20 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”like structures called chromosomes. (In bacteria, there is no distinct
nucleus, so the DNA is loosely packed in a structure called a “nucle-
oid.” Further, bacterial DNA is distinct from higher forms of DNA in
that it has a circular form.) Present in the cytoplasm of all cells are
tiny protein-making factories called ribosomes, which are composites
Fig. 3.1. Drawing of a typical animal cell showing its major components. The entire
cell is enclosed by a membrane called the plasma membrane. The most prominent struc-
ture inside the cell is a nucleus containing the genetic material (DNA) in the form of
chromatin. In the nucleus, the nucleolus is the part of chromatin that is dedicated to the
synthesis of ribosomes. The nucleus is surrounded by a membrane with openings permit-
ting the entry and exit of materials. Inside the nucleus, messenger RNAs are synthesized
according to the information in the DNA. In the cytoplasm, proteins are synthesized on
ribosomes according to the information in the messenger RNA. Endoplasmic reticulum
(ER) is the membrane in the cytoplasm that is continuous with the nuclear membrane.
Ribosomes engaged in protein synthesis are adhered to the ER, forming “rough ER”.
Nascent proteins destined for export are channeled through a stack of smooth ER called
Golgi apparatus. Also prominent in the cytoplasm are the football-like structures called
mitochondria, which are responsible for harnessing energy (in the form of ATP) from
food. Lysosomes contain digestive enzymes for breaking down cellular wastes. Other
structures inside the cell, including peroxisomes, actin fibers, microtubules, and cen-
trosome, are not shown.