Cells Carry Out Many Processes 57
• Several processes allow substances to cross membranes.
Diffusion and osmosis involve the movement of substances
from areas of higher concentration to areas of lower con-
centration. Facilitated diffusion involves the movement of
substances through membrane channels and sometimes
involves carriers like the one illustrated in the figure.
Faciliated diffusion • Figure 3.12c
• Active transport, which moves substances against concen-
tration gradients with energy from various sources, utilizes
pumps and exchangers. Substances that are too large to
take advantage of other transport mechanisms move across
membranes via endocytosis, exocytosis, phagocytosis, and
• Information for making proteins lies within a cell's DNA.
This information gets transcribed by RNA polymerase into
mRNA; the mRNA is translated into protein sequences
through the actions of ribosomes and tRNA.
• Cells divide through the processes of mitosis and meiosis.
Somatic cells undergo mitosis, which produces two identical
cells with a genetic makeup identical to that of the starting
cell. In contrast, gametes undergo meiosis, which produce
four cells; each cell has half the amount of genetic material
present in the starting cell. The mechanics of mitosis and
meiosis are similar, but there are also crucial differences that
permit genetic variation from generation to generation.
Cells Specialize into Various Tissues 71
• Epithelium covers and lines body cavities. The types of
epithelium are based on cell shapes (squamous, cuboidal,
columnar) and arrangements (simple, stratified, pseudostrat-
ified); simple cuboidal epithelium is shown here. Various
types are located throughout the body and serve specific
functions, based on their structures.
Epithelial tissue • Figure 3.22
• Connective tissue binds together, supports, protects, and in-
sulates body tissues. Its extracellular matrix contains many
types of fibers and proteins, and the makeup of this matrix
varies with the type of connective tissue.
• Muscular tissues generate force for movement, maintain
posture, and generate heat. There are three types of muscu-
lar tissue: skeletal, cardiac, and smooth.
• Nerve cells conduct nerve impulses, which transmit informa-
tion to and from various other cells. Nervous tissue consists
of neurons, which transmit the nerve impulses, and neuro-
glia, which support the neurons.
• Membranes cover organs and line various body cavities.
Epithelial membranes (mucous, serous) consist of epithelium
overlying connective tissue. Synovial membranes line joint
cavities and have no epithelial layer.
Aging Affects Cells and Tissues 84
• Aging slows and may even stop cell division, and it induces
loss of genetic material through shortening of telomeres
(shown here), cross-linking of glycoproteins, buildup of free
radicals, and development of autoimmunity.
vessels. Aging of blood vessels can lead to hypertension,
atherosclerosis, cardiovascular disease, and stroke.