Chloroplasts are the sites of photosynthesis in eukaryotic cells. They capture sunlight and store the light energy in the form of adenosine triphosphate and nicotinamide adenine dinucleotide phosphate molecules. Chloroplasts play a role in carbon fixation, fatty acid synthesis, amino acid synthesis and the immune response of plants.
The energy of sunlight striking chlorophyll causes chloroplasts to synthesize sugars, starches and free oxygen. Plants and animals take advantage of these sugars for food and energy, while animals use oxygen for survival. The stroma of chloroplasts produces carbon fixation reactions known as "dark reactions," while the thylakoids contain chlorophyll in their membranes. The stroma contains various enzymes that carry out reactions using ATP, NADPH and carbon dioxide that lead to the formation of sugars, which are stored as starch and used either in respiration or in cellulose production. Thylakoids are arranged into a stack called a granum, and several grana are connected by stromal lamellae. The lamellae maintain the integrity of the chloroplast by keeping a safe distance between the grana, and they increase the efficiency of the organelle. The main pigment that converts light energy into chemical energy is chlorophyll a. The other pigments are chlorophyll b, carotene and xanthophyll.