why do plants already have ATP and NADPH for chemical energy from lights reactions of photosynthesis but they still go on to manufacture energy-rich sugars?

Plants perform two main reactions during photosynthesis to convert light energy into chemical energy: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).

During the light-dependent reactions, plants use light energy to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) as energy carriers. These molecules act as "energy currencies" within the cell, providing the necessary energy and reducing power for the subsequent chemical reactions.

However, ATP and NADPH themselves are not stable in the long term and cannot be stored for extended periods. Consequently, plants utilize these energy carriers to fuel the light-independent reactions, or the Calvin cycle. In the Calvin cycle, plants utilize ATP and NADPH along with carbon dioxide (CO2) to synthesize energy-rich sugars such as glucose.

The light-independent reactions require a constant supply of ATP and NADPH to sustain the production of sugars. As a result, plants continuously undergo photosynthesis to replenish these energy carriers, ensuring a steady source of chemical energy for growth, maintenance, and reproduction.

In summary, while ATP and NADPH are crucial products of the light-dependent reactions, plants still need to synthesize energy-rich sugars through the light-independent reactions to store energy for later use.