Chapter 2: Basic Biological Principles
Lesson 2.1: 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
• development
• eukaryote
• eukaryotic cell
• growth
 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?
30
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 consists of cell(s) that are organized.
2. It grows and develops.
3. It obtains and uses energy to power all life processes through metabolism.
4. It goes through reproduction: produces offspring (asexually or sexually).
5. It as a universal genetic code (DNA).
6. It has the ability to change over time (evolution).
7. It maintains homeostasis.
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 (recall this is one part of the cell theory from
chapter 1). Living things may appear very different from one another on the outside, but their cells are
very similar. Compare the human cells in Figure 2.2 and onion cells in Figure 2.3. How are they similar?
You
should
view
the
animation
titled
Parts
of
an
Animal
Cell
at:
http://www.youtube.com/watch?v=Fzj6TRnXmps.
Figure 2.2: Human Cells. If you looked
at human cells under a microscope, this
is what you might see.
Figure 2.3: 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 2.4. 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
31
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 2.4: Complex levels of organization in multicellular organisms.
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 2.5. How will the tadpoles change as they grow and
develop into adult frogs? This process of change is called metamorphosis.
Figure 2.5: Tadpoles go through many changes to become adult frogs.
32
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 for use within their cells. 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"; by converting light energy into
chemical energy. The energy they make is used for cellular respiration. Animals and fungi are
heterotrophs (consumers) and capture their food for energy in a variety of ways. Heterotrophs can
either use cellular respiration or fermentation to get energy from their food, you will learn more about
all of these processes in the chapter called Cells and Cell Processes.
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.
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 (parents) of the
same species, producing genetically different offspring. In asexually reproducing organisms hereditary
information does not come from two different organisms to the offspring, it comes from one organism
(parent). Thus 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.
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. DNA is
made up of four different nucleotides; the order in which the nucleotides are assembled determines the
protein that is produced. This is important because proteins give you and every other organism its
genetic traits that make living things unique. The genetic code information stored in DNA is copied and
passed from parent to offspring during cell reproduction.
Ability to Change Over Time (Evolution)
Evolution occurs by a process called natural selection according to the theory of evolution
developed by Charles Darwin in the mid to late 1800s. 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. The genes that they pass on usually contain a vast variety of characteristics. This variety of
characteristics that get passed from one generation to the next can lead to evolution. Evolution is a
change in the characteristics of populations of living things over time. The changes in population
characteristics arise from genetic mutations which create genetic variations or genetic diversity enabling
a population to adapt to changes in their environments and survive. 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. This
33
ability to adapt and survive is known as an organisms’ fitness. When we talk about fitness in terms of
evolution we are talking about an organisms’ ability to survive and reproduce fertile offspring, hence
evolution has often been referred to as “survival of the fittest”.
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.
Maintaining Homeostasis
All living things are able to maintain a more-or-less stable 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 homeostatic 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 or osmoregulation), and
uptake of nutrients by our cells. Organisms maintain homeostasis through feedback loops that regulate
their physiological and behavioral responses. A feedback loop utilizes three components in order for
homeostatic regulation to occur. The three components are receptors, a control center, and an effector.
The receptors monitor and respond to changes in an organism’s environment. The control center is the
area in the feedback loop that receives information from the receptor. When the control center receives
the information it determines if the variable sent by the receptors needs to monitored and adjustments
made. In humans, the brain is the control center. Finally the effectors receive information from the
control center and correct or made adjustments to maintain homeostasis using either positive or
negative feedback loops. 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. The receptors in your body alert the control center that there is a drop of body
temperature in this example and the effectors response by causing you to shiver, which generates body
heat. We will look at more in-depth at homeostatic feedback loops in the chapter called Cells and Cell
Processes.
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.
34
Lesson Summary








Living things are distinguished from nonliving things on the basis of seven characteristics: cells
and organization, growth and development, obtains and use energy (metabolism), reproduction,
universal genetic code, ability to change over time (evolution), and homeostasis.
A cell is the basic unit of the structure and function of living things capable of performing all the
activities of life.
Growth is the physical change in an organism’s size and weight, while development is the genedirected process by which an organism matures.
Metabolic processes are the sum of all chemical reactions that take in and transform energy and
materials from an organism’s environment.
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.
Living things are based on a universal genetic code.
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.
All living things detect changes in their environment and respond to them. Maintaining stable
internal conditions through homeostatic mechanisms is a matter of life or death for living
organisms.
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>
Textbook resource granted through licensure agreement with the CK-12 Foundation at
www.ck-12.org
CK-12 Foundation
3430 W. Bayshore Rd., Suite 101
Palo Alto, CA 94303
USA
http://www.ck12.org/saythanks
Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available
to Users in accordance with the Creative Commons Attribution/Non-Commercial/Share Alike 3.0
Unported (CC-by-NC-SA) License (http://creativecommons.org/licenses/by-nc-sa/3.0/), as amended and
updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by
this reference. Complete terms can be found at http://www.ck12.org/terms.
35