Biology (Holt)

58 CHAPTER 3Cell Structure

Mitochondrion

Microfilaments

Lysosome

Ribosomes

Golgi

apparatus

Smooth ER

Rough ER

Cell membrane

Microtubules

Nuclear pore

Nuclear envelope

Nucleus

Nucleolus

Figure 7 Animal cell. Like
all eukaryotic cells, animal cells
contain a cell membrane, a
nucleus, and other organelles.

Eukaryotic Cells

The first cells with internal compartments were primitive eukaryotic

cells, which evolved about 1.5 billion years ago. A (yoo

KAIR ee oht) is an organism whose cells have a nucleus. The

(NOO klee uhs) is an internal compartment that houses the

cell’s DNA. Other internal compartments, or organelles, enable

eukaryotic cells to function in ways different from prokaryotes. An

is a structure that carries out specific activities in the cell.

The major organelles in an animal cell are shown in Figure 7. The

cytoplasm includes everything inside the cell membrane but outside

the nucleus. A complex system of internal membranes connects the

organelles within the cytoplasm. These membranes provide chan-

nels that guide the distribution of substances within the cell. The

membranes also form envelopes called vesiclesthat move proteins

and other molecules from one organelle to another.

Many single-celled eukaryotes use flagella for movement. Short

hairlike structures called (SIL ee uh) protrude from the surface

of some eukaryotic cells. Flagella or cilia propel some cells through

their environment. In other cells, cilia and flagella move substances

across the cell’s surface. For example, cilia on cells of the human res-

piratory system, shown in Figure 8,sweep mucus and other debris

out of the lungs.

A web of protein fibers, shown in Figure 9, makes up the

cytoskeleton. The cytoskeleton holds the cell together and keeps the

cell’s membranes from collapsing. The fluid surrounding the cyto-

plasm’s organelles, internal membranes, and cytoskeleton fibers is

called the cytosol.

cilia

organelle

nucleus

eukaryote

Figure 8 Cilia.Cilia on
cells lining the respiratory
system remove debris from
air passages.