The plasma membrane (also called the cell membrane) has several functions. For example, it allows only certain molecules to enter or leave the cell. It separates internal metabolic reactions from the external environment. In addition, the plasma membrane allows the cell to excrete wastes and to interact with its environment. Membrane Lipids The plasma membrane, as well as the membranes of cell organelles, is made primarily of phospholipids. Phospholipids have a polar, hydrophilic (“water-loving”) phosphate head and two nonpolar, hydrophobic (“water-fearing”) fatty acid tails. Water molecules sur- round the plasma membrane. The phospholipids line up so that their heads point outward toward the water and their tails point inward, away from water. The result is a double layer called a phospholipid bilayer, as shown in Figure 4-10. The cell membranes of eukaryotes also contain lipids, called sterols, between the tails of the phospho- lipids. The major membrane sterol in animal cells is cholesterol. Sterols in the plasma membrane make the membrane more firm and prevent the membrane from freezing at low temperatures. SECTION 3 OBJECTIVES ● Describe the structure and function of a cell’s plasma membrane. ● Summarize the role of the nucleus. ● List the major organelles found in the cytosol, and describe their roles. ● Identify the characteristics of mitochondria. ● Describe the structure and function of the cytoskeleton. VOCABULARY phospholipid bilayer chromosome nuclear envelope nucleolus ribosome mitochondrion endoplasmic reticulum Golgi apparatus lysosome cytoskeleton microtubule microfilament cilium flagellum centriole Cell membranes are made of a phospholipid bilayer. Each phospholipid molecule has a polar “head” and a two-part nonpolar “tail.” FIGURE 4-10 Copyright © by Holt, Rinehart and Winston. All rights reserved. 78 CHAPTER 4 OUTSIDE OF CELL INSIDE OF CELL 1. Cell-surface marker: Glycoprotein that identifies cell type 3. Enzyme: Assists chemical reactions inside the cell 2. Receptor protein: Recognizes and binds to substances outside the cell 4. Transport protein: Helps substances move across cell membrane Carbohydrate portion Protein portion Phospholipid heads Phospholipid tails Phospholipid Cholesterol bilayer Membrane Proteins Plasma membranes often contain specific proteins embedded within the lipid bilayer. These proteins are called integral proteins. Figure 4-11 shows that some integral proteins, such as cell surface markers, emerge from only one side of the membrane. Others, such as receptor proteins and transport proteins, extend across the plasma membrane and are exposed to both the cell’s interior and exterior environments. Proteins that extend across the plasma membrane are able to detect environmental signals and transmit them to the inside of the cell. Peripheral proteins, such as the enzyme shown in Figure 4-11, lie on only one side of the membrane and are not embedded in it. As Figure 4-11 shows, integral proteins exposed to the cell’s external environment often have carbohydrates attached. These carbohydrates can act as labels on cell surfaces. Some labels help cells recognize each other and stick together. Viruses can use these labels as docks for entering and infecting cells. Integral proteins play important roles in actively transporting molecules into the cell. Some act as channels or pores that allow certain substances to pass. Other integral proteins bind to a mol- ecule on the outside of the cell and then transport it through the membrane. Still others act as sites where chemical messengers such as hormones can attach. Fluid Mosaic Model A cell’s plasma membrane is surprisingly dynamic. Scientists describe the cell membrane as a fluid mosaic. The fluid mosaic model states that the phospholipid bilayer behaves like a fluid more than it behaves like a solid. The membrane’s lipids and pro- teins can move laterally within the bilayer, like a boat on the ocean. As a result of such lateral movement, the pattern, or “mosaic,” of lipids and proteins in the cell membrane constantly changes.