Characteristics of Life:
How do scientists determine if something is alive
or not? This sounds like an easy task, but it is harder than you think. Based on
observations and experimentation by many scientists the characteristics of
living organisms have been devised.
Living things are
If you analyze how a living organism is put
together you will find many levels of complexity. However, all of this
complexity is actually put together in a very organized manner. The cell is the
most basic unit of life. As the cell theory states, all living things are made
up of cells. Some organisms spend their entire life as just a single cell while
others are made up of trillions of cells working together to make the
When we look at the cell we can see that it is made
up of various organized levels.
Cell > organelles (small parts with various jobs
inside the cell) > Macromolecules (large organic molecules such as lipids and
proteins) > Atoms (basic unit from which matter is made) > Subatomic
particles (electrons/ protons/ and neutrons)
When we look larger than the cell we see
organization as well.
Cell < tissue (many cells working together) <
organ (many tissues working together) < organ system (many organ working
together) < multi-cellular organism
So no matter how you look at it, living things are
organized at all levels. Organization is a requirement (characteristic) of life.
Living things acquire
energy and materials.
All living things must obtain energy and materials
(basic building blocks) in order to survive, grow, and reproduce. This is a
requirement (characteristic) of life. The way in which a living organism
acquires energy and materials may be different from organism to organism. Living
organisms can be split into three major groups depending on how they obtain
energy and materials. Let’s look at how energy is obtained by each group
obtain energy from the sun. They use the sun’s light energy to power
photosynthesis reactions. Photosynthesis is a type of chemical reaction that
will use sun light energy to combine carbon dioxide and water to make a sugar.
The sugar can then be broken down as an energy source by the organism’s cells.
Plants fall into this category of being a producer. So do algae and a special
group of bacteria known as the cyanobacteria.
(heterotroph) cannot make their own
energy source using sunlight. They must consume (ingest) energy. They obtain
energy by eating producers or other consumers. I am sure you can guess that all
animals fall into this category, so do protozoa (we will learn more about this
type of organism later).
obtain energy by absorbing it from dead matter. When plants die or animals die
or make waste products, the energy tied up in the dead matter can be absorbed by
the decomposer group. Bacteria and fungi fall into this category.
So as you can see the flow of energy is from the
sun to the producers to the consumers and ultimately to the decomposers. Energy,
however, cannot recycle back to the sun or the producer group. Recall that
energy must be able to power metabolic reactions. Heat is released at each step
in the energy transfer and heat cannot be used by living cells to power
metabolic reactions. What this means is that if the sun disappeared all energy
would eventually stop flowing through the ecosystem and ultimately life would
disappear as well.
Now let’s look at how materials move
through the ecosystem from group to group. Recall that materials are the basic
building blocks of life. They are the elements we are all made up of such as
carbon, hydrogen, oxygen, phosphorus, and sulfur to name a few. Producers obtain
these materials from the atmosphere and the soil. When consumers eat producers
or other consumers they obtain materials along with energy. Since everything
eventually dies, the decomposers will obtain materials from the dead matter they
are breaking down. Some of the materials go to form the decomposers themselves,
much of the materials are returned to the atmosphere and soil where it can be
used by the producer group again. What this means, is that as long as you have
the decomposers around materials will be able to recycle through the ecosystem.
Thank- you decomposers.
Living things respond to
the environment and maintain homeostasis.
Homeostasis is the
ability to maintain a constant internal environment. Cells must maintain their
internal environment in a very narrow range of variables. Cells need a certain
amount of water, materials, and energy to survive. They also have strict
temperature requirements for survival as well. All cells including those of a
multi-cellular organism must maintain their homeostatic state in order to
survive. Due to this fact living things will respond to their environment and
make changes as needed in response to the environment in order to maintain
homeostasis. For example, to maintain its body temperature a reptile will move
toward an environment with proper temperature for survival. If it is cold it
will move toward the sun, if it is too warm it will hide in the shade. Our body
cells can deal with temperature regulation in a much more efficient manner. When
we are cold our cells speed up metabolism to generate more heat and when we are
too hot we sweat. As our sweat evaporates off of our skin this helps remove body
heat. In either case the living organism is responding to the environment and
making the necessary changes to help insure homeostasis is maintained.
Living things reproduce
All living organisms contain a molecule known as DNA
(deoxyribonucleic acid) in their cells. DNA is the blueprint for life; it is the
instructions for making all of the proteins in your cells. These proteins help
define you. They determine your hair color, eye color, the types of food you can
digest, etc. In order to make more living organisms DNA must be passed onto your
offspring. All living things must be able to pass their DNA on, what is commonly
called reproduction. We will go into some detail in a later unit on how cells
reproduce themselves. Once a multi-cellular organism has passed DNA onto an
offspring cell, that cell must divide and become a new multi-cellular organism.
In other words the organism must develop. As I am sure you know you came from
the union of an egg cell from your mother and a sperm cell from your father.
That first single cell, known as a zygote, went through many many rounds of cell
division and development to make you. We will also learn in a later unit how the
cell can turn on different segments of the DNA molecule to make proteins
specific to different cell types which is necessary to produce a multi-cellular
Living things adapt to
A characteristic of the DNA molecule itself is that
it can change. Changes in the DNA molecule, known as mutation, can change
the instructions on how a protein is made. Thus changes in the DNA can change to
organism. Some of the changes can be passed onto offspring and over time
organisms and even entire populations can change. Changes in populations over
time can lead to production of new species, something we commonly call
evolution. So since all living things contain DNA and DNA can change so can
living things change over time. Changes in a population that allows the members
to fit in better with their environment are known as adaptations. Evolution
works to keep organisms adapted to their environment.
The above described criteria are the
characteristics of life that scientist use. Practice using them to explain why
certain things like your lab table are not alive while other things like
bacteria are considered alive.
With the abundance of different kinds of living organisms
how can we organize them all to study them further?
The science of classification. Taxonomy helps organize
living things into groups based on common characteristics. The broadest most
diverse group is known as the Domain level while the smallest most unique group
is known as the species level. In between these two levels are various groups.
Going from largest to smallest the taxonomic groups are called: Domain >
Kingdom > Phylum (division) > Class > Order > Family > Genus >
With regard to this course we will limit our discussion
to the Domain and the Kingdom level of living organisms. If you go on and take
further biology courses you will be introduced to the remaining levels of
Organisms can be grouped into three major categories,
known as Domains. The current domain names are:
Bacteria: These are the majority of the bacteria
that you are familiar with. They are simple single cells with no nucleus
(membrane sac enclosing the DNA).
Archae: These are a special group of
bacteria that are believed to be the most ancient of the bacteria. They are also
simple single cells with no nucleus. You will learn more about this group if you
take a microbiology class.
Eukarya: The rest of the living organisms belong to
this grouping. All plants, animals, fungi, algae, and protozoa fit into this
domain. Eukarya cells are complex and have a nucleus.
Let’s look closer at the various Kingdoms. There are
currently six accepted Kingdoms, we will study five. They best way to study them
is to chart them out. We will name the kingdom, describe the member’s
organization (single cell, colonial, or multi-cellular), name the member’s
mode of obtaining nutrients (photosynthesis, ingest, or absorb), and give a
common example of a member from the kingdom.
Kingdom Monera also
called Prokaryotae (note the specific name for this kingdom is still under
discussion). These cells belong to the domain bacteria.
Single cell or colonial. Nutrition through absorption or photosynthesis.
Example would be the bacteria and cyanobacteria. (We will not discuss the
kingdom for the domain archaea due to the fact that much debate is still going
on over their taxonomy.)
Single cell or colonial. Nutrition through ingestion, absorption, or
photosynthesis. Example would be protozoa and algae.
also called Mycetae. Members can be single cells, colonial, or multi-cellular.
Nutrition through absorption. Examples would be molds and mushrooms and yeast.
Multi-cellular ingestors. Example would be animals (hydra to elephants)
Multicellular photosynthetic. Example would be plants (moss to trees)
The Scientific Method is a step-by-step procedure
to help you find an answer to a problem. It is not limited to science. We use
the scientific method every day without knowing it. Just finding this classroom
may have required you to use many steps in the scientific method.
Let’s look closer at the steps in the scientific
method. It is best to describe each step and then give an example. We will look
at a both a scientific and an everyday life example.
You wouldn’t need to use the scientific method if you didn’t have some sort
of problem or question that you needed an answer to. As our somewhat scientific
example, you can wonder if vitamin C helps reduce the duration of a cold. For an
everyday example, you can wonder why your car is making a funny noise under to
hood. Both of these are valid questions that you may want the answer to.
This is a fancy way of saying what is already known about the problem at hand.
As a scientist you might refer to old scientific journal articles about vitamin
C, you might go to library or use the internet to find important background
information on vitamin C itself or its use; you can even talk to other scientist
to find out what they know about the problem. We would do basically the same
thing with our everyday example of our car making a noise. We can consult the
owner’s manual, check the library or internet for information on the make and
model of car, or even talk to family or friends about the problem.
Once you have collected some accumulated scientific data you can make an
educated guess as to what the answer to your problem may be. This is known as
forming a hypothesis. You don’t know if you are right or not with you guess,
but you are basing it on the background information you collected. Let’s say
that after reviewing the literature on vitamin C you hypothesize that increased
vitamin C levels during a cold should shorten the duration of the cold. With our
car example you hypothesize that tightening the bla bla bla should make the
noise go away. You could be wrong, but you are basing your guess on solid
background information. Time will tell if you hypothesis is supported or not.
Now we must test out your hypothesis to see if it can be supported. We do that
through observation and experimentation. With our car example we would tighten
the bla bla bla and listen to see if the noise under the car hood goes away.
Things are a little more complicated with our scientific hypothesis. In order
for a scientific experiment to be valid and believable we must have both an
experimental group along with a control group.
The group receiving what you are testing. In our case, receiving vitamin C. We
can only test one thing at time to have a valid experiment. We could not test
vitamin C and vitamin E at the same time and draw conclusion about how vitamin C
alone works. We can test various levels of the vitamin C though; these levels
are known as variables. For example, we can have three experimental (variable)
groups. Group 1 receives 50 mg vitamin C per day, group 2 receives 100 mg
vitamin C per day, and group 3 receives 1000 mg vitamin C per day. All groups
are receiving the same thing, vitamin C; the only difference is the amount that
they are receiving.
The control group does not receive what you are testing. The role of the control
group is to act as a comparison to the experimental group. You need to know what
would happen if the substance being tested was not administered. Our control
group would receive 0 mg of vitamin C.
The data section of the scientific method includes the results of your
experimentation or observation. Results mean facts only, not why the results
occurred or what they mean. You can present your results in many forms. You can
write them up in a paragraph, put them into a table, or present
them in a graph form. Again results do not try to explain what happed,
they just present the facts. With our car example your results might be written
out. After you tightened the bla bla bla the car no longer made a noise
underneath the hood. With our vitamin C experiment we might like to see the
results in a table. You must clearly title your table so everyone knows what the
information in the table represents.
cold symptoms for individual on vitamin C therapy
As you can clearly see, we are presenting results
only. What these results mean, will be explained later.
The conclusion section of the scientific method attempts to explain what your
results mean. With our car example due to the fact that the noise under the hood
went away when we tightened the bla bla bla we can safely conclude that the bla
bla bla must have needed tightening. There are several conclusions we can make
when we look at our scientific example. Compared to the control, all vitamin C
takers reported a lesser duration of cold symptoms. So it does appear that
vitamin C is working to somehow limit the duration of the cold. We can also
conclude that 1000mg appears to be the best dosage since individuals in that
group only had cold symptoms for 3 days (the shortest time period of symptoms).
What happens if the hypothesis is not supported?
With the above examples the original hypothesis was supported. That is not
always the case in science. For example all vitamin C groups could have had
their cold symptoms for the same duration as the control group. When the
hypothesis is not supported that doesn’t automatically mean you did something
wrong. You should run your experimental tests again and if the same results
appear, you need to rethink and modify your original hypothesis. Maybe you would
need more or less vitamin C to observe lessened cold symptom duration or maybe
vitamin C has no effect on cold symptoms at all. There will be plenty of times
in class that what you thought was going to happen with an experiment didn’t
happen. This is the way science works, hypotheses are not always supported. In
fact finding out what doesn't work is just as important as finding out what does
to a scientist.
A few words on terminology.
Recall that the hypothesis is basically an educated guess. Even if you
hypothesis is supported by your experiment you don’t change your hypothesis
and now call it a theory. The term theory is reserved for hypotheses that
have been supported over a long period of time by many people and have never
been disproved. Very few problems in science have been solved and are called
theories. An example of a theory is the cell theory. It sounds simple enough,
all living things are made up of cells, however when it was first hypothesized
it was a radical way of viewing the world. To date all living things are made up
of cells so we can safely conclude that this most likely is true and call it a