Physiologically, the Chapter has lost two of the organs particular to Space Marines: the Betcher's Gland, which allows the Marine to produce acidic spittle, and the Sus-an Membrane, which allows the Marine to enter a state of suspended animation. http://wh40k.lexicanum.com/wiki/Imperial_Fists#Geneseed
http://wh40k.lexicanum.com/wiki/Imperial_Fists#Geneseed