94
UNIT 1
Organization of the Body
3
Table 3.3
Parts of the Cell: Structure and Function
CELL PART*
STRUCTURE
FUNCTIONS
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
sugar groups.
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.
Cytoplasm
Cellular region between the nuclear and plasma membranes. Consists of fluid
cytosol
containing
dissolved solutes,
organelles
(the metabolic machinery of the cytoplasm), and
inclusions
(stored
nutrients, secretory products, pigment granules).
Organelles
 
 
Mitochondria (Figure 3.17)
Rodlike, double-membrane structures; inner
membrane folded into projections called
cristae.
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
(Figure 3.18)
Membranous system of sacs and tubules; free
of ribosomes.
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
membrane.
Peroxisomes (Figure 3.2)
Membranous sacs of catalase and oxidase
enzymes.
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.
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