The Nature of Life
have you ever dropped a pellet of
dry ice (frozen carbon dioxide) into a pan of water and watched what happens?
The solid pellet darts randomly about the surface, looking like a highly ener-
getic bug waterskiing, as the warmer water rap- idly converts it to a gas. Does
all that motion make the dry ice alive? Hardly; yet one of the attributes of
living things is the capacity to move. But if living things move, what about
plants? If a tree remains fixed in one place and doesn’t crawl down the
sidewalk, does that mean it isn’t alive? Again the answer is no, but these
ques- tions do serve to point out some of the difficulties encoun- tered in
defining life. In fact, some argue that there is no such thing as life—only
living organisms—and that life is a concept based on the collective attributes
of living organisms.
ATTRIBUTES OF LIVING ORGANISMS
Composition and Structure
The activities of living
organisms originate in tiny structural units called cells, which consist of
cytoplasm (a souplike fluid) bounded by a very thin membrane. All living cells
contain genetic material that controls their development and activities. In the
cells of many organisms, this genetic mate- rial, known as DNA (an abbreviation
for deoxyribonucleic acid), is housed in a somewhat spherical structure called
the nucleus, which is suspended in the cytoplasm. In bacteria and other simple
cells, however, the DNA is distributed directly in the cytoplasm. The cells of
plants, algae, fungi, and many simpler organisms have a cell wall outside of
the membrane that bounds the cytoplasm. The cell wall pro- vides support and
rigidity.
The activities of living
organisms originate in tiny structural units called cells, which consist of
cytoplasm (a souplike fluid) bounded by a very thin membrane. All living cells
contain genetic material that controls their development and activities. In the
cells of many organisms, this genetic mate- rial, known as DNA (an abbreviation
for deoxyribonucleic acid), is housed in a somewhat spherical structure called
the nucleus, which is suspended in the cytoplasm. In bacteria and other simple
cells, however, the DNA is distributed directly in the cytoplasm. The cells of
plants, algae, fungi, and many simpler organisms have a cell wall outside of
the membrane that bounds the cytoplasm. The cell wall pro- vides support and
rigidity.
Growth
Some have described growth as
simply an increase in mass (a body of matter—the basic “stuff” of the
universe), usually accompanied by an increase in volume. Most growth results
from the production of new cells and includes variations in form—some the
result of inheri- tance, some the result of response to the environment. What
happens, for example, if you plant two apple seeds of the same variety in poor
soil but don’t give them the same care? If you water one just enough to allow
it to germinate and grow, while you water the other one freely and work
fertilizers and conditioners into the soil around it, you might expect the
second one to grow larger and produce more fruit than the first. In other
words, although the two apple trees grew from the same variety of apple seed,
they differ in form, following patterns of growth dictated by the DNA and the
environment.
Reproduction
Dinosaurs were abundant 160
million years ago, but none exist today. Hundreds of mammals, birds, reptiles,
plants, and other organisms are now listed as endangered or threat- ened
species, and many of them will become extinct within the next decade or two.
All these once-living or living things have one feature in common: it became
impossible or it has become difficult for them to reproduce. Reproduction is
such an obvious feature of living organ- isms that we take it for granted—until
it no longer takes place. When organisms reproduce, the offspring always
resemble the parents: guppies never have puppies—just more guppies—and a
petunia seed, when planted, will not develop into a pineapple plant. Also,
offspring of one kind tend to resemble their parent more than they do other
individuals of the same kind.
Response to Stimuli
If you stick a pin into a pillow,
you certainly don’t expect any reaction from the pillow, but if you stick the
same pin into a friend, you know your friend will react immediately (assuming
he or she is conscious) because responding to stimuli is a major characteristic
of all living things. You might argue, however, that when you stuck a pin into
your house plant, nothing happened, even though you were fairly certain the
plant was alive. You might not have been aware that the house plant did indeed
respond but in a manner very different from that of a human. Plant responses to
stimuli are generally much slower than those of animals and usually are of a
different nature. If the house plant’s food-conducting tissue was pierced, it
probably responded by producing a plugging substance called callose in the
affected cells. Some studies have shown that callose may form within as little
as 5 seconds after wounding. Also, an unorganized tissue called callus, which
forms much more slowly, may be produced at the site of the wound.
Metabolism
Definitions of metabolism vary
somewhat but are mostly based on the observation that metabolism is the
collective product of all the biochemical reactions taking place within an
organism. All living organisms undergo various meta- bolic activities, which
include the production of new cyto- plasm, the repair of damage, and normal
cell maintenance. The most important activities include respiration, an
energy-releasing process that takes place in all living things; photosynthesis,
an energy-harnessing process in green cells that is, in turn, associated with
energy storage; digestion, the conversion of large or insoluble food mole-
cules to smaller soluble ones; and assimilation, the conver- sion of raw
materials into cytoplasm and other cell substances.
Movement
At the beginning of this chapter,
we mentioned that plants generally don’t move from one place to another
(although their reproductive cells may do so). This does not mean, however,
that plants do not exhibit movement, a universal characteristic of living
things. The leaves of sensitive plants (Mimosa pudica) fold within a few
seconds after being disturbed or subjected to sudden environmental changes, and
the tiny underwater traps of bladderworts (Utricularia) snap shut in less than
one-hundredth of a sec- ond. But most plant movements, when compared with those
of animals, are slow and imperceptible and are mostly related to growth
phenomena. They become obvi- ous only when demonstrated experimentally or when
shown by time-lapse photography. Time-lapse photo- graphy often reveals many
types and directions of motion, particularly in young organs. Movement is not
confined to the organism as a whole but occurs down to the cellular level. For
example, the cytoplasm of living cells constantly flows like a river within
cells; this streaming motion is called cyclosis, or cytoplasmic streaming.
Cyclosis usually appears to run clockwise or counterclockwise within the
boundaries of each cell, but movement may actually be in various directions.
Complexity of Organization
The cells of living organisms are
composed of large num- bers of molecules (the smallest unit of an element or
com- pound retaining its own identity). Typically there are more than 1
trillion molecules in a single cell. The molecules are not simply mixed, like
the ingredients of a cake or the concrete in a sidewalk, but are organized into
compart- ments, membranes, and other structures within cells and tissues. Even
the most complex nonliving object has only a tiny fraction of the types of
molecules of the simplest living organism. Furthermore, the arrangements of
these molecules in living organisms are highly structured and complex. Bacteria,
for example, are considered to have the simplest cells known, yet each cell
contains a mini- mum of 600 different kinds of protein as well as hundreds of
other substances, with each component having a spe- cific place or being a part
of a specific structure within the cell. When flowering plants and other larger
living objects are examined, the complexity of organization is over- whelming,
and the number of molecule types can run into the millions.
Reference :
Stern-jansky-bidlack introductory
plant biology ninth edition. The Nature of Life. The McGraw-Hill companies.2003
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