8
U N I T 1
Cell and Tissue Function
Actin Microfilaments and Intermediate
Filaments
In addition to microtubules, two other cytoskeletal
structures exist: microfilaments and intermediate fila-
ments. Microfilaments are composed of actin, whereas
intermediate filaments are a heterogeneous group of
filaments with diameter sizes between those of microtu-
bules and actin filaments.
Actin, which can exist in globular and filamentous
forms, is of central importance to cellular biology. It
contributes to cell motility, positioning of organelles in
the cell, and cell shape and polarity. Many functions of
actin filaments are performed in association with myo-
sin motor proteins. Contractile activities involving actin
microfilaments and associated thick myosin filaments
contribute to muscle contraction.
The intermediate filaments include the cytokeratins,
vimentin, and neurofilaments. They have structural
and maintenance functions that are important in tissue,
cellular, developmental, and differentiation processes.
They are also very responsive to cellular stresses, such
as heat, radiation, toxins, pathogens and oxidation.
Neurofibrillary tangles
found in the brain in Alzheimer
disease contain microtubule-associated proteins and
neurofilaments, evidence of a disrupted neuronal
cytoskeleton.
Cell Metabolism and Energy
Storage
Energy metabolism refers to the chemical processes
involved in converting carbohydrates, fats, and pro-
teins from the foods we eat into the energy needed
for cell functions. Cells use oxygen to transform the
breakdown products of the foods we eat into the
energy needed for muscle contraction; the transport
of ions and other molecules across cell membranes;
and the synthesis of enzymes, hormones, and other
macromolecules.
The special “unit of currency” for transferring energy
in living cells is ATP. Adenosine triphosphate molecules
consist of adenosine, a nitrogenous base; ribose, a five-
carbon sugar; and three phosphate groups (Fig. 1-8). The
last two phosphate groups are attached to the remainder
of the molecule by two high-energy bonds. Each bond
releases a large amount of energy when hydrolyzed.
Adenosine triphosphate is hydrolyzed to form adenosine
in the cell.The endoplasmic reticulum (ER)
functions as a tubular communication system
through which substances can be transported
from one part of the cell to another and as the
site of protein (rough ER), carbohydrate, and
lipid (smooth ER) synthesis.The Golgi apparatus
modifies materials synthesized in the ER and
packages them into secretory granules for
transport within the cell or export from the cell.
Lysosomes, which can be viewed as the digestive
system of the cell, contain hydrolytic enzymes
that digest worn-out cell parts and foreign
materials.The mitochondria serve as power plants
for the cell because they transform food energy
into ATP, which is used to power cell activities.
They contain their own extrachromosomal DNA,
which is used in the synthesis of certain proteins
required for mitochondrial function.
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In addition to its organelles, the cytoplasm
also contains a network of microtubules, actin
microfilaments, and intermediate filaments
called the cytoskeleton. Microtubules are slender,
stiff tubular structures that influence cell shape,
provide a means of moving organelles through
the cytoplasm, and form cilia, flagella, and
centrioles. Actin microfilaments are dynamic,
thin, threadlike cytoplasmic structures that
are important in cell movement and organelle
positioning. Many types of intermediate filaments
function in supporting and maintaining the shape
of cells and participate in numerous cellular
processes.
SUMMARY CONCEPTS
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Cells are the smallest functional unit of the
body.They are autonomous units that contain
structures that are strikingly similar to those
needed to maintain total body function.
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The cell membrane is a protein-studded lipid
bilayer that surrounds the cell and separates
it from its surrounding external environment.
It contains receptors for hormones and other
biologically active substances, participates in the
electrical events that occur in nerve and muscle
cells, and aids in the regulation of cell growth
and proliferation.The cell surface is surrounded
by a fuzzy-looking layer called the cell coat or
glycocalyx.The cell coat participates in cell-to-cell
recognition and adhesion, and it contains tissue
transplant antigens.
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The nucleus is the control center for the cell. It
contains deoxyribonucleic acid (DNA), which
provides the information necessary for the
synthesis of the various proteins that the cell
must produce to stay alive and to transmit
information from one generation to another.
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The cytoplasm contains the cell’s organelles.
Ribosomes serve as sites for protein synthesis
