1. Biology: Life on Earth Lecture for Chapter 5
Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 5
Membrane Structure and Function
Copyright © 2008 Pearson Prentice Hall, Inc.

2. Chapter 5 Opener
A rattlesnake prepares to strike.

3. Chapter 5 Outline
5.1 How Is the Structure of a Membrane Related to Its Function, p. 82
5.2 How Do Substances Move Across Membranes? p. 85
5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? p. 95

4. Section 5.1 Outline
5.1 How is the Structure of a Membrane Related to Its Function?
The Plasma Membrane Isolates the Cell While Allowing Communication with Its Surroundings
Membranes Are “Fluid Mosaics” in Which Proteins Move Within Layers of Lipids
The Phospholipid Bilayer Is the Fluid Portion of the Membrane
A Mosaic of Proteins Is Embedded in the Membrane

5. Plasma Membrane Functions of the plasma membrane
Isolates the cell’s contents from environment
Regulates exchange of essential substances
Communicates with other cells
Creates attachments within and between other cells
Regulates biochemical reactions

6. Membranes Are “Fluid Mosaics”
Membranes are dynamic, ever-changing structures

7. Membranes Are “Fluid Mosaics”
“Fluid mosaic” model of a membrane proposed in 1972
A lumpy, constantly shifting mosaic of “tiles” or proteins
Proteins float around in a sea of phospholipids

8. FIGURE 5-1 The plasma membrane
The plasma membrane is a bilayer of phospholipids that form a fluid matrix in which various proteins (blue) are embedded. Many proteins have carbohydrates attached to them, forming glycoproteins. Three of the five major types of membrane proteins are illustrated here: recognition, receptor, and transport proteins.

9. The Phospholipid Bilayer
Phospholipids are the basis of membrane structure
Polar, hydrophilic head
Two non-polar, hydrophobic tails

10. FIGURE 5-2 Phospholipid

11. The Phospholipid Bilayer
The cell exterior and interior face watery environments

12. The Phospholipid Bilayer
Hydrophobic and hydrophilic interactions drive phospholipids into bilayers
Double row of phospholipids
Polar heads face outward and inward
Non-polar tails mingle within the membrane
Cholesterol in animal membranes keeps them flexible

13. FIGURE 5-3 Phospholipid bilayer of cell membrane

14. The Phospholipid Bilayer
Phospholipid bilayer is a flexible, fluid membrane to allow for cellular shape changes

15. The Phospholipid Bilayer
Individual phospholipid molecules are not bonded to one another
Some of the phospholipids have unsaturated fatty acids, whose double bonds introduce “kinks” into their “tails”
The above features make the membrane fluid

16. FIGURE 5-4 (part 1) Kinks in phospholipid tails increase membrane fluidity

17. FIGURE 5-4 (part 2) Kinks in phospholipid tails increase membrane fluidity

18. Membrane Proteins Form a Mosaic
Proteins are embedded in the phospholipid bilayer
Some proteins can float and drift
Other proteins are anchored by protein filaments in the cytoplasm
Many proteins have attached carbohydrates (glycoproteins)

19. FIGURE 5-1 The plasma membrane
The plasma membrane is a bilayer of phospholipids that form a fluid matrix in which various proteins (blue) are embedded. Many proteins have carbohydrates attached to them, forming glycoproteins. Three of the five major types of membrane proteins are illustrated here: recognition, receptor, and transport proteins.

20. Membrane Proteins Form a Mosaic
Categories of membrane proteins
Receptor Proteins
Recognition Proteins
Enzymatic Proteins
Attachment Proteins
Transport Proteins

21. Membrane Proteins Form a Mosaic
Receptor Proteins
Trigger cellular responses upon binding specific molecules, e.g. hormones
Recognition Proteins
Serve as identification tags on the surface of a cell

22. FIGURE 5-5 Receptor activation

23. Membrane Proteins Form a Mosaic
Enzymes
Promote chemical reactions that synthesize or break apart biological molecules
Attachment Proteins
Anchor the cell membrane to inner cytoskeleton, to proteins outside the cell, and to other cells

24. Membrane Proteins Form a Mosaic
Transport Proteins
Include channel and carrier proteins
Regulate import/export of hydrophilic molecules

25. Section 5.2 Outline 5.2 How Do Substances Move Across Membranes?
Molecules in Fluids Move in Response to Gradients
Movement Across Membranes Occurs by Both Passive and Active Transport
Passive Transport Includes Simple Diffusion, Facilitated Diffusion, and Osmosis
Osmosis Plays an Important Role in Cells

26. Section 5.2 Outline
5.2 How Do Substances Move Across Membranes? continued
Active Transport Uses Energy to Move Molecules Against Their Concentration Gradients
Cells Engulf Particles or Fluids by Endocytosis
Exocytosis Moves Material Out of the Cell
Exchange of Materials Across Membranes Influences Cell Size and Shape

27. Movement of Molecules in Fluids
Definitions relevant to substance movement
A fluid is a substance that can move or change shape in response to external forces
A solute is a substance that can be dissolved (dispersed as ions or molecules) in a solvent
A solvent is a fluid capable of dissolving a solute

28. Movement of Molecules in Fluids
Definitions relevant to substance movement (continued)
The concentration of molecules is the number of them in a given volume unit
A gradient is a physical difference in temperature, pressure, charge, or concentration in two adjacent regions

29. Movement of Molecules in Fluids
Why molecules move from one place to another
Substances move in response to a concentration gradient
Molecules move from high to low concentration (diffusion) until dynamic equilibrium is reached

30. FIGURE 5-6 (part 1) Diffusion of a dye in water

31. FIGURE 5-6 (part 2) Diffusion of a dye in water

32. FIGURE 5-6 (part 3) Diffusion of a dye in water

33. Movement of Molecules in Fluids
The greater the concentration gradient, the faster the rate of diffusion
Diffusion cannot move molecules rapidly over long distances

34. Movement Across Membranes
Concentration gradients of ions and molecules exist across the plasma membranes of all cells
There are two types of movement across the plasma membrane
Passive transport
Energy-requiring transport

35. Table 5-1 Transport Across Membranes

36. Movement Across Membranes
Passive transport
Substances move down their concentration gradients across a membrane
No energy is expended
Membrane proteins and phospholipids may limit which molecules can cross, but not the direction of movement

37. Movement Across Membranes
Energy-requiring transport
Substances are driven against their concentration gradients
Energy is expended

38. Passive Transport Plasma membranes are selectively permeable
Different molecules move across at different locations and rates
A concentration gradient drives all three types of passive transport: simple diffusion, facilitated diffusion, and osmosis

39. Passive Transport Simple diffusion
Lipid soluble molecules (e.g. vitamins A and E, gases) and very small molecules diffuse directly across the phospsholipid bilayer

40. FIGURE 5-7a Diffusion through the plasma membrane
(a) Simple diffusion: gases such as oxygen and carbon dioxide and lipid-soluble molecules can diffuse directly through the phospholipids.

41. Passive Transport Facilitated diffusion
Water soluble molecules like ions, amino acids, and sugars diffuse with the aid of channel and carrier transport proteins

42. FIGURE 5-7b (part 1) Diffusion through the plasma membrane
(b) Facilitated diffusion through a channel protein: channels (pores) allow passage of some water-soluble molecules, principally ions, that cannot diffuse directly through the bilayer.

43. FIGURE 5-7b (part 2) Diffusion through the plasma membrane
(b) Facilitated diffusion through a channel protein: channels (pores) allow passage of some water-soluble molecules, principally ions, that cannot diffuse directly through the bilayer.

44. FIGURE 5-7c (part 1) Diffusion through the plasma membrane
(c) Facilitated diffusion through a carrier protein.

45. FIGURE 5-7c (part 2) Diffusion through the plasma membrane
(c) Facilitated diffusion through a carrier protein.

46. FIGURE 5-7c (part 3) Diffusion through the plasma membrane
(c) Facilitated diffusion through a carrier protein.

47. FIGURE 5-7c (part 4) Diffusion through the plasma membrane
(c) Facilitated diffusion through a carrier protein.

48. Passive Transport Osmosis – the special case of water diffusion
Water diffuses from high concentration (high purity) to low concentration (low purity) across a membrane
Dissolved substances reduce the concentration of free water molecules (and hence the purity of water) in a solution

49. Passive Transport
The flow of water across a membrane depends on the concentration of water in the internal or external solutions

50. FIGURE 5-8 Isotonic solution

51. Passive Transport
Comparison terms for solutions on either side of a membrane
Isotonic solutions have equal concentrations of water and equal concentrations of dissolved substances
No net water movement occurs across the membrane

52. FIGURE 5-8 Isotonic solution

53. Passive Transport
Comparison terms for solutions on either side of a membrane (continued)
A hypertonic solution is one with lower water concentration or higher dissolved particle concentration
Water moves across a membrane towards the hypertonic solution

54. Passive Transport
Comparison terms for solutions on either side of a membrane (continued)
A hypotonic solution is one with higher water concentration or lower dissolved particle concentration
Water moves across a membrane away from the hypotonic solution

55. FIGURE 5-9 Hypotonic solution

56. Passive Transport
The effects of osmosis are illustrated when red blood cells are placed in various solutions

57. FIGURE 5-10 The effects of osmosis
(a) If red blood cells are immersed in an isotonic salt solution, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape. (b) A hypertonic solution, with more salt than is in the cells, causes water to leave the cells and the cells to shrivel. (c) A hypotonic solution, with less salt than is in the cells, causes water to enter; the cells swell, and may burst.

58. FIGURE 5-10a The effects of osmosis
(a) If red blood cells are immersed in an isotonic salt solution, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape.

59. FIGURE 5-10b The effects of osmosis
(b) A hypertonic solution, with more salt than is in the cells, causes water to leave the cells and the cells to shrivel.

60. FIGURE 5-10c The effects of osmosis
(c) A hypotonic solution, with less salt than is in the cells, causes water to enter; the cells swell, and may burst.

61. Passive Transport
Osmosis explains why fresh water protists have contractile vacuoles
Water leaks in continuously because the cytosol is hypertonic to fresh water
Salts are pumped into the vacuoles, making them hypertonic to the cytosol
Water follows by osmosis and is then expelled by contraction

62. FIGURE 4-16 (part 1) Contractile vacuoles
Many freshwater protists contain contractile vacuoles. (a) Water constantly enters the cell by osmosis. In the cell, water is taken up by collecting ducts and drains into the central reservoir of the vacuole. (b) When full, the reservoir contracts, expelling the water through a pore in the plasma membrane

63. Active Transport
Cells need to move some substances against their concentration gradients
Figure: 19-2 part a
Title:
Viral structure and replication part a
Caption:
(a) A cross section of the virus that causes AIDS. Inside, genetic material is surrounded by a protein coat and molecules of reverse transcriptase, an enzyme that catalyzes the transcription of DNA from the viral RNA template after the virus enters the host cell. This virus is among those that also have an outer envelope that is formed from the host cell's plasma membrane. Spikes made of glycoprotein (protein and carbohydrate) project from the envelope and help the virus attach to its host cell.

64. Active Transport
Active-transport membrane proteins move molecules across using ATP
Proteins span the entire membrane
Often have a molecule binding site and an ATP binding site
Often referred to as pumps
Figure: 19-2 part a
Title:
Viral structure and replication part a
Caption:
(a) A cross section of the virus that causes AIDS. Inside, genetic material is surrounded by a protein coat and molecules of reverse transcriptase, an enzyme that catalyzes the transcription of DNA from the viral RNA template after the virus enters the host cell. This virus is among those that also have an outer envelope that is formed from the host cell's plasma membrane. Spikes made of glycoprotein (protein and carbohydrate) project from the envelope and help the virus attach to its host cell.

65. FIGURE 5-12 Active transport
Active transport uses cellular energy to move molecules across the plasma membrane against a concentration gradient. A transport protein (blue) has an ATP binding site and a recognition site for the molecules to be transported, in this case calcium ions (Ca2+)Notice that when ATP donates its energy, it loses its third phosphate group and becomes ADP + P.

66. Endocytosis
Cells import large particles or substances via endocytosis

67. Endocytosis
Plasma membrane pinches off to form a vesicle in endocytosis
Types of endocytosis
Pinocytosis
Receptor-mediated endocytosis
Phagocytosis

68. Endocytosis Types of endocytosis
Pinocytosis (“cell drinking”) brings in droplet of extracellular fluid

69. FIGURE 5-13a Pinocytosis
The circled numbers correspond to both the diagram and the electron micrograph.

70. FIGURE 5-13b Pinocytosis
The circled numbers correspond to both the diagram and the electron micrograph.

71. Endocytosis Types of endocytosis
Receptor-mediated endocytosis moves specific molecules into the cell

72. FIGURE 5-14 Receptor-mediated endocytosis
The circled numbers correspond to both the diagram and the electron micrograph.

73. FIGURE 5-14 (part 1) Receptor-mediated endocytosis
The circled numbers correspond to both the diagram and the electron micrograph.

74. FIGURE 5-14 (part 2) Receptor-mediated endocytosis
The circled numbers correspond to both the diagram and the electron micrograph.

75. Endocytosis Types of endocytosis
Phagocytosis (“cell eating”) moves large particles or whole organisms into the cell

76. FIGURE 5-15a Phagocytosis

77. FIGURE 5-15b Phagocytosis

78. FIGURE 5-15c Phagocytosis

79. Exocytosis Exocytosis
Vesicles join the membrane, dumping out contents in exocytosis

80. FIGURE 5-16 Exocytosis
Exocytosis is functionally the reverse of endocytosis.

81. Cell Size and Shape Exchange affects cell size and shape
As a spherical cell enlarges, its innermost parts get farther away from the plasma membrane
Also, its volume increases more rapidly than its surface area
A larger cell has a relatively smaller area of membrane for nutrition exchange than a small cell

82. FIGURE 5-17 Surface area and volume relationships

83. Section 5.3 Outline
5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate?
Desmosomes Attach Cells Together
Tight Junctions Make Cell Attachments Leakproof
Gap junctions and Plasmodesmata Allow Direct Communication Between Cells

84. Desomosomes Desmosomes attach cells together
Found where cells need to adhere tightly together under the stresses of movement (e.g. the skin)

85. FIGURE 5-18a (part 1) Cell attachment structures
(a) Cells lining the small intestine are attached by desmosomes. Protein filaments bound to the inside surface of each desmosome extend into the cytosol and attach to other filaments inside the cell, strengthening the connection between cells.

86. FIGURE 5-18a (part 2) Cell attachment structures
(a) Cells lining the small intestine are attached by desmosomes. Protein filaments bound to the inside surface of each desmosome extend into the cytosol and attach to other filaments inside the cell, strengthening the connection between cells.

87. Tight Junctions Tight junctions make the cell leakproof
Found where tubes and sacs must hold contents without leaking (e.g. the urinary bladder)

88. FIGURE 5-18b (part 1) Cell attachment structures
(b) Tight junctions prevent leakage between cells, as between cells of the urinary bladder.

89. FIGURE 5-18b (part 2) Cell attachment structures
(b) Tight junctions prevent leakage between cells, as between cells of the urinary bladder.

90. Gap Junctions and Plasmodesmata
Gap junctions and plasmodesmata allow for communication
Cell-to-cell channels allowing for passage of hormones, nutrients, and ions in animal cells are gap junctions
Plant cells have cytoplasmic connections called plasmodesmata

91. FIGURE 5-19 Cell communication structures
(a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytosol of adjacent cells. (b) Plant cells are interconnected by plasmodesmata, which form cytosolic connections through the walls of adjacent cells.

92. FIGURE 5-19a (part 1) Cell communication structures
(a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytosol of adjacent cells.

93. FIGURE 5-19a (part 2) Cell communication structures
(a) Gap junctions, such as those between cells of the liver, contain cell-to-cell channels that interconnect the cytosol of adjacent cells.

94. FIGURE 5-19b (part 1) Cell communication structures
(b) Plant cells are interconnected by plasmodesmata, which form cytosolic connections through the walls of adjacent cells.

95. FIGURE 5-19b (part 2) Cell communication structures
(b) Plant cells are interconnected by plasmodesmata, which form cytosolic connections through the walls of adjacent cells.

96. Caribous Legs and Membranes
Membrane function varies between organisms

97. Caribous Legs and Membranes
Plasma membrane phospholipids in caribous legs adapted for cold
Cold areas near hooves have more membrane unsaturated fatty acids to keep cell membranes fluid

98. FIGURE 5-20 Caribou browse on the frozen Alaskan tundra
The lipid composition of the membranes in the cells of a caribou's legs varies with distance from the animal's trunk. Unsaturated phospholipids predominate in the lower leg; more saturated phospholipids are found in the upper leg.

99. Viscous Venoms
Snake and spider venoms contain phospholipases, enzymes that break down phospholipids
Venoms attack cell membranes, causing cells to rupture and die
Cell death destroys tissue around bite

100. FIGURE 5-21a Phospholipases in venoms can destroy cells
(a) A brown recluse spider bite on a person's forearm.

101. FIGURE 5-21b Phospholipases in venoms can destroy cells
(b) A rattlesnake bite on the forearm. Both show extensive tissue destruction caused by phospholipases.