Light Dependent Reaction

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• There are two photosystems at the thylakoid membranes of the chloroplast.
• Photosynthetic pigments (eg. chlorophyll and carotenoids) are organized into the two photosystems, PSII and PSI.
• Photosynthetic pigments absorb light energy which cause the excitation of their electrons (photoactivation).
• Excited electrons are ejected from PSI and PSII due to their high energy.
• The excited electrons produce adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using ATP synthase.
• Nicotinamide adenine dinucleotide phosphate (NADPH) is also produced from nicotinamide adenine dinucleotide phosphate ion (NADP+).
• The electron transport chain and the ATP synthase complex at the thylakoid membrane are involved in the production of ATP and NADPH.
• As light is involved in the production of ATP, it is known as photophosphorylation.
• At PSII, the electrons used to produced ATP and NADPH are replenished by electrons from water molecules. 
• Splitting of water (photolysis) results in the release of oxygen gas.

In simpler terms, when light falls on the thylakoid membrane,

• Light causes photoactivation, which is the excitement of electrons by absorption of light energy, at the photosynthetic pigments at PSII and PSI.
• Excited electrons are ejected from the reaction centers of PSII and PSI.
• Ejected electrons are used to produce ATP and NADPH.
• Electrons lost are replenished from photolysis of water.
• Oxygen gas is released in the process.