Cells Teaming Up

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Tissues and organs work together in related groups called systems. Different organs perform specific jobs within a system. All those jobs add up to a more general service the system performs for the organism.

Scientists classify the systems of the human body in various ways. Eleven organ systems of the human body are pictured below. You can blend different systems together. Add each system by moving its slider to the right.


All the systems working together make a whole multicellular organism—in this case a human being. Other multicellular organisms (from mice to mushrooms to maple trees) have different cells, tissues, organs, and systems. Some are more different than others. Mice have hearts and lungs roughly similar to ours. But maple trees have very different kinds of cells, and their organs include things like roots and leaves.

An organism has all the parts it needs to live. A single human cell, or a piece of tissue, or an organ, or an organ system, cannot normally live by itself for long. Those parts depend on each other. Alone, they would die. But the whole multicellular organism, with all of its parts working together, can perform all the basic biological functions it needs to survive.

So can a single-celled organism, like a yeast cell or an amoeba. A yeast cell in a blob of bread dough does not depend on its neighboring yeast cells the way the cells of a human body depend on each other. True, a yeast cell can’t do all the interesting things a human can do. But the yeast cell’s needs are relatively simple, and it is able to cope on its own.

Single-celled living and multicellular living are two strategies for solving the basic problems of how to stay alive. Both strategies accomplish the same four basic necessities:

  1. storing and following instructions on how to function
  2. getting energy
  3. getting building materials
  4. disposing of waste

All the cells in a multicellular organism need to be able to meet these four basic needs at a cellular level. Their cooperative organization makes it easier for each cell to meet those necessities. The systems of the organism work together to create an internal environment where nutrients are delivered to each cell for building materials and energy, and where waste is carried away so it doesn’t build up in and around cells.

The digestive system breaks down food and absorbs nutrients (and disposes of the unusable portion of food as feces). The respiratory system brings in oxygen for the chemical reactions that get energy out of nutrients in the cells, and disposes of waste carbon dioxide. The cardiovascular system picks up oxygen from the respiratory system and nutrients from the digestive system and delivers them to cells throughout the body. And the cardiovascular system also picks up waste carbon dioxide from cells and delivers it to the respiratory system for exhalation, and picks up waste products from processed nutrients and delivers them to the urinary system for elimination from the body. All of this activity and more is supported by the other systems in the body.


Turn and Talk

A single yeast cell and a whole human being are both considered organisms. But a single human cell is not considered an organism. Why is a yeast cell an organism while a human cell is not?

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