Why is it necessary to have six co2 entering the chloroplast?

In order to produce the 6-carbon glucose molecule and regenerate RUBP.

Explanation:

Plants undergo two stages in photosynthesis, one which is the light dependent reaction is used to produce ATP and NADPH needed for the next stage, which is the light independent stage or C3 or Calvin cycle.

In the Calvin cycle, Carbon is fixed when CO2, which enters via the stomata is accepted by Ribulosephosphate (RUBP) to produce an unstable 6-car-on compound which disintegrates into 3-phosphoglycerate (PGA). The PGA undergoes reduction and phosphorylation by ATP and NADPH to produce Glyceraldehyde-3-phosphate (G3P). The G-3-P either continues the cycle in order to regenerate RUBP or leaves the cycle to produce glucose.

CO2 can only be fixed one at a time. Since G-3-P is a 3-carbon compound, it takes 3 molecules of CO2 to enter the cycle in order to produce 1 G3P molecule that will leave the cycle i.e 3 molecules of CO2 enters the cycle to produce 6 molecules of G3P, one molecule leaves the cell and 5 molecules are used to regenerate RUBP.

Hence, since, glucose is a 6-carbon compound, 2 molecules of 3-carbon G3P will be required. To get 2 molecules of G3P that will leave the cycle to produce glucose, 6 molecules of CO2 will need to be fixed in the stroma of the CHLOROPLAST.