1. Cell Membranes

2. 6.1 What Is the Structure of a Biological Membrane?
6 Cell Membranes
6.1 What Is the Structure of a Biological Membrane?
6.2 How Is the Plasma Membrane Involved in Cell Adhesion and Recognition?
6.3 What Are the Passive Processes of Membrane Transport?
6.4 What Are the Active Processes of Membrane Transport?
6.5 How Do Large Molecules Enter and Leave a Cell?

3. 6 Cell Membranes
The cell membrane regulates what enters and leaves the cytoplasm. Some cell membranes have pores called aquaporins that allow water to pass freely.
Opening Question:
Water purity is a worldwide problem. Can aquaporin membrane channels be used in water purification?

4. 6.1 What Is the Structure of a Biological Membrane?
The general structure of biological 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 6.1 The Fluid Mosaic Model
Figure 6.1 The Fluid Mosaic Model The general molecular structure of a biological membrane is a continuous phospholipid bilayer which has proteins embedded in it or associated with it.

6. 6.1 What Is the Structure of a Biological Membrane?
Phospholipids have a polar, hydrophilic “head” and hydrophobic fatty acid “tails.” In an aqueous environment, phospholipids form a bilayer.

7. Figure 3.22 Phospholipids (Part 1)
Figure Phospholipids (A) Phosphatidylcholine (lecithin) demonstrates the structure of a phospholipid molecule. In other phospholipids, the amino acid serine, the sugar alcohol inositol, or other compounds replace choline. (B) In an aqueous environment, hydrophobic interactions bring the “tails” of phospholipids together in the interior of a bilayer. The hydrophilic “heads” face outward on both sides of the bilayer, where they interact with the surrounding water molecules.

8. Figure 6.2 A Phospholipid Bilayer
Figure 6.2 A Phospholipid Bilayer The phospholipid bilayer separates two aqueous regions. The eight phospholipid molecules shown on the right represent a small cross section of a membrane bilayer.

9. 6.1 What Is the Structure of a Biological Membrane?
Artificial bilayers can be made in the laboratory.
Lipids maintain a bilayer organization spontaneously. This helps membranes fuse during phagocytosis, vesicle formation, etc.

10. 6.1 What Is the Structure of a Biological Membrane?
Lipid composition of membranes vary.
Phospholipids vary in fatty acid chain length, degree of saturation, and phosphate groups.

11. 6.1 What Is the Structure of a Biological Membrane?
Animal cell membranes may be up to 25% cholesterol, which is important for membrane integrity.

12. 6.1 What Is the Structure of a Biological Membrane?
The fatty acid tails make the interior somewhat fluid, allowing lateral movement of molecules.
Fluidity depends on temperature and lipid composition.

13. 6.1 What Is the Structure of a Biological Membrane?
Cholesterol and long-chain, saturated fatty acids pack tightly, making a less- fluid membrane. As temperature decreases, movement of molecules and cellular processes slow. Some organisms change the lipid content of the cell membranes when they get cold.

14. 6.1 What Is the Structure of a Biological Membrane?
Membranes also contain proteins; the number varies depending on membrane function.
Peripheral membrane proteins lack exposed hydrophobic groups and do not penetrate the bilayer.

15. 6.1 What Is the Structure of a Biological Membrane?
Integral membrane proteins have hydrophobic and hydrophilic regions or domains.
Some extend across the lipid bilayer; others are partially embedded.

16. Figure 6.3 Interactions of Integral Membrane Proteins
Figure 6.3 Interactions of Integral Membrane Proteins An integral membrane protein is held in the membrane by the distribution of the hydrophilic and hydrophobic side chains on its amino acids. The hydrophilic parts of the protein extend into the aqueous cell exterior and the internal cytoplasm. The hydrophobic side chains interact with the hydrophobic lipid core of the membrane.

17. 6.1 What Is the Structure of a Biological Membrane?
Freeze-fracturing is a technique that reveals proteins embedded in the phospholipid bilayers of cellular membranes.

18. Figure 6.4 Membrane Proteins Revealed by the Freeze-Fracture Technique
Figure 6.4 Membrane Proteins Revealed by the Freeze-Fracture Technique This HeLa cell (a human cell) membrane was first frozen to immobilize the lipids and proteins, and then fractured so that the bilayer was split open.

19. 6.1 What Is the Structure of a Biological Membrane?
The proteins and lipids interact noncovalently. But some membrane proteins have lipid groups covalently attached and are tethered to the lipid bilayer.

20. 6.1 What Is the Structure of a Biological Membrane?
Transmembrane proteins extend all the way through the phospholipid bilayer.
They have one or more transmembrane domains, and the domains on the inner and outer sides of the membrane can have specific functions.
Peripheral membrane proteins are located on one side of the membrane.

21. 6.1 What Is the Structure of a Biological Membrane?
Some membrane proteins can move freely within the bilayer, while some are anchored to a specific region. When cells are fused experimentally, some proteins from each cell distribute themselves uniformly around the membrane.

22. Figure 6.5 Rapid Diffusion of Membrane Proteins
Figure 6.5 Rapid Diffusion of Membrane Proteins Two animal cells can be fused together in the laboratory, forming a single large cell (heterokaryon). This phenomenon was used to test whether membrane proteins can diffuse independently in the plane of the plasma membrane.

23. Figure 6.5 Rapid Diffusion of Membrane Proteins (Part 1)
Figure 6.5 Rapid Diffusion of Membrane Proteins Two animal cells can be fused together in the laboratory, forming a single large cell (heterokaryon). This phenomenon was used to test whether membrane proteins can diffuse independently in the plane of the plasma membrane.

24. Figure 6.5 Rapid Diffusion of Membrane Proteins (Part 2)
Figure 6.5 Rapid Diffusion of Membrane Proteins Two animal cells can be fused together in the laboratory, forming a single large cell (heterokaryon). This phenomenon was used to test whether membrane proteins can diffuse independently in the plane of the plasma membrane.

25. 6.1 What Is the Structure of a Biological Membrane?
Membranes are dynamic and are constantly forming, transforming, fusing, and breaking down.

26. Figure 5.9 The Endomembrane System (Part 2)
Figure 5.9 The Endomembrane System Membranes of the nucleus, endoplasmic reticulum, and Golgi form a network connected by vesicles.

27. 6.1 What Is the Structure of a Biological Membrane?
Membranes also have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules. Glycolipids—carbohydrate + lipid Glycoproteins—carbohydrate + protein

28. Working with Data
A key experiment providing evidence for the fluid mosaic model used the technique of cell fusion to show that membrane proteins rapidly diffuse within the cell membrane.

29. Working with Data 6.1, Table 1

30. Working with Data 6.1: Rapid Diffusion of Membrane Proteins
Question 1: Plot the percentage of fully mixed cells over time. How long did it take for complete mixing?

31. Working with Data 6.1: Rapid Diffusion of Membrane Proteins
Question 2: What does your answer to Question 1 indicate about the rate of diffusion of the mouse and human proteins?

32. 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cells arrange themselves in groups by cell recognition and cell adhesion. These processes can be studied in sponge cells—the cells are easily separated and will come back together again.

33. Figure 6.6 Cell Recognition and Adhesion
Figure 6.6 Cell Recognition and Adhesion In most cases (including the aggregation of animal cells into tissues), the binding between molecules is homotypic (same to same).

34. 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Molecules involved in cell recognition and binding are glycoproteins. Binding of cells is usually homotypic: The same molecule sticks out from both cells and forms a bond. Some binding is heterotypic: The cells have different proteins.

35. 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cell junctions are specialized structures that hold cells together: • Tight junctions • Desmosomes • Gap junctions

36. Figure 6.7 Junctions Link Animal Cells Together (Part 1)
Tight junctions help ensure directional movement of materials.

37. Figure 6.7 Junctions Link Animal Cells Together (Part 2)
Desmosomes are like “spot welds.”

38. Figure 6.7 Junctions Link Animal Cells Together (Part 3)
Gap junctions allow communication.

39. 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cell membranes also adhere to the extracellular matrix. The transmembrane protein integrin binds to the matrix outside epithelial cells, and to actin filaments inside the cells. The binding is noncovalent and reversible.

40. 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cells can move within a tissue by the binding and reattaching of integrin to the extracellular matrix. This is important for cell movement within developing embryos and for the spread of cancer cells.

41. Figure 6.8 Integrins and the Extracellular Matrix
Figure 6.8 Integrins and the Extracellular Matrix (A) Integrins mediate the attachment of cells to the extracellular matrix. (B) Cell movements are mediated by integrin attachment.

42. 6.3 What Are the Passive Processes of Membrane Transport?
Membranes have selective permeability—some substances can pass through, but not others. Passive transport—no outside energy required (diffusion). Active transport—energy required.

43. 6.3 What Are the Passive Processes of Membrane Transport?
Energy for passive transport comes from the concentration gradient: the difference in concentration between one side of the membrane and the other.

44. 6.3 What Are the Passive Processes of Membrane Transport?
Particles in a solution move randomly until they are evenly distributed. At equilibrium, the particles continue to move, but there is no net change in distribution.

45. In-Text Art, Ch. 6, p. 113

46. 6.3 What Are the Passive Processes of Membrane Transport?
Diffusion: the process of random movement toward equilibrium. Net movement is directional until equilibrium is reached. Diffusion is the net movement from regions of greater concentration to regions of lesser concentration.

47. 6.3 What Are the Passive Processes of Membrane Transport?
Diffusion rate depends on:
Diameter of the molecules or ions
Temperature of the solution
Concentration gradient

48. 6.3 What Are the Passive Processes of Membrane Transport?
Diffusion works very well over short distances (e.g., within a cell). Membrane properties affect the diffusion of solutes. A membrane is permeable to solutes that move easily across it; impermeable to those that cannot.

49. 6.3 What Are the Passive Processes of Membrane Transport?
Simple diffusion: Small molecules pass through the lipid bilayer. Water and lipid-soluble molecules can diffuse across the membrane. Electrically charged and polar molecules can not pass through easily.

50. 6.3 What Are the Passive Processes of Membrane Transport?
Osmosis: the diffusion of water.
It depends on the relative concentrations of water molecules on each side of the membrane.
Hypertonic: higher solute concentration
Isotonic: equal solute concentrations
Hypotonic: lower solute concentration

51. Figure 6.9 Osmosis Can Modify the Shapes of Cells (Part 1)
Figure 6.9 Osmosis Can Modify the Shapes of Cells In a solution that is isotonic with the cytoplasm (B), a plant or animal cell maintains a consistent, characteristic shape because there is no net movement of water into or out of the cell. In a solution that is hypotonic to the cytoplasm (C), water enters the cell. An environment that is hypertonic to the cytoplasm (A) draws water out of the cell.

52. Figure 6.9 Osmosis Can Modify the Shapes of Cells (Part 2)
Figure 6.9 Osmosis Can Modify the Shapes of Cells In a solution that is isotonic with the cytoplasm (B), a plant or animal cell maintains a consistent, characteristic shape because there is no net movement of water into or out of the cell. In a solution that is hypotonic to the cytoplasm (C), water enters the cell. An environment that is hypertonic to the cytoplasm (A) draws water out of the cell.

53. Figure 6.9 Osmosis Can Modify the Shapes of Cells (Part 3)
Figure 6.9 Osmosis Can Modify the Shapes of Cells In a solution that is isotonic with the cytoplasm (B), a plant or animal cell maintains a consistent, characteristic shape because there is no net movement of water into or out of the cell. In a solution that is hypotonic to the cytoplasm (C), water enters the cell. An environment that is hypertonic to the cytoplasm (A) draws water out of the cell.

54. 6.3 What Are the Passive Processes of Membrane Transport?
If two solutions are separated by a membrane that allows water, but not solutes, to pass through:
Water will diffuse from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solute concentration).

55. 6.3 What Are the Passive Processes of Membrane Transport?
Water will diffuse (net movement) from a hypotonic solution across a membrane to a hypertonic solution.
Animal cells may burst when placed in a hypotonic solution.
Plant cells with rigid cell walls build up internal pressure that keeps more water from entering—turgor pressure.

56. 6.3 What Are the Passive Processes of Membrane Transport?
Facilitated diffusion of polar molecules (passive):
Channel proteins—integral membrane proteins that form a channel.
Carrier proteins—membrane proteins that bind some substances and speed their diffusion through the bilayer.

57. 6.3 What Are the Passive Processes of Membrane Transport?
Ion channels: Channel proteins with hydrophilic pores. Most are gated—can be closed or open to ion passage. Gate opens when protein is stimulated to change shape by a chemical signal (ligand) or an electrical charge difference (voltage-gated).

58. Figure 6.10 A Gated Channel Protein Opens in Response to a Stimulus
Figure A Gated Channel Protein Opens in Response to a Stimulus The channel protein has a pore of polar amino acids and water. It is anchored in the hydrophobic bilayer by its outer coating of nonpolar R groups. The protein changes its three-dimensional shape when a stimulus molecule (ligand) binds to it, opening the pore so that specific hydrophilic substances can pass through. Other (voltage) gated channels open in response to an electrical potential (voltage).

59. 6.3 What Are the Passive Processes of Membrane Transport?
The potassium channel allows K+ in the unhydrated state to pass through, but hydrated Na+ is too large to pass.

60. In-Text Art, Ch. 6, p. 116

61. 6.3 What Are the Passive Processes of Membrane Transport?
Water can cross a membrane by moving through special water channels called aquaporins. The function of these proteins was determined by injecting the aquaporin proteins into a frog oocyte.

62. Figure 6.11 Aquaporins Increase Membrane Permeability to Water
Figure Aquaporins Increase Membrane Permeability to Water A protein was isolated from the membranes of cells in which water diffuses rapidly across the membranes. When the protein was inserted into oocytes, which do not normally have it, the water permeability of the oocytes was greatly increased.

63. 6.3 What Are the Passive Processes of Membrane Transport?
Carrier proteins transport polar molecules such as glucose across membranes in both directions. Glucose binds to the protein, causing it to change shape and release the glucose on the other side.

64. Figure 6.12 A Carrier Protein Facilitates Diffusion (Part 1)
Figure A Carrier Protein Facilitates Diffusion The glucose transporter is a carrier protein that allows glucose to enter the cell at a faster rate than would be possible by simple diffusion. (A) The transporter binds to glucose, brings it into the membrane interior, then changes shape, releasing glucose into the cell cytoplasm. (B) The graph shows the rate of glucose entry via a carrier versus the concentration of glucose outside the cell. As the glucose concentration increases, the rate of diffusion increases until the point at which all the available transporters are being used (the system is saturated).

65. 6.3 What Are the Passive Processes of Membrane Transport?
In carrier-mediated transport, the rate of diffusion is limited by the number of carrier proteins in the cell membrane. When all carriers are loaded with solute, the diffusion system is saturated. Cells that need lots of energy (e.g., muscle cells) have many glucose transporters.

66. Figure 6.12 A Carrier Protein Facilitates Diffusion (Part 2)
Figure A Carrier Protein Facilitates Diffusion The glucose transporter is a carrier protein that allows glucose to enter the cell at a faster rate than would be possible by simple diffusion. (A) The transporter binds to glucose, brings it into the membrane interior, then changes shape, releasing glucose into the cell cytoplasm. (B) The graph shows the rate of glucose entry via a carrier versus the concentration of glucose outside the cell. As the glucose concentration increases, the rate of diffusion increases until the point at which all the available transporters are being used (the system is saturated).

67. 6.4 What Are the Active Processes of Membrane Transport?
Active transport: moves substances against a concentration and/or electrical gradient. Requires energy. The energy source is often adenosine triphosphate (ATP).

68. Table 6.1

69. 6.4 What Are the Active Processes of Membrane Transport?
Active transport is directional. It involves three kinds of proteins:
Uniporter—moves one substance in one direction
Symporter—moves two substances in one direction
Antiporter—moves two substances in opposite directions

70. Figure 6.13 Three Types of Proteins for Active Transport
Figure Three Types of Proteins for Active Transport Note that in each of the three cases, transport is directional. Symporters and antiporters are examples of coupled transporters. All three types of transporters are coupled to energy sources in order to move substances against their concentration gradients.

71. 6.4 What Are the Active Processes of Membrane Transport?
Primary active transport: requires direct hydrolysis of ATP. Secondary active transport: energy comes from an ion concentration gradient that is established by primary active transport.

72. 6.4 What Are the Active Processes of Membrane Transport?
The sodium–potassium (Na+–K+) pump is primary active transport. Found in all animal cells. The pump is an integral membrane glycoprotein (an antiporter).

73. Figure 6.14 Primary Active Transport: The Sodium–Potassium Pump
Figure Primary Active Transport: The Sodium–Potassium Pump In active transport, energy is used to move a solute against its concentration gradient. Here, energy from ATP is used to move Na+ and K+ against their concentration gradients.

74. 6.4 What Are the Active Processes of Membrane Transport?
In secondary active transport, energy can be “regained” by letting ions move across a membrane with the concentration gradient.
Aids in uptake of amino acids and sugars.
Uses symporters and antiporters.

75. Figure 6.15 Secondary Active Transport
Figure Secondary Active Transport The Na+ concentration gradient established by primary active transport (left) powers the secondary active transport of glucose (right). A symporter protein couples the movement of glucose across the membrane against its concentration gradient to the passive movement of Na+ into the cell.

76. 6.5 How Do Large Molecules Enter and Leave a Cell?
Macromolecules (proteins, polysaccharides, nucleic acids) are too large to cross the membrane. They can be taken in or secreted by means of membrane vesicles.

77. 6.5 How Do Large Molecules Enter and Leave a Cell?
Endocytosis: processes that brings molecules and cells into a eukaryotic cell. The plasma membrane folds in or invaginates around the material, forming a vesicle.

78. Figure 6.16 Endocytosis and Exocytosis
Figure Endocytosis and Exocytosis Endocytosis (A) and exocytosis (B) are used by eukaryotic cells to take up and release fluids, large molecules, and particles. Smaller cells, such as invading bacteria, can be taken up by endocytosis.

79. 6.5 How Do Large Molecules Enter and Leave a Cell?
Phagocytosis: molecules or entire cells are engulfed. Some protists feed in this way. Some white blood cells engulf foreign substances in this way. A food vacuole or phagosome forms, which fuses with a lysosome.

80. 6.5 How Do Large Molecules Enter and Leave a Cell?
Pinocytosis: a vesicle forms to bring small dissolved substances or fluids into a cell. Vesicles are much smaller than in phagocytosis. Pinocytosis is constant in endothelial (capillary) cells.

81. 6.5 How Do Large Molecules Enter and Leave a Cell?
Receptor mediated endocytosis is highly specific:
Depends on receptor proteins— integral membrane proteins—to bind to specific substances.
Sites are called coated pits—coated with other proteins such as clathrin.

82. Figure 6.17 Receptor-Mediated Endocytosis
Figure Receptor-Mediated Endocytosis The receptor proteins in a coated pit bind specific macromolecules, which are then carried into the cell by a coated vesicle.

83. 6.5 How Do Large Molecules Enter and Leave a Cell?
Mammalian cells take in cholesterol by receptor-mediated endocytosis. In the liver, cholesterol is packaged into low-density lipoprotein, or LDL, and secreted to the bloodstream. Cells that need cholesterol have receptors for the LDLs in clathrin- coated pits.

84. 6.5 How Do Large Molecules Enter and Leave a Cell?
Exocytosis: material in vesicles is expelled from a cell. Indigestible materials are expelled. Other materials leave cells such as digestive enzymes and neurotransmitters.

85. Table 6.2

86. 6 Answer to Opening Question
Aquaporins in both animals and plants are similar in structure. Aquaporins are being inserted into synthetic membranes to purify drinking water—they allow only water to pass through, not solutes.