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Life comes in an enormous range of sizes. It can be as small as a bacterium, which is invisible to the unaided eye. Or it can be large enough to fit half a dozen humans in its jaws alone, such as the school-bus-sized, prehistoric Megalodon shark. Scientists can only observe Megalodon through the fossils it left behind. Due to its huge size, however, it was made up of hundreds of trillions of cells. A single-celled organism relies on the organelles within its one cell to carry out its life functions. A multi-celled organism requires a greater degree of organization to survive. Most of the cells in a multi-celled organism have different structures and abilities that help them carry out specific functions. But even these specialized cells are limited in what they can do on their own. As a result, the survival of a multi-celled organism relies on teamwork. Working together, specialized cells carry out life functions, such as digestion, breathing, and circulation. Such organization is key to developing complex life forms such as Megalodon and you. 1) All cells begin alike and differentiate into specialized cells. 2) Specialized cells have different structures that allow them to perform unique functions. 3) Groups of cells working together form tissues. 4) Groups of tissues working together form organs, which work together in systems. - All cells begin alike and differentiate into specialized cells. Imagine that you are building a shelving unit. You cut the wood, join the shelves together, and mount the unit on the wall. Now imagine that you have only one tool, such as a hammer, to work with. Will you have a harder time completing the job? After all a hammer is great for pounding nails but is inefficient for cutting wood. To build a shelving unit successfully, you need tools that are specialized for the different tasks you must complete. As the construction task becomes more complex, such as building a shed, the tools you need become more specialized. In this sense, organisms have a lot in common with construction jobs—as they become more complex, more specialized tools (or cells) are required to perform different functions. Cell Specialization and Differentiation We talk about specialized cells and tools, but what does it mean to become specialized? If you are a mechanic who specializes in Harley Davidson motorcycles, you have to learn a lot about these motorcycles and how to fix them. You have to gain the knowledge you need to work on the motorcycles, as well as obtain the tools. Similarly, a cell that transmits messages to your brain, such as the one shown in Figure 1.13, has to have both the structures and the ability to perform this task. All cells begin alike and differentiate into specialized cells. Imagine that you are building a shelving unit. You cut the wood, join the shelves together, and mount the unit on the wall. Now imagine that you have only one tool, such as a hammer, to work with. Will you have a harder time completing the job? After all a hammer is great for pounding nails but is inefficient for cutting wood. To build a shelving unit successfully, you need tools that are specialized for the diff erent tasks you must complete. As the construction task becomes more complex, such as building a shed, the tools you need become more specialized. In this sense, organisms have a lot in common with construction jobs—as they become more complex, more specialized tools (or cells) are required to perform different functions. Cell Specialization and Differentiation We talk about specialized cells and tools, but what does it mean to become specialized? If you are a mechanic who specializes in Harley Davidson motorcycles, you have to learn a lot about these motorcycles and how to fi x them. You have to gain the knowledge you need to work on the motorcycles, as well as obtain the tools. Cell specialization refers to the fact that diff erent types of cells have diff erent structures and abilities to perform their functions efficiently. All cells start their lives as identical cells called stem cells. Each stem cell has the potential to become a specialized cell, such as a muscle cell or nerve cell, as it matures. During this process, the cell becomes altered to suit its specific functions. The series of events through which stem cells develop into specialized cells is called cell differentiation. Scientists are very interested in studying how stem cells differentiate. These cells offer the potential for treating many diseases if scientists can trigger them to develop into cells that can repair damaged tissues. Groups of cells working together form tissues. Cells are the basic unit of life. But even specialized cells can only accomplish so much on their own. This is where teamwork becomes important, as you discovered in the topic opener. When specialized cells work together to perform a specific function, they are known as a tissue. There are four basic types of tissues: muscle, epithelial, connective, and nervous tissue. Groups of tissues working together form organs, which work together in systems. Different tissues perform unique, individual functions. However, like specialized cells, they can only do so much on their own. Tissues working together overcome this limitation to form organs. An organ is made up of different tissues working together to perform a specific task. An organ contains at least two different types of tissues. For example, your heart is an organ. Within your heart, muscle tissue contracts and relaxes to pump blood through valves made of connective tissue. The heart also contains nerve tissue and a protective layer of epithelial tissue. organ, definition: different tissues working together to perform a specific task. brain, function: controls all body activities lungs, function: draw oxygen-rich air into the body and remove carbon-dioxide rich air from the body heart: pumps blood through the body liver: cleans the blood, stores substances such as certain vitamins and minerals kidneys: produce urine by filtering wastes and excess water from the blood stomach: churns and digests food Just as tissues working together form organs, organs working together form organ systems. Th e organs in an organ system interact with each other to perform a common task. organ system: a group of organs that interact with each other to perform a common task