What is Life?
Introduction
In this chapter we will learn how living organisms change as they become
better adapted to their environment. Over billions of years, these changes
have produced a large number of different kinds of organisms. It has been
estimated that there are currently 30 million species of organisms living on
earth.
Living organisms are comprised of the same chemical elements
that make up nonliving things. They obey the same laws of physics and chemistry as
nonliving objects. We can better understand what distinguishes living from
nonliving by examining characteristics that all living organisms have in common.
Some of these characteristics are discussed below. The student is
encouraged to add to or modify these criteria so that they can develop a better
understanding of life.
Characteristics Common to All Living Organisms
Living things are composed of cells
Small organisms such as bacteria and many protists are composed of a single
cell. Larger organisms are composed of many cells. These organisms are
multicellular.
The list below shows increasing levels of biological organization.
atoms
molecules
macromolecules
organelles
cells
¬ The smallest unit of life is the cell.
tissues
organs
organ systems
individual organism
population
community
ecosystem
The first three items on this list (atoms, molecules,
and macromolecules) will be discussed further in the chapter on
chemistry.
Cells are considered to be the smallest structure that is
alive. They are often too small to see without the aid of a microscope. All
living organisms are composed of cells. The smallest organisms are composed of a
single cell; larger organisms are composed of more than one cell.
Similar kinds of cells may be arranged together to form a tissue.
Tissues have specific properties and functions. For example muscle tissue is
composed of muscle cells. It functions to move body components.
Two or more tissues that form a structure with a specific function is an organ.
For example, the heart is an organ formed from muscle tissue, nervous
tissue, connective tissue, and epithelial tissue. It functions to pump
blood.
An organ system consists of two or more organs which perform a
specific task. Some organ systems are: the integumentary, nervous, sensory,
endocrine, skeletal, muscular, circulatory, immune, digestive, respiratory,
excretory, and reproductive systems.
A
population is an interbreeding group of organisms (the same species) that occupies a
particular area.
Two or more populations form a community.
The word community refers to the organisms. The word ecosystem
refers to the organisms and the nonliving environment.
Organisms need nutrients and energy for their activities, growth,
reproduction, and maintenance.
Chemical reactions are needed to store and release energy and to synthesize compounds
needed by the organism. The word metabolism refers to the chemical reactions
that occur within a cell.
Energy cannot be created or destroyed, but it can be transformed from one
form to another. For example, photosynthetic organisms such as plants are able
to transform radiant (solar) energy to chemical energy.
Plants, some algae, and some bacteria obtain
their energy from light. The energy is used to bond molecules of carbon
dioxide together to form sugar (glucose). Energy is stored in glucose. When
a cell needs energy, chemical reactions within the cell are able to release
this stored energy for it's needs. The energy stored in glucose can be used
to form other chemicals. The new chemicals now contain the energy. Whenever
energy is transferred from one chemical to another, a little is lost as
heat. Animals that eat plants obtain their energy from the chemicals in the
plants. As with plants, chemical reactions within the animal cells release
the energy stored in their food and make it available for the cell.
Organisms must sense, interact with, and respond to their environment
because they need nutrients and energy from the environment.
Organisms need to protect themselves; other organisms would like their energy.
The internal environment of a cell fluctuates less than the external
environment. For example the temperature of some organisms remains fairly
constant even though the outside temperature fluctuates. The maintenance of
constant internal conditions is called homeostasis.
Living things contain DNA
The genetic instructions of all living organisms is contained in molecules
of deoxyribonucleic acid.
Living things reproduce
Within a multicellular organism cells reproduce to enable growth and tissue
repair.
Individual organisms reproduce.
Asexual Reproduction
The advantage of asexual reproduction is that it can produce
large numbers of offspring very rapidly and it does not require a mate.
Asexual
reproduction, however, produces offspring which are identical to the parent. Populations
in which all of the individuals are identical are more likely to go extinct if the
environment fluctuates. Moreover, these populations are less likely to
change over time in response to environmental change.
Sexual Reproduction
Sexual reproduction requires two parents and thus promotes
genetic variation. Populations which show variability are more likely to survive
environmental fluctuations because there is an increased likelihood that at least some
individuals are going to be able to survive due to their being better adapted.
Populations of living things evolve
Evolution involves changes in the genetic composition of a
population. This changes the characteristics of organisms.
A mutation is a change in the genetic instructions (DNA) of an individual.
The change is usually harmful but occasionally it is beneficial.
Any beneficial mutations that occur are likely to spread within a population because
individuals that possess the mutations will have higher reproductive output and they will
reproduce the mutation. Beneficial mutations are therefore likely to result in
evolutionary change.
Evolutionary change has led to diversity among organisms. To date,
approximately 1.8 million different species of organisms have been
identified. Biologists estimate that there are between 10 and 200 million
species on earth.
|