1. Membrane Structure and Membrane Proteins
Presenter:
Mrs. Knopke
FUHS Science Dept.

2. Fluid Mosaic Model of a Biological Membrane

3. Summary Section 2 – pages 175-178
Plasma Membrane
Water
Summary Section 2 – pages

4. Phospholipids:
Hydrophobic Molecules fatty acid tails are sheltered from water
Hydrophilic molecules phosphate groups interact with water.

5. Summary Section 2 – pages 175-178
Structure of the Plasma Membrane
The plasma membrane is composed of two layers of phospholipids back-to-back.
Phospholipids are lipids with a phosphate attached to them.
Summary Section 2 – pages

6. Phospholipid Bi-layer
The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water.

7. Summary Section 2 – pages 175-178
Phosphate Group
The lipids in a plasma membrane have a glycerol backbone, two fatty acid chains, and a phosphate group.
Glycerol Backbone
Two Fatty Acid Chains
Summary Section 2 – pages

8. Summary Section 2 – pages 175-178
Makeup of the phospholipid bilayer
The phosphate group is critical for the formation and function of the plasma membrane.
Phosphate Group
Summary Section 2 – pages

9. Summary Section 2 – pages 175-178
Makeup of the phospholipid bilayer
The fluid mosaic model
Summary Section 2 – pages

10. Fluidity of Membranes
Phospholipids in membranes are in a fluid state.

11. Summary Section 2 – pages 175-178
Other components of the plasma membrane: Cholesterol plays the important role of preventing the fatty acid chains of the phospholipids from sticking together.
Cholesterol
Molecule
Summary Section 2 – pages

12. Membrane fluidity is influenced by temperature and by its constituents.
As temperatures cool, it maintains fluidity by preventing tight packing.
Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails prevent tight packing.
Fig. 8.4b
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13. The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animals cells.
At warm temperatures, it restrains the movement of phospholipids and reduces fluidity.
At cool temperatures, it maintains fluidity by preventing tight packing.
Fig. 8.4c
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14. Membranes are fluid Membrane molecules are held in place by relatively
weak hydrophobic interactions.
Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other.
Fig. 8.4a
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15. Membrane Proteins and the Fluid Mosaic Model
Some electron micrographs show the positionof proteins within membranes.
The proteins are seen to be dotted over the membrane
This gives the membranes the appearance of a mosaic.
Because the protein molecules float in the fluid phospholipid bilayer.

16. A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholid bilayer prior to electron microscopy.
This shows protein particles interspersed with a smooth matrix, supporting the fluid mosaic model.
Fig. 8.3
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17. To work properly with active enzymes and appropriate permeability, membrane must be fluid, about as fluid as salad oil.
Cells can alter the lipid composition of membranes to compensate for changes in fluidity caused by changing temperatures.
For example, cold-adapted organisms, such as winter wheat, increase the percentage of unsaturated phospholipids in the autumn.
This allows these organisms to prevent their membranes from solidifying during winter.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

18. Summary Section 2 – pages 175-178
Other components of the plasma membrane:
Transport proteins allow needed substances or waste materials to move through the plasma membrane.
Click image to view movie.
Summary Section 2 – pages

19. Proteins determine most of the membrane’s specific functions.
The plasma membrane and the membranes of the various organelles each have unique collections of proteins.
There are two populations of membrane proteins.
Peripheral proteins are not embedded in the lipid bilayer at all.
Instead, they are loosely bounded to the surface of the protein, often connected to the other population of membrane proteins.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

20. Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein).
Where they contact the core, they have hydrophobic regions with with nonpolar amino acids, often coiled into alpha helices.
Where they are in contact with the aqueous environment, they have hydrophilic regions of amino acids.
Fig. 8.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

21. Membranes have distinctive inside and outside faces.
The two layers may differ in lipid composition, and proteins in the membrane have a clear direction.
The outer surface also has carbohydrates.
This asymmetrical orientation begins during synthesis of new membrane in the endoplasmic reticulum.
Fig. 8.8
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22. The proteins in the plasma membrane may provide a variety of major cell functions.
Fig. 8.9
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23. Membrane carbohydrates are important for cell-cell recognition
The membrane plays the key role in cell-cell recognition.
Cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another.
This attribute is important in cell sorting and organization as tissues and organs in development.
It is also the basis for rejection of foreign cells by the immune system.
Cells recognize other cells by keying on surface molecules, often carbohydrates, on the plasma membrane.

24. Fluid Mosaic Model of a Biological Membrane

25. Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units.
They may be covalently bonded either to lipids, forming glycolipids, or, more commonly, to proteins, forming glycoproteins.
The oligosaccharides on the external side of the plasma membrane vary from species to species,
individual to individual, and even from cell type to cell type within the same individual.
This variation marks each cell type as distinct.
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

26. Question 1 A. protein synthesis B. selective permeability
Which of the following best describes the plasma membrane's mechanism in maintaining homeostasis?
A. protein synthesis
B. selective permeability
C. fluid composition
D. structural protein attachment
CA: Biology/Life Sciences 1a
Section 2 Check

27. The answer is B. Selective permeability is the process in which the membrane allows some molecules to pass through, while keeping others out.
CA: Biology/Life Sciences 1a
Section 2 Check

28. Question 2 Describe the structure of the plasma membrane.
CA: Biology/Life Sciences 1a
Section 2 Check

29. The plasma membrane is composed of a phospholipid bilayer, which has two layers of phospholipids back-to-back. The polar heads of phospholipid molecules contain phosphate groups and face outward.
CA: Biology/Life Sciences 1a
Section 2 Check

30. Question 3 Phospholipid molecule
Why is the phosphate group of a phospholipid important to the plasma membrane?
Polar head (includes phosphate group)
Nonpolar tails (fatty acids)
CA: Biology/Life Sciences 1a
Section 2 Check

31. When phospholipid molecules form a bilayer, the phosphate groups lie to the outside. Because phosphate groups are polar, they allow the cell membrane to interact with its watery (polar) environments inside and outside the cell.
Phospholipid molecule
Polar head (includes phosphate group)
Nonpolar tails (fatty acids)
CA: Biology/Life Sciences 1a
Section 2 Check

32. Question 4
Explain why the model of the plasma membrane is called the fluid mosaic model.
CA: Biology/Life Sciences 1a
Section 2 Check

33. It is fluid because the phospholipid molecules move within the membrane. Proteins in the membrane that move among the phospholipids create the mosaic pattern.
CA: Biology/Life Sciences 1a
Section 2 Check

34. Summary Section 2 – pages 152-156
Early observations: Bownian motion
In 1827, Scottish scientist Robert Brown used a microscope to observe pollen grains suspended in water. He noticed that the grains moved constantly in little jerks, as if being struck by invisible objects.
This motion is now called Brownian motion.
Today we know that Brown was observing evidence of the random motion of atoms and molecules.
Summary Section 2 – pages

35. Kinetic Energy
Heat is kinetic energy while temperature is a measure of the average kinetic energy per particle
Ek= ½ MV2
where Ek = kinetic energy M= mass of the particle and V= the velocity of the particle.

36. Brownian motion
Heat motion of small particles in water

37. Summary Section 2 – pages 152-156
The process of diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
Diffusion results because of the random movement of particles (Brownian motion).
Three key factors—concentration, temperature, and pressure—affect the rate of diffusion.
Summary Section 2 – pages

38. Terms
CONCENTRAION of molecules in a fluid is the number of molecules of solute in a given unit of volume
GRADIENT is the physical difference between two regions of space such that molecules tend to move from one region to the other. Ex. = pressure, concentration, and electrical charge

39. Summary Section 2 – pages 152-156
The results of diffusion
When a cell is in dynamic equilibrium with its environment, materials move into and out of the cell at equal rates.
As a result, there is no net change in concentration inside or outside the cell.
Material moving out of cell equals material moving into cell
Summary Section 2 – pages

40. Summary Section 2 – pages 152-156
Diffusion in living systems
The difference in concentration of a substance across space is called a concentration gradient.
Ions and molecules diffuse from an area of higher concentration to an area of lower concentration, moving with the gradient.
Dynamic equilibrium occurs when there is no longer a concentration gradient.
Summary Section 2 – pages

41. Diffusion - the movement of particles from an area of high concentration to an area of low concentration

42. Factors that affect speed of diffusion
Molecular Mass
Type of medium in which diffusion is occurring
Temperature
Steepness of the concentration gradient

43. Section 8.1 Summary – pages 195 - 200
Osmosis: Diffusion of Water
The diffusion of water across a selectively permeable membrane is called osmosis.
Regulating the water flow through the plasma membrane is an important factor in maintaining homeostasis within a cell.
Section 8.1 Summary – pages

44. In which direction will water flow?
                                      

45. Tonicity

46. Section 8.1 Summary – pages 195 - 200
What controls osmosis?
Unequal distribution of particles, called a concentration gradient, is one factor that controls osmosis.
Before
Osmosis
After
Osmosis
Water molecule
Sugar molecule
Selectively permeable membrane
Section 8.1 Summary – pages

47. Cell survival depends on balancing water uptake and loss
An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane.
Water flows across the membrane, but at the same rate in both directions.
The volume the cell is no net movement of water across its plasma membrane.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

48. Section 8.1 Summary – pages 195 - 200
Cells in an isotonic solution
Most cells whether in multicellular or unicellular organisms, are subject to osmosis because they are surrounded by water solutions.
H2O
H2O
Water Molecule
Dissolved Molecule
Section 8.1 Summary – pages

49. A cell in a hypotonic solution will gain water, swell, and burst.
The same cell is a hypertonic environment will loose water, shrivel, and probably die.
A cell in a hypotonic solution will gain water, swell, and burst.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

50. For a cell living in an isotonic environment (for example, many marine invertebrates) osmosis is not a problem.
Similarly, the cells of most land animals are bathed in an extracellular fluid that is isotonic to the cells.
Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation to maintain their internal environment.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

51. Section 8.1 Summary – pages 195 - 200
Cells in an isotonic solution
In an isotonic solution, the concentration of dissolved substances in the solution is the same as the concentration of dissolved substances inside the cell.
H2O
H2O
Water Molecule
Dissolved Molecule
Section 8.1 Summary – pages

52. Section 8.1 Summary – pages 195 - 200
Cells in an isotonic solution
In an isotonic solution, water molecules move into and out of the cell at the same rate, and cells retain their normal shape.
H2O
H2O
Water Molecule
Dissolved Molecule
Section 8.1 Summary – pages

53. Section 8.1 Summary – pages 195 - 200
Cells in an isotonic solution
A plant cell has its normal shape and pressure in an isotonic solution.
Section 8.1 Summary – pages

54. For example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives.
In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell.
To solve this problem, Paramecium have a specialized organelle, the contractile vacuole, that functions as a bilge pump to force water out of the cell.
Fig. 8.13
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55. The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell’s water balance.
An animal cell in a hypotonic solution will swell until the elastic wall opposes further uptake.
At this point the cell is turgid, a healthy state for most plant cells.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

56. Section 8.1 Summary – pages 195 - 200
Cells in a hypotonic solution
In a hypotonic solution, water enters a cell by osmosis, causing the cell to swell.
H2O
H2O
Water Molecule
Dissolved Molecule
Section 8.1 Summary – pages

57. Section 8.1 Summary – pages 195 - 200
Cells in a hypotonic solution
Plant cells swell beyond their normal size as pressure increases.
Section 8.1 Summary – pages

58. Turgid cells contribute to the mechanical support of the plant.
If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell flaccid and the plant may wilt.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

59. Section 8.1 Summary – pages 195 - 200
Cells in a hypertonic solution
In a hypertonic solution, water leaves a cell by osmosis, causing the cell to shrink.
H2O
H2O
Water Molecule
Dissolved Molecule
Section 8.1 Summary – pages

60. In a hypertonic solution, a cell wall has no advantages.
As the plant cell looses water, its volume shrinks.
Eventually, the plasma membrane pulls away from the wall.
This plasmolysis is usually lethal.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

61. Section 8.1 Summary – pages 195 - 200
Cells in a hypertonic solution
Plant cells lose pressure as the plasma membrane shrinks away from the cell wall.
Section 8.1 Summary – pages

62. Turgor Pressure
Turgor pressure occurs when the central vacuole fills and applies pressure to the cell wall.

63. Section 8.1 Summary – pages 195 - 200
Passive Transport
When a cell uses no energy to move particles across a membrane passive transport occurs.
Concentration gradient
Plasma membrane
Section 8.1 Summary – pages

64. Section 8.1 Summary – pages 195 - 200
Passive Transport by proteins
Passive transport of materials across the membrane using transport proteins is called facilitated diffusion.
Channel proteins
Concentration gradient
Plasma membrane
Section 8.1 Summary – pages

65. Section 8.1 Summary – pages 195 - 200
Passive Transport by proteins
Some transport proteins, called channel proteins, form channels that allow specific molecules to flow through.
Channel proteins
Concentration gradient
Plasma membrane
Section 8.1 Summary – pages

66. Section 8.1 Summary – pages 195 - 200
Passive transport by proteins
The movement is with the concentration gradient, and requires no energy input from the cell.
Carrier proteins
Concentration gradient
Plasma membrane
Step 1
Step 2
Section 8.1 Summary – pages

67. Section 8.1 Summary – pages 195 - 200
Passive transport by proteins
Carrier proteins change shape to allow a substance to pass through the plasma membrane.
Carrier proteins
Concentration gradient
Plasma membrane
Step 1
Step 2
Section 8.1 Summary – pages

68. Section 8.1 Summary – pages 195 - 200
Passive transport by proteins
In facilitated diffusion by carrier protein, the movement is with the concentration gradient and requires no energy input from the cell.
Carrier proteins
Concentration gradient
Plasma membrane
Step 1
Step 2
Section 8.1 Summary – pages

69. Section 8.1 Summary – pages 195 - 200
Active Transport
Movement of materials through a membrane against a concentration gradient is called active transport and requires energy from the cell.
Carrier proteins
Plasma membrane
Concentration gradient
Cellular energy
Step 1
Step 2
Section 8.1 Summary – pages

70. Section 8.1 Summary – pages 195 - 200
How active transport occurs
In active transport, a transport protein called a carrier protein first binds with a particle of the substance to be transported.
Carrier proteins
Concentration gradient
Plasma membrane
Cellular energy
Step 1
Step 2
Section 8.1 Summary – pages

71. Section 8.1 Summary – pages 195 - 200
How active transport occurs
Click image to view movie.
Section 8.1 Summary – pages

72. Section 8.1 Summary – pages 195 - 200
How active transport occurs
Each type of carrier protein has a shape that fits a specific molecule or ion.
Carrier proteins
Concentration gradient
Plasma membrane
Cellular energy
Step 1
Step 2
Section 8.1 Summary – pages

73. Section 8.1 Summary – pages 195 - 200
How active transport occurs
When the proper molecule binds with the protein, chemical energy allows the cell to change the shape of the carrier protein so that the particle to be moved is released on the other side of the membrane.
Carrier proteins
Concentration gradient
Plasma membrane
Cellular energy
Step 1
Step 2
Section 8.1 Summary – pages

74. Section 8.1 Summary – pages 195 - 200
How active transport occurs
Once the particle is released, the protein’s original shape is restored.
Active transport allows particle movement into or out of a cell against a concentration gradient.
Carrier proteins
Concentration gradient
Plasma membrane
Cellular energy
Step 1
Step 2
Section 8.1 Summary – pages

75. Section 8.1 Summary – pages 195 - 200
How active transport occurs
Click image to view movie.
Section 8.1 Summary – pages

76. Section 8.1 Summary – pages 195 - 200
Transport of Large Particles
Endocytosis is a process by which a cell surrounds and takes in material from its environment.
Nucleus
Wastes
Digestion
Endocytosis
Exocytosis
Section 8.1 Summary – pages

77. Section 8.1 Summary – pages 195 - 200
Transport of Large Particles
The material is engulfed and enclosed by a portion of the cell’s plasma membrane.
Nucleus
Wastes
Digestion
Endocytosis
Exocytosis
Section 8.1 Summary – pages

78. Section 8.1 Summary – pages 195 - 200
Transport of Large Particles
The resulting vacuole with its contents moves to the inside of the cell.
Nucleus
Wastes
Digestion
Endocytosis
Exocytosis
Section 8.1 Summary – pages

79. Section 8.1 Summary – pages 195 - 200
Transport of Large Particles
Exocytosis is the expulsion or secretion of materials from a cell.
Nucleus
Wastes
Digestion
Exocytosis
Endocytosis
Section 8.1 Summary – pages

80. Section 8.1 Summary – pages 195 - 200
Transport of Large Particles
Endocytosis and exocytosis both move masses of material and both require energy.
Nucleus
Wastes
Digestion
Endocytosis
Exocytosis
Section 8.1 Summary – pages

81. Question 1 A. active transport B. endocytosis
The diffusion of water across a selectively permeable membrane is called __________.
Water molecule
Selectively permeable membrane
Sugar molecule
A. active transport
B. endocytosis
CA: Biology/Life Sciences 1a
Section 1 Check

82. Question 1 C. exocytosis D. osmosis
The diffusion of water across a selectively permeable membrane is called __________.
Water molecule
Selectively permeable membrane
Sugar molecule
C. exocytosis
D. osmosis
CA: Biology/Life Sciences 1a
Section 1 Check

83. The answer is D, osmosis. Regulating the water flow through the plasma membrane is an important factor in maintaining homeostasis within the cell.
Before osmosis
After osmosis
Water molecule
Selectively permeable membrane
Sugar molecule
CA: Biology/Life Sciences 1a
Section 1 Check

84. Question 2 A. The cell shrivels up. C. The cell swells up.
What is the expected result of having an animal cell in a hypertonic solution?
A. The cell shrivels up.
B. The plasma membrane shrinks away from
the cell wall.
C. The cell swells up.
D. The cell retains its normal shape.
CA: Biology/Life Sciences 1a
Section 1 Check

85. The answer is A. In a hypertonic solution, cells experience osmosis of water out of the cell. Animal cells shrivel because of decreased pressure in the cells.
H2O
H2O
Water molecule
Sugar molecule
CA: Biology/Life Sciences 1a
Section 1 Check

86. Question 3 A. isotonic B. hypotonic C. hypertonic D. exotonic
A grocer mists the celery display with water to keep it looking fresh. What type of solution is the celery now in?
A. isotonic
B. hypotonic
C. hypertonic
D. exotonic
CA: Biology/Life Sciences 1a, 1j
Section 1 Check

87. The answer is B. Plant cells contain a rigid cell wall and do not burst even in a hypotonic solution.
CA: Biology/Life Sciences 1a, 1j
Section 1 Check

88. Concentration gradient
Question 4
Transport of materials across the plasma membrane that does not require energy from the cell but does use transport proteins is called __________.
Channel proteins
A. osmosis
B. simple diffusion
Concentration gradient
Plasma membrane
CA: Biology/Life Sciences 1a
Section 1 Check

89. Concentration gradient
Question 4
Transport of materials across the plasma membrane that does not require energy from the cell but does use transport proteins is called __________.
Channel proteins
C. facilitated diffusion
Concentration gradient
Plasma membrane
D. active transport
CA: Biology/Life Sciences 1a
Section 1 Check

90. Concentration gradient
The answer is C. Facilitated diffusion is a type of passive transport and requires no energy from the cell.
Channel proteins
Concentration gradient
Plasma membrane
CA: Biology/Life Sciences 1a
Section 1 Check