Chapter 2: Basic Biological Principles
Lesson 1: Characteristics of Life
Is this picture a colorful work of abstract art, or is it something else? Imagine shrinking down to
a tiny size, so small you could enter a blood vessel. The picture shows what you might see rushing
toward you. Do you know what the red objects are? If you guessed red blood cells, you are right. What
about the knobby green objects? Watch out for these! They are viruses that have invaded the blood.
When you read this book, you will take an exciting journey into the realm of blood cells, viruses, and just
about everything else that is related to life. You will learn how your own body works, what makes living
things unique, and what you and viruses have in common.
Lesson Objectives



Explain the characteristics of life common to all organisms.
Identify the parts that all cells have in common.
Compare and contrast prokaryotic and eukaryotic cells.
Vocabulary
• biology
 cell
• development
• eukaryote
• eukaryotic cell
• growth
• homeostasis
 homeostatic mechanism
 metabolism
• prokaryote
• prokaryotic cell
• reproduction
• virus
Introduction
In this lesson, you will learn about one particular branch of science, the branch called biology.
Biology is the science of life. Do you know what life is? Can you define it? Do you know the
characteristics of life common to all organisms? Can you describe the characteristics of life that are
shared by prokaryotes and eukaryotes?
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CHARACTERISTICS OF LIFE
Look at the duck decoy in Figure 2.1. It looks very similar to a real duck. Of course, real ducks are
living things. What about the decoy duck? It looks like a duck, but it is actually made of wood. The decoy
duck doesn’t have all the characteristics of a living thing. What characteristics set the real ducks apart
from the decoy duck? What are the characteristics of living things?
Figure 2.1: This duck decoy looks like it
To be classified as a living thing, an object must have all of the following characteristics:
1. It responds to the environment.
2. It grows and develops.
3. It goes through reproduction: produces offspring (asexually or sexually).
4. It maintains homeostasis.
5. It obtains and uses energy to power all life processes through metabolism
6. It consists of cell(s) that are organized.
7. It has the ability to change over time (evolution).
8. It as a universal genetic code (DNA).
Response to the Environment
All living things detect changes in their environment and respond to them. What happens if you
step on a rock? Nothing; the rock doesn’t respond because it isn’t alive. The rock is only a stimulus that
elicits a response from you. But what if you think you are stepping on a rock and actually step on a turtle
shell? The turtle is likely to respond by moving—it may even snap at you! In this case you are the
stimulus that the turtle responds to.
Some of an organism’s responses are instinctive behaviors and others are learned behaviors.
Instinctive behavior is inborn behavior that does not depend on experience, it happens on instinct, or
automatically. Some examples of this are; frightened animals will either flee the situation or stand and
fight, or newly hatched sea turtles on a beach automatically move toward the ocean. Learned behaviors
depend on experience or observing other organisms. For example, you can teach a dog to roll over by
giving it a treat or children whining until they get what they want.
Growth and Development
All living things grow and develop. Growth is the physical change in an organism’s size and
weight, while development is the gene-directed process by which an organism matures. Growth occurs
through cell division and enlargement of cells, development occurs though cell differentiation (cell
becoming different from each other; i.e. some cells become blood cells and others become bone cells)
and specialization (adaptation of a cell to perform a specific function). For example, a plant seed may
look like a lifeless pebble, but under the right conditions it will grow and develop into a plant. Animals
also grow and develop. Look at the animals in Figure 2.2. How will the tadpoles change as they grow and
develop into adult frogs? This process of change is called metamorphosis.
Figure 2.2: Tadpoles go through many changes to become adult frogs.
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Reproduction
All living things are capable of reproduction. Reproduction is the process by which living things
give rise to offspring and transmit hereditary information (coded in DNA, deoxyribonucleic acid). In
sexually reproducing organisms hereditary information recombines from two organisms of the same
species, producing genetically different offspring. In asexually reproducing organisms hereditary
information does not come from different organisms to the offspring, the offspring is genetically
identical to the parent.
Reproduction is not essential for the survival of an individual organism but it is essential in order
for a species to continue and survive as a whole. Reproducing may be as simple as a single cell dividing
to form two daughter cells. Generally, however, it is much more complicated. Nonetheless, whether a
living thing is a huge whale or a microscopic bacterium, it is capable of reproduction.
Maintaining Homeostasis
All living things are able to maintain a more-or-less constant internal environment. They keep
things relatively stable on the inside regardless of the conditions around them. The process of
maintaining a stable internal environment is called homeostasis, without homeostasis maintenance an
organism could die. In human beings, homeostatic mechanisms, regulatory mechanisms that contribute
to maintaining a state of equilibrium help us maintain a stable internal body temperature
(thermoregulation), regulate our body’s water content (water regulation), and uptake of nutrients by
our cells. For example, if you go outside when the air temperature is below freezing, your body doesn’t
freeze. Instead, by shivering and other means, it maintains a stable internal temperature.
Homeostasis, or keeping things constant, is not just a characteristic of living things. It also
applies to nature as a whole. Consider the concentration of oxygen in Earth’s atmosphere. Oxygen
makes up 21% of the atmosphere, and this concentration is fairly constant. What keeps the
concentration of oxygen constant? The answer is living things. Most living things need oxygen to survive,
and when they breathe, they remove oxygen from the atmosphere. On the other hand, many living
things, including plants, give off oxygen when they make food, and this adds oxygen to the atmosphere.
The concentration of oxygen in the atmosphere is maintained mainly by the balance between these two
processes.
A
quick
overview
of
homeostasis
can
be
viewed
at
http://www.youtube.com/watch?v=DFyt7FJn-UM
Obtaining and Using Energy/ Metabolism
All living things have an ability to acquire materials for energy to power life processes; such as,
cellular repair, growth, and movement. Metabolic processes are the sum of all chemical reactions that
take in and transform energy and materials from an organism’s environment. In other words,
metabolism is the sum of the chemical reactions through which living things break down or build up
materials. Most of us call this eating! We have to be able to convert our food; a form of energy, to
chemicals our cells can use through metabolism. Some organisms like plants, algae, and some
microorganisms are autotrophs (self-feeders, producers). The autotrophs we are most familiar with are
the green plants that use photosynthesis to make their own "food"; energy. Animals and fungi are
heterotrophs (consumers) and capture their food for energy in a variety of ways.
The ability to acquire and use energy is extremely important. Keep in mind without a constant
input of usable energy, organisms would quickly become "disorganized" and die.
Cells and Organization
All forms of life are built of cells. A cell is the basic unit of the structure and function of living
things capable of performing all the activities of life. Living things may appear very different from one
another on the outside, but their cells are very similar. Compare the human cells in Figure 2.3 and onion
cells in Figure 2.4. How are they similar? You should view the animation titled Parts of an Animal Cell at:
http://www.youtube.com/watch?v=Fzj6TRnXmps.
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Figure 2.3: Human Cells. If you looked
at human cells under a microscope, this
is what you might see.
Figure 2.4: Onion Cells. If you looked at
onion cells under a microscope, this is what
you might see.
Some organisms are made up of only one cell and are called unicellular; others are composed of
two or more cells and are called multicellular. Complex multicellular organisms have levels of
organization as shown in Figure 2.5 below. Organism is the highest level of organization, made up of
organ systems (groups of specialized parts that carry out a certain function in an organism). Organ
systems are made up of organs; structures that carry out specialized jobs within an organ system.
Organs are made up of tissues; groups of cells that have similar abilities and allow an organ to function.
Tissues are made up of cells; these are covered by a membrane, contain genetic information needed for
replication, and carry out all cell functions. Within cells we find organelles, tiny structures that carry out
functions necessary for a cell to stay alive. Organelles contain biological molecules, chemical compounds
that provide physical structure, fuel movement, organize energy use, and other cellular functions.
Biological molecules are made up of atoms; which are the simplest particle of an element.
Figure 2.5: Complex levels of organization in multicellular organisms.
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Ability to Change Over Time (Evolution)
Evolution is a change in the characteristics of populations of living things over time; which
creates genetic variations enabling a population to adapt to changes in their environments and survive.
Evolution occurs by a process called natural selection. In natural selection, some populations of living
things produce more offspring than others, so they pass more genes to the next generation than others
do. Over many generations, this can lead to major changes in the characteristics of the population of
living things. Evolution explains how populations living things are changing today and how modern living
things have descended from ancient life forms that no longer exist on Earth. As populations of living
things evolve, they generally become better suited for their environment. This is because they evolve
adaptations. An adaptation is a characteristic that helps a population of living thing survive and
reproduce in a given environment. Look at the mole in Figure 2.6. It has tentacles around its nose that it
uses to sense things by touch. The mole lives underground in the soil where it is always dark. However,
by using its touch organ, it can detect even tiny food items in the soil in total darkness. The touch organ
is an adaptation because it helps the mole survive in its dark, underground environment. A cartoon
depicting
the
evolution
of
Homer
(Simpson)
can
be
viewed
at:
http://www.youtube.com/watch?v=faRlFsYmkeY.
Figure 2.6: This mole uses its star-shaped nose organ to sense food by touch in the dark.
Universal Genetic Code (DNA)
Living things are based on a universal genetic code, with very small differences between
different living things. All organisms store the information they need to carry out life processes and
reproduce in this universal genetic code contained in a DNA (deoxyribonucleic acid) molecule. The
genetic code information stored in DNA is copied and passed from parent to offspring during
reproduction.
Lesson Summary




Living things are distinguished from nonliving things on the basis of eight characteristics:
response to the environment, growth and development, reproduction, homeostasis, obtains
and use energy (metabolism), cells and organization, ability to change over time (evolution), and
universal genetic code.
All living things detect changes in their environment and respond to them.
Growth is the physical change in an organism’s size and weight, while development is the genedirected process by which an organism matures.
Reproduction is the process by which living things give rise to offspring and transmit hereditary
information (coded in DNA, deoxyribonucleic acid). In sexually reproducing organisms hereditary
information recombines from two organisms of the same species, producing genetically
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




different offspring. In asexually reproducing organisms hereditary information does not come
from different organisms to the offspring, the offspring is genetically identical to the parent.
Maintaining stable internal conditions through homeostatic mechanisms is a matter of life or
death for living organisms.
Metabolic processes are the sum of all chemical reactions that take in and transform energy and
materials from an organism’s environment.
A cell is the basic unit of the structure and function of living things capable of performing all the
activities of life.
Evolution is a change in the characteristics of populations of living things over time; which
creates genetic variations enabling a population to adapt to changes in their environments and
survive.
Living things are based on a universal genetic code.
References/ Multimedia Resources
Opening image copyright Kirsty Pargeter, 2010. http://www.shutterstock.com. Used under license from
Shutterstock.com.
"Homeostasis." YouTube. YouTube, 07 Jan. 2008. Web. Summer 2013.
<http://www.youtube.com/watch?v=DFyt7FJn-UM>
"Parts of an Animal Cell." YouTube. YouTube, 24 Nov. 2009. Web. Summer 2013.
<http://www.youtube.com/watch?v=Fzj6TRnXmps>
"The Simpsons - Homer Evolution." YouTube. YouTube, 23 May 2008. Web. Summer 2013.
<http://www.youtube.com/watch?v=faRlFsYmkeY>
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