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1.2: Biology: The Study of Life

Difficulty Level: At Grade Created by: CK-12
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Key Concept

Living things have certain characteristics, such as being composed of cells and being able to respond to the environment, grow, and reproduce. Four basic principles underlying all of biology are cell theory, gene theory, homeostasis, and evolution. The great diversity of life on Earth today is the result of 4 billion years of evolution.


  • CA.9–12.LS.6.a
  • NSES.9–12.C.1.1; NSES.9–12.C.3.1, 2, 3, 4; NSES.9–12.C.4.3; NSES.9–12.D.3.4; NSES.9–12.E.2.3; NSES.9–12.G.1.3; NSES.9–12.G.3.3, 4
  • AAAS.9–12.1.C.1; AAAS.9–12.5.A.2; AAAS.9–12.5.B.3; AAAS.9–12.5.F.1, 3, 6, 7, 8; AAAS.9–12.10.H.3, 4, 5, 6; AAAS.9–12.12.A.1
  • McREL.9–

Lesson Objectives

  • List the characteristics of all living things.
  • State four unifying principles of biology.
  • Describe how living things interact.
  • Explain how life on Earth evolves.

Lesson Vocabulary

  • adaptation: characteristic that helps living things survive and reproduce in a given environment
  • biodiversity: number of different species in an area or in the entire biosphere
  • 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
  • evolution: change in the characteristics of living things over time, the change in species over time
  • 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
  • natural selection: evolutionary process in which some living things produce more offspring than others so the characteristics of organisms change over time
  • 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

Teaching Strategies

Introducing the Lesson

Ask students to name examples of living things. Write their responses on the board. Record the names of at least 10 different organisms. Then challenge students to identify ways all of the living things are alike. Tell the class they will learn six defining characteristics of living things in this lesson.

Using Visuals

Make sure students appreciate Earth’s biodiversity. Have them compare the kingdoms in FlexBook® textbook Figure 1.16 (reproduced below). Ask volunteers to find and share additional pictures of organisms in each kingdom. With the class, brainstorm possible characteristics that were used to divide living things into these kingdoms.

Differentiated Instruction

Students should focus on the sections headed Characteristics of Life and Unifying Principles of Biology. Create cloze sentences based on these two sections by leaving out the boldfaced terms in the relevant sentences. Have students complete the sentences as they read the sections. ELL, LPR


Ask a few volunteers to create a room-sized scale model based on the diagram in FlexBook® textbook Figure 1.17 (reproduced below). The model should use linear distances marked on the floor or a wall of the classroom to represent the time intervals between major events in the evolution of life. The model will help the class appreciate the vastness of geologic time. It will be useful for later chapters as well.

This timeline shows the history of life on Earth. In the entire span of the time, humans are a relatively new addition.

Science Inquiry

Have groups of students look for evidence of life by doing the investigation “Is Yeast Alive” at the link below. They will test dried baker’s yeast for metabolism and growth.

Real-World Connection

Point out that the theory of evolution is widely accepted today by scientists but not by everyone else. Have students ask classmates, friends, and relatives their views about evolution. Ask volunteers to share some of the responses. The responses should remain anonymous. Discuss with the class possible reasons why some people still reject the theory of evolution.

Reinforce and Review

Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.

Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.

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?

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