Cells: The Living Units
aging are “programmed into our genes.” One interesting notion
here is that a
determines the number of times a
cell can divide.
piece) are non-
sensical strings of nucleotides that cap the ends of chromosomes
and provide protection from fraying, fusing with other chromo-
somes, inappropriate repair, and losing important genetic infor-
mation, much like plastic caps preserve the ends of shoelaces.
In human telomeres, the base sequence TTAGGG is repeated
a thousand times or more (like a DNA stutter). ±ough telo-
meres carry no genes, they appear to be vital for chromosomal
survival, because each time DNA is replicated, 100 to 200 of
the end nucleotides are lost and the telomeres get a bit shorter.
When telomeres reach a certain minimum length, it is thought
that the stop-division signal is given.
±e idea that cell longevity depends on telomere integrity
was supported by the discovery of
, an enzyme that
lengthens the previously shortened telomeres. Pegged as the
“immortality enzyme,” telomerase is found in germ line cells
(cells that give rise to sperm and ova), but it is absent or barely
detectable in other adult cell types.
Check Your Understanding
What is apoptosis and what is its importance in the body?
What is the wear-and-tear theory of aging?
For answers, see Appendix H.
In this chapter we have described the structure and func-
tion of the generalized cell. One of the wonders of the cell is the
disparity between its minute size and its extraordinary activ-
ity, which reﬂects the diversity of its organelles. ±e evidence
for division of labor and functional specialization among or-
ganelles is inescapable. Only ribosomes synthesize proteins,
while protein packaging is the bailiwick of the Golgi apparatus.
Membranes compartmentalize most organelles, allowing them
to work without hindering or being hindered by other cell ac-
tivities, and the plasma membrane regulates molecular traﬃc
across the cell’s boundary. Now that you know what cells have in
common, you are ready to explore how they diﬀer in the various
body tissues, the topic of Chapter 4.
Cell aging is a complicated process with many causes, but to be
perfectly frank, the precise reason an otherwise healthy person
grows old and dies is still a mystery.
attributes aging to little chemi-
cal insults and formation of free radicals, both of which have
cumulative eﬀects. For example, environmental toxins such as
pesticides, alcohol, and bacterial toxins may damage cell mem-
branes, poison enzyme systems, or cause “mistakes” in DNA
replication. Temporary lack of oxygen, which occurs increas-
ingly with age as our blood vessels clog with fatty materials,
accelerates rates of cell death throughout the body.
According to the
mitochondrial theory of aging
, free radicals
deserve most of the blame. Most free radicals are produced in the
mitochondria, because these organelles have the highest meta-
bolic rate. ±is ﬁnding implies that diminished energy production
by radical-damaged mitochondria, due perhaps to an increasing
burden of mutations in mitochondrial DNA, weakens (and ages)
cells. X rays and other types of radiation, and some chemicals, also
generate huge numbers of free radicals, which can overwhelm the
peroxisomal enzymes. Vitamins C and E act as antioxidants in
the body and may help to absorb or defuse free radicals. With
age, glucose (blood sugar) becomes party to cross-linking proteins
together, a condition that severely disrupts protein function and
accelerates the course of atherosclerosis.
Another theory attributes cell aging to progressive disorders
in the immune system. According to this theory, cell damage
results from (1) autoimmune responses, which means the im-
mune system turns against one’s own tissues, and (2) a progres-
sive weakening of the immune response, so that the body is less
able to get rid of cell-damaging pathogens.
±e idea that cell aging and chronic illnesses result from
inﬂammation (and an overactive immune system) has been
kicking around since the 1800s. In the 1990s evidence did in
fact emerge to link inﬂammation to aging. C-reactive protein,
a protein that indicates acute inﬂammation, is elevated in heart
disease and is an amazingly accurate predictor of future heart
attacks. It seems that as we age, acute episodes of inﬂammation
tend to become chronic.
Perhaps the most widely accepted theory of cell aging is the
, which suggests that cessation of mitosis and cell
The Cellular Basis of Life
All living organisms are composed of cells—the basic structural
and functional units of life. Cells vary widely in both shape
±e principle of complementarity states that the biochemical
activity of cells reﬂects the operation of their organelles.
±e generalized cell is a concept that typiﬁes all cells. ±e
generalized cell has three major regions—the nucleus, cytoplasm,
and plasma membrane.
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