Cells: The Living Units
Lysosomes (Figure 3.21)
Membranous sacs containing acid hydrolases.
Sites of intracellular digestion.
Microtubules (Figures 3.23–3.25)
Cylindrical structures made of tubulin proteins.
Support the cell and give it shape. Involved
in intracellular and cellular movements. Form
centrioles and cilia and ﬂagella, if present.
Microﬁlaments (Figures 3.23, 3.24)
Fine ﬁlaments composed of the protein actin.
Involved in muscle contraction and other types
of intracellular movement, help form the cell’s
Intermediate ﬁlaments (Figure 3.23)
Protein ﬁbers; composition varies.
The stable cytoskeletal elements; resist
mechanical forces acting on the cell.
Centrioles (Figure 3.25)
Paired cylindrical bodies, each composed of
nine triplets of microtubules.
Organize a microtubule network during
mitosis (cell division) to form the spindle and
asters. Form the bases of cilia and ﬂagella.
Varied; includes stored nutrients such as lipid
droplets and glycogen granules, protein
crystals, pigment granules.
Storage for nutrients, wastes, and cell
Cilia (Figures 3.26, 3.27)
Short cell-surface projections; each cilium
composed of nine pairs of microtubules
surrounding a central pair.
Coordinated movement creates a
unidirectional current that propels substances
across cell surfaces.
Like a cilium, but longer; only example in
humans is the sperm tail.
Propels the cell.
Microvilli (Figure 3.28)
Tubular extensions of the plasma membrane;
contain a bundle of actin ﬁlaments.
Increase surface area for absorption.
Nucleus (Figures 3.2, 3.29)
Largest organelle. Surrounded by the nuclear
envelope; contains ﬂuid nucleoplasm, nucleoli,
Control center of the cell; responsible
for transmitting genetic information and
providing the instructions for protein