2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Plasma Membrane Cells Basic Parts of a Cell The three basic parts of a cell are the plasma membrane, the cytoplasm, and the nucleus.

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 All living cells must maintain a balance regardless of internal and external conditions. Survival depends on the cell’s ability to maintain the proper conditions within itself.

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Click below to watch the Visual Concept. Visual Concept Internal Organization of a Cell Plasma Membrane

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Plasma Membrane Cells Plasma Membrane –The cell’s outer boundary, called the plasma membrane (or the cell membrane), covers a cell’s surface and acts as a barrier between the inside and the outside of a cell.

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 It is the plasma membrane’s job to: allow waste and other products to leave the cell. remove excess amounts of these nutrients when levels get so high that they are harmful. allow a steady supply of glucose, amino acids, and lipids to come into the cell no matter what the external conditions are.

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 Water Plasma Membrane

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 Plasma Membrane & Homeostasis This process of maintaining the cell’s environment is called homeostasis. Selective permeability is a process used to maintain homeostasis in which the plasma membrane allows some molecules into the cell while keeping others out.

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Plasma Membrane Membrane Lipids –Cell membranes consist of a phospholipid bilayer. Plasma Membrane

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Structure of Lipid Bilayer Plasma Membrane

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 Structure of the Plasma Membrane The phospholipid bilayer is composed of two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate attached to them.

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Cell Membrane Plasma Membrane Plasma Membrane MovieMembrane

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Fluid Mosaic Model –The fluid mosaic model states that the phospholipid bilayer behaves like a fluid more than it behaves like a solid. Plasma Membrane

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 The fluid mosaic model describes the plasma membrane as a flexible boundary of a cell. The phospholipids move within the membrane.

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Plasma Membrane

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 Other components of the plasma membrane: Cholesterol plays the important role of preventing the fatty acid chains of the phospholipids from sticking together. Cholesterol Molecule

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Cells Membrane Proteins –Cell membranes often contain proteins embedded within the phospholipid bilayer. Plasma Membrane

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Summary Section 2 – pages 175-178 Transport proteins allow needed substances or waste materials to move through the plasma membrane. Click image to view movie.

19. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check Which of the following best describes the plasma membrane's mechanism in maintaining homeostasis? A. protein synthesis B. selective permeability C. fluid composition D. structural protein attachment

20. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check The answer is B. Selective permeability is the process in which the membrane allows some molecules to pass through, while keeping others out.

21. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check Describe the structure of the plasma membrane.

22. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check The plasma membrane is composed of a phospholipid bilayer, which has two layers of phospholipids back-to-back. The polar heads of phospholipid molecules contain phosphate groups and face outward.

23. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check Why is the phosphate group of a phospholipid important to the plasma membrane? Polar head (includes phosphate group) Nonpolar tails (fatty acids) Phospholipid molecule

24. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check When phospholipid molecules form a bilayer, the phosphate groups lie to the outside. Because phosphate groups are polar, they allow the cell membrane to interact with its watery (polar) environments inside and outside the cell. Polar head (includes phosphate group) Nonpolar tails (fatty acids) Phospholipid molecule

25. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check Explain why the model of the plasma membrane is called the fluid mosaic model.

26. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Check It is fluid because the phospholipid molecules move within the membrane. Proteins in the membrane that move among the phospholipids create the mosaic pattern.