Unit 2: Characteristics of Living Things Lesson 20: Cell Membrane

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1 Name Unit 2: Characteristics of Living Things Lesson 20: Cell Membrane Date Objective: Students will be able to prove why a simple defect in a cell membrane protein can make a life or- death difference. Essential Questions: 1. How does the cell membrane help a cell maintain homeostasis? 2. How does the cell membrane restrict the exchange of substances? 3. What are some functions of membrane proteins? Vocabulary Words of the Day: Phospholipid, Lipid Bilayer Why It Matters: Every cell is surrounded by a cell membrane. The cell membrane protects the cell and helps move substances and messages in and out of the cell. By regulating transport, the membrane helps the cell maintain constancy and order. Do Now 1. Two types of human cells are shown in the diagram to the right. Cell A causes the cells at B to contract. This activity would be most useful for: a. Lifting a book from a bookshelf. b. Coordinating the functions of organelles. c. Digesting food in the small intestine. d. Carrying out the process of protein synthesis.

2 LIPID BILAYER 5. The cell membrane is made of a sea of phospholipids. Ø What is a phospholipid? ü Answer: A phospholipid is a specialized lipid made of a phosphate head and two fatty acid tails. 6. The phosphate head is polar and is attracted to water. In contrast, the fatty acid tails are nonpolar and are repelled by water. 7. Because there is water inside and outside the cell, the phospholipids form a double layer called the lipid bilayer. 8. The non polar tails, repelled by water, make up the interior of the lipid bilayer. 9. The polar heads, attracted by water, so they point toward the surfaces of the lipid bilayer. 10. One layer of polar heads faces the cytoplasm, while the other layer is in contact with the cell s immediate surroundings. 11. Only certain substances can pass through the lipid bilayer. 12. The phospholipids form a barrier through which only small, nonpolar substances can pass. 13. Ions and most polar molecules are repelled by the nonpolar interior of the lipid bilayer.

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4 MEMBRANE PROTEINS 14. Various proteins can be found in the cell membrane. Some proteins face inside the cell, and some face outside, Other proteins may stretch across the lipid bilayer and face both inside and outside. Ø Question: What are proteins made of? ü Answer: Proteins are made of amino acids. 15. Some amino acids are polar, and others are nonpolar. 16. Nonpolar portions of a protein are attracted to the interior of the lipid bilayer but are repelled by water on either side of the membrane. in contrast. 17. Polar parts of the protein are attracted to the water on both sides of the lipid bilayer. 18. These opposing attractions help hold the protein in the membrane.

5 TYPES OF PROTEINS 19. Proteins in the cell membrane include cell-surface markers, receptor proteins, enzymes, and transport proteins. 20. Cell surface markers are like a name tag, a chain of sugars acts as a marker to identify each type of cell. These sugars (carbohydrates) are attached to the cell surface by proteins called glycoproteins. Glycoproteins help cells work together. 21. Receptor Proteins enable a cell to sense its surroundings by binding to a certain substances outside of the cell. When this happens, it causes changes inside the cell. 22. Enzymes are many proteins in the cell membrane that help with important biochemical reactions inside the cell. 23. Transport Proteins aid the movement of many substances that the cell needs but cannot pass through the lipid bilayer (channel protein).