Most books and other items in the library are organized by the Library of Congress Call Number system. In that system, the basic sciences (biology, chemistry, physics, etc.) are under Q.
Biology is split among the QH's (natural history and general biology), QK (botany), QL (zoology), QM (human anatomy), QP (physiology) and QR (microbiology). You'll find biology materials in the STACKS (3rd floor), Oversized Books (3rd floor), Media (1st floor), and Reference (1st floor).