ing the ow of energy through living systems. In the photosynthetic reaction,
the water molecule is split by energy absorbed from sunlight via the
chlorophyll of green plants. Part of the split molecule yields oxygen; the rest
combines with carbon dioxide to form carbohydrates (which are organic com—
pounds). The organic products of this reaction may then become a source of
energy for the animals who eat the plants and, indirectly, for other animals
farther up the food chain, including humans, who eat animals nourished on
the plants.
Water also plays an important role in transporting many of the materials
necessary to the life processes
because it has such an exceptional ability to
dissolve a variety of substances. In man, food and oxygen are brought to the
cells via the bloodstream; chemical messages,
in the form of hormones, are
also carried in the bloodstream. In higher plants, nutrients are transported
from the roots to the leaves in water moving through tubular structures called
xylem. In the same way, waste products are removed by means of water; the
kidneys remove urea, a waste product of protein metabolism, in urine. To
those organisms capable of sexual reproduction, water is the means by which
the sperm, mobile only in water, can reach the egg
to fertilize it.
Water thus is not only ubiquitous in nature, but is a necessary ingredient
for all life. People, no less than protozoa, are dependent upon
water. The hu—
man embryo is approximately 90 percent water, and adults are 65 to 70 per—
cent water; the percentage of water then decreases in old age.
More than
one—half of the water in humans is found in the muscles and in the skin; carti—
lage and bone contain little water. The body very successfully regulates the
amount of uid it contains. For example, water intake depends in part on
thirst, which is regulated by the nervous system. Another point of control is at
the kidneys: Of the approximately 170 liters of water which pass through the
kidneys per day, 168.5 are reabsorbed [7]. The basic biological need for water
has strongly affected the history of human Civilization. Man has been able to
settle only near sources of water which are so essential not only for him but
for all the other plant and animal Species on which he depends.
Life’s dependence on water is the result of millions of years of evolution
during which living cells became adapted to such special properties of water
as its ability to moderate temperature, to dissolve a great number of sub—
to exist as a gas, a liquid, and a solid at temperatures common to
the eartn s surface. These properties are all a reection of the unique molecu—
lar structure of
water. L. ]. Henderson’s famous book about the suitability of
these properties
for the support of life is called, appropriately, The Fitness Of
the Environment [8]. Understanding the molecular structure of. water is there—
fore helpful m understanding the environmental role of water.
The Chemical Structure of Water
utnfzËmiliar
to people who have not studied
water. There is more to be undeÊËcr
0
eVeryday language as a synonym
for
OOd about the structure of water, however,
36
An Uncommon Friend
the water molecule is split by energy absorbed from sunlight via the
chlorophyll of green plants. Part of the split molecule yields oxygen; the rest
combines with carbon dioxide to form carbohydrates (which are organic com—
pounds). The organic products of this reaction may then become a source of
energy for the animals who eat the plants and, indirectly, for other animals
farther up the food chain, including humans, who eat animals nourished on
the plants.
Water also plays an important role in transporting many of the materials
necessary to the life processes
because it has such an exceptional ability to
dissolve a variety of substances. In man, food and oxygen are brought to the
cells via the bloodstream; chemical messages,
in the form of hormones, are
also carried in the bloodstream. In higher plants, nutrients are transported
from the roots to the leaves in water moving through tubular structures called
xylem. In the same way, waste products are removed by means of water; the
kidneys remove urea, a waste product of protein metabolism, in urine. To
those organisms capable of sexual reproduction, water is the means by which
the sperm, mobile only in water, can reach the egg
to fertilize it.
Water thus is not only ubiquitous in nature, but is a necessary ingredient
for all life. People, no less than protozoa, are dependent upon
water. The hu—
man embryo is approximately 90 percent water, and adults are 65 to 70 per—
cent water; the percentage of water then decreases in old age.
More than
one—half of the water in humans is found in the muscles and in the skin; carti—
lage and bone contain little water. The body very successfully regulates the
amount of uid it contains. For example, water intake depends in part on
thirst, which is regulated by the nervous system. Another point of control is at
the kidneys: Of the approximately 170 liters of water which pass through the
kidneys per day, 168.5 are reabsorbed [7]. The basic biological need for water
has strongly affected the history of human Civilization. Man has been able to
settle only near sources of water which are so essential not only for him but
for all the other plant and animal Species on which he depends.
Life’s dependence on water is the result of millions of years of evolution
during which living cells became adapted to such special properties of water
as its ability to moderate temperature, to dissolve a great number of sub—
to exist as a gas, a liquid, and a solid at temperatures common to
the eartn s surface. These properties are all a reection of the unique molecu—
lar structure of
water. L. ]. Henderson’s famous book about the suitability of
these properties
for the support of life is called, appropriately, The Fitness Of
the Environment [8]. Understanding the molecular structure of. water is there—
fore helpful m understanding the environmental role of water.
The Chemical Structure of Water
utnfzËmiliar
to people who have not studied
water. There is more to be undeÊËcr
0
eVeryday language as a synonym
for
OOd about the structure of water, however,
36
An Uncommon Friend