Nuclear
poresNuclear envelopeNucleolusSection 3 Cell Organelles
62 CHAPTER 3Cell Structure
The Nucleus
Most functions of a eukaryotic cell are controlled by the cell’s
nucleus. As shown in Figure 13,the nucleus is surrounded by a
double membrane called the nuclear envelope, also called the
nuclear membrane. The nuclear envelope is made of two lipid
bilayers that separate the nucleus from the cytoplasm.
Scattered over the surface of the nuclear envelope are many
small channels through the envelope called nuclear pores.
Substances that are made in the nucleus, including ribosomal
proteins and RNA, move into the cytoplasm by passing through
the nuclear pores. Ribosomes are partially assembled in a region
of the nucleus called the nucleolus, which is also shown in Figure- Recall from Section 2 that ribosomes are the structures on
which proteins are made.
The hereditary information of a eukaryotic cell is coded in the
cell’s DNA, which is stored in the nucleus. Eukaryotic DNA is
wound tightly around proteins. Most of the time, DNA exists as
elongated and thin strands, which appear as a dark mass under
magnification. When a cell is about to divide, however, the DNA
strands wind up into a more compact form and appear as dense,
rod-shaped structures called chromosomes. The number of chro-
mosomes in a eukaryotic cell differs between species. Human
body cells have 46 chromosomes, while the cells of garden
peas have 14 chromosomes. You will learn more about DNA and
chromosomes later in this book.
Objectives
Describethe role of
the nucleus in cell
activities.
Analyzethe role of internal
membranes in protein
production.
Summarizethe importance
of mitochondria in eukaryotic
cells.
Identifythree structures in
plant cells that are absent
from animal cells.
Key Terms
endoplasmic reticulum
vesicle
Golgi apparatus
lysosome
mitochondrion
chloroplast
central vacuoleThe nucleus is surrounded by a double membrane called the nuclear envelope.Figure 13 Nucleus4A 4B4A 4B4A4A 4B