C H A P T E R 7
Introduction to cell physiology
75
Cytoplasm
The cell
cytoplasm
lies within the cell membrane and
outside the nucleus and is the site of activities of cellular
metabolism and special cellular functions. The cyto-
plasm contains many
organelles
, which are structures
with specific functions such as producing proteins and
energy. The organelles within the cytoplasm include
the mitochondria, the endoplasmic reticulum, free ribo-
somes, the Golgi apparatus and the lysosomes.
Mitochondria
Mitochondria
are rod-shaped “power plants” within
each cell that produce energy in the form of ATP, which
allows the cell to function. Mitochondria are plentiful
in very active cells such as muscle cells and are relatively
scarce in inactive cells such as bone cells. Mitochon-
dria, which can reproduce when a cell is very active, are
always very abundant in cells that consume energy. For
example, cardiac muscle cells, which must work con-
tinually to keep the heart contracting, contain a great
number of mitochondria. Milk-producing cells in breast
tissue, which are normally quite dormant, contain
very few mitochondria. If a woman is breastfeeding,
however, the mitochondria become more abundant to
meet the demands of the milk-producing cells. The mito-
chondria can take carbohydrates, fats and proteins from
the cytoplasm and make ATP via the Krebs cycle, which
depends on oxygen. Cells use the ATP to maintain
homeostasis, produce proteins and carry out specific
functions. If oxygen is not available, lactic acid builds
up as a by-product of cellular respiration. Lactic acid
leaves the cell and is transported to the liver for conver-
sion to glycogen and carbon dioxide.
Endoplasmic reticulum
Much of the cytoplasm of a cell is made up of a fine
network of interconnected channels known as cisternae,
which form the
endoplasmic reticulum
. The undulating
surface of the endoplasmic reticulum provides a large
surface for chemical reactions within the cell. Many
granules that contain enzymes and ribosomes, which
produce protein, are scattered over the surface of the
rough endoplasmic reticulum. Production of proteins,
phospholipids and cholesterol takes place in the rough
endoplasmic reticulum. The smooth endoplasmic reticu-
lum is the site of further lipid and cholesterol production
and the production of cell products, such as hormones.
The breakdown of many toxic substances may also
occur here in particular cells.
Free ribosomes
Other ribosomes that are not bound to the surface of the
endoplasmic reticulum exist throughout the cytoplasm.
These free-floating ribosomes produce proteins that are
important to the structure of the cell and some of the
enzymes that are necessary for cellular activity.
Golgi apparatus
The
Golgi apparatus
is a series of flattened sacs that may
be part of the endoplasmic reticulum. These structures
prepare hormones or other substances for secretion by
processing them and packaging them in vesicles to be
moved to the cell membrane for excretion from the cell.
In addition, the Golgi apparatus may produce lysosomes
and store other synthesised proteins and enzymes until
they are needed.
Lysosomes
Lysosomes
are membrane-covered organelles that
contain specific digestive enzymes that can break down
proteins, nucleic acids, carbohydrates and lipids and are
responsible for digesting worn or damaged sections of
a cell when the membrane ruptures and the cell dies.
Lysosomes form a membrane around any substance that
needs to be digested and secrete the digestive enzymes
directly into the isolated area, protecting the rest of
the cytoplasm from injury. This phenomenon can be
seen with old lettuce in the refrigerator. The side of the
lettuce head that has been “lying down” for a prolonged
period becomes brown and wet as the lettuce cells die
and self-digest when their lysosomes are released. If
the lettuce is not used, the released lysosomes begin to
digest any healthy lettuce that remains, with eventual
destruction of the entire head. Lysosomes are important
in ecology. Dead trees, animals and other organisms
self-digest.
KEY POINTS
■■
The cytoplasm of the cell contains various organelles
that are important for cellular function.
■■
The mitochondria produce energy for the cell; the
endoplasmic reticulum contains ribosomes that
produce proteins; the Golgi apparatus packages
proteins; and lysosomes contain protein-dissolving
enzymes that are important for digestion and the
recycling of organisms in nature.
CELL PROPERTIES
Cells have certain properties that allow them to survive.
Endocytosis
involves incorporation of material into the
cell by extending the cell membrane around the sub-
stance.
Pinocytosis
, a form of endocytosis, refers to
the engulfing of specific substances that have reacted
with a receptor site on the cell membrane. This process
allows cells to absorb nutrients, enzymes and other
materials.
Phagocytosis
is a similar process; it allows
the cell (white blood cell), usually a neutrophil or mac-
rophage, to engulf a bacterium or a foreign protein and
destroy it within the cell by secreting digestive enzymes
KEY POINTS
