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. 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.”

5. Figure 5.1 Membrane Molecular Structure

6. 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

7. 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

8. 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

9. 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.

10. 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.

11. 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

12. 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.

13. 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

14. 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

15. 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.

16. 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.

17. 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.

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

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

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

21. 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.

22. 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.

23. 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.

24. 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.

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

26. 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.

27. 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.

28. 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.

29. 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.

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

31. 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.

32. 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.

33. Figure 5.8 Endocytosis and Exocytosis (Part 1)

34. Figure 5.8 Endocytosis and Exocytosis (Part 2)

35. 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.

36. 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.

37. Concept 5.4 Large Molecules Cross the Membrane via Vesicles In pinocytosis (“cellular drinking”), vesicles also form. The vesicles are smaller and bring in fluids and dissolved substances, as in the endothelium near blood vessels.

38. Concept 5.4 Large Molecules Cross the Membrane via Vesicles Receptor–mediated endocytosis depends on receptors to bind to specific molecules (their ligands). The receptors are integral membrane proteins located in regions called coated pits. The cytoplasmic surface is coated by another protein (often clathrin).

39. Concept 5.4 Large Molecules Cross the Membrane via Vesicles When receptors bind to their ligands, the coated pit invaginates and forms a coated vesicle. The clathrin stabilizes the vesicle as it carries the macromolecules into the cytoplasm. Once inside, the vesicle loses its clathrin coat and the substance is digested.

40. Figure 5.9 Receptor-Mediated Endocytosis (Part 1)

41. Figure 5.9 Receptor-Mediated Endocytosis (Part 2)

42. Concept 5.4 Large Molecules Cross the Membrane via Vesicles Exocytosis moves materials out of the cell in vesicles. The vesicle membrane fuses with the plasma membrane and the contents are released into the cellular environment. Exocytosis is important in the secretion of substances made in the cell.

43. Synopsis of Cellular Transport Bozeman Biology

44. Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Cells can respond to many signals if they have a specific receptor for that signal. A signal transduction pathway is a sequence of molecular events and chemical reactions that lead to a cellular response, following the receptor’s activation by a signal.

45. Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Cells are exposed to many signals and may have different responses: Autocrine signals affect the same cells that release them. Paracrine signals diffuse to and affect nearby cells. Hormones travel to distant cells.

46. Figure 5.10 Chemical Signaling Concepts

47. Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Only cells with the necessary receptors can respond to a signal—the target cell must be able to sense it and respond to it. A signal transduction pathway involves a signal, a receptor, and a response.

48. Figure 5.11 Signal Transduction Concepts

49. Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals A common mechanism of signal transduction is allosteric regulation. This involves an alteration in a protein’s shape as a result of a molecule binding to it. A signal transduction pathway may produce short or long term responses.

50. Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals A signal molecule, or ligand, fits into a three-dimensional site on the receptor protein. Binding of the ligand causes the receptor to change its three-dimensional shape. The change in shape initiates a cellular response.