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P A R T 2
Chemotherapeutic agents
neurotransmitters, enzymes and other substances
produced within a cell are excreted into the body by this
process (see Figure 7.3).
Homeostasis
The main goal of a cell is to maintain homeostasis,
which means keeping the cytoplasm stable within the
cell membrane. Each cell uses a series of active and
passive transport systems to achieve homeostasis; the
exact system used depends on the type of cell and its
reactions with the immediate environment. For a cell to
produce the energy needed to carry out cellular metab-
olism and other processes, the cell must have a means
to obtain necessary elements from the outside environ
ment. In addition, it must have a way to dispose of
waste products that could be toxic to its cytoplasm. To
accomplish this, the cell moves substances across the
cell membrane, either by passive transport or by active
(energy-requiring) transport (see Figure 7.4).
Passive transport
Passive transport happens without the expenditure
of energy and can occur across any semipermeable
membrane. There are essentially three types of passive
transport: diffusion, osmosis and facilitated diffusion.
Diffusion
Diffusion
is the movement of a substance from a region
of higher concentration to a region of lower concentra-
tion. The difference between the concentrations of the
substance in the two regions is called the
concentra-
tion gradient
of the substance; usually, the greater the
concentration gradient, the faster the substance moves.
Movement into and out of a cell is regulated by the cell
membrane. Some substances move through channels
or pores in the cell membrane. Small substances and
Solute
particle
Higher solute
concentration
Concentration gradient
Lower solute
concentration
Transporter
Cytoplasm
Extracellular fluid
A
B
C
P
l
a
s
m
a
m
e
m
b
r
a
n
e
FIGURE 7.4
Schematic representation of transport across a
cell membrane
(A)
, which includes
diffusion
through the
cell membrane
(B)
and
pore diffusion
through a protein
channel
(C)
.
Extracellular fluid
Cytoplasm
Vesicle
Stored material
A
Release of
stored material
Fusion with
plasma membrane
P
l
a
s
m
a
m
e
m
b
r
a
n
e
FIGURE 7.3
Schematic representation of endocytosis and exocytosis.
A.
Exocytosis is the movement of substances (waste products,
hormones, neurotransmitters) out of the cell.
B.
Endocytosis involves
the destruction of engulfed proteins or bacteria.
Particle
Extracellular
fluid
Cytoplasm
Phagocytic
vesicle
P
l
a
s
m
a
B
into the area.
Exocytosis
is the opposite of endocyto-
sis and involves removing substances from a cell by
pushing them through the cell membrane. Hormones,
