The organization of the body's cells into tissues, organs, and organ systems creates a division of labor among those cells that allows the organism to maintain homeostasis. Specialization and interdependence are two of the remarkable attributes of living things. Appreciating these characteristics is an important step in understanding the nature of living things.

Cellular Communication Cells in a large organism communicate by means of chemical signals that are passed from one cell to another. These cellular signals can speed up or slow down the activities of the cells that receive them and can even cause a cell to change what it is doing in a most dramatic way.

Certain cells, including those in the heart and liver, form connections, or cellular junctions, to neighboring cells. Some of these junctions, like those in Figure 7–25, hold cells together firmly. Others allow small molecules carrying chemical messages or signals to pass directly from one cell to the next. To respond to one of these chemical signals, a cell must have a receptor to which the signaling molecule can bind. Some receptors are on the cell membrane; receptors for other types of signals are inside the cytoplasm. The chemical signals sent by various types of cells can cause important changes in cellular activity. For example, the electrical signal that causes heart muscle cells to contract begins in a region of the muscle known as the pacemaker. Ions carry that electrical signal from cell to cell through a special connection known as a gap junction, enabling millions of heart muscle cells to contract as one in a single heartbeat. Other junctions hold the cells together, so the force of contraction does not tear the muscle tissue. Both types of junctions are essential for the heart to pump blood effectively.

FIGURE 7–25 Cellular Junctions Some junctions, like the one seen in brown in this micrograph of capillary cells in the gas bladder of a toadfish, hold cells together in tight formations (TEM 21,600X).

7.4 Assessment

Review Key Concepts

1. • Review What is homeostasis?

   • Explain What do unicellular organisms do to maintain homeostasis?

   • Apply Concepts The contractile vacuole is an organelle found in paramecia, a group of unicellular organisms. Contractile vacuoles pump out fresh water that accumulates in the organisms by osmosis. Explain how this is an example of the way paramecia maintain homeostasis.

2. • Review What is cellular specialization?

   • Explain How do cellular junctions and receptors help an organism maintain homeostasis?

   • Predict Using what you know about the ways muscles move, predict which organelles would be most common in muscle cells.

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3. Use an area in your life—such as school, sports, or extracurricular activities—to construct an analogy that explains why specialization and communication are necessary for you to function well.

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