McDougal Littell Science Cells and Heredity 

Table of Contents
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A multicellular organism is a community of cells.

Cells in a multicellular organism are specialized. The ways in which the cells work together and interact depend on the organism. You can think of the cells of an organism as members of a community. The size and complexity of the community differ from organism to organism.

A sponge is an animal that is fairly simple in its organization. It spends its life attached to the ocean floor, filtering food and other nutrients from the water. Like all animals, the sponge is organized at a cellular level. Different types of cells in its body perform different functions. For example, certain cells take in food, and other cells digest it. However, cells in a sponge are not very highly specialized. A piece broken from a living sponge will actually regenerate itself as new cells replace the lost ones.

In more complex organisms, such as plants and animals, cells are not only specialized but grouped together in tissues. A tissue is a group of similar cells that are organized to do a specific job. If you look at your hand, you will see the top layer of tissue in your skin. Humans have two layers of skin tissue, layered one on top of the other. Together these skin tissues provide protection and support.


In what way is a tissue an organization of cells?

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Levels of Organization

Levels of organization in multicellular organisms include cells, tissues, organs, organ systems, and the organism itself.


Different tissues working together to perform a particular function represent another level of organization, the organ. The eye is an organ that functions with the tarsier's brain to allow sight. A leaf is an organ that provides a plant with energy and materials. It has tissue that brings in water and nutrients, tissue that uses the Sun's energy to make sugar, and tissue that moves sugar to other parts of the plant.


What is the relationship between tissues and organs?

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Different organs and tissues working together form an organ system. An organism may have only a few organ systems. The organ systems of plants include roots, stems, and leaves. Other organisms have many organ systems. Humans have 11 major organ systems, made up of about 40 organs and over 200 types of tissue. The human nervous system, for example, includes the brain, the spinal cord, nerves, and sensory organs, such as the ears and eyes.

An organism itself represents the highest level of organization. It is at this level that we see all the characteristics we associate with life. If an organism is a complex organism—a human, for example—it will consist of trillions of cells grouped into tissues, organs, and organ systems. However, a simple organism, like a sponge, meets its needs with a body made up of only a few types of specialized cells.


What level of organization is an organism? What do we see at this level of organization?


What are some of the limitations of using a model to represent a cell?

  2. 1 Work with a partner to choose a type of cell to model and to determine the types of organelles to include.
  3. 2 Using the poster board as a base, construct the model from available supplies. Make the model as accurate as you can.
  4. 3 Use a marker to label each organelle, and include a description of its function.
  5. 4 Compare your cell model with those made by your classmates.
  • What are some of the limitations of using a model to represent a cell?
  • What are some of the benefits of making a three-dimensional model of a cell?

CHALLENGE Think of something to which you might compare the activities of a cell—perhaps the activities of a factory or a school. Add labels to your model to show how the comparison applies to each of the cell's structures.