1.2: Biology: The Study of Life

Lesson Objectives

  • List the characteristics of all living things.
  • State three unifying principles of biology.
  • Describe how living things interact.


Chapter 1.2 workbook pages

Get the workbook here: https://guesthollow.com/store/free-high-school-biology-workbook/


  • biodiversity
    • the variety of life and its processes; including the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur
  • biology
    • science of life, study of life
  • biome
    • group of similar ecosystems with the same general type of physical environment
  • biosphere
    • part of Earth where all life exists, including land, water, and air
  • cell
    • basic unit of structure and function of living things
  • cell theory
    • theory that all living things are made up of cells, all life functions occur within cells, and all cells come from already existing cells
  • community
    • all of the populations of different species that live in the same area
  • competition
    • relationship between living things that depend on the same resources in the same place and at the same time
  • ecosystem
    • all the living things in a given area together with the physical factors of the nonliving environment
  • gene theory
    • theory that the characteristics of living things are controlled by genes that are passed from parents to offspring
  • homeostasis
    • process of maintaining a stable environment inside a cell or an entire organism
  • organ
    • structure composed of more than one type of tissue that performs a particular function
  • organ system
    • group of organs that work together to do a certain job
  • organism
    • an individual living thing
  • population
    • all the organisms of the same species that live in the same area
  • reproduction
    • process by which living things give rise to offspring
  • symbiosis
    • close relationship between organisms of different species in which at least one of the organisms benefits from the relationship
  • tissue
    • group of cells of the same kind that perform a particular function in an organism


In this book, you will learn about one particular branch of science, the branch called biology. Biology is the science of life. OK. That answered the chapter’s question. We’re done.

Just kidding. You knew it couldn’t be THAT short and simple, right?     icon_wink

What exactly is life? What makes something alive? Watch http://vimeo.com/15407847 to begin your journey into the study of life.

Characteristics of Life

Look at the duck decoy in Figure below. 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. *Sheesh, did the person who wrote this even LOOK at the picture? I’d say the duck below is probably made from plastic.  Ok, back to reading about the wood, plastic, whatever duck:

 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?

This duck decoy looks like it’s alive. It even fools real ducks. Why isn’t it a living thing?

To be classified as a living thing, an object must have all six of the following characteristics:

  1. It responds to the environment.
  2. It grows and develops.
  3. It produces offspring. – Offspring is just a fancy word for babies or little copies of itself or something like that.
  4. It maintains homeostasis. – Don’t worry, I’ll explain what that means in a minute.
  5. It has complex chemistry.
  6. It consists of cells.

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. “But wait!” you say! “The rock DID move when I stepped on it. Isn’t that responding?” Well, the rock didn’t move by itself. You’re getting into the realm of physics now and not biology: an object tends to stay at rest unless acted upon by an outside force (your foot). Get it? The rock didn’t make a decision to get out of the way. It can’t do that. It’s a rock. We all know rocks aren’t alive. Now, what if you think you are stepping on a rock and actually step on a turtle? The turtle is likely to respond by moving—it may even snap at you!

“Dude! Get off my shell!” That’s what the response means in turtle-speak. The turtle can respond because it’s alive. If you stepped on a dead turtle, well, it wouldn’t move. Therefore, you would hopefully come to the conclusion that this particular turtle isn’t alive, as a response to the environment is one of the characteristics of living things.

Growth and Development

All living things grow and develop. 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 below. How will the tadpoles change as they grow and develop into adult frogs?

frog life cycle
Tadpoles go through many changes to become adult frogs.


All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. 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.

Keeping Things Constant

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.


Human beings, for example, maintain a stable internal body temperature. 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.

Complex Chemistry

All living things—even the simplest life forms—have complex chemistry. Living things consist of large, complex molecules, and they also undergo many complicated chemical changes to stay alive. Complex chemistry is needed to carry out all the functions of life.


All forms of life are built of cells. A cell is the basic unit of the structure and function of living things. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in Figure below. How are they similar?

Human Cells (left). Onion Cells (right). If you looked at human and onion cells under a microscope, this is what you might see.

 When you see how complex just a SINGLE cell is, you can better understand how something like that just couldn’t happen by chance.

Unifying Principles of Biology

Three unifying principles form the basis of biology. Whether biologists are interested in the life of bacteria, or how humans could live on the moon, they base their overall understanding of biology on these principles:

  1. cell theory
  2. gene theory
  3. homeostasis

The Cell Theory

According to the cell theory, all living things are made up of cells, and living cells always come from other living cells. In fact, each living thing begins life as a single cell. Some living things, such as bacteria, remain single-celled. Other living things, including plants and animals, grow and develop into many cells. Your own body is made up of an amazing 100 trillion cells! But even you—like all other living things—began life as a single cell. More of the cell theory will be discussed in a later chapter.

The Gene Theory

The gene theory is the idea that the characteristics of living things are controlled by genes, which are passed from parents to their offspring. Genes are located on larger structures, called chromosomes, that are found inside every cell. Chromosomes, in turn, contain large molecules known as DNA (deoxyribonucleic acid). Molecules of DNA are encoded with instructions that tell cells what to do. To see how this happens, click on the animation titled Journey into DNA at the link below. http://www.pbs.org/wgbh/nova/genome/dna.html


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.

Amoeba Sisters: Homeostasis

Interdependence of Living Things

All living things depend on their environment to supply them with what they need, including food, water, and shelter. Their environment consists of physical factors—such as soil, air, and temperature—and also of other organisms. An organism is an individual living thing. Many living things interact with other organisms in their environment. In fact, they may need other organisms in order to survive. For example, living things that cannot make their own food must eat other organisms for food. Other interactions between living things include symbiosis and competition.



Symbiosis is a close relationship between organisms of different species in which at least one of the organisms benefits. The other organism may also benefit, or it may be unaffected or harmed by the relationship. Figure below shows an example of symbiosis. The birds in the picture are able to pick out food from the fur of the deer. The deer won’t eat the birds. In fact, the deer knowingly lets the birds rest on it. What, if anything, do you think the deer gets out of the relationship?

deer and birds
A flock of starlings looks out, before searching for parasites on a red deer stag.

Read this article about symbiosis:

Symbiosis: Creatures that Need Each Others

by Dave Miller, Ph.D.

The very idea that a lizard came from a fish, or a human came from an ape-like ancestor is silly. Even millions of years of gradual changes would not cause such a thing. So, even on the face of it, the theory of evolution is untrue. In addition, there are many proofs that show evolution is false. One of those proofs is symbiosis (sim-bee-OH-sus). Symbiosis refers to the relationship between two or more plants or animals of different species that depend on each other to survive. Each provides a necessary service to the other. For example, hummingbirds fly to flowers to get the nectar they need to live, but in the process they collect pollen and take it to other plants so those plants can be pollinated and also live.

Evolutionists try hard (without success) to explain the existence of symbiosis in plant and animal species. They say that species that depend on each other for survival must have “co-evolved.” Darwin talked about his belief that “a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by the continued preservation of individuals presenting mutual and slightly favourable deviations of structure.” But this thinking is silly. The “continued preservation of individuals” is what is at stake. Not only could flowers and hummingbirds not have evolved in the first place (since the Law of Biogenesis forbids such), they could not have evolved together. There would be no time for them to do so. If the plant was not pollinated by the hummingbird, the plant would soon die! If the plant’s flower did not provide the hummingbird with nectar, the bird would soon die of starvation—and that would be the end of those species! No time for “evolution”!

In fact, all species of insects, animals, and plants whose lives depend on other creatures would have only a matter of days, weeks, or months at the most before they would die. No organism could have survived for millions of years as it waited for “gradual changes” to occur so that the symbiosis relationship could “slowly become” workable. Both parties in a symbiotic relationship would have to be in existence together, each fully operational and doing its job, for both to survive.

Consider, for example, the Oceanic Whitetip Shark—a competitive, fearless predator that does not avoid trouble in favor of an easier meal. Famed oceanographer Jacques Cousteau described the Oceanic Whitetip as “the most dangerous of all sharks.” This shark feeds on bony fishes including lancetfish, oarfish, barracuda, jacks, dolphinfish, marlin, tuna, mackerels—and even garbage. If other species of sharks are encountered, the Whitetip becomes aggressive and dominates over them. It will bite into schools of bony fishes, and swim through schools of feeding tuna with wide-open jaws, scooping up the tuna as they unknowingly swim into the shark’s mouth.

Of course, consuming all those smaller fish causes bits of food and parasites to collect around their teeth. Food particles can produce disease or a build-up of matter that can hinder eating. So God created this ferocious predator to allow Pilotfish to swim into its mouth to clean away food particles from between its teeth! God created the Pilotfish to act as a biological toothbrush! The shark is relieved of painful parasites and the Pilotfish gains protection by getting to hang around a fierce predator. Their symbiotic relationship is proof of God!

Another amazing example of symbiosis involving completely different species of sea life is the “Watchman Goby” and the shrimp. The shrimp digs and maintains a burrow in the sand for itself and the fish to live in. The constantly moving ocean water continually shifts the sand around and would fill up the burrow if not for the relentless efforts of the shrimp. In the meantime, the Goby keeps watch for danger, and actually warns the almost blind shrimp of possible predators. The shrimp uses its antennae to keep in contact with the fish, and the Goby flicks the shrimp with its tail when alarmed by a possible threat. Both benefit from this amazing symbiotic relationship: the shrimp gets a warning of approaching danger, and the goby gets a safe home and a place to lay its eggs.

Symbiosis is proof of God—and proof that evolution is a myth. God designed hundreds of thousands of His living organisms to speak to humans—if we will listen—that the Creator exists. Indeed, “There is no speech nor language where their voice is not heard” (Psalm 19:3).

 Copyright © 2010 Apologetics Press, Inc. All rights reserved.


Want to read some more Christian articles about symbiosis? Check out these urls for some great proofs that God created our world:

Ant Farmers and their Aphids


Yahweh, Yuccas, and a Young Earth


This video shows cleaner fish and cleaner shrimp cleaning other marine animals much like the symbiosis discussed above between the shark and pilot fish. Notice the trust the shrimp shows as it climbs right into the eel’s mouth.



Competition is a relationship between living things that depend on the same resources. The resources may be food, water, or anything else they both need. Competition occurs whenever they both try to get the same resources in the same place and at the same time. You know, like when you and your sister both try to get the leftover hamburger from last night’s dinner AT THE SAME TIME… There’s going to be a bit of competition. 

Levels of Organization

The living world can be organized into different levels. For example, many individual organisms can be organized into the following levels:

  • Cell: basic unit of all living things
  • Tissue: a white thing to blow your nose on…OOPS, I mean…a group of cells of the same kind
  • Organ: something you play music on – JUST KIDDING (at least in this context)! It’s actually a structure composed of one or more types of tissues.
  • Organ system: group of organs that work together to do a certain job
  • Organism: individual living thing that may be made up of one or more organ systems – like, you know, an animal, plant, creature, or single-celled life-form

Examples of these levels of organization are shown in Figure below.

An individual mouse is made up of several organ systems
An individual mouse is made up of several organ systems. The system shown here is the digestive system, which breaks down food to a form that cells can use. One of the organs of the digestive system is the stomach. The stomach, in turn, consists of different types of tissues. Each type of tissue is made up of cells of the same type.

There are also levels of organization above the individual organism. These levels are illustrated in Figure below.

  • Organisms of the same species that live in the same area make up a population. For example, all of the goldfish living in the same area make up a goldfish population.
  • All of the populations that live in the same area make up a community. The community that includes the goldfish population also includes the populations of other fish, coral and other organisms.
  • An ecosystem consists of all the living things in a given area, together with the nonliving environment. The nonliving environment includes water, sunlight, and other physical factors.
  • A group of similar ecosystems with the same general type of physical environment is called a biome.
  • The biosphere is the part of Earth where all life exists, including all the land, water, and air where living things can be found. The biosphere consists of many different biomes.
This picture shows the levels of organization in nature, from the individual organism to the biosphere.

Diversity of Life

Life on Earth is very diverse. The diversity of living things is called biodiversity.


A measure of Earth’s biodiversity is the number of different species of organisms that live on Earth. At least 10 million different species live on Earth today. They are commonly grouped into six different kingdoms. Examples of organisms within each kingdom are shown in Figure below.

Diversity of life from Archaebacteria to Plants and Animals

 Lesson Summary

  • Living things are distinguished from nonliving things on the basis of six characteristics: response to the environment, growth and development, reproduction, homeostasis, complex chemistry, and cells.
  • Three underlying principles form the basis of biology. They are cell theory, gene theory, and homeostasis.
  • Many living things interact with one another in some way. The interactions are often necessary for their survival.
  • The great diversity of life on Earth today is the result of God’s creation. He created everything to work together perfectly.

Lesson Review Questions


1. List the six characteristics of all living things.

2. Identify three unifying principles of modern biology.

3. Outline the levels of organization of a complex, multicellular organism such as a mouse, starting with the cell.

4. What is homeostasis? Give an example.

Apply Concepts

5. Describe examples of ways that you depend on other living things.

6. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you determine if it is a living thing?

Think Critically

7. Compare and contrast symbiosis and competition.

8. Explain how a population differs from a community.

Points to Consider

In this lesson, you learned that living things have complex chemistry.

  • Do you know which chemicals make up living things?
  • All living things need energy to carry out the processes of life. Where do you think this energy comes from? For example, where do you get the energy you need to get through your day?

Opening image copyright Kirsty Pargeter, 2010. http://www.shutterstock.com. Used under license from Shutterstock.com.

Symbiosis: Creatures that Need Each Others

from Discovery Magazine 9/1/2010:

Apologetics Press
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Phone (334) 272-8558


Previous: Science and the Natural World

Next: Matter and Organic Compounds

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Changes/edits were made to the original ck12 biology text by Guest Hollow. Changes are not endorsed by ck12 in any way.

6 thoughts on “1.2: Biology: The Study of Life

  1. Loved this lesson! 🙂

    1. So glad you enjoyed it! Thank you so much for taking the time to leave a comment. We love feedback! <3

  2. This lesson is so amazing.

    1. So glad you enjoyed it! 🙂 We love your feedback! <3

  3. you guys make science fun!

    1. Awww that makes me really happy to read your comment! Thank you for posting! 🙂

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