Biology (Holt)

Nuclearpores

Nuclear envelope

Nucleolus

Section 3 Cell Organelles

62 CHAPTER 3Cell Structure

The Nucleus

Most functions of a eukaryotic cell are controlled by the cell’snucleus. As shown in Figure 13,the nucleus is surrounded by a

double membrane called the nuclear envelope, also called thenuclear membrane. The nuclear envelope is made of two lipid

bilayers that separate the nucleus from the cytoplasm.

small channels through the envelope called nuclear pores.Scattered over the surface of the nuclear envelope are many

Substances that are made in the nucleus, including ribosomalproteins and RNA, move into the cytoplasm by passing through

the nuclear pores. Ribosomes are partially assembled in a regionof the nucleus called the nucleolus, which is also shown in Figure

13. 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 aselongated and thin strands, which appear as a dark mass under
    magnification. When a cell is about to divide, however, the DNAstrands 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 andchromosomes later in this book.

Objectives

Describethe nucleus in cell the role of

activities.

Analyzemembranes in proteinthe role of internal

production.

Summarizeof mitochondria in eukaryoticthe importance

cells.

Identifyplant cells that are absentthree structures in

from animal cells.

Key Terms

endoplasmic reticulumvesicle

Golgi apparatuslysosome

mitochondrionchloroplast

central vacuole

The nucleus is surrounded by a double membrane called the nuclear envelope.

Figure 13Nucleus

4A 4B

4A 4B

4A

4A 4B

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