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Chapter 1 The Science of Biology
The goal of science is to investigate and understand the natural
world, to explain events in the natural world, and to use those
explanations to make useful predictions.
Science is different from other human works:
• Science deals only with the natural world.
• Scientists collect and organize information in a careful,
orderly way. They look for patterns and connections
among events.
• Scientists propose explanations that can be tested by looking
carefully at evidence.
So science is defined as an organized way of using evidence to
learn about the natural world. Work in science usually follows a
path that includes these steps:
1. Make observations using the senses. The information gathered
is called data. Scientists use data to make inferences. An
inference is a logical explanation based on knowledge
or experience.
2. Suggest one or more hypotheses. A hypothesis is a likely explanation
for a set of observations. Scientists form hypotheses using
knowledge, inference, and informed imagination.
3. Test the hypothesis. Some hypotheses are tested by doing controlled
experiments. Others are tested by gathering more data.
4. Draw valid conclusions from the data. To be valid, a conclusion
must be based on logical analysis of reliable data.
1–2 How Scientists Work
A scientific investigation has several steps. These include:
• asking a question
• forming a hypothesis
• setting up a controlled experiment
• recording and analyzing results
• drawing a conclusion
Summary

1–1 What Is Science?

Whenever possible, a hypothesis should be tested by an
experiment in which only one variable is changed at a time.
All other variables should be kept unchanged, or controlled.
This type of experiment is called a controlled experiment.
• The variable that is changed is called the manipulated
variable.
• A variable that changes in response to the manipulated
variable is called the responding variable.
A key idea in science is that experimental results can be
repeated. As evidence from many studies builds up, a hypothesis
may be so well supported that scientists regard it as a theory. In
science, the word theory applies to a well-tested explanation that
unifies a broad range of observations. Scientists use theories to
predict what will happen in new situations.

1–3 Studying Life
Although all living things may look different, they all have
certain things in common. All living things share the following
characteristics:
1. Living things are made up of cells. A cell is living matter
enclosed in a barrier. The barrier separates the cell from its
surroundings. A cell is the smallest unit of an organism that
can be thought of as alive.
2. Living things reproduce and produce new offspring. In sexual
reproduction, two cells from different parents join to form the
first cell of the new organism. In asexual reproduction, a unicellular
organism divides and forms two new organisms.
3. Living things are based on a universal genetic code. The
directions for inheritance are carried by a molecule called DNA.
4. Living things grow and develop. All living things grow. Many
multicellular organisms go through a process called development
in which cells divide and change in shape and structure.
5. Living things obtain and use materials and energy. Living
things use energy and materials to stay alive, grow, develop,
and reproduce. Metabolism is all the chemical reactions by
which an organism builds up or breaks down materials as it
carries out its life processes.
6. Living things respond to their environment. A stimulus is a
signal to which an organism responds. Organisms detect and
respond to stimuli from their environment.
7. Living things maintain a stable internal environment. The
process by which they do this is called homeostasis.
8. Taken as a group, living things change over time. Evolution
is change over time in living things.
Scientists study life at different levels. These levels include
molecules, cells, organisms, populations of a single kind of
organism, communities of different organisms in an area, and
the biosphere. At all these levels, smaller living systems are
found within larger systems.

1–4 Tools and Procedures
Most scientists use the metric system when collecting data and
performing experiments. The metric system is a decimal system
of measurement. Its units are based on standards and are scaled
on multiples of 10.
A microscope is a device that forms a magnified image of
structures too small to see with the unaided eye. Light microscopes
and electron microscopes are two kinds of microscopes.
• Light microscopes produce magnified images by focusing
visible light rays. A compound light microscope uses two
lenses to form an image as they allow light to pass through
the specimen. Most school microscopes are compound light
microscopes.
• Electron microscopes produce magnified images by
focusing beams of electrons. These microscopes produce
more detailed images than light microscopes. There are two
types of electron microscopes—scanning electron microscopes
(SEMs) and transmission electron microscopes
(TEMs). Scanning electron microscopes scan a narrow
beam of electrons back and forth across the surface of an
object. SEMs produce images of the surfaces of objects.
Transmission electron microscopes shine a beam of electrons
through a thin specimen. TEMs can be used to see structures
inside cells.
Safety rules are important in the biology laboratory. The most
important safety rule is simple: Always follow your teacher’s
instructions and the textbook directions exactly. It’s also important
to thoroughly wash your hands after every scientific activity.