Pea seeds undergo cellular respiration during germination in order to grow and develop. Germination describes the beginnings of its growth from a seed.
The deep cellular respiration is a form of breathing, similar to what animals do. Breathing for all things, of any size, enables the being to take in and process nutrients through metabolic activity. In the case of pea seeds, the ability to photosynthesize or take in nutrients for energy through leaves does not exist, because leaves are not present. So, the actual cells of the seed "breathe." With the energy obtained from cellular respiration, the pea seeds can develop into a plant and eventually produce leaves.