How will the level of carbon in the atmosphere change if humans do nothing to reduce their impact on the carbon cycle?

It will keep rising above 400 parts per million. Carbon is released into the atmosphere through the burning of fossil fuels. It will likely not go down for decades.

the cell theory states that all living things including animals, as well as plants, are made of cells.

gametes or germ cells are haploid cells (example: sperm and ova) containing only one set (or n) number of chromosomes and autosomal or somatic cells are diploid cells containing 2n number of chromosomes.