Which “Characteristic of Life” (section 1.1 in the textbook) do you think is the most important AND why?
The Characteristics of Life
LEARNING OUTCOMES
Upon completion of this section, you should be able to
1. Identify the basic characteristics of life.
2. Distinguish between the levels of biological organization.
3. Recognize the importance of adaptation and evolution to life.
Life. Everywhere we look, from the deepest trenches of the oceans to the geysers of Yellowstone, we find that planet Earth is teeming with life. Without
life, our planet would be nothing but a barren rock hurtling through space. The variety of life on Earth is staggering, recent estimates suggest that
there are around 8.7 million species on the planet, and humans are a part of it. The variety of living organisms ranges in size from bacteria, much too
small to be seen by the naked eye, all the way up to giant sequoia trees that can reach heights of 100 meters (m) or more (Fig. 1.1).
[D]Figure1.1Life on planet Earth.If aliens ever visit our corner of the universe, they will be amazed at the diversity of life on our planet. Yet
despite its diversity, all life shares some common characteristics.
The diversity of life seems overwhelming, and yet all living organisms have certain characteristics in common. Taken together, these characteristics
give us insight into the nature of life and help us distinguish living organisms from nonliving things. All life generally shares the following
characteristics (1) is organized, (2) requires materials and energy, (3) has the ability to reproduce and develop, (4) responds to stimuli, (5) is
homeostatic, and (6) has the capacity to adapt to their environment. In the next sections we explore each of these characteristics in more detail.
Life is Organized
Life can be organized in a hierarchy of levels (Fig. 1.2). In trees, humans, and all other organisms, atoms join together to form molecules, such as DNA
molecules that occur within cells. A cell is Page 4the smallest unit of life, and some organisms are single-celled. In multicellular organisms, a cell
is the smallest structural and functional unit. For example, a human nerve cell is responsible for conducting electrical impulses to other nerve cells.
A tissue is a group of similar cells that perform a particular function. Nervous tissue is composed of millions of nerve cells that transmit signals to
all parts of the body. Several tissues then join together to form an organ. The main organ that receives signals from nerves is the brain. Organs then
work together to form an organ system. In the nervous system, the brain sends messages to the spinal cord, which in turn sends them to body parts
through spinal nerves. Complex organisms such as trees and humans are a collection of organ systems.
The levels of biological organization extend beyond the individual. All the members of one species (a group of interbreeding organisms) in a particular
area belong to a population. A tropical grassland may have a population of zebras, acacia trees, and humans, for example. The interacting populations of
the grasslands make up a community. The community of populations interacts with the physical environment to form an ecosystem. Finally, all the Earth’s
ecosystems collectively make up the biosphere.
Figure1.2Levels of biological organization.Life is connected from the atomic level to the biosphere. While the cell is the basic unit of life, it
comprises molecules and atoms. The sum of all life on the planet is called the biosphere.
Life Requires Materials and Energy
Living organisms need an outside source of materials and energy to maintain their organization and carry on life’s other activities. Plants, such as
trees, use carbon dioxide, water, and solar energy to make their own food. Humans and other animals acquire materials and energy by eating food.
The food we eat provides nutrients, which cells use as building blocks or for energy—the capacity to do work. Cells use energy from nutrients to carry
out everyday activities. Some nutrients are broken down completely by chemical reactions to provide the necessary energy to carry out other reactions,
such as building proteins. The term metabolism is used to describe all of the chemical reactions that occur in a cell. Cells need energy to perform
their metabolic functions, and it takes work to maintain the organization of a cell as well as an organism.
The ultimate source of energy for nearly all life on Earth is the sun. Plants and certain other organisms are able to capture solar energy and carry on
photosynthesis, a process that transforms solar energy into the chemical energy of organic nutrient molecules. All life on Earth acquires energy by
metabolizing nutrient molecules made by photosynthesizers. This applies even to plants themselves.
The energy and chemical flow between organisms also defines how an ecosystem functions (Fig. 1.3). Within an ecosystem, chemical cycling and energy flow
begin when producers, such as grasses, take in solar energy and inorganic nutrients to produce food (organic nutrients) by photosynthesis. Chemical
cycling (aqua arrows in Fig. 1.3) occurs as chemicals move from one population to another in a food chain, until death and decomposition allow inorganic
nutrients to be returned to the producers once again. Energy (red arrows), on the other hand, flows from the sun through plants and the other members of
the food chain as they feed on one another. The energy gradually dissipates and returns to the atmosphere as heat. Because energy does not cycle,
ecosystems could not stay in existence without solar energy and the ability of photosynthetic organisms to absorb it.
Figure 1.3Chemical Cycling and Energy Flow in an Ecosystem. In an ecosystem, chemical cycling (aqua arrows) and energy flow (red arrows) begin when
plants use solar energy and inorganic nutrients to produce their own food. Chemicals and energy are passed from one population to another in a food
chain. Eventually, energy dissipates as heat. With the death and decomposition of organisms, chemicals are returned to living plants once more.
Energy flow and nutrient cycling in an ecosystem climate largely determine not only where different ecosystems are found in the biosphere but also what
communities are found in the ecosystem. For example, deserts exist in areas of minimal rain, while forests require much rain. The two most biologically
diverse ecosystems—tropical rain forests and coral reefs—occur where solar energy is most abundant. One example of an ecosystem in North America is the
grasslands, which are inhabited by populations of rabbits, hawks, and various types of grasses, among many others. These populations interact with each
other by forming food chains in which one population feeds on another. For example, rabbits feed on grasses, while hawks feed on rabbits and other
organisms.
Living Organisms Reproduce and Develop
Life comes only from life. All forms of life have the capability of reproduction, or to make another organism like itself. Bacteria, protists, and other
single-celled organisms simply split in two. In most multicellular organisms, the reproductive process begins with the pairing of a sperm from one
partner and an egg from the other partner. The union of sperm and egg (Fig 1.4), followed by Page 5many cell divisions, results in an immature stage,
which proceeds through stages of development, or change, to become an adult.
Figure 1.4Growth and development define life. Following the (a) fertilization of an egg cell by a sperm cell (b) humans grow and develop. All life
exhibits the characteristics of growth and development.
When living organisms reproduce, their genes, or genetic instructions, are passed on to the next generation. Random combinations of sperm and egg, each
of which contains a unique collection of genes, ensure that the offspring has new and different characteristics. An embryo develops into a whale, a
yellow daffodil, or a human because of the specific set of genes it inherits from its parents. In all organisms, the genes are made of longDNA
(deoxyribonucleic acid) molecules. DNA provides the blueprint, or instructions, for the organization and metabolism of the particular organism. All
cells in a multicellular organism contain the same set of genes, but only certain genes are turned on in each type of specialized cell. You may notice
that not all members of a species, including humans, are exactly the same, and that there are obvious differences between species. These differences are
the result of mutations, or inheritable changes in the genetic information. Mutation provides an important source of variation in the genetic
information. However, not all mutations are bad—the observable differences in eye and hair color are examples of mutations.
Living Organisms Respond to Stimuli
Organisms respond to external stimuli, often by moving toward or away from a stimulus, such as the smell of food. Right now, your eyes and ears are
receiving stimuli from the external environment. Movement in animals, including humans, is dependent upon their nervous and musculoskeletal systems.
Other living organisms use a variety of mechanisms in order to move. The leaves of plants track the passage of the sun during the day, and when a
houseplant is placed near a window, hormones help its stem bend to face the sun.
The movement of an organism, whether self-directed or in response to a stimulus, constitutes a large part of its behavior. Behavior is largely directed
toward minimizing injury, acquiring food, and reproducing.
Living Organisms Are Homeostatic
Homeostasis means “staying the same.” Actually, the internal environment of an organism stays relatively constant. For example, human body temperature
will show only a slight fluctuation throughout the day. Also, the body’s ability to maintain a normal internal temperature is somewhat dependent on the
external temperature—we will die if the external temperature becomes too hot or cold.
Organisms have intricate feedback and control mechanisms that do not require any conscious activity. These mechanisms may be controlled by one or more
tissues themselves or by the nervous system. When you are studying and forget to eat lunch, your liver releases stored sugar to keep blood sugar levels
within normal limits. Many organisms depend on behavior to regulate their internal environment. In animals, these behaviors are controlled by the
nervous system and are usually not consciously controlled. For example, a lizard may raise its internal temperature by basking in the sun, or cool down
by moving into the shade.
Organisms Have the Capacity to Adapt
Throughout the nearly 4 billion years that life has been on Earth, the environment has constantly been changing. For example, glaciers that once covered
much of the world’s surface 10,000–15,000 years ago have since receded, and many areas that were once covered by ice are now habitable. On a smaller
scale, a hurricane or fire could drastically change the landscape in an area quite rapidly.
As the environment changes, some individuals of a species (a group of organisms that can successfully interbreed and produce fertile offspring) may
possess certain features that make them better suited to the new environment. We call such features adaptations. For example, consider a hawk, which can
catch and eat a rabbit. A hawk, like other birds, can fly because it has hollow bones, which is an adaptation. Similarly, its strong feet can take the
shock of a landing after a hunting dive, and its sharp claws can grab and hold onto prey. As is presented in the Scientific Inquiry feature, “Adapting
to Life at High Elevations,” humans also exhibit adaptations to their environment.
Individuals of a species that are better adapted to their environment tend to live longer and produce more offspring than other individuals. This
differential reproductive success, called natural selection, results in changes in the characteristics of a population (all the members of a species
within a particular area) through time. That is, adaptations that result in higher reproductive success tend to increase in frequency in a population
from one generation to the next. This change in the frequency of traits in populations and species is called evolution.
Evolution explains both the unity and diversity of life. As stated at the beginning of this chapter, all organisms share the same basic characteristics
of life because we all share a common ancestor—the first cell or cells—that arose nearly 4 billion years ago. During the past 4 billion years, the
Earth’s environment has changed drastically, and the diversity of life has been shaped by the evolutionary responses of organisms to these changes.
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Humans, like all other organisms, have an evolutionary history. This means not only that we share common ancestors with other animals but also that over
time we demonstrate adaptations to changing environmental conditions. One study of populations living in the high-elevation mountains of Tibet (Fig. 1A)
demonstrates how the processes of evolution and adaptation influence humans.
Figure 1AHumans’ adaptations to their environments.Humans have adaptations that allow them to live at high altitudes, such as these individuals in
Tibet.
Normally, if a person moves to a higher altitude, his or her body responds by making more hemoglobin, the component of blood that carries oxygen, which
thickens the blood. For minor elevation changes, this does not present much of a problem. But for people who live at extreme elevations (some people in
the Himalayas can live at elevations of over 13,000 ft, or close to 4,000 m), this can present a number of health problems, including chronic mountain
sickness, a disease that affects people who live at high altitudes for extended periods of time. The problem is that, as the amount of hemoglobin
increases, the blood thickens and becomes more viscous. This can cause elevated blood pressure, or hypertension, and an increase in the formation of
blood clots, both of which have negative physiological effects.
Because high hemoglobin levels would be a detriment to people at high elevations, it makes sense that natural selection would favor individuals who
produced less hemoglobin at high elevations. Such is the case with the Tibetans in this study. Researchers have identified an allele of a gene that
reduces hemoglobin production at high elevations. Comparisons between Tibetans at both high and low elevations strongly suggest that selection has
played a role in the prevalence of the high-elevation allele.
The gene is EPSA1, located on chromosome 2 of humans. EPSA1 produces a transcription factor, which basically regulates which genes are turned on and off
in the body, a process called gene expression. The transcription factor produced by EPSA1 has a number of functions in the body. For example, in
addition to controlling the amount of hemoglobin in the blood, this transcription factor also regulates other genes that direct how the body uses
oxygen.
When the researchers examined the variations in EPSA1 in the Tibetan population, they discovered that their version greatly reduces the production of
hemoglobin. Therefore, the Tibetan population has lower hemoglobin levels than people living at lower altitudes, allowing these individuals to escape
the consequences of thick blood.
How long did it take for the original population to adapt to living at higher elevations? Initially, the comparison of variations in these genes between
high-elevation and low-elevation Tibetan populations suggested that the event may have occurred over a 3,000-year period. But researchers were skeptical
of that data since it represented a relatively rapid rate of evolutionary change. Additional studies of genetic databases yielded an interesting
finding—the EPSA1 gene in Tibetans was identical to a similar gene found in an ancient group of humans called the Denisovans (see Chapter 32).
Scientists now believe that the EPSA1 gene entered the Tibetan population around 40,000 years ago, either through interbreeding between early Tibetans
and Denisovans, or from one of the immediate ancestors of this lost group of early humans.
Questions to Consider
1. What other environments do you think could be studied to look for examples of human adaptation?
2. In addition to hemoglobin levels, do you think that people at high elevations may exhibit other adaptations?
1. List the common characteristics of all living organisms.
2. Trace the organization of life from the cell to the biosphere.
3. Explain how adaptations relate to evolutionary change.
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