How would you explain the significance of NADPH in photosynthesis and NADH in cellular respiration?
NAD+/NADH play a role as redox agent (mainly by abstracting hydrogen ions (H+ or H-) or transferring hydrogen ions). They act on oxidation processes and reduction processes in metabolism of sugar.
The chemicals in glycolysis and TCA cycle undergo a series of oxidation reaction going from glucose to carbon dioxide. This means that electrons is ultimately transferred to oxygen (which subsequently reduces it to H2O)
Electron is ultimately transported to O2 in electron transport chain (a reduction process
The process of conversion from C6H12O6 to CO2 do not occur in a single step but through a series of redox processes.
Remember that these enzymes are located far away from each other. So the removal of proton must be done with the help of a molecule that can carry the proton between different enzymes. Thiw is done using NADH (reduced state) or NAD+ (oxidized state)
Fig. Source ref 
Fig. Source: Ref 
You can see how H- is removed from OH group
In TCA cycle , it occurs in 4 different processe as highlighted in red.
ELECTRON TRANSPORT CHAIN
 Glycolysis: Energy Generation Without an Oxygen Requirement Glucose Biofuel Prominence: Low-reactive ring-form minimizes protein glycosylation. Published by May Butler http://slideplayer.com/slide/8259865/