1. Lesson Overview Lesson Overview Homeostasis and Cells Lesson Overview 7.4 Homeostasis and Cells

2. Lesson Overview Lesson Overview Homeostasis and Cells THINK ABOUT IT The diversity of life is so great that you might have to remind yourself that all living things are composed of cells, use the same basic chemistry, and even contain the same kinds of organelles. This does not mean that all living things are the same. Differences arise from the ways in which cells are specialized and the ways in which cells associate with one another to form multicellular organisms.

3. Lesson Overview Lesson Overview Homeostasis and Cells The Cell as an Organism In terms of their numbers, unicellular organisms dominate life on Earth. Unicellular organisms include both prokaryotes and eukaryotes. Prokaryotes, especially bacteria, are remarkably adaptable and live almost everywhere. Eukaryotes include algae and yeasts, or unicellular fungi. Yeasts play an important role in breaking down complex nutrients, making them available for other organisms.

4. Lesson Overview Lesson Overview Homeostasis and Cells The Cell as an Organism A single-celled, or unicellular, organism does everything you would expect a living thing to do. Every unicellular organism needs to find sources of energy or food, keep concentrations of water and minerals within certain levels, and respond quickly to changes in its environment. Just like other living things, unicellular organisms must achieve homeostasis, relatively constant internal physical and chemical conditions. Whether a prokaryote or a eukaryote, homeostasis is an issue for each unicellular organism.

5. Lesson Overview Lesson Overview Homeostasis and Cells Multicellular Life The cells of multicellular organisms become specialized for particular tasks and communicate with one another to maintain homeostasis. No matter what the role, each specialized cell contributes to the overall homeostasis of the organism. For example, specialized animal cells act like street sweepers to keep particles of dust, smoke, and bacteria out of the lungs. These cells are full of mitochondria, which provide a steady supply of the ATP that powers the cilia on their upper surfaces. Pollen grains are highly specialized cells that are tiny and light, with thick cell walls to protect the cell’s contents. Pine pollen grains have two tiny wings that enable the slightest breeze to carry them great distances.

6. Lesson Overview Lesson Overview Homeostasis and Cells Levels of Organization The specialized cells of multicellular organisms are organized into tissues, then into organs, and finally into organ systems.

7. Lesson Overview Lesson Overview Homeostasis and Cells Levels of Organization A tissue is a group of similar cells that performs a particular function.

8. Lesson Overview Lesson Overview Homeostasis and Cells Levels of Organization To perform complicated tasks, many groups of tissues work together as an organ. Each type of tissue performs an essential task to help the organ function. In most cases, an organ completes a series of specialized tasks.

9. Lesson Overview Lesson Overview Homeostasis and Cells A group of organs that work together to perform a specific function is called an organ system. For example, the stomach, pancreas, and intestines work together as the digestive system. Levels of Organization

10. Lesson Overview Lesson Overview Homeostasis and Cells Levels of Organization 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.

11. Lesson Overview Lesson Overview Homeostasis and Cells 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 cause a cell to change what it is doing.

12. Lesson Overview Lesson Overview Homeostasis and Cells Cellular Communication Some cells form connections, or cellular junctions, to neighboring cells. Some junctions hold cells firmly together.

13. Lesson Overview Lesson Overview Homeostasis and Cells Cellular Communication Other junctions allow small molecules carrying chemical messages 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. Sometimes these receptors are on the cell membrane, although the receptors for certain 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, such junctions enable the cells of the heart muscle to contract in a coordinated fashion.