Plants need sunlight to complete photosynthesis. During photosynthesis plants use light energy from the sun, water and carbon dioxide to create energy; plants store their energy as starch.
Sunlight is necessary for the light reactions of photosynthesis. Photosynthesis occurs in the chloroplasts of plant cells and the light reactions, specifically, happen in the thylakoid membranes of the chloroplast. Thylakoids contain two photosystems for gathering light from the sun. These photosystems utilize pigment molecules to absorb light energy, which passes between molecules until it reaches a reaction center. In the reaction center, energy from sunlight excites an electron from the pigment chlorophyll and electron transport allows for the creation of two high energy molecules: adenosine triphosphate and nicotinamide adenine dinucleotide phosphate-oxidase. Also during the light reactions, water loses hydrogen and electrons to produce oxygen, which the plant expels through its leaves.
Adenosine triphosphate and nicotinamide adenine dinucleotide phosphate-oxidase from the light reactions are essential for the other half of photosynthesis, the dark reactions. These reactions require carbon dioxide but do not use any light energy. During the dark reactions, adenosine triphosphate and nicotinamide adenine dinucleotide phosphate-oxidase catalyze the synthesis of various sugars, including glucose, from an existing five carbon sugar, Ribulose bisphosphate. As the dark reactions conclude, Ribulose bisphosphate reforms for reuse during the next round of reactions.