During the process of food-making of plants(photosynthesis),there are two reactions that occur---the light-dependent reaction and the light-independent reaction. These two reactions happen in the different parts of the chloroplast. Light-independent occurs at the chloroplast's stroma while the light-dependent reaction takes place in the thylakoid of the chloroplast. In the light-dependent reaction, ATP (Adenosine Triphosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate) are formed. Therefore the answer is, " During photosynthesis, ATP production or synthesis happens because of the light-dependent reaction happening in the thylakoid".