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Chapter 3
Cellular Form and Function
I. Concepts of Cellular Structure (pp. 88–91)
A. Cytology was born in 1665 when Robert Hooke observed the empty cell walls of cork and
called them cellulae. (p. 88)
1. In the 1800s, Theodor Schwann concluded that all animals are made of cells, and by
the end of the nineteenth century, it had been established that all cells arise only from
other cells.
2. Biochemical advances in the late 1800s and early 1900s led to the development of the
modern cell theory.
a. All organisms are composed of cells and cell products.
b. The cell is the simplest structural and function unit of life.
c. An organism’s structure and functions are ultimately due to cellular activity.
d. Cells come only from preexisting cells.
e. The cells of all species have many fundamental similarities.
B. Cells can be classified according to their shape and size. (pp. 88–90) (Fig. 3.1)
1. Squamous cells are thin and flat and may bulge where the nucleus lies.
2. Polygonal cells have angular shapes with four, five, or more sides.
3. Stellate cells, such as nerve cells, have a starlike shape.
4. Cuboidal cells are squarish and are roughly as tall as they are wide.
5. Columnar cells are markedly longer than wide.
6. Spheroid and ovoid cells are round to oval.
7. Discoid describes the shape of red blood cells.
8. Fusiform cells, such as smooth muscle cells, are thick in the middle and tapered to the
ends.
9. Fibrous cells, such as skeletal muscle cells, have a threadlike shape.
C. Cell size is limited partly due to the surface area to volume relationship. (p. 90–91)
1. Volume increases much faster than surface area as cells enlarge. (Fig. 3.2)
2. Waste excretion and nutrient uptake requires exchange through the surface, so as cell
size increases, the ability of a cell to support its activities decreases.
D. All cells have similarities of structure. (p. 90–91)
1. The invention of the transmission electron microscope (TEM) in the mid-1900s
allowed viewing of cells’ ultrastructure. (Fig. 3.3)
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a. The TEM allowed not only greatly increased magnification but also greatly
increased resolution, so that images were sharp.(Fig. 3.4)
2. The scanning electron microscope (SEM) produces three-dimensional images but can
only view surface features. (Fig. 3.11a)
3. The resolution of the TEM is 5 nm, compared to the light microscope’s resolution of
200 nm and the eye’s resolution of 70–100 μm. (Table 3.1)
4. The cell’s components include the plasma membrane, a number of specialized
organelles, and the cytoskeleton. (Fig. 3.5)
a. The cytoskeleton and organelles are embedded in a gel-like solution called
cytosol or intracellular fluid (ICF).
b. The fluid outside the cell is extracellular fluid (ECF).
II. The Cell Surface (pp. 91–100)
A. Many physiological processes, including immune response, binding of egg and sperm, cell
signaling, and detection of taste and smell, occur at the surface of the cell. (p. 91)
B. The plasma membrane is the unit membrane at the cell surface; the side that faces the
cytoplasm is the intracellular face, and the side that faces outward is the extracellular face. (p. 91–
95) (Fig. 3.6a)
1. The plasma membrane consists of an oily film of lipids with diverse embedded
proteins. (Fig. 3.6b)
a. 75% of the lipids are phospholipids arranged in a bilayer, with their phosphate
heads facing the watery region on each side of the membrane and their
hydrophobic tails directed toward the center of the bilayer.
b. Cholesterol molecules constitute about 20% of the membrane lipids.
c. The remaining 5% consists of glycolipids that help form the glycocalyx.
2. Proteins are a small portion of the molecules in the membrane but make up about 50%
of membrane weight.
a. Some are transmembrane proteins that span the bilayer. (Fig. 3.7)
i. Most are glycoproteins conjugated with oligosaccharides that face the
extracellular side of the membrane.
ii. Many drift about freely; others are anchored to the cytoskeleton.
b. Peripheral proteins adhere to one face of the membrane and are typically
associated with transmembrane proteins.
3. Membrane proteins serve at least seven functions.
a. Some proteins are receptors to which signaling molecules, called messengers,
can attach. (Fig. 3.8a)
b. Some play key roles in second-messenger systems, in which binding of a
signaling molecule causes release of a second molecule in the cytoplasm.
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c. Others are enzymes acting at the cell’s surface. (Fig. 3.8b)
d. Some serve as ion channels, allowing water and dissolved ions to pass
through the membrane. (Fig. 3.8c)
i. Channels may have gates that open and close depending on the
stimulus. (Fig. 3.8d)
ii. Ligand-regulated gates respond to chemical messengers; voltageregulated gates respond to changes in electrical potential; mechanically
regulated gates respond to physical changes such as stretch and
pressure.
e. Carriers bind to target molecules and take them across the membrane; pumps
are carriers that use ATP energy. (Fig. 3.18, 3.19)
f. Some glycoproteins are cell-identity markers, allowing the recognition of cells
as “self.” (Fig. 3.8e)
g. Some proteins are cell-adhesion molecules, holding cells to one another. (Fig.
3.8f)
4. Second messengers are of importance to hormone and neurotransmitter action.
a. A first messenger, like epinephrine, cannot pass through the membrane, but
binds to a surface receptor.
b. The receptor is linked inside the membrane to a G protein. (Fig. 3.9)
c. Upon activation by binding of the first messenger to the receptor, the G
protein relays the signal to adenylate cyclase.
e. Adenylate cyclase coverts ATP to cyclic AMP, the second messenger.
f. Cyclic AMP activates kinases that add phosphates to enzymes, which may
activate some, but inactivate others, to carry out the cell’s physiological
response.
Insight 3.1 Calcium Channel blockers
C. All animal cells have a glycocalyx external to the plasma membrane, consisting of the
carbohydrate moieties of glycolipids and glycoproteins. (p. 95) (Fig. 3.10) (Table 3.2)
1. Human blood types are determined by glycolipids.
D. Cells may have surface extensions called microvilli, cilia, and flagella, which aid in absorption,
movement, and sensory processes. (p. 95–100)
1. Microvilli are extensions of the plasma membrane that serve to increase a cell’s
surface area. (Fig. 3.10, 3.11a, c)
a. On some cells these are dense and appear as a fringe called a brush border.
b. Some microvilli have a bundle of actin filaments that extend from its tip into
the cell and attach to the terminal web; these filaments can cause the microvillus
to contract toward the cell.
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2. Cilia are hairlike processes; nearly every human cell has a single, nonmotile primary
cilium a few micrometers long. (Fig. 3.11)
a. These cilia are thought to be sensory and to play a role in the inner ear, the
retina, and the kidney tubules.
b. Motile cilia occur in the respiratory tract, the uterine tubes, spaces in the
brain, and ducts of the testes, where they move materials in a single direction,
generally toward the outside of the body.
c. Motile cilia in a single tissue exhibit coordinated action, with a power stroke
in one direction and a passive recovery stroke in the other. (Fig. 3.12)
d. Cilia beat within a water layer at the cell’s surface, which is produced by
chloride ions pumped out of the cell; mucus floats above this water layer.
e. The basis for movement is a core called the axoneme composed of
microtubules, arranged in a 9 + 2 structure; the central 2 tubules connect to a
basal body that anchors the cilium. (Fig. 3.11d)
f. Dynein is a motor protein responsible for cilia movement; it uses ATP to
move along the tubules, causing motion.
3. Flagella are whiplike structures longer than cilia but with an identical axoneme; in
humans, flagella occur only as the tails of sperm cells.
Insight 3.2 Cystic Fibrosis
III. Membrane Transport (pp. 100–110) (Table 3.3)
A. The plasma membrane is selectively permeable; materials pass through the membrane via
passive or active mechanisms that may be carrier mediated or not. (p.100)
B. Filtration is the process by which particles are driven through the membrane by hydrostatic
pressure. (p. 100)
1. The most important example is in the blood capillaries, where materials are forced
through gaps by the blood pressure. (Fig. 3.13)
2. This is also how the kidneys filter waste materials from the blood.
C. Simple diffusion is the net movement of particles from a place of high concentration to a place
of lower concentration. (pp. 100–101) (Fig. 3.14)
1. A concentration gradient exists when the concentration of a substance in one point
differs from that at another point.
2. Diffusion occurs readily in air or water; when a membrane is present that is permeable
to a substance, that substance will move from one side of the membrane to the other,
down or with the gradient.
3. Substances have diffusion rates based on five factors.
a. An increase in temperature increases the rate of diffusion.
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b. Particles with greater molecular weight, such as large proteins, diffuse more
slowly.
c. A steep gradient, that is, a gradient with a large difference from one side of a
membrane to the other, increases the rate of diffusion.
d. An increased membrane surface area increases the rate of diffusion.
e. A greater permeability to a substance means that the substance diffuses at a
higher rate; cells can adjust their permeability to different materials.
D. Osmosis is the diffusion of water down a concentration gradient through a selectively
permeable membrane. (pp. 101–102) (Fig. 3.14)
1. Significant amounts of water enter many cells through channel proteins called
aquaporins, which may be increased or decreased in number.
2. The direction of osmosis is from a more dilute solution, in which there is more water,
to a more concentrated solution, in which there is less water.
a. In reverse osmosis, water is forced through a membrane under pressure
against its concentration gradient.
3. If on one side of a membrane, a solution contains molecules that are nonpermeating,
water will cross the membrane toward the side with the nonpermeating molecules. (Fig.
13.15)
a. The water level falls on the side without the nonpermeating molecules, and
rises on the side with them.
b. The levels become stable when the osmotic pressure on both sides is in
balance.
4. Equilibrium between osmosis and filtration is an important consideration in fluid
exchange through capillaries.
E. Osmotic concentration is measured in the osmole, which is 1 mole of dissolved particles; a
solution of 1 molar (M) glucose is also 1 osm/L. (pp. 102–103)
1. Osmolality is the number of osmoles per kilogram of water, and osmolarity is the
number of osmoles per liter of solution.
2. Physiological concentrations are usually expressed in milliosmoles per liter (mOsm/L).
3. Tonicity is the ability of a solution to affect fluid volume and pressure in a cell.
a. A hypotonic solution has a lower concentration of nonpermeating solutes than
does the intracellular fluid; water flows into the cell. (Fig. 3.16a)
b. A hypertonic solution has a higher concentration of nonpermeating solutes
than does the intracellular fluid; water flows out of the cell. (Fig. 3.16c)
c. An isotonic solution has a total concentration of nonpermeating solutes equal
to that of the intracellular fluid; as much water flows into the cell as flows out.
(Fig. 3.16b)
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F. Transport proteins are responsible for carrier-mediated transport of substances across the cell
membrane. (pp. 104–106)
1. Carriers are similar to enzymes in that the solute acts as a ligand that binds to a
receptor site on the carrier.
a. Carriers thus exhibit specificity for a particular solute, such as glucose.
b. Carriers also exhibit saturation; at a certain point, all carrier sites are filled
with ligand molecules, giving a rate of transport called the transport maximum.
(Fig. 3.17)
2. Uniports carry only one solute at a time.
3. Symports carry two or more solutes simultaneously in the same direction; also called
cotransport.
4. Antiports carry two or more solutes in opposite directions; also called countertransport.
5. Facilitated diffusion is carrier-mediated transport that moves a solute down its
diffusion gradient.
a. No ATP is consumed in facilitated diffusion.
6. Active transport is carrier-mediated transport the moves a solute up (against) its
diffusion gradient.
a. ATP energy is consumed in active transport.
7. The sodium-potassium (Na+–K+ pump) is an example of active transport.
a. Each cycle of the pump hydrolyzes one ATP and exchanges 3 NA + for 2 K+.
b. The sodium-potassium pump compensates for continual leakage of these ions.
8. About half the calories consumed in a day are used to keep the Na +–K+ pumps going to
help accomplish four main functions.
a. Cell volume is regulated by the Na+–K+ pump’s removal of excess cations
drawn into the cell by large anionic molecules
b. Secondary active transport is facilitated by the ion gradient maintained by the
Na+–K+ pump.
i. In kidney cells, this gradient helps drive the sodium–glucose
transport protein that prevents glucose from being excreted in the urine.
(Fig. 3.20)
c. When the temperature drops, thyroid hormone stimulates cells to produce
more Na+–K+ pumps, which release heat as they consume ATP.
d. The Na+–K+ pump maintains the membrane potential essential for nervous
system function.
G. Vesicular transport processes move large particles, droplets of fluid, or bundles of molecules
through the membrane packaged in vesicles. (pp. 106–110)
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1. Endocytosis brings materials into a cell, whereas exocytosis releases material to
outside the cell; both processes employ motor proteins powered by ATP.
2. Phagocytosis is the process by which cells engulf, or “eat,” particles such as bacteria,
dust, and cellular debris.
a. Neutrophils are a class of white blood cells that phagocytize bacteria by
extending pseudopods and trapping a bacterium in a phagosome (vesicle). (Fig.
3.21)
b. The phagosome is merged with a lysosome, which contains enzymes that
destroy the invader.
3. Pinocytosis is the process of taking in droplets of the extracellular fluid containing
useful molecules.
a. Pits form in the cell membrane that become pinocytotic vesicles inside the
cell.
4. Receptor-mediated endocytosis is a selective process by which cells take in specific
molecules with a minimum of unnecessary fluid. (Fig. 3.22)
a. Receptors bind to particles from the ECF and then cluster together.
b. The membrane sinks in at this point, and the pit becomes coated inside the
cell with the protein clathrin; it then buds off into the cells as a vesicle.
5. Low density lipoprotein (LDL) is one example of a substance taken up by receptormediated endocytosis.
Insight 3.3 Familial Hypercholesterolemia
6. Endothelial cells imbibe insulin from the blood and pass it through the cytoplasm,
releasing it on the other side; this is called transcytosis. (Fig. 3.23)
7. Exocytosis, the process of discharging material from a cell, occurs in many cells such
as mammary gland cells, gland cells, and so on; a vesicle containing the material to be
discharged merges with the cell membrane, releasing the material to the extracellular
space. (Fig. 3.24)
IV. The Cytoplasm (pp. 110–119) (Table 3.4)
A. Structures in the cytoplasm include organelles, the cytoskeleton, and inclusions, all embedded
in the cytosol (p. 110)
B. Organelles are internal structures in a cell that carry out specialized metabolic tasks. (pp. 110–
115)
1. The nucleus is the largest organelle.
a. Most cells have a single nucleus, but red blood cells are anucleate and a few
cell types are multinucleate.
b. The nucleus is surrounded by a double membrane that forms the nuclear
envelope, which is perforated with nuclear pores.
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c. The material in the nucleus is called nucleoplasm and includes chromatin and
nucleoli.
2. The endoplasmic reticulum (ER) is a system of interconnected cisternae enclosed by a
single (unit) membrane. (Fig. 3.26)
a. Rough endoplasmic reticulum consists of flattened sacs covered with
ribosomes.
b. Smooth endoplasmic reticulum consists of more tubular cisternae and lacks
ribosomes.
c. Rough ER synthesizes steroids and lipids, detoxifies alcohol and other drugs,
and manufactures cell membranes.
d. Smooth ER is more active in detoxification and also stores calcium in muscle
cells.
3. Ribosomes are small granules of protein and RNA that translate messenger RNA into
protein.
4. The Golgi complex is a system of cisternae resembling a stack of pita bread; the
complex synthesizes carbohydrates and completes proteins and glycoprotein synthesis.
(Fig. 3.27)
a. The Golgi complex receives completed proteins from rough ER, sorts them,
and packages them into Golgi vesicles.
b. Some are secretory vesicles that store a cell product.
5. Lysosomes are packages of enzymes surrounded by a unit membrane; they are
involved in phagocytosis, digestion of “worn out” organelles, and programmed cell death.
(Fig. 3.28a)
6. Peroxisomes are like lysosomes, but are produced differently; they use oxygen to
oxidize organic molecules, producing hydrogen peroxide that is then used to oxidize
other molecules. (Fig. 3.28b)
7. Mitochondria are specialized to synthesize ATP. (Fig. 3.29)
a. Mitochondria have a double unit membrane; the inner membrane has folds
called cristae.
b. The matrix, between the cristae, contains ribosomes, enzymes, and
mitochondrial DNA (mtDNA).
c. The mitochondria are the powerhouses of the cell.
8. A Centriole is a short cylindrical assembly of microtubules in nine groups of three.
(Fig. 3.30)
a. Two centrioles lie together in a clear area called the centrosome.
b. This pair of centrioles plays a role in cell division.
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C. The cytoskeleton is a collection of protein filaments and cylinders that determine the shape of a
cell, lend it structural support, organize its contents, move substances through the cell, and
contribute to movement of the entire cell. (pp. 115–117) (Fig.3.31)
1. The cytoskeleton is composed of microfilaments, intermediate filaments, and
microtubules.
a. Microfilaments are thin (6 nm) and made of actin; they form a network on the
inside of the plasma membrane called the membrane skeleton.
b. Intermediate filaments (8–10 nm) resist stress and participate in junctions;
they are composed of keratin.
c. Microtubules (25 nm) are cylinders made of 13 parallel strands called
protofilaments, which are made of tubulin; they radiate from the centrosome and
hold organelles in place, form structural bundles, guide organelles and
molecules, and form the axonemes of cilia and flagella. (Fig. 3.32)
d. Microtubules also form the mitotic spindle that guides chromosomes during
cell division.
D. Inclusions are of two kinds: stored cellular products such as glycogen granules, and foreign
bodies such as dust particles, viruses, and intracellular bacteria; they are never bounded by a unit
membrane. (Fig. 3.26b)
Insight 3.4 Mitochondria—Evolution and Clinical Significance
Cross References
Additional information on topics mentioned in Chapter 3 can be found in the chapters listed below.
App. C: Units of measurement.
Chapter 4: Function of the nucleus.
Chapter 4: Function of the endoplasmic reticulum
Chapter 4: Function of ribosomes.
Chapter 4: Role of the Golgi complex.
Chapter 4: Cell division.
Chapter 5: Programmed cell death.
Chapter 20: Osmosis and filtration in blood capillaries.
Chapter 21: Phagocytic cells.
Chapter 26: Role of mitochondria in ATP synthesis.