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1.7 Cells in Their Environment Imagine if you had to live inside a sealed plastic bag. How long would you survive? Well, not long without holes so oxygen could enter. Soon, you would need a way to get water and food through the plastic. Even this would not be enough. You would also need a way of removing wastes, such as carbon dioxide and urine. In some ways, the cell membrane is like that plastic bag. However, the cell membrane is also much more complex, as you can see in Figure 1. Cell Membranes Figure 1 Cells allow some materials to enter or leave, but not others. They are said to be permeable to some materials and impermeable to others. Permeable means permitting passage, and impermeable means not permitting passage. In general, small molecules pass easily through the cell membrane, medium-sized molecules move through less easily, and large ones cannot pass through without help from the cell. Because it allows certain substances to enter or leave, but not others, the cell membrane is said to be selectively permeable. The cell membrane has two layers of fat (lipid). Embedded in the fat layers are protein molecules (coloured blobs) and pores made of protein. There are pores of several different sizes. protein pore lipids Diffusion In Figure 2 a blob of ink gradually spreads out and colours the whole beaker of water. Why doesn’t the ink remain as a small blob? What causes it to move outward? The molecules of the ink blob are constantly moving and colliding with other ink molecules and with the molecules of the water. When they collide, they bounce off each other. This causes molecules that are concentrated in one area to gradually spread outward. Diffusion is the movement of molecules from an area of high concentration to an area of lower concentration. 28 Unit 1 Figure 2 Ink diffusing in water. Diffusion and Cells Diffusion is one of the ways substances move into and out of cells. A substance that a cell uses up, such as oxygen, will be in low concentration inside the cell. Outside the cell, the concentration of the substance will be higher. The molecules of the substance will diffuse across the cell membrane into the cell. Diffusion will continue until the concentration of the substance is the same inside and outside the cell. Waste products, such as carbon dioxide, tend to become more concentrated inside the cell than outside, so they will diffuse out of the cell. Understanding Concepts 1. Explain in your own words the process of diffusion. 2. Explain what is meant by impermeable, permeable, and selectively permeable materials. 3. What type of membrane do cells have? Explain why. 4. Hypothesize why the pores in the cell membrane are of different sizes. Making Connections Models of Membranes Look at Figure 3. Compare the permeability of the three materials—glass, mesh, and cloth—covering the jars. 1. Which covering is impermeable to all three substances? 2. Which covering is permeable to all three substances? 3. Which covering is impermeable to some substances, but permeable to others? 4. Name two other materials that are permeable to some of the substances shown, but impermeable to the others. Test the permeability of the substances for yourself. Figure 3 Three “membranes” — glass, wire or plastic mesh, and cloth. 5. The pores of cells aren’t always open. Sometimes they are locked. In humans and many other animals, insulin acts like a key to a lock. Insulin is a protein released by the pancreas into the blood. Once in the blood, insulin molecules can reach all the cells of the body. An insulin molecule is able to attach itself to the cell membrane of cells. When it does, one kind of pore opens. Sugar from the blood can then enter the cell through the open pore. (a) Imagine a mechanism that would allow insulin to open a pore. Draw a diagram showing how insulin would open the pore. (b) People who are unable to produce insulin have diabetes. What problem would be caused by not producing insulin? (c) What organelles within the cell would be most affected by a lack of insulin? Explain your choice. sugar-water solution water sugar Cells, Tissues, Organs, and Systems 29