Essentials of Anatomy and Physiology

are not. These mechanisms of cellular transport will
be covered later in this chapter. The cell membrane is
of particular importance for muscle cells and nerve
cells because it carries electrical impulses. This will be
covered in Chapters 7 and 8.

NUCLEUS

With the exception of mature red blood cells, all
human cells have a nucleus. The nucleusis within the
cytoplasm and is bounded by a double-layered nuclear
membranewith many pores. It contains one or more
nucleoli and the chromosomes of the cell (Fig. 3–2).
A nucleolusis a small sphere made of DNA, RNA,
and protein. The nucleoli form a type of RNA called
ribosomal RNA, which becomes part of ribosomes
(a cell organelle) and is involved in protein synthesis.

The nucleus is the control center of the cell

because it contains the chromosomes. Even with a

microscope, the 46 chromosomesof a human cell are

usually not visible; they are long threads called chro-

matin. Before a cell divides, however, the chromatin

coils extensively into visible chromosomes. Chromo-

somes are made of DNA and protein. Some chromo-

somal proteins provide the structural framework for

the coiling of the chromatin into chromosomes so

that cell division can take place. Other chromosomal

proteins help regulate the activity of the DNA.

Remember from our earlier discussion that the DNA

is the genetic code for the characteristics and activities

of the cell. Although the DNA in the nucleus of each

cell contains all of the genetic information for all

human traits, only a small number of genes (a geneis

the genetic code for one protein) are actually active, or

Cells 49

Receptor site

Outside of cell Oligosaccharide antigens

Cholesterol

Inside of cell

Protein forming a pore

Phospholipid

bilayer

Figure 3–1. The cell (plasma) membrane depicting the types of molecules present.

QUESTION:The receptor site shown is probably what type of organic molecule?