Biology is the science of life. Biologists
study all living things, or organisms, on
earth. They study all the plants and
animals that you see around you every
day. They even study the tiny
microscopic organisms that you can't
see.
Some biologists study the way living
things interact with the environment.
This means that sometimes biologists
think about the non-living things on
earth too.
Can you tell which of the things in the
picture are alive and which are not?
You probably know that plants and
animals are living things and that rocks
and water are not. Do you know how
a biologist tells the difference?
Characteristics of Life
You are a living thing. Can you list the
characteristics that a biologist would
use to determine why you are a living
thing?
Is it because you are made up of
chemical elements?
No. All things, living and non-living,
are made of chemical elements.
Is it because you breathe oxygen?
No. Many microorganisms can live
without oxygen, and many organisms
do not breathe.
Is it because you eat food and drink
water and move around from place to
place?
No. Most plants do not eat food and
they cannot move around the way
Biologists have found that all living
things have these characteristics in
common:
Click on the image to enlarge it.
6) All living things maintain stable
internal conditions.
7) All living things respond to stimuli.
1) All living things are made of one or
more cells.
2) All living things grow and develop.
3) All living things use energy.
4) All living things can reproduce.
5) All living things contain molecules
that contain the blueprint for life called
DNA.
For something to be considered living,
all these characteristics must be true.
For example, a car uses energy, but it
cannot reproduce. A wooden desk is
made of dead plant cells, but it does
not use energy. A machine might
maintain stable conditions, would
probably use energy and might even
respond to stimuli; but it is not made
of cells and does not contain DNA.
Cells
All living things are made of one or
more cells.
Inside that membrane is a watery
solution of chemicals and DNA. Some
cells are very simple, and others are
more complex.
Some organisms are made of only one
cell. They are called unicellular
organisms. All bacteria and many
simple organisms called protists are
unicellular.
A cell is a mass of living material
enclosed in an oily membrane.
Growth and Development
All living things grow and
develop.
Cells grow by using energy to
produce more of the materials
from which they are composed.
There is a limit to how big a cell
can grow. Eventually, most cells
reproduce, and eventually all
cells die. In unicellular organisms
such as bacteria, the
characteristic of growth is only
related to increasing in size.
All plants and animals are multicellular.
They are made of many cells. Your
body is made of trillions of very small
cells. (A trillion has twelve zeros and
is a thousand billion. A billion is a
thousand million.)
Development is all the changes that take
place during the life of an organism, including
growth.
For unicellular organisms, development
involves reproductive changes and aging
Complex, multicellular organisms such
as plants and animals grow by
producing more cells.
However, complex organisms such
as human beings are not simply a
mass of several trillion identical cells.
Obviously, something else is going on.
Click on the picture below to see a
larger version. As you can see from the
picture, development in plants and
animals involves much more than just
getting bigger.
Development in complex organisms
like humans involves a process called
differentiation. Differentiation is the
production of specialized cells.
Click on the image to enlarge it.
Energy
All living things use energy.
Energy is the ability to do work. Every
cell in your body needs energy to do
work. Cells use energy to enable you
to move, to think, to breathe, and to
digest food. You obtain energy for
your cells from the food you eat.
Without energy, an organism will die.
Some organisms are autotrophic. That
means they are able to make their own
food. All plants and some bacteria are
autotrophic. Plants use the sun's
energy to make their own food.
Other organisms are heterotrophic.
That means they cannot make their
own food. Heterotrophic organisms
must obtain energy from other living
things. You are a heterotrophic
organism.
Ultimately, all energy in living things on
Reproduction
All living things can reproduce.
All organisms come from other
organisms of the same kind. Seeds from
plants develop into new plants. A single
bacterium given plenty of food can
produce millions of new, identical
bacteria in a few days. Human babies
are born from human adults. Cells
reproduce to form new cells, allowing
multicellular organisms to grow.
Reproduction may be asexual or sexual.
DNA
All living things contain the blueprint for
life in a molecule called
deoxyribonucleic acid or DNA for short.
Just how did those two tiny cells from
your parents make the amazing
organism that is you? Each one carried
half of a blueprint on a DNA molecule
for how to make a human being. All
living cells contain this blueprint.
The blueprint for life is passed from
parents to offspring on DNA. Bacteria
carry DNA that holds instructions for
building new bacteria.
Bacteria and the cells in your body
reproduce asexually. That means
that one parent cell produces new
cells exactly like themselves in every
way. An identical new cell or
organism is called a clone. Many
plants and a few animals are also
able to reproduce asexually.
Sexual reproduction involves two
cells coming together to make a new
cell or organism. You were produced
from one cell from your father and
one cell from your mother. Organisms
produced by sexual reproduction are
unique, unlike any other organism in
the world. The only exception is
That same DNA carries instructions
for all the functions of a bacteria cell.
It tells the cell how to use energy and
how to reproduce. It helps the cell
maintain stable conditions, and it
helps the cell respond to stimuli.
Human DNA carries the same kinds
of information. Your blueprint
determined what you look like and
how your body works. The reason
that people in the same family tend to
resemble each other is because their
DNA is very similar. Later you will
find that the more DNA two
organisms share, the more similar
they are to each other.
Homeostasis
All living things maintain stable internal
conditions.
The world is an ever changing place.
Human organisms experience day and
night, warmth and cold. Like all living
things, humans must maintain
homeostasis. Homeostasis means that
most conditions inside living things
change very little relative to the
surrounding environment. A bacterial
cell on your skin might be surrounded
by air one minute and surrounded by
water the next. The cell works to keep
conditions inside the same.
Stimulus
All living things respond to stimuli.
When you are warm, your body
produces sweat. When a dog chases
a cat, the cat might run up a tree.
When a bacterial cell encounters a
food source, it produces chemicals to
help break that food down. When a
houseplant senses light coming in a
window, it grows toward the light. All
these things involve an organism doing
something in response to a stimulus.
All living things are capable of
responding to stimuli in both their
Think about human body temperature.
Our bodies are maintained at about 99
degrees F (37 degrees C). When the
body senses that it is hot outside, a
message sent to the brain stimulates
perspiration that helps cool the body.
When it is cold outside, a message
sent to the brain causes shivering.
Shivering is caused by muscle
contractions, which generates heat in
our bodies.
Regulation of body temperature is just
one example of homeostasis. All
organisms have similar events going
on all the time. Much of the energy
used by living things goes toward
Some responses, such as sweating,
involve maintenance of homeostasis.
Other responses involve obtaining
food, avoiding danger, or looking for
mates.
Responding to stimuli, maintaining
homeostasis, acquiring energy, and
being successful in passing on DNA
through reproduction is what "being
a living thing" is all about.
Understanding how organisms do
these things is what studying biology
is all about.
Organization of Living Systems
The cells that make up living things
have internal organization.
The smallest particles of matter are
called atoms. Atoms form bonds to
make larger, more complex bits of
matter called molecules.
Cells use molecules to make working
parts called organelles. A complete
system of organelles working together
to maintain homeostasis makes a living
cell.
In complex organisms, cells come
together to form tissues. Tissues come
together to form organs, and organs
work together in organ systems.
A group of organ systems working
Muscle is one kind of tissue in humans.
The human heart is an organ made of
muscle tissue. The heart is part of the
larger cardiovascular system.
Organisms interact in an organized
way. A group of similar organisms
living in a specific area is called a
population.
Several populations of different kinds of
organisms living in the same area are
called a community. A community within
an environment makes up an
ecosystem. All the ecosystems on earth
make up the biosphere.
Just as a living cell is organized, so is
the living system of the earth's
The Branches of Biology
During this course, you will learn
about the various types of biological
studies. There are many diverse
types of organisms, and within each
organism there are a variety of
different processes in progress.
Although many biologists study only
one particular kind of organism,
different organisms constantly
interact with each other in nature.
As you will see, scientists also
interact with each other regardless
of their field of study.
Microorganisms can also be helpful to
humans. Bacteria are used in genetic
engineering to make drugs like human
insulin to treat diabetes. Bacteria are
also used to help clean up the
environment. Simple fungi are used to
help make bread, beer, and wine, and
are also important recyclers in nature.
Microbiology is the study of organisms
too small to be seen with the naked
eye. This includes bacteria and other
single celled organisms. Microbiologists
also study nonliving things called
viruses.
How is the work of a microbiologist
important? Many of the organisms that
are studied by microbiologists cause
human disease. Any time you have
been sick, it has probably been due to
an infection by a bacteria or virus.
Other microorganisms can also cause
disease. The organisms that cause
diseases such as malaria and certain
Learning how to grow plants for food
is certainly one way botanists help us.
But we use plants for many things,
probably more than you've ever
imagined.
Through the work of microbiologists,
we understand how microorganisms
cause disease and how they can help
us.
Many of the drugs prescribed by
doctors are made from plants. We also
grow plants to produce clothing,
wood, and other fibrous materials.
Plants make the environment more
pleasant, both by looking good and by
cleaning the air.
Botany is the study of plants. Within
the field of botany, a scientist may
study a particular kind of plant or a
At the same time, some plants are
poison to living things. Studies in the
field of botany help us know how to
Mycology is the study of fungi.
Mushrooms and mold are two
examples of fungi. Some mycologists
are also microbiologists, because
certain types of fungi are very tiny.
Other ways botany helps us are less
obvious. The father of genetics,
Gregor Mendel, was a botanist who
studied heredity in pea plants. Much of
our understanding of humans started
with understanding how other
organisms work.
Fungi can also be used as
food. When you have
mushrooms on your pizza you
are eating a fungus. The
antibiotic penicillin is made
from a kind of fungus. Fungi
are used in industry to help
produce food and other
products.
Some fungi form very close
relationships, or symbioses,
with other organisms. A study
of these relationships has
helped in our understanding of
evolution. Almost all that is
known about fungi is because
Fungi can cause diseases in many
plants and animals, including humans.
Fungal diseases in plants are a major
problem for farmers growing food
crops. Fungal diseases in humans can
be very mild, such as athlete's foot, or
very severe or even fatal. The
presence of mold in buildings can
cause serious allergies, and the cost
of cleaning and removing the mold can
be very high.
Zoology is the study of animals. Like botany and
mycology, zoology has many different areas of
study.
Many biologists do not study any particular
organism but rather examine a particular
biological process or system.
Cell Biology and Genetics
A cell biologist studies the activities
that go on inside the individual cells
of all kinds of organisms. The cells
of all living things perform many
fuctions in the same way. That is
because they are all made from
similar blueprints of DNA.
Genetics is the study of heredity
and DNA. The information in a DNA
blueprint is carried in units called
genes.
Genetic engineering is an exciting
new field of biology that you will
Because cells are made and function
from DNA blueprints, the fields of cell
biology and genetics go hand in hand.
Now that we understand the blueprints,
we can better understand the cellular
machinery.
We now can manufacture human insulin,
just the way our bodies make it, by
manipulating DNA. We can even create
The study of cells and genetics has
helped doctors and scientists
understand human health more than
any other kind of study in recent years.
By understanding what is going on in
cells, scientists are exploring the very
core of what goes wrong with bodies
that have diseases such as cancer or
diabetes.
Cell biology and genetics have told us
about ourselves and much, much more.
Ecology and Evolution
Ecologists and evolutionary biologists
look at an even bigger picture. They
also study how organisms interact
with their nonliving environment. They
try to understand where humans and
other organisms fit in the big picture of
life on Earth.
As in the other fields of biology, there
is a lot of overlap in such
understanding.
Ecologists study how organisms
interact with each other and in
environments. Those interactions have
helped shape evolution.
Some ecologists study human and
environmental interactions.
The scientists who study such
interactions are just as vital to our
future as those who study health and
disease. It won't do us much good to
have healthy bodies if we don't have a
Evolution is the process by which life on
earth has changed over time. As life has
changed, so has the environment. Both
fields have helped us understand our place
in nature and our connection to other living
things.
Evolutionary biologists are interested in how
living things have changed over time. They
often work closely with taxonomists, who
are interested in how organisms are related
to each other.
These are just a few of the many branches
of the study of biology. Within each branch
mentioned there are many more detailed
fields of study. The one thing they all have in
The Importance of Biology
Suppose someone tells you that she is
a botanist. You would know that
means she studies plants, but you
might not know what that means to
your everyday life.
If you thought about it for awhile, you
would probably realize that studying
plants might help contribute to farmers
being able to grow better food crops.
Still, that is only one small part of why
biology is important to people.
Pieces of the Puzzle
You might think that biology is a very
broad subject to study, and you would
be right. There is so much diversity
among living things that it may seem
impossible to learn much about them in
one school year.
For a very long time, biology was
studied in bits and pieces. Around 150
years ago, all those bits and pieces
started to come together. Like the
pieces of a jigsaw puzzle, the big
picture of biology became clearer and
clearer as we discovered where the
pieces joined together.
Doctors must study biology to
understand how the human body
works. The knowledge that one doctor
has represents the work of thousands
of biologists studying cells, genes, and
organisms.
The work a doctor does is a kind of
applied science. Applied science is a
science that is used for practical
purposes. Many scientists study what
is called basic science. They study the
fundamental processes of how living
things work. Basic science is the
foundation for applied science.
When scientists learned about DNA
around fifty years ago, the pieces of
the puzzle finally seemed to fit. The
picture of life formed by the puzzle of
biology looks like this:
All living things are made of cells. Cells
of all living things operate in a very
similar way.
All cells contain DNA, which carries
the instructions on how to build cells
and how they operate.
All living things come from other living
things. Reproduction passes along the
information carried on a DNA blueprint.
All living things are related to one
another over a 3.5 billion-year-old tree
of life. There is a unity of life on earth
in the blueprint of DNA.
There is also diversity. The diversity of
life on earth is the result of evolution
acting on the DNA blueprint.
Throughout this course, you will learn
how scientists have worked to put the
fantastic puzzle of life together
through the science of biology.