Source: http://www.ests.um5.ac.ma/cours/Dipl%C3%B4me%20Universitaire%20de%20Technologie/G%C3%A9nie%20Bio-Industriel/Semestre%2002/Biologie%20g%C3%A9n%C3%A9rale/S+-%C2%ABance%202-3-Biologie%20G%20II%20.pdf