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 Some Substances Can Cross the Membrane by Diffusion • 5.3 Some Substances Require Energy to Cross the Membrane

3. Chapter 5 Cell Membranes and Signaling 5.4 Large Molecules Cross the Membrane via Vesicles 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 structure of membranes is known as the fluid mosaic model. Phospholipids form a bilayer which is like a “lake” in which a variety of proteins “float.”

6. Figure 5.1 Membrane Molecular Structure

7. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Lipids form the hydrophobic core of the membrane. Most lipid molecules are phospholipids with two regions: • Hydrophilic regions—electrically charged “heads” that 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. Bilayer organization helps membranes fuse during vesicle formation and phagocytosis.

9. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes may differ in lipid composition as 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 Two important factors in membrane fluidity: Lipid composition—types of fatty acids can increase or decrease fluidity Temperature—membrane fluidity decreases in colder conditions

11. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Biological membranes contain proteins, with varying ratios of phospholipids. Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer. Integral membrane proteins are partly embedded in the phospholipid bilayer.

12. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Anchored membrane proteins have lipid components that anchor them in the bilayer. Proteins are asymmetrically distributed on the inner and outer membrane surfaces. A transmembrane protein extends through the bilayer on both sides, and may have different functions in its external and transmembrane domains.

13. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Some membrane proteins can move within the phosopholipid bilayer, while others are restricted. Proteins inside the cell can restrict movement of membrane proteins, as can attachments to the cytoskeleton.

14. Figure 5.2 Rapid Diffusion of Membrane Proteins

15. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Plasma membrane carbohydrates are located on the outer membrane and can serve as recognition sites. Glycolipid—a carbohydrate bonded to a lipid Glycoprotein—a carbohydrate bonded to a protein

16. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes are constantly changing by 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.

17. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Biological membranes allow some substances, and not others, to pass. This is known as selective permeability. Two processes of transport: Passive transport does not require metabolic energy. Active transport requires input of metabolic energy.

18. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Passive transport of a substance can occur through two types of diffusion: Simple diffusion through the phospholipid bilayer Facilitated diffusion through channel proteins or aided by carrier proteins

19. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Diffusion is the process of random movement toward equilibrium. 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

20. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion The concentration gradient in the system—the greater the concentration gradient in a system, the faster a substance will diffuse A higher concentration inside the cell causes the solute to diffuse out, and a higher concentration outside causes the solute to diffuse in, for many molecules.

21. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Simple diffusion takes place through the phospholipid bilayer. A molecule that is hydrophobic and soluble in lipids can pass through the membrane. Polar molecules do not pass through—they are not soluble in the hydrophilic interior and form bonds instead in the aqueous environment near the membrane.

22. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Osmosis is the diffusion of water across membranes. It depends on the concentration of solute molecules on either side of the membrane. Water passes through special membrane channels.

23. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion 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.

24. Figure 5.3A Osmosis Can Modify the Shapes of Cells

25. Figure 5.3B Osmosis Can Modify the Shapes of Cells

26. Figure 5.3C Osmosis Can Modify the Shapes of Cells

27. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion The concentration of solutes in the environment determines the direction of osmosis in all animal cells. In other organisms, cell walls limit the volume 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.

28. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Diffusion may be aided by channel proteins. Channel proteins are integral membrane proteins that form channels across the membrane. Substances can also bind to carrier proteins to speed up diffusion. Both are forms of facilitated diffusion.

29. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Ion channels are a type of channel protein—most are gated, and can be opened or closed to ion passage. A gated channel opens when a stimulus causes the channel to change shape. The stimulus may be a ligand, a chemical signal.

30. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Aligand-gated channel responds to its ligand. A voltage-gated channel opens or closes in response to a change in the voltage across the membrane.

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

32. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Water crosses membranes at a faster rate than simple diffusion. It may “hitchhike” with ions such as Na+ as they pass through channels. Aquaporins are specific channels that allow large amounts of water to move along its concentration gradient.

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

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

35. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion 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.

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

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

38. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Transport by carrier proteins differs from simple diffusion, though both are driven by the concentration gradient. The facilitated diffusion system can become saturated—when all of the carrier molecules are bound, the rate of diffusion reaches its maximum.

39. Concept 5.3 Some Substances Require Energy to Cross the Membrane Active transport requires the input of energy to move substances against their concentration gradients. Active transport is used to overcome concentration imbalances that are maintained by proteins in the membrane.

40. Table 5.1 Membrane Transport Mechanisms

41. Concept 5.3 Some Substances Require Energy to Cross the Membrane The energy source for active transport is often ATP. Active transport is directional and moves a substance against its concentration gradient. A substance moves in the direction of the cell’s needs, usually by means of a specific carrier protein.

42. Concept 5.3 Some Substances Require Energy to Cross the Membrane Two types of active transport: Primary active transport involves hydrolysis of ATP for energy. Secondary active transport uses the energy from an ion concentration gradient, or an electrical gradient.

43. Concept 5.3 Some Substances Require Energy to Cross the Membrane 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.

44. Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump

45. Concept 5.3 Some Substances Require Energy to Cross the Membrane Secondary active transport uses energy that is “regained,” by letting ions move across the membrane with their concentration gradients. Secondary active transport may begin with passive diffusion of a few ions, or may involve a carrier protein that transports both a substance and ions.

46. Concept 5.4 Large Molecules Cross the Membrane via Vesicles Macromolecules are too large or too charged to pass through biological membranes and instead pass through vesicles. To take up or to secrete macromolecules, cells must use endocytosis or exocytosis.

47. Figure 5.8 Endocytosis and Exocytosis (Part 1)

48. Figure 5.8 Endocytosis and Exocytosis (Part 2)

49. Concept 5.4 Large Molecules Cross the Membrane via Vesicles Three types of endocytosis brings molecules into the cell: phagocytosis, pinocytosis, and receptor–mediated endocytosis. In all three, the membrane invaginates, or folds around the molecules and forms a vesicle. The vesicle then separates from the membrane.

50. Concept 5.4 Large Molecules Cross the Membrane via Vesicles In phagocytosis (“cellular eating”), part of the membrane engulfs a large particle or cell. A food vacuole (phagosome) forms and usually fuses with a lysosome, where contents are digested.