What is the main reason that energy is harvested in stages in cellular respiration?

In pea plants, yellow seed color (y) is dominant and green seed color (y) is recessive. based on the punnett squares, what are the chances that the offspring in the second generation will have green seeds? first generation y y y yy yy y yy yy second generation y y y yy yy y yy yy there is a % chance that the offspring will have green seeds.