Membrane Structure and Function

1. Membrane Structure and Function Chapter 5

2. Membrane Models • Lipid-soluble molecules enter cells more rapidly than water soluble molecules. • Lipids are a component of the plasma membrane.

3. Fluid-Mosaic Model • Fluid-mosaic model—model for the plasma membrane based on the changing location and pattern of protein molecules in a fluid phospholipid bilayer.

4. Plasma Membrane Structure and Function • Plasma membrane separates the internal environment of the cell from the external environment. • Regulates the entrance and exit of molecules into the cell. • Helps the organism maintain a steady internal environment.

5. Plasma Membrane Structure and Function • Plasma membrane is made up of a phospholipid bilayer with embedded proteins. • Proteins form a mosaic pattern. • Hydrophilic (water loving) polar heads of the phospholipid molecules face the outside and inside of the cell where water is found.

6. Plasma Membrane Structure and Function • Hydrophobic (water-fearing) nonpolar tails face each other. • Cholesterol—one of the major lipids found in animal plasma membranes • Makes the membrane impermeable to many molecules. • Stiffens and strengthens the membrane by helping regulate its fluidity.

9. Plasma Membrane Structure and Function • Glycoproteins—protein in the plasma membranes that bears a carbohydrate chain. • Phospholipids and proteins attached to carbohydrate chains.

10. Plasma Membrane Structure and Function • Carbohydrate Chains • Carbohydrate chains of proteins give the cell a “sugar coat”, called the glycocalyx. • Glycocalyx protects the cell, facilitates adhesion between cells, reception of signal molecules, and cell-to-cell recognition. • In humans, carbohydrate chains are the basis for the A, B, and O blood groups.

11. Functions of Proteins within the Plasma Membrane • Channel proteins • Involved in the passage of molecules through the membrane. • Carrier proteins • Transports sodium and potassium ions across a nerve cell membrane. • Carrier protein is essential in nerve conduction.

12. Functions of Proteins within the Plasma Membrane • Cell recognition proteins • Glycoproteins • Help the body recognize when it is being invaded by pathogens so that an immune reaction can occur. • Receptor proteins • Have a shape that allows a specific molecule to bind to it. • The binding of this molecule causes the protein to change its shape, and resulting in a cellular response.

13. Functions of Proteins within the Plasma Membrane • Enzymatic proteins • Carry out metabolic reactions directly. • Without enzymatic proteins degradation and synthetic reaction would not occur.

14. Permeability of the Plasma Membrane

15. Permeability of the Plasma Membrane • Differentially (selectively) permeable—ability of plasma membranes to regulate the passage of substances into and out of the cell. • Allowing some to pass through and preventing the passage of others.

16. Permeability of the Plasma Membrane • Passive transport • Involves diffusion or facilitated transport does require a carrier protein. • Does not require chemical energy. • Diffusion occurs without benefit of a carrier protein, whereas facilitated transport does require a carrier protein.

17. Permeability of the Plasma Membrane • Active Transport • Requires a carrier protein and chemical energy. • Vesicle formation can take a molecule out of a cell (called exocytosis) or into a cell (called endocytosis).

18. Permeability of the Plasma Membrane • Concentration gradient—gradual change in chemical concentration from one point to another. • Concentration of a substance moves from a high concentration area to an area where their concentration is low.

19. Permeability of the Plasma Membrane • Diffusion—movement of molecules from a higher to a lower concentration until equilibrium is achieved. • Spontaneous • Requires no chemical energy • Solution—contains both a solute and a solvent. • Solute—substance being dissolved within the solvent. • Solvent—substance in which the solute is dissolved within.

20. Permeability of the Plasma Membrane • Gas exchange in lungs. • Oxygen diffuses into the capillaries of the lungs because there is a higher concentration of oxygen in the alveoli (air sacs) than in the capillaries. • After inhalation, the concentration of oxygen in the alveoli is higher than that in the blood; oxygen diffuses into the blood.

21. Permeability of the Plasma Membrane • Osmosis—diffusion of water through a selectively permeable membrane. • Osmosis pressure—measure of the tendency of water to move across a selectively permeable membrane. • Visible as an increase in liquid on the side of the membrane with higher solute concentration.

22. Permeability of the Plasma Membrane • Osmosis • Isotonic solution—solution that is equal in solute concentration to that of the cytoplasm of a cell. • Causes cell to neither lose nor gain water by osmosis. • Tonicity—osmalarity of a solution compared to that of a cell. • If the solution is isotonic to the cell, there is no net movement of water. • If the solution is hypotonic the cell gains water. • If the solution is hypertonic the cell loses water.

23. Permeability of the Plasma Membrane • Osmosis • Hypotonic solution—lower solute concentration than the cytoplasm of a cell. • Causes cell to gain water by osmosis. • Lower concentration of solute. • Turgor pressure—pressure of the cell contents against the cell wall. • In plant cells, determined by the water content of the vacuole and provides internal support.

24. Permeability of the Plasma Membrane • Osmosis • Hypertonic solution—higher solute concentration than the cytoplasm of a cell. • Causes cell to lose water by osmosis. • Higher percentage of solute • Crenation—shriveling of the cell due to water leaving the cell when the environment is hypertonic. • Plasmolysis—contraction of the cell contents due to the loss of water.

25. Permeability of the Plasma Membrane • Transport by Carrier Proteins • Carrier proteins are specific; each can combine with only a certain type of molecule or ion, which is then transported through the membrane. • Carrier proteins are REQUIRED for both facilitated transport and active transport.

26. Permeability of the Plasma Membrane • Facilitated Transport—passive transfer of a substance into or out of a cell along a concentration gradient by a process that requires a carrier. • During facilitated transport, a carrier protein speeds the rate at which the solute crosses the plasma membrane toward a lower concentration. • Carrier protein undergoes a change in shape as it moves a solute across the membrane.

27. Permeability of the Plasma Membrane • Active Transport—use of a plasma membrane carrier protein to move a molecule or ion from a region of lower concentration to one of higher concentration. • It opposes equilibrium and requires energy. • Opposite to the process of diffusion.

28. Permeability of the Plasma Membrane • Active Transport • Proteins involved in active transport often are called pumps. • One type of pump in animal cells, moves sodium ions to the outside of the cell and potassium ions to the inside of the cell. • These two event are linked, and the carrier protein is called a sodium-potassium pump.

29. Permeability of the Plasma Membrane • Sodium-potassium pump • Steps • Carrier has a shape that allows it to take up 3Na ions. • ATP is split, and phosphate group attached to carrier. • Change in shape results and causes carrier to release 3Na ions outside the cell. • Carrier protein now has a shape that allows it to take up 2K ions.

30. Permeability of the Plasma Membrane • Sodium-potassium pump • Steps • Phosphate group is released from the carrier. • Phosphate group is donated by ATP when it is broken down by the carrier. • Change in shape results and causes carrier to release 2K ions inside the cell. • Sodium-potassium pump results in both a concentration gradient and an electrical gradient for these ions across the plasma membrane.

31. Permeability of the Plasma Membrane • Membrane-Assisted Transport • Macromolecules are too large to be transported by carrier proteins. • Vesicles form to transport into or out of the cell by vesicle formation. • Vesicle formation is an energy-requiring process, which include exocytosis and endocytosis.

32. Permeability of the Plasma Membrane • Exocytosis—process in which an intracellular vesicle fuses with the plasma membrane so that the vesicle’s contents are released outside the cell (secretion occurs). • Often these vesicles have been produced by the Golgi Apparatus and contain proteins.

33. Permeability of the Plasma Membrane • Exocytosis • The membrane of the exocytosis vesicle becomes part of the plasma membrane. • Exocytosis occurs automatically during cell growth. • The proteins are then released from the vesicle adhere to the cell surface or become incorporated in an extracellular matrix.

34. Permeability of the Plasma Membrane • Endocytosis—process where cells take in substances by vesicle formation. • Endocytosis can occur in 3 different ways: • Phagocytosis • Pinocytosis • Receptor-mediated endocytosis

35. Permeability of the Plasma Membrane • Endocytosis • Phagocytosis—when the material or large substances are engulfed, forming an intracellular vacuole. • Common in unicellular organisms. • Example: amoebas

36. Permeability of the Plasma Membrane • Endocytosis • Pinocytosis—vesicles form around a liquid or around very small particles and brings them into the cell. • Involves a significant amount of the plasma membrane because it occurs continuously. • The loss of the membrane due to pinocytosis is balanced by the occurrence of exocytosis.

37. Permeability of the Plasma Membrane • Endocytosis • Receptor-Mediated Endocytosis—selective uptake of molecules into a cell by vacuole formation after they bind to specific receptor proteins in the plasma membrane. • Type of pinocytosis but is more efficient. • The receptors for these substances are found at one location in the plasma membrane. • This location is called a coated pitbecause it isa layer of protein on the cytoplasmic side of the pit.

38. Modification of Cell Surfaces

39. Modification of Cell Surfaces • Cell Surfaces in Animals • 2 different types of animal cell surface features: • Junctions between cells • The extracellular matrix

40. Modification of Cell Surfaces • Junction between cells • 3 types of junctions between cells • Adhesion junctions • Tight junctions • Gap junctions

41. Modification of Cell Surfaces • Junctions between cells • Adhesion junctions—adjacent plasma membranes do not touch but are held together by intercellular filaments attached to buttonlike thickenings.

42. Modification of Cell Surfaces • Junctions between cells • Tight junctions—adjacent plasma membrane proteins join to form an impermeable barrier. • Produces a zipperlike fastening. • Examples: intestines and kidney tubules

43. Modification of Cell Surfaces • Junctions between cells • Gap junctions—formed by the joining of two adjacent plasma membranes. • It lends strength and allows ions, sugars, and small molecules to pass between cells. • It allows cells to communicate. • Important in the heart muscle and smooth muscle because they permit a flow of ions that is required for the cells to contract as a unit.

44. Modification of Cell Surfaces • Extracellular Matrix—meshwork of polysaccharides and proteins in close association with the cell that produced them. • Collagen is a structural proteins that gives the matrix strength. • Elastin fibers are structural proteins that gives the matrix resilience.

45. Modification of Cell Surfaces • Extracellular matrix • Fibronectins and laminins are adhesive proteins which influence the behavior of the cells. • They form “highways” that direct the migration of cells during development. • Permit communication between the extracellular matrix and cytoplasm of the cell.

46. Question 1 • A phospholipid molecule has a head and two tails. The tails are found • At the surfaces of the membrane. • In the interior of the membrane. • Spanning the membrane. • Where the environment is hydrophilic. • Both a and b are correct.

47. Question 2 2. During diffusion, • A solvents move from the area of higher to lower concentration, but solutes do not. • There is a net movement of molecules from the area of higher to lower concentration. • A cell must be present for any movement of molecules to occur. • Molecules move against their concentration gradient if they are small and charged. • All of these are correct.

48. Question 3 • When a cell is placed in a hypotonic solution, • solute exits the cell to equalize the concentration on both sides of the membrane. • Water exits the cell toward the area of lower solute concentration. • Water enters the cell toward the area of higher solute concentration. • Solute exits and water enters the cell. • Both c and d are correct.

49. Question 4 • When a cell is placed in a hypertonic solution, • solute exits the cell to equalize the concentration on both sides of the membrane. • Water exits the cell toward the area of lower solute concentration. • Water enters the cell toward the area of higher solute concentration. • Solute exits and water enters the cell. • Both c and d are correct.

50. Question 5 • Active transport • Requires a carrier protein. • Moves a molecule against its concentration gradient. • Requires a supply of chemical energy. • Does not occur during facilitated transport. • All of these are correct.