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How does the food you eat provide energy to cells in your toes? Why do doctors measure their patients' blood pressure? r Y In your Science Log. describe how a horse obtains water and how a tree obtains water. Then describe the system you think each organism has inside its body to deliver water to all its cells. At the end of this chapter, look back to see how your ideas have changed. ().St cell,, of nlulticell(1lar organisms Such as a horse Or a ho\\ biolO,^ists think of cells as I)ein0, tree or \'oursclf are not in direct Con - ortianir,ed into S\Stenls. "I he interac- tact with the Olllsille e11v ii.Ollllle1lt. ti0 tions of-s\-stenls OI cell,, maintain a h()\\, (10 the cells in the hind legs of a stable en\vironnlent inside nlulticellu- horse get oxvgen= I IO\\. do cells in Iar organisnls, even though con- the )-()()is of a tree het food (sugars)-- ditions Illav Change outside their 'li) think about this I)roblenl, bodies. \Vhat happens \\ hen a system inr l gin e that all the cell.,, of an animal does not \\orl: 1)rol)crl} = I)rohlenl,, in ()1. 1)I:illt al•c orga niz ed into (hll ercllt the (rgalll/atloll and Iunct1On ()f Cells s\:sten l s. Filc h s \'St elll has a particular and sy stem s can cause ill ness. I'u11ct1OIl to 1)el'101'lll. l 'Or exa mple, c hapt er on e S\' s t e i n 66 (:ells kind I''u(ly System, In this chalner, you will learn c arries oxygen thl'Oll`^'h- also explores ho\\ h ea lthy lifestyle Choices ca n help 1)1-c\cllt the out the body to every' cell. Other O nset systems (hake sure that each cell chal)t er b\ taking your pulse under receives food, and so on. "lo-ether, difl cre nt conditions. 'I'h e pulse is one the systems keep all the cells of the g u ide to h()\\- well body functioning I)rol)erI}. s \' st eill is \\ ()I•klllg. of Illness and disease. Start this yo ur circulatory The Advantages of Being Multicellular Imagine von are a microSCopiC, unicellular organism. lour whole body is one cell. This one cell must carry out all the functions needed to keep you alive. It must be able to move, obtain food, reproduce, and respond to the environment. "There are many living organisms that consist of only one cell. What disadvantages do you think they have, compared With muulticellular organisms? In the last chapter, you learned of one disadvantage. Unicellular organisms cannot grow very lark. Also, because they must take in all the materials they need through their cell membranes, most unicellular organisms can only live in watery, food-rich surroundings. Multicellular organisms have several advantages compared to unicellular living things. they can live ill a wide variety of environments. "They are able to grow very large - as large as a whale or a Douglas fir. Multicellular animals can obtain their energy from a wide variety of foods. "Their bodies are more complex. By specializing ill particular functions, each cell in a multicellular organism can work much more efficiently than the cell of a unicellular organism, which must do every Job itself In multicellular organisms, speciali zed cells of a similar kind work closely together, and are usually found grouped closely together ill the body. Groups of specialized cells, ill turn, work ill harmony With other groups. You will investigate this organi zation of cells ill the next chapter. Check Your Understanding 1. Why do cells ill your body need to be specialized? 2. Why do nerve cells have long fibres, whereas red blood cells are thin and disklike? 3. 1V'hv do unicellular organisms live mainly ill a watery environment? 4. Apply Most people think of the skin as just a body covering. How do you think skin cells are important to other body cells= 62 Cells and Body Sytitenx 2.4 Speci ized Cells Imagine an orchestra made up of only a hundred trumpet players or a hundred violins. Such an orchestra would be very limited! To play every kind of music, an orchestra needs a variety of musical instruments - some flutes, some oboes, a piano, drums, and so on. In the same way, a multicellular organism cannot be made up only of identical cells. As Figure 2.13 shows, although multicellular organisms grow from single cells that repeatedly divide, their cells are not all the same. red blood cell ti• Figure 2 .13 Different cells have different shapes and functions. Like the instruments in an orchestra, different cells have different appearances and perform different jobs. They are said to be specialized for particular tasks. For example, your muscle cells are shaped to move parts of your body, and your skin cells are built to protect your body from the drying rays of the Sun. Human s have about a hundred different types of cells, each with their own particular structu re and functions. A nerve fibre in the neck of a giraffe can be up to 1 m in length. However, the main part of the cell from which it comes is about the same size as a human nerve cell. 60 Cells and Body Systems Look at the examples of plant and animal cells in Figure 2.13. How do their shapes relate to their functions? Nerve cells have long, branched fibres running from the main part of the cell, shaped to carry nerve signals from one part of the body to another. Red blood cells, which carry oxygen in the bloodstream, have a thin, disklike shape. This gives them a large surface area to pick up large amount s of oxygen. The water-conducting cells of a plant are tubelike, with thick walls and a network of holes that lets water pass easily through them. Onion skin cells are flat and brickshaped, so they can fit closely together to form a continuous protective layer.