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Transcript
E X P E C TAT I O N S Identify the structure and function of phospholipids. Describe the fluid-mosaic structure of the cell membrane. Figure 1.23 From an altitude of 10 000 m, a city may look quiet and still. From 1000 m, it becomes clear that buses, trucks, and cars are moving. Airplanes fly into its airport. Ships and boats come and go from its harbour. How does this city resemble a cell? When viewed with even the most powerful light microscope, the cell membrane looks like nothing more than a thin, dark line. Yet if the cell membrane functioned only as a barrier separating the inside of the cell from its external environment, how could the cell survive? How would the cell get the raw materials it needs to build macromolecules? The cell membrane must also regulate the movement of materials from one environment to the other. The efficient operation of a city such as the one pictured in Figure 1.23 would soon grind to a halt without adequate routes for the flow of people and things in and out. Similarly, the activities of a living cell depend on the ability of its membrane to transport raw materials into the cell transport manufactured products and wastes out of the cell prevent the entry of unwanted matter into the cell prevent the escape of the matter needed to perform the cellular functions Getting the Cell Membrane in Focus Figure 1.24 What was the original purpose of this wall around the old part of Québec City? How did its original function resemble that of a cell membrane? The development of the electron microscope gave scientists the information they needed to begin exploring how the cell membrane performs its regulatory functions. An electron microscope uses beams of electrons instead of light to produce images. Electron microscopes and other devices separate electrons from their atoms and focus them into a beam. For example, the image on a TV set is formed by electron beams that cause the inner coating on the screen to glow. Compared to light, an electron beam has a very short wavelength — so short that it can pass between two cell features less than 0.2 µm apart and form an image of them that shows two distinct and separate points. Exploring the Micro-universe of the Cell • MHR 21 Figure 1.25 James Hillier was in his early twenties when his professor asked him to help build a practical electron microscope. The microscope that Hiller and Albert Prebus built is now on display at the Ontario Science Centre in Toronto. It has 7000x magnification. The first really usable electron microscope was built in 1938 at the University of Toronto by two graduate students, James Hillier (1915–) and Albert Prebus (1913–1997). Their microscope revealed that what look like “grains” under the light microscope are complex cellular structures. In Chapter 2, you will learn more about these structures. This section continues the story of research into the cell membrane. When electron microscopy finally yielded a more detailed view, microscopists saw that the cell membrane is in fact a bilayer, or a structure consisting of two layers of molecules. Chemical analysis revealed that this bilayer is composed mainly of phospholipid molecules, a type of lipid. Phospholipids have two fatty acids bonded to a glycerol “backbone.” The third glycerol reaction site is bonded to a chain containing phosphorus, and in some cases nitrogen as well. This makes the shape and properties of a phospholipid quite different from those of a triglyceride. The phosphate chain forms a “head,” while the two fatty acids form two “tails.” The electric charge in the molecule is unevenly distributed, as shown in Figure 1.27: the molecule has a polar head and nonpolar tails. The polar head of a phospholipid molecule is attracted to water molecules, which are also polar. This makes the phosphorus end of a phospholipid water soluble. The hydrocarbon chains in the fattyacid tails of the phospholipid are not attracted to water molecules. They are, however, compatible with other lipids. Earlier in this chapter, you learned that hydro means water. Many textbooks use the terms hydrophobic and hydrophilic to describe the way that molecules interact with water. Write a definition for each of these words, including the word soluble in one definition and insoluble in the other. Which end of a phospholipid is hydrophobic and which is hydrophilic? Figure 1.28 shows what can happen when a film of phospholipid molecules is spread in a water sample. Through a combination of attraction and repulsion, the phospholipids spontaneously arrange themselves into a spherical, cage-like bilayer. Their water-attracting polar heads face both the inside and the outside of the sphere, while Figure 1.26 Electron microscopy showed that the cell membranes of both plant and animal cells have a two-layered structure. This gave scientists the clue they needed to begin unravelling the mystery of how the cell membrane works. 22 MHR • Cellular Functions Figure 1.28 The molecular structure of a phospholipid bilayer. Unlike the cell membrane of a living cell, this bilayer contains only water inside it. Figure 1.27 Constructed much like a triglyceride (fat), phospholipids contain a phosphate group and sometimes also a nitrogen group. their water-averse, nonpolar lipid tails face each other. This sandwich-like phospholipid structure, called a phospholipid bilayer, forms the basis of the cell membrane. The ability of phospholipids to spontaneously form a spherical bilayer in water likely played a key role in the formation of the first cells about 3.8 billion years ago. The Fluid-Mosaic Membrane Model The fact that lipids do not dissolve in water creates a border around the cell. The phosphate edges of this border help to define and contain the more fluid lipid centre. However, there is much more to a cell membrane than its phospholipid bilayer. Based on intensive research by biochemists and electron microscopists, biologists have inferred that the cell membrane also contains a mosaic of different components scattered throughout it, much like raisins in a slice of raisin bread. For example, numerous protein molecules stud the phospholipid bilayer. The phospholipid molecules and some of these proteins can drift sideways in the bilayer, a phenomenon which supports the idea that the phospholipid bilayer has a fluid consistency. Thus, this description of the cell membrane is called the fluid-mosaic membrane model. Figure 1.29 on the next page shows how proteins and phospholipids fit together in the continuous mosaic of an animal cell membrane. Note that this cell membrane also contains another type of lipid: cholesterol molecules. Cholesterol allows animal cell membranes to function in a wide range of temperatures. At high temperatures, it helps maintain rigidity in the oily membrane bilayer. At low temperatures, its keeps the membrane fluid, flexible, and functional — preventing cell death from a frozen membrane. Cholesterol also makes the membrane less permeable to most biological molecules. Plants have a different lipid that serves a similar function in their cell membranes. The shapes of the membrane proteins vary according to their function, and each type of cell has a characteristic arrangement of proteins in its membrane. For example, the membrane of a human red blood cell includes 50 different protein types arranged in a pattern that only other cells from humans with the same blood type can “recognize.” Exploring the Micro-universe of the Cell • MHR 23 Figure 1.29 Fluid-mosaic model of membrane structure. Notice that many lipids and proteins facing the exterior of the cell have carbohydrate chains attached to them, while on the interior of the cell, parts of the cell’s skeleton (called 1. List the functions of the cell membrane. 7. 2. Compare the structures of a phospholipid and a fatty acid using a simple diagram of each type of molecule. Label any differences in polarity. Why does your body manufacture cholesterol even if you do not eat any foods that contain cholesterol? 8. Make a model cell membrane that shows the different components. Include a legend that makes your model easy to understand. Explain why the electron microscope is better than the light microscope for looking at the cell membrane. 9. What other cellular structures might the electron microscope provide useful information about that a light microscope could not? 3. 4. 24 its cytoskeleton) support the membrane. Each type of cell has its own unique “fingerprint” of carbohydrate chains that distinguish it from other kinds of cells. Cells are organized differently from the world outside the cell membrane. Draw a diagram of a predator cell, showing how this organization inside the cell is different from the material outside the cell. Then make a second diagram to show the impact that opening a hole in the cell membrane would have on the cell. 5. Identify the component(s) of the cell membrane that give it a fluid consistency. 6. Why does the cell membrane require a fluid consistency? MHR • Cellular Functions 10. Oil acts as an organic solvent. What kinds of problems would organisms coming into contact with an oil spill have?