Cell Membranes and Signaling

1. 5 Cell Membranes and Signaling

2. Chapter 5 Cell Membranes and Signaling • Key Concepts • 5.1 Biological Membranes Have a Common Structure and Are Fluid • 5.2 Passive Transport across Membranes Requires No Input of Energy • 5.3 Active Transport Moves Solutes against Their Concentration Gradients • 5.4 Large Molecules Cross Membranes via Vesicles

3. Chapter 5 Cell Membranes and Signaling 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals 5.6 Signal Transduction Allows the Cell to Respond to Its Environment

4. Chapter 5 Opening Question What role does the cell membrane play in the body’s response to caffeine?

5. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid • A membrane’s structure and functions are determined by its constituents: lipids, proteins, and carbohydrates. • The general design of membranes is known as the fluid mosaic model. • Phospholipids form a continuous bilayer which is like a “lake” in which a variety of proteins “float.”

6. Figure 5.1 Membrane Structure

7. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid • The lipid molecules are usually phospholipids with two regions: • Hydrophilic regions—electrically charged “heads” associate with water molecules • Hydrophobic regions—nonpolar fatty acid “tails” that do not dissolve in water

8. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid A bilayer is formed when the fatty acid “tails” associate with each other and the polar “heads” face the aqueous environment.

9. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes may differ in lipid composition; there are many types of phospholipids. Phospholipids may differ in: Fatty acid chain length Degree of saturation Kinds of polar groups present

10. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Cholesterol is an important component of animal cell membranes. Hydroxyl groups interact with the polar heads of phospholipids. Cholesterol is important in modulating membrane fluidity; other steroids function as hormones.

11. In-Text Art, Chapter 5, p. 84 (2)

12. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid The fatty acids make the membrane somewhat fluid. This allows some molecules to move laterally within the membrane. Membrane fluidity is influenced by: Lipid composition—short, unsaturated chains increase fluidity Temperature—fluidity decreases in colder conditions

13. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid All biological membranes contain proteins; the ratio of proteins to phospholipids varies. Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer. Integral membrane proteins are at least partly embedded in the phospholipid bilayer.

14. In-Text Art, Chapter 5, p. 85

15. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Anchored membrane proteins have hydrophobic lipid components that anchor them in the bilayer. Proteins are asymmetrically distributed on the inner and outer membrane surfaces. Transmembrane proteins extend through the bilayer; they may have domains with different functions on the inner and outer sides of the membrane.

16. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Some membrane proteins can move within the phosopholipid bilayer; others are restricted. Cell fusion experiments illustrate this migration. Proteins inside the cell can restrict movement of membrane proteins, as can attachments to the cytoskeleton.

17. Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 1)

18. Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 2)

19. Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 3)

20. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Diverse carbohydrates are located on the outer cell membrane and play a role in communication. Glycolipid—carbohydrate covalently bonded to a lipid Glycoprotein—one or more oligosaccharides covalently bonded to a protein Proteoglycan—protein with more and longer carbohydrates bonded to it

21. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Cells can adhere to one another through interactions between cell surface carbohydrates and proteins.

22. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes are constantly forming, transforming into other types, fusing, and breaking down. Though membranes appear similar, there are major chemical differences among the membranes of even a single cell.

23. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Selective permeability: biological membranes allow some substances, but not others, to pass Two processes of transport across membranes: 1. Passive transport does not require metabolic energy. A substance moves down its concentration gradient.

24. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy 2. Active transport does require input of metabolic energy. A substance moves against its concentration gradient.

25. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Passive transport can occur by: Simple diffusion through the phospholipid bilayer Facilitated diffusion through channel proteins or aided by carrier proteins

26. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Diffusion is the process of random movement toward equilibrium; a net movement from regions of greater concentration to regions of lesser concentration.

27. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Speed of diffusion depends on three factors: Diameter of the molecules—smaller molecules diffuse faster. Temperature of the solution—higher temperatures lead to faster diffusion. Concentration gradient—the greater the concentration gradient, the faster a substance will diffuse.

28. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Cell cytoplasm is an aqueous solution, as is the surrounding environment. Diffusion of each solute depends only on its own concentration. A higher concentration inside the cell causes the solute to diffuse out; higher concentration outside causes the solute to diffuse in.

29. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Some molecules cross the phospholipid bilayer by simple diffusion: O2, CO2, and small, nonpolar, lipid-soluble molecules. Polar (hydrophilic) molecules do not pass through—they are not soluble in the hydrophobic interior of the membrane. Amino acids, sugars, ions, water

30. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Osmosis is the diffusion of water across membranes through special channels. It depends on the concentration of water molecules on either side of the membrane—water moves down its concentration gradient. The higher the total solute concentration, the lower the concentration of water molecules.

31. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Osmotic pressure: pressure that must be applied to a solution to prevent flow of water across a membrane by osmosis Π = cRT c = total solute concentration R = the gas constant T = absolute temperature

32. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy The higher concentration of a substance on one side of a membrane represents stored energy. If a membrane allows water, but not solutes, to pass through, the net movement of water molecules will be toward the solution with the higher solute concentration and the lower concentration of water molecules.

33. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy When comparing two solutions separated by a membrane: A hypertonic solution has a higher solute concentration. Isotonic solutions have equal solute concentrations. A hypotonic solution has a lower solute concentration.

34. Figure 5.3 Osmosis Can Modify the Shapes of Cells

35. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Concentration of solutes in the environment determines the direction of osmosis in all animal cells. In other organisms, cell walls limit the volume of water that can be taken up. Turgor pressure is the internal pressure against the cell wall—as it builds up, it prevents more water from entering.

36. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Facilitated diffusion: Channel proteins are integral membrane proteins that form channels across the membrane through which some substances can pass. Substances can also bind to carrier proteins to speed up diffusion. Both processes operate in either direction.

37. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Ion channels:channel proteins that allow specific ions to pass through Most are gated channels—they open when a stimulus causes the protein to change shape. Ligand-gated—the stimulus is a ligand, a chemical signal. Voltage-gated—the stimulus is a change in electrical charge difference across the membrane.

38. Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus

39. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Water crosses membranes at a faster rate than simple diffusion. It may “hitchhike” with ions such as Na+ as they pass through ion channels. Aquaporins are channels that allow large amounts of water to move along its concentration gradient.

40. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)

41. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 2)

42. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 3)

43. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Carrier proteins in the membrane facilitate diffusion by binding substances. Glucose transporters are carrier proteins in mammalian cells. Glucose molecules bind to the carrier protein and cause the protein to change shape—it releases glucose on the other side of the membrane.

44. Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)

45. Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Glucose is quickly broken down in the cell, so there is always a strong concentration gradient that favors glucose uptake. But the system can become saturated—when all of the carrier molecules are bound, the rate of diffusion reaches a maximum.

46. Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 2)

47. Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients Cells maintain an internal environment with a different composition than the outside environment. This requires work—energy from ATP is needed to move substances against their concentration gradients (active transport). Specific carrier proteins move substances in only one direction, either into or out of the cell.

48. Table 5.1

49. Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients Two types of active transport: Primary active transport involves direct hydrolysis of ATP for energy. Secondary active transport uses the energy from an ion concentration gradient or an electrical gradient. The gradients are established by primary active transport.

50. Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients The sodium–potassium (Na+–K+) pump is an integral membrane protein that pumps Na+ out of a cell and K+ in. One molecule of ATP moves two K+ and three Na+ ions.