthesis takes place in the stroma of chloroplasts. Chloroplasts are adapted to carrying out the LIR of photosynthesis, in the following ways:

> The fluid of the stroma contains all the enzymes needed to carry out the LIR.

> The stroma fluid surrounds the grana so the products of the LDR in the grana can readily diffuse into the stroma.

> Contains both DNA and ribosomes so it can quickly & easily manufacture some of the proteins needed for the LIR.

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