ATP and NADPH formed during light reaction of photosynthesis are used in dark reaction for fixing $\mathrm{CO}_2$ and to form glucose molecule.

The basis for designating $\mathrm{C}_3$ and $\mathrm{C}_4$ pathways of photosynthesis is as follows

$\mathrm{C}_3$ Pathway	$\mathrm{C}_4$ Pathway
In Calvin cycle or $C_3$ cycle (dark reaction), carbon dioxide is fixed into first stable compound called 3-PGA, (3-phorphoglyceric acid) which is a 3 carbon compund.	In $C_4$ plants, the Calvin cycle $C_3$ cycle occurs in bundle sheath cells. Prior to reaching there, the $\mathrm{CO}_2$ is trapped by mesophyll cells and fixed into a 4-carbon stable product called oxalo acetic acid.

Succulent plants grow in dry and xeric conditions so, they have to shut down the stomata during day time, to prevent water loss through transpiration. So and the gaseous exchange does not take place. Thus plants have developed the mechanism to fix $\mathrm{CO}_2$ during night in the form of malic acid, which is a 4 carbon compound and store $\mathrm{CO}_2$, release it during day, inside the photosynthetic cells.

Accessory pigments are also photosynthetic pigments. These are chlorophyll-b, xanthophyll and carotenoids. These are not directly involved in emission of excited electrons, but they help in harvesting solar radiation and pass it on to chlorophyll-a. This pigment itself absorbs maximum radiation at blue and red region. So, Chlorophylla is the chief pigment of photosynthesis and others (i.e., chlorophyll-b, xanthophyll and carotinoion) are accessory pegment.

$\mathrm{CO}_2$ is reduced through various biochemical reactions to produce $\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$ (glucose) which does not need light. But they depend on the products formed during light reactions, i.e., $\mathrm{NADPH}_2$ and ATP.