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The Cellular Level
of Organization
Did you know...?
Cell structure tells a lot about cell functions. In
laboratories, cells are collected and examined
microscopically to detect abnormal cells.
Learning Outcomes
After completing this chapter, you should be able to do the following:
3-1
List the functions of the plasma membrane and the structural
features that enable it to perform those functions.
3-2
Describe the organelles of a typical cell, and indicate the specific
functions of each.
3-3
Explain the functions of the cell nucleus and discuss the nature
and importance of the genetic code.
3-4
Summarize the role of DNA in protein synthesis, cell structure,
and cell function.
3-5
Describe the processes of cellular diffusion and osmosis, and
explain their role in physiological systems.
3-6
Describe carrier-mediated transport and vesicular transport
mechanisms, which cells use to facilitate the absorption or
removal of specific substances.
3-7
Explain the origin and significance of the transmembrane
potential.
3-8
Describe the stages of the cell life cycle, including mitosis,
interphase, and cytokinesis, and explain their significance.
3-9
Discuss the regulation of the cell life cycle.
3-10
Discuss the relationship between cell division and cancer.
3-11
Define differentiation, and explain its importance.
Clinical Notes
Inheritable Mitochondrial Disorders p. 81
DNA Fingerprinting p. 84
Mutations p. 88
Drugs and the Plasma Membrane p. 91
Telomerase, Aging, and Cancer p. 106
Parkinson Disease p. 107
67
Chapter 3
The Cellular Level of Organization
An Introduction to Cells
In the rest of this chapter, we describe the structure of a
typical somatic cell, consider some of the ways in which cells
interact with their environment, and discuss how somatic
cells reproduce. It is important to keep in mind that the “typ-
ical” somatic cell is like the “average” person: Any description
masks enormous individual variations. Our model cell,
shown in
Figure 3–1,
shares features with most cells of the
body, without being identical to any one. Table 3–1 summa-
rizes the structures and functions of the cell’s parts.
Our model cell is surrounded by a watery medium known
as the
extracellular fluid
. The extracellular fluid in most tis-
sues is called
interstitial
(in-ter-STISH-ul)
fluid
(
interstitium
,
something standing between). A
plasma membrane (cell mem-
brane)
separates the cell contents, or
cytoplasm
, from the ex-
tracellular fluid. The cytoplasm can itself be subdivided into
(1) the
cytosol
, a liquid, and (2) intracellular structures collec-
tively known as
organelles
(or-ga-NELZ; “little organs”).
This chapter relates how combinations of chemicals form
cells
, the smallest living units in the human body. It also de-
scribes the chemical events that sustain life, most of which
occur inside cells.
Cells are very small indeed—a typical cell is only about
0.1 mm in diameter. As a result, no one could actually ex-
amine the structure of a cell until relatively effective micro-
scopes were invented in the 17th century. In 1665, Robert
Hooke inspected thin slices of cork and found that they
consisted of millions of small, irregular units. In describing
his observations, Hooke used the term
cell
because the
many small, bare spaces he saw reminded him of the rooms,
or cells, in a monastery or prison. Although Hooke saw only
the outlines of the cells, and not the cells themselves, he
stimulated considerable interest in the microscopic world
and in the nature of cellular life. The research that he began
more than 342 years ago has, over time, produced the
cell
theory
in its current form. The basic concepts of this theory
can be summarized as follows:
3-1
The plasma membrane
separates the cell from its
surrounding environment and
performs various functions
•
Cells are the building blocks of all plants and animals.
•
All cells come from the division of preexisting cells.
•
Cells are the smallest units that perform all vital
physiological functions.
•
Each cell maintains homeostasis at the cellular level.
We begin our look at the anatomy of cells by discussing the first
structure you encounter when viewing cells through a micro-
scope. The outer boundary of the cell is the
plasma membrane
,
also called the
cell membrane
or
plasmalemma
(
lemma
, husk).
Its general functions include the following:
•
Physical Isolation.
The plasma membrane is a physical
barrier that separates the inside of the cell from the
surrounding extracellular fluid. Conditions inside and
outside the cell are very different, and those differences
must be maintained to preserve homeostasis. For
example, the plasma membrane keeps enzymes and
structural proteins inside the cell.
•
Regulation of Exchange with the Environment.
The plasma
membrane controls the entry of ions and nutrients, such as
glucose; the elimination of wastes; and the release of
secretions.
•
Sensitivity to the Environment.
The plasma membrane is the
first part of the cell affected by changes in the composition,
concentration, or pH of the extracellular fluid. It also
contains a variety of receptors that allow the cell to
recognize and respond to specific molecules in its
environment. For instance, the plasma membrane may
receive chemical signals from other cells. The binding of
just one molecule may trigger the activation or
deactivation of enzymes that affect many cellular activities.
Homeostasis at the level of the tissue, organ, organ sys-
tem, and organism reflects the combined and coordinated ac-
tions of many cells.
The human body contains trillions of cells, and all our ac-
tivities—from running to thinking—result from the combined
and coordinated responses of millions or even billions of cells.
Many insights into human physiology arose from studies of the
functioning of individual cells. What we have learned over the
last 50 years has given us a new understanding of cellular phys-
iology and the mechanisms of homeostatic control. Today, the
study of cellular structure and function, or
cytology
, is part of
the broader discipline of
cell biology
, which incorporates as-
pects of biology, chemistry, and physics.
The human body contains two general classes of cells:
sex cells and somatic cells.
Sex cells
(also called
germ cells
or
reproductive cells
) are either the
sperm
of males or the
oocytes
of females. The fusion of a sperm and an oocyte at fertiliza-
tion is the first step in the creation of a new individual.
Somatic cells
(
soma
, body) include all the other cells in the
human body. In this chapter, we focus on somatic cells; we
will discuss sex cells in Chapters 28 and 29, which describe
the reproductive system and development, respectively.
68
Unit 1
Levels of Organization
Microvilli
Secretory
vesicles
Cytosol
Golgi apparatus
Lysosome
Mitochondrion
Centrosome
Centriole
Peroxisome
Chromatin
Proteasomes
Nucleoplasm
Nuclear pores
Nucleolus
Smooth
endoplasmic
reticulum
Nuclear envelope
surrounding nucleus
Rough
endoplasmic
reticulum
Fixed ribosomes
Cytoskeleton
Free ribosomes
Plasma membrane
Figure 3–1
The Anatomy of a Model Cell.
See Table 3–1 for a summary of the functions associated with the various cell structures.
69
Chapter 3
The Cellular Level of Organization
TABLE 3–1
Organelles of a Representative Cell
Appearance
Structure
Composition
Function(s)
PLASMA MEMBRANE
Lipid bilayer containing phospholipids,
steroids, proteins, and carbohydrates
Isolation; protection; sensitivity; support;
controls entry and exit of materials
Fluid component of cytoplasm
Distributes materials by diffusion
CYTOSOL
NONMEMBRANOUS ORGANELLES
Cytoskeleton
Microtubule
Microfilament
Proteins organized in fine filaments or
slender tubes
Strength and support; movement of cellular
structures and materials
Membrane extensions containing
microfilaments
Increase surface area to facilitate absorption
of extracellular materials
Microvilli
Centrosome
Centrioles
Cytoplasm containing two centrioles
at right angles; each centriole is
composed of 9 microtubule triplets in
a 9
Essential for movement of chromosomes
during cell division; organization of
microtubules in cytoskeleton
0 array
Membrane extensions containing
microtubule doublets in a 9
Movement of materials over cell surface
Cilia
2 array
RNA
proteins; fixed ribosomes bound
to rough endoplasmic reticulum, free
ribosomes scattered in cytoplasm
Hollow cylinders of proteolytic enzymes
with regulatory proteins at ends
Protein synthesis
Ribosomes
Proteasomes
Breakdown and recycling of damaged or
abnormal intracellular proteins
MEMBRANOUS ORGANELLES
Mitochondria
Double membrane, with inner
membrane folds (cristae) enclosing
important metabolic enzymes
Produce 95% of the ATP required by the cell
Network of membranous channels
extending throughout the cytoplasm
Synthesis of secretory products;
intracellular storage and transport
Endoplasmic
reticulum (ER)
Has ribosomes bound to membranes
Modification and packaging of newly
synthesized proteins
Lipid and carbohydrate synthesis
Rough ER
Lacks attached ribosomes
Smooth ER
Stacks of flattened membranes
(cisternae) containing chambers
Storage, alteration, and packaging of
secretory products and lysosomal enzymes
Golgi apparatus
Vesicles containing digestive enzymes
Intracellular removal of damaged organelles
or pathogens
Catabolism of fats and other organic
compounds; neutralization of toxic compounds
generated in the process
Control of metabolism; storage and
processing of genetic information; control of
protein synthesis
Lysosome
Vesicles containing degradative
enzymes
Peroxisome
Nucleoplasm containing nucleotides,
enzymes, nucleoproteins, and
chromatin; surrounded by double
membrane (nuclear envelope)
NUCLEUS
Nuclear envelope
Nuclear pore
Dense region in nucleoplasm
containing DNA and RNA
Site of rRNA synthesis and assembly of
ribosomal subunits
Nucleolus
70
Unit 1
Levels of Organization
•
Structural Support.
Specialized connections between
plasma membranes, or between membranes and
extracellular materials, give tissues stability. For example,
the cells at the surface of the skin are bound together,
while those in the deepest layers are attached to
extracellular protein fibers in underlying tissues.
interstitial fluid on the outside and the cytosol on the inside—
and the hydrophobic tails form the interior of the membrane.
The lipid bilayer also contains cholesterol and small quantities
of other lipids, but these have relatively little effect on the gen-
eral properties of the plasma membrane.
Notice the similarities in lipid organization between the
plasma membrane and a micelle (see
Figure 2–17c,
p. 52). Ions
and water-soluble compounds cannot enter the interior of a
micelle, because the lipid tails of the phospholipid molecules
are hydrophobic and will not associate with water molecules.
For the same reason, water and solutes cannot cross the lipid
portion of the plasma membrane. Thus, the hydrophobic
compounds in the center of the membrane isolate the cyto-
plasm from the surrounding fluid environment. Such isola-
tion is important because the composition of cytoplasm is
very different from that of extracellular fluid, and the cell can-
not survive if the differences are eliminated.
The plasma membrane is extremely thin and delicate,
ranging from 6 to 10 nm in thickness (
Figure 3–2
). This mem-
brane contains lipids, proteins, and carbohydrates.
Membrane Lipids
Although lipids form most of the surface area of the plasma
membrane, they account for only about 42 percent of its
weight. The plasma membrane is called a
phospholipid bi-
layer
, because the phospholipid molecules in it form two lay-
ers. Recall from Chapter 2 that a phospholipid has both a
hydrophilic end (the phosphate portion) and a hydrophobic
end (the lipid portion).
l
p. 51
In each half of the bilayer,
the phospholipids lie with their hydrophilic heads at the mem-
brane surface and their hydrophobic tails on the inside. Thus,
the hydrophilic heads of the two layers are in contact with the
aqueous environments on either side of the membrane—the
Membrane Proteins
Proteins, which are much denser than lipids, account for
roughly 55 percent of the weight of a plasma membrane.
There are two general structural classes of membrane pro-
EXTRACELLULAR FLUID
Glycolipids
of glycocalyx
Phospholipid
bilayer
Integral protein
with channel
Integral
glycoproteins
Hydrophobic
tails
Plasma
membrane
Cholesterol
Hydrophilic
heads
Peripheral
proteins
Gated
channel
CYTOPLASM
Cytoskeleton
(Microfilaments)
= 2 nm
Figure 3–2
The Plasma Membrane.
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