Life’s Classification and Structure

Life's Classification and Structure

Classifying Living Things

What do you think? Read the three statements below and decide

whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you've read this lesson, reread the statements to see if you have changed your mind.

Before

Statement

After

1. All living things are made of cells.

2. A group of organs that work together and perform a function is called a tissue.

3. Living things are classified based on similar characteristics.

Key Concepts

? What are living things?

? What do living things need?

? How are living things classified?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

What are living things?

It might be easy to tell whether a bird, a tree, or a person is alive. But it is harder to tell whether some organisms are even living things. For example, think about moldy bread. Is the bread a living thing? What about the mold on the bread? All living things have six characteristics in common:

? Living things are made of cells.

? Living things are organized.

? Living things grow and develop.

? Living things respond to their environment.

? Living things reproduce.

? Living things use energy.

In the moldy bread example, the bread is not living, but the molds growing on the bread are living things. Mold is a type of fungus. If you looked at the mold using a microscope, you would see that the mold is made of cells. Mold cells respond to their environment by growing and reproducing. The mold cells obtain energy, which they need to grow, from the bread.

3TUDY#OACH

Create a Quiz Create a 5-question quiz about classifying living things. Exchange quizzes with a partner. After taking the quizzes, discuss your answers.

Key Concept Check 1. Identify What are living things?

Reading Essentials

Life's Classification and Structure 97

REVIEW VOCABULARY

macromolecule substance in a cell that forms by joining many small molecules together

Reading Check 2. Name the four macromolecules in cells.

ACADEMIC VOCABULARY

unique (adjective) without an equal, distinctive

Living things are organized.

Marching bands are made up of rows of people playing different instruments. Some rows are made up of people playing flutes, and other rows are filled with drummers. Although marching bands are organized into different rows, all band members work together to play a song.

Like marching bands, living things are organized. Some living things are more complex than others, but all organisms are made of cells. In all cells, macromolecules are organized into different structures that help cells function.

You might recall that cells have four macromolecules-- nucleic acids, lipids, proteins, and carbohydrates. Nucleic acids, such as DNA, store information. Lipids are the main component of cell membranes and provide structure. Some proteins are enzymes, and others provide structure. Carbohydrates are used for energy.

Unicellular Organisms Some living things are unicellular, which means they are made up of only one cell. In fact, most living things on Earth are unicellular organisms. Unicellular organisms are the oldest forms of life.

There are many groups of unicellular organisms, each with unique characteristics. Bacteria, amoebas (uh MEE buhz), and paramecia (per uh MEE see ah) are unicellular organisms. A unicellular organism has everything needed to obtain and use energy, reproduce, and grow.

Some unicellular organisms are tiny and cannot be seen without a microscope. Other unicellular organisms are large. For example, the plasmodial (plaz MOH dee ul) slime mold is a huge cell formed by many cells that joined together and formed one cell.

Multicellular Organisms Soccer teams are made up of many types of players, including goalkeepers, forwards, and fullbacks. Each team member has a specific job, but all the team members work together when playing a game.

Many living things are made of more than one cell and are called multicellular organisms. Like the different types of players on a soccer team, multicellular organisms have different types of cells that carry out specialized functions. One example is the ladybug. A ladybug has cells that form wings and other cells that form eyes.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

98 Life's Classification and Structure

Reading Essentials

Organization in Multicellular Organisms Multicellular organisms have different levels of organization. Groups of cells that work together and perform a specific function are called tissues. Tissues that work together and carry out a specific function are called organs. Organs that work together and perform a specific function are called organ systems. Organ systems work together and perform all the functions an organism needs to survive.

Living things grow, develop, and reproduce.

During their lifetimes, living things grow, or increase in size. For a unicellular organism, the size of its cell increases. For a multicellular organism, the number of its cells increases.

Development Living things also develop, or change, during their lifetimes. For some organisms, it is easy to see the changes that happen as they grow and develop. As shown in the figure below, ladybug larva grow into pupae (PYEW pee; singular, pupa), an intermediate stage, before developing into adults.

Ladybug Larva and Pupa

Reading Check 3. Select Tissues are

that work together and perform a specific function. (Circle the correct answer.)

a. organs b. organ symptoms c. groups of cells

Reading Check 4. Describe How does a multicellular organism grow?

Visual Check

Larva

5. Differentiate What

differences do you see

between the two stages?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Pupa Reading Essentials

Life's Classification and Structure 99

Reading Check 6. Relate What are the two ways reproduction can occur?

Reading Check 7. Explain How do some autotrophs use energy from light?

Reading Check 8. Define What is a stimulus?

Reproduction Once an organism is an adult, it can reproduce asexually or sexually and form new organisms. Unicellular organisms, such as bacteria, reproduce asexually when one cell divides and forms two new organisms. Some multicellular organisms also can reproduce asexually; one parent organism produces offspring when body cells replicate and divide.

Sexual reproduction occurs when the reproductive cells of one or two parent organisms join and form a new organism. Multicellular organisms, such as humans and other mammals, reproduce sexually. Some organisms such as yeast can reproduce both asexually and sexually.

Living things use energy.

All living things need energy to survive. Some organisms are able to convert light energy to chemical energy that is used for many cellular processes. Organisms that convert light energy to usable energy are called autotrophs (AW tuh trohfs).

Many autotrophs use energy from light and convert carbon dioxide and water into carbohydrates, or sugars. Autotrophs use the carbohydrates for energy. Plants and algae are autotrophs.

Other autotrophs are called chemoautotrophs (kee moh AW tuh trohfs). Chemoautotrophs use energy released by chemical reactions of inorganic substances such as sulfur and ammonia. Many chemoautotrophs are bacteria that live in extreme environments such as deep in the ocean or in hot sulfur springs.

Heterotrophs (HE tuh roh trohfs) are organisms that obtain energy from other organisms. Heterotrophs eat autotrophs or other heterotrophs to obtain energy. Animals and fungi are examples of heterotrophs.

Living things respond to stimuli.

All living things sense their environments. When an organism detects a change in its external environment, it will respond to that change. A change in an organism's environment is called a stimulus (STIHM yuh lus; plural, stimuli).

Responding to a stimulus might help an organism protect itself. For example, an octopus responds to a stimulus such as a predator by releasing a black liquid. The black ink hides the octopus while it escapes. In many organisms, nerve cells detect the environment, process the information, and coordinate a response.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

100 Life's Classification and Structure

Reading Essentials

What do living things need?

You just read that all living things need energy to survive. Some organisms obtain energy from food. What else do living things need to survive?

Living things also need water and a place to live. Organisms live in environments specific to their needs where they are protected, can obtain food and water, and can be sheltered.

A Place to Live

Living things are everywhere. Organisms live in the soil, in lakes, and in caves. Some living things live on or in other organisms. For example, bacteria live in your intestines and on other body surfaces. A specific environment where an organism lives is its habitat.

Most organisms can survive in only a few habitats. A land iguana lives in warm, tropical environments and would not survive in cold places such as the Arctic.

Food and Water

Living things also need food and water. Food is used for energy. Water is essential for survival. In Lesson 2, you will read about how water is in all cells and helps them function.

The type of food that an organism eats depends on the habitat in which it lives. Marine iguanas live near the ocean and eat algae. Land iguanas live in hot, dry areas and eat cactus fruits and leaves. The food is processed to obtain energy. Plants and some bacteria use energy from light and produce chemical energy for use in cells.

How are living things classified?

You might have a notebook with different sections. Each section might contain notes from a different class. This organizes information and makes it easy to find notes on different subjects.

Scientists use a classification system to group organisms with similar traits. Classifying living things makes it easier to organize organisms and to recall how they are similar and how they differ.

Make a vertical three-column chart book to organize your notes about living things, their needs, and classification criteria.

LDiveifnoignfiTathioinng

Survival Requirements

ClaCssrifitiecraitaion

9. Consider Why would a

land iguana have difficulty surviving in the Arctic?

Key Concept Check 10. Express What do living things need?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Life's Classification and Structure 101

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