What's the difference between the light dependent and independent reactions?

The light dependent and light independent reactions are two stages of photosynthesis that overall utilise light energy to fix carbon dioxide into sugar molecules. There are however major differences between the two stages.

Light requirement: As the name suggests, light dependent stage requires the absorption of light, and is the first stage of the reaction. The second stage is the light independent reaction, otherwise named as the Calvin cycle, which does not require sunlight.

Reaction site: Light dependent reaction takes place within the thylakoid membrane of chloroplasts because the necessary machineries required are embedded within the membrane, including PSI, PSII, ATP synthase and various electron transport chain proteins. The light independent reaction takes place within the stroma of chloroplasts, where all the necessary enzymes are found.

Reactants and products: The reactants required for the light dependent reactions (not counting light) are ADP+ Pi, which will be converted into ATP and NADP, which will be reduced into reduced NADP. Additionally, water is also required for photolysis, with oxygen being produced as a waste product. The light independent reaction uses the ATP and reduced NADP from the light dependent reaction as reactants, to produce G3P by fixing carbon dioxide. RuBP, a 5 carbon compound is also required as a reactant for the light independent reaction, but will eventually be regenerated as a product.

Steps involved: The light dependent reaction is mostly a linear process that harvests light energy using pigments from both PSI and PSII, which results in excited electrons being passed through an electron transport chain. In certain cases, chloroplasts can also switch to a cyclic process, where only PSI is used and only ATP (no reduced NADP) will be produced as products. The light independent reaction is a cyclic process consisting of three major steps: carbon fixation, reduction, and regeneration.