Aging Affects Cells and Tissues
LEARNING OBJECTIVE
T elom eres • Figure 3.29
Describe
the cellular and tissue changes that occur
with aging.
s we get older, our wounds take longer to heal,
skin becomes less pliable, joints stiffen, and
arteries become less elastic. Many of these
changes can be explained by the effects of in-
creasing age on cells:
A
Cellular metabolism slows, so cells do not make new
materials as fast as they once did, and healing occurs
more slowly.
Because cells do not divide as quickly as they once did
and some may stop dividing altogether, worn-out cells
do not get replaced.
Cellular DNA erodes after many cell divisions. The
protective DNA sequences at the ends of chromosomes,
called telomeres (TEL-o-merz) (Figure 3.29), shorten
and disappear after many divisions, leading to loss of
functional DNA and cell death.
Increasingly, glucose gets added to proteins both
inside
and
outside
the
cell.
The
glucose
units
cross-link to each other, binding adjacent proteins
and stiffening cell membranes and tissues such as
skin and connective
tissues. Loss of elasticity in
connective tissue fibers such as collagen and elastin
can also lead to wrinkles, stiffness in blood vessels,
Telomeres are sequences of repeating DNA bases located at the
tips of chromosomes that protect the chromosomes.
high blood pressure, atherosclerosis, cardiovascular
disease, and stroke.
Chemical reactions within the cell produce highly
reactive oxygen free radicals, which damage lipids,
proteins, and nucleic acids.
Cells lose identity markers on the plasma membrane
and become susceptible to attack by the body’s immune
system.
CONCEPT CHECK
Why
does connective tissue stiffen with age?
THE PLANNER
Summary
The nucleus • Figure 3.10
Chromatin
Nucleolus
Cells Have Distinct Parts 50
• A cell has a plasma membrane that separates the inside from
the outside and controls the movement of substances across it.
Within the membrane, the cytoplasm contains fluid and numer-
ous organelles that carry out various cell functions. As shown,
the largest organelle is the nucleus, which houses the
cell's DNA. DNA controls and coordinates cell functions
and reproduction. •
Rough endoplasmic
reticulum
Nuclear envelope
Nuclear pore
Polyribosome
• The cell's organelles include a plasma membrane, cyto-
skeleton, centrosomes, ribosomes, rough ER, smooth ER, Golgi
complex, lysomomes, mitochondria, and a nucleus. Each organ-
elle carries out specific functions.
84 CHAPTER 3
Cells and Tissues
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