Cilia and flagella
Microtubules form the main parts
of cilia and flagella. Cilia (SIL-e-a; singular is
short, hair-like projections of the plasma membrane that
sweep material across the surface of the cell (See Figure
3.1). Cells may have hundreds of cilia with coordinated
movements. For example, cells that line the respiratory
passages have cilia that help sweep mucus containing for-
eign particles out of the lungs to keep the airways clear.
Flagella (fla-JEL-a; singular is
whip-like structures than cilia (see Figure 3.1). In contrast
to cilia, which move material along the cell surface, a fla-
gellum moves the entire cell. In humans, the sperm cell
is the only type of cell with a flagellum, which allows it to
swim through the uterine tube to fertilize an egg cell dur-
ing the process of sexual reproduction.
Ribosomes (Rl-bo-somz) are made of RNA
and proteins. They consist of a large subunit and a small
subunit, both of which are made in the nucleolus inside
the cell’s nucleus (
). The subunits leave the
nucleus and are assembled in the cytosol. Some ribosomes
are associated with the rough endoplasmic reticulum,
some are located within mitochondria, and some are free-
Ribosomes • Figure 3.5
Ribosome subunits combine to make proteins in the cytosol,
rough endoplasmic reticulum, and mitochondria.
Functions of the Ribosomes
1. Ribosomes associated with endoplasmic
reticulum synthesize proteins destined
for insertion in the plasma membrane or
secretion from the cell.
2. Free ribosomes synthesize proteins used
in the cytosol.
Endoplasmic reticulum (ER) • Figure 3.6
ER is an extensive membrane system that participates in protein
synthesis (RER), detoxifying substances (SER), making fatty acids and
steroids (SER), and storing calcium for intracellular signaling (SER).
Functions of the Endoplasmic Reticulum
1. Rough ER synthesizes glycoproteins and
phospholipids that are transferred into the plasma
membrane, or secreted during exocytosis.
2. Smooth ER synthesizes fatty acids and steroids,
such as estrogens and testosterone; inactivates or
detoxifies drugs and other potentially harmful
substances; removes the phosphate group from
glucose-6-phosphate; and stores and releases
calcium ions that trigger contraction in muscle cells.
floating. Whether free-floating or associated with organ-
elles, ribosomes are involved in making new proteins.
The endoplasmic reticu-
lum (en'-do-PLAS-mik re-TIK-u-lum) (ER) is a large mem-
brane system that extends outward from the outer nuclear
membrane throughout the cytoplasm (
ER makes up half of the membranes within the cyto-
plasm. ER comes in two forms:
Rough ER (RER) —RER is a portion of the ER that
extends immediately from the nuclear envelope and is
studded with ribosomes. Proteins made within RER and
inserted into the RER membrane are destined for the
plasma membrane or the membranes of other organelles.
Proteins inserted through the RER membrane into its
lumen (space inside) will be secreted from the cell.
54 CHAPTER 3
Cells and Tissues