Inclusions

Inclusions are of two kinds: stored cellular products such as glycogen granules, pigments, and fat droplets (see fig. 3.26b), and foreign bodies such as dust particles, viruses, and intracellular bacteria. Inclusions are never enclosed in a unit membrane, and unlike the organelles and cytoskeleton, they are not essential to cell survival.

The major features of a cell are summarized in table 3.4.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Distinguish between organelles and inclusions. State two examples of each.
  2. Briefly state how each of the following cell components can be recognized in electron micrographs: the nucleus, a mitochondrion, a lysosome, and a centriole. What is the primary function of each?
  3. What three organelles are involved in protein synthesis?
  4. In what ways do rough and smooth endoplasmic reticulum differ?
  5. Define centriole, microtubule, cytoskeleton, and axoneme. How are these structures related to each other?

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Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

122 Part One Organization of the Body

Plasma Membrane Diagram Labeled

Plasma membrane

Cell Cross Section

Figure 3.31 The Cytoskeleton. (a) Electron micrograph of a cell of the testis showing numerous microtubules in longitudinal section and cross section. (b) Fluorescent antibodies are used here to label the fibrous cytoskeleton of a cell and make it visible through a fluorescence microscope. (c) Diagram of the cytoskeleton.

  • Endoplasmic reticulum
  • Intermediate filaments

Ribosome Mitochondrion

Microtubule

Microfilament

Figure 3.31 The Cytoskeleton. (a) Electron micrograph of a cell of the testis showing numerous microtubules in longitudinal section and cross section. (b) Fluorescent antibodies are used here to label the fibrous cytoskeleton of a cell and make it visible through a fluorescence microscope. (c) Diagram of the cytoskeleton.

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 3 Cellular Form and Function 123

Chapter 3 Cellular Form and Function 123

Tubulin

Figure 3.32 Microtubules. (a) A microtubule is composed of 13 protofilaments. Each protofilament is a spiral chain of globular proteins called tubulin. (b) One of the nine microtubule pairs that form the axonemes of cilia and flagella. (c) One of the nine microtubule triplets that form a centriole.

Table 3.4 Summary of Organelles and Other Cellular Structures

Structure

Appearance to TEM

Function

Plasma membrane

Two dark lines at cell surface, separated by narrow

Prevents escape of cell contents; regulates exchange of

(figs. 3.3 and 3.6)

light space

materials between cytoplasm and extracellular fluid;

involved in intercellular communication

Microvilli

Short, densely spaced, hairlike processes or scattered

Increase absorptive surface area; some sensory roles

(figs. 3.10 and 3.11 a-b)

bumps on cell surface; interior featureless or with bundle

(hearing, equilibrium, taste)

of microfilaments

Cilia

Long hairlike projections of apical cell surface; axoneme

Move substances along cell surface; some sensory roles

(figs. 3.11c-e and 3.12)

with 9 + 2 array of microtubules

(hearing, equilibrium, smell, vision)

Flagellum

Long, single, whiplike process with axoneme

Sperm motility

Nucleus

Largest organelle in most cells, surrounded by double unit

Genetic control center of cell; directs protein synthesis

(figs. 3.3 and 3.25)

membrane with nuclear pores

Rough ER

Extensive sheets of parallel unit membranes with

Protein synthesis and manufacture of cellular membranes

(fig. 3.26a)

ribosomes on outer surface

Smooth ER

Branching network of tubules with smooth surface

Lipid synthesis, detoxification, calcium storage

(fig. 3.26b)

(no ribosomes); usually broken into numerous small

segments in TEM photos

Ribosomes

Small dark granules free in cytosol or on surface of

Interpret the genetic code and synthesize polypeptides

(fig. 3.26a)

rough ER

Golgi complex

Several closely spaced, parallel cisternae with thick edges,

Receives and modifies newly synthesized polypeptides,

(fig. 3.27)

usually near nucleus, often with many Golgi vesicles nearby

synthesizes carbohydrates, adds carbohydrates to

glycoproteins; packages cell products into Golgi vesicles

Golgi vesicles

Round to irregular sacs near Golgi complex, usually with

Become secretory vesicles and carry cell products to

(fig. 3.27)

light, featureless contents

apical surface for exocytosis, or become lysosomes

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

124 Part One Organization of the Body

Table 3.4 Summary of Organelles and Other Cellular Structures, (continued)

Structure

Appearance to TEM

Function

Lysosomes

Round to oval sacs with single unit membrane, often a dark

Contain enzymes for intracellular digestion, autophagy,

(fig. 3.28a)

featureless interior but sometimes with protein layers

programmed cell death, and glucose mobilization

or crystals

Peroxisomes

Similar to lysosomes; often lighter in color

Contain enzymes for detoxification of free radicals,

(fig. 3.286)

alcohol, and other drugs; oxidize fatty acids

Mitochondria

Round, rod-shaped, bean-shaped, or threadlike structures

ATP synthesis

(fig. 3.29)

with double unit membrane and shelflike infoldings called

cristae

Centrioles

Short cylindrical bodies, each composed of a circle of nine

Form mitotic spindle during cell division; unpaired

(fig. 3.30)

triplets of microtubules

centrioles form basal bodies of cilia and flagella

Centrosome

Clear area near nucleus containing a pair of centrioles

Organizing center for formation of microtubules of

(fig. 3.5)

cytoskeleton and mitotic spindle

Basal body

Unpaired centriole at the base of a cilium or flagellum

Point of origin, growth, and anchorage of a cilium or

(fig. 3.11 e)

flagellum; produces axoneme

Microfilaments

Thin protein filaments (6 nm diameter), often in parallel

Support microvilli; involved in muscle contraction and

(figs. 3.10 and 3.31)

bundles or dense networks in cytoplasm

other cell motility, endocytosis, and cell division

Intermediate filaments

Thicker protein filaments (8-10 nm diameter) extending

Give shape and physical support to cell; anchor cells to

(fig. 3.31)

throughout cytoplasm or concentrated at cell-to-cell

each other and to extracellular material; compartmentalize

junctions

cell contents

Microtubules

Hollow protein cylinders (25 nm diameter)

Form axonemes of cilia and flagella, centrioles, basal

(figs. 3.31 and 3.32)

bodies, and mitotic spindles; enable motility of cell parts;

direct organelles and macromolecules to their

destinations within a cell

Inclusions

Highly variable—fat droplets, glycogen granules, protein

Storage products or other products of cellular metabolism,

(fig. 3.266)

crystals, dust, bacteria, viruses; never enclosed in unit

or foreign matter retained in cytoplasm

membranes

Insight 3.4 Evolutionary Medicine

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