Organization of the Body
Parts of the Cell: Structure and Function
Plasma Membrane (Figure 3.3)
Membrane made of a double layer of lipids
(phospholipids, cholesterol, and so on) within
which proteins are embedded. Proteins may
extend entirely through the lipid bilayer or
protrude on only one face. Most externally
facing proteins and some lipids have attached
Serves as an external cell barrier, and acts in
transport of substances into or out of the cell.
Maintains a resting potential that is essential
for functioning of excitable cells. Externally
facing proteins act as receptors (for hormones,
neurotransmitters, and so on), transport
proteins, and in cell-to-cell recognition.
Cellular region between the nuclear and plasma membranes. Consists of ﬂuid
(the metabolic machinery of the cytoplasm), and
nutrients, secretory products, pigment granules).
Mitochondria (Figure 3.17)
Rodlike, double-membrane structures; inner
membrane folded into projections called
Site of ATP synthesis; powerhouse of the cell.
Ribosomes (Figures 3.18, 3.37–3.39)
Dense particles consisting of two subunits, each
composed of ribosomal RNA and protein. Free
or attached to rough endoplasmic reticulum.
The sites of protein synthesis.
Rough endoplasmic reticulum
(Figures 3.18, 3.39)
Membranous system enclosing a cavity, the
cistern, and coiling through the cytoplasm.
Externally studded with ribosomes.
Sugar groups are attached to proteins within
the cisterns. Proteins are bound in vesicles for
transport to the Golgi apparatus and other
sites. External face synthesizes phospholipids.
Smooth endoplasmic reticulum
Membranous system of sacs and tubules; free
Site of lipid and steroid (cholesterol) synthesis,
lipid metabolism, and drug detoxiﬁcation.
Golgi apparatus (Figures 3.19, 3.20)
A stack of ﬂattened membranes and associated
vesicles close to the nucleus.
Packages, modiﬁes, and segregates proteins
for secretion from the cell, inclusion in
lysosomes, and incorporation into the plasma
Peroxisomes (Figure 3.2)
Membranous sacs of catalase and oxidase
The enzymes detoxify a number of toxic
substances. The most important enzyme,
catalase, breaks down hydrogen peroxide.
* Individual cellular structures are not drawn to scale.