2. What are living things like? What does it mean to be alive? Any living thing is called an organism. They have different behaviors and food needs. In spite of these differences, all organisms have similar traits. Living Things 2 2

3. Living Things Are Organized A cell is the smallest unit of an organism that carries on the functions of life. Cells take in materials from their surroundings and use them in complex ways. Each cell has an orderly structure and contains hereditary material. Living Things 2 2 All organisms are either unicellular (made of one cell) or multicellular (made of many cells).

4. Living Things Respond Living things interact with their surroundings. Anything that causes some change in an organism is a stimulus (plural, stimuli). Living Things 2 2 The reaction to a stimulus is a response. Often that response results in movement.

5. Living Things Respond Living things also respond to stimuli that occur inside them. Living Things 2 2 An organism’s ability to keep the proper conditions inside no matter what is going on outside the organism is called homeostasis. Homeostasis is a trait of all living things.

6. Living Things Use Energy The energy used by most organisms comes either directly or indirectly from the Sun. Living Things 2 2 Plants and some other organisms use the Sun’s energy and the raw materials carbon dioxide and water to make food.

7. Living Things Use Energy You and most other organisms can’t use the energy of sunlight directly. Instead, you take in and use food as a source of energy. Living Things 2 2

8. Living Things Use Energy Some bacteria live at the bottom of the oceans and in other areas where sunlight cannot reach. Living Things 2 2 They can’t use the Sun’s energy to produce food. Instead, the bacteria use energy stored in some chemical compounds and the raw material carbon dioxide to make food.

9. Living Things Grow and Develop Growth of many-celled organisms is mostly due to an increase in the number of cells. Living Things 2 2 In one-celled organisms, growth is due to an increase in the size of the cell. Organisms change as they grow. The changes that take place during the life of an organism (life span) are called development.

10. Living Things Reproduce Living things reproduce themselves in many different ways. Living Things 2 2 Cats, dogs, alligators, fish, birds, bees, and trees eventually reproduce. Beetles, like most insects, reproduce by laying eggs.

11. Living Things Reproduce Without reproduction, living things would not exist to replace those individuals that die. Living Things 2 2 An individual cat can live its entire life without reproducing. However, if cats never reproduced, all cats soon would disappear.

12. What do living things need? To survive, all living things need a place to live and raw materials. Living Things 2 2 The raw materials that they require and the exact place where they live can vary.

13. A Place to Live The environment limits where organisms can live. Living Things 2 2 Not many kinds of organisms can live in extremely hot or extremely cold environments. Most cannot live at the bottom of the ocean or on the tops of mountains. An organism’s surroundings must provide for all of its needs.

14. Raw Materials Water is important for all living things. Living Things 2 2 Plants and animals take in and give off large amounts of water each day. Most organisms are composed of more than 50 percent water. You are made of 60 to 70 percent water.

15. Raw Materials Living things are made up of substance such as proteins, fats, and sugars. Living Things 2 2 Animals take in most of these substances from the foods they eat. Plants and some bacteria make them using raw materials from their surroundings.

16. Raw Materials When organisms die, substances in their bodies are broken down and released into the soil or air. Living Things 2 2 The substances can then be used again by other living organisms.

17. 2 2 Section Check Question 1 What is the smallest unit in a cat that carries on the functions of life? Answer The cell is the smallest unit. All organisms are composed of cells. Cells take in materials from their surroundings and use them in complex ways.

18. 2 2 Section Check Question 2 Pulling your hand back when you accidentally touch a hot pan on the stove is an example of what characteristic of living things? A. growth and development B. organization C. reproduction D. response to stimuli

19. 2 2 Section Check Answer The correct answer is D. The hot pan is an example of a stimulus. Living things can respond to both external and internal stimuli.

20. 2 2 Section Check Question 3 Give an example of two raw materials that you need to survive. Answer Some possible answers include: water, food, and oxygen. Some bacteria and plants can make food from raw materials in their surroundings.