Products of the light-dependent reaction
The products carried over to the light-independent stage of photosynthesis from the light-dependent stage are ATP and NADPH. These products are both used in the reduction of glycerate-3-phosphate (GP) into triose phosphate (TP). The ATP is broken down into ADP + Pi, which return to the light- dependent reaction. The NADPH is broken down into NADP and also returns to the light-dependent reaction.
The Calvin cycle
1. Carbon dioxide from the atmosphere diffuses into the leaf through the stomata and dissolves in water around the walls of mesophyll cells. It then diffuses into the stroma of the chloroplasts.
2. The carbon dioxide reacts with the 5-C compound RuBP, and this reaction is catalysed by an enzyme called rubisco.
3. The 6-C molecule produced is unstable, so the reaction instead produces two molecules of the 3-C GP.
4. NADPH is used to reduce the GP to TP, using energy supplied by ATP.
5. NADP and ADP + Pi are reformed in this reaction, and return to the light-dependent reaction.
6. Some molecules of TP are converted to organic substances such as starch, cellulose, lipids, glucose, amino acids, and nucleotides.
7. Most TP molecules are used to regenerate RuBP using ATP from the light-dependent reaction.
Site of the light-independent reaction
The chloroplast is adapted to carrying out the light-dependent recation in the following ways:
1. The fluid in the stroma has all of the neccessary enzymes for the light-independent reaction to be carried out. This fluid is membrane-bound; this allows it to have a much higher concentration of enzymes and substrates than the cytoplasm.
2. The fluid in the stroma surrounds the grana so the productsfrom the light dependent reaction, such as ATP and NADPH can diffuse into it quickly.
3. The fluid contains DNA and ribosomes to enable it to quickly produce proteins involved in the light-independent reaction.