I believe they cannot start with big trees and bushes because the there is not enough nutrients in the soil, so instead pioneer species pave the way for primary succession.
Melanin provides pigmentation to the skin and it also provides UV ray protection (which is its role in the epidermis).
Ribosomes although the mitochondria is similar its ribosomes.
Some activities that cell requires energy may include cell division, active transport, protein synthesis etc.
In human, Cells get their energy by the food we eat. The nutrients in the food breaks down into soluble and simple molecules in stomach and small intestine and it is absorbed through the small intestine and assimilated into the cells, becoming part of it, providing the uses for each type of nutrient. And of course, many of them is the energy source.
