1. Chapter 04 Membrane Structure and Function
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2. 4.1 Plasma Membrane Structure and Function (1)
The plasma membrane separates the internal environment of the cell from its external environment.
It regulates the entrance and exit of molecules into and out of the cell.
The steady internal environment maintained is called homeostasis.

3. 4.1 Plasma Membrane Structure and Function (2)
Phospholipid bilayer with embedded proteins
Hydrophilic (water-loving) polar heads
Face inside and outside of cell (water present)
Hydrophobic (water-fearing) nonpolar tails
Face each other, away from water
Cholesterol (animal cells) controls excess fluidity

4. 4.1 Plasma Membrane Structure and Function (3)
Membrane proteins throughout membrane may be:
Peripheral proteins – associated with only one side of membrane
Integral proteins – span the membrane
Can protrude from one or both sides
Embedded within the membrane
Able to move laterally

5. 4.1 Plasma Membrane Structure and Function (4)
Both phospholipids and protein can have attached carbohydrate chains.
Glycolipids are lipids attached to carbohydrates.
Glycoproteins are proteins attached to carbohydrates.

6. Figure 4.1
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7. Functions of Membrane Proteins (1)
Channel proteins are involved in the passage of solutes through the membrane.
Substances simply move across the membrane.
Some may contain a gate that must be opened in response to a signal.
Carrier proteins allow the passage of a solute by combining with it and helping it to move across the membrane.

8. Functions of Membrane Proteins (2)
Cell recognition proteins are glycoproteins.
These proteins help the body recognize when it is being invaded by pathogens.
Receptor proteins have a shape that allows a specific molecule to bind.
The binding causes the receptor to change shape to initiate a cellular response.

9. Functions of Membrane Proteins (3)
Enzymatic proteins carry out metabolic reactions directly.
Example: The proteins of the electron transport chain, which carry out the final steps of aerobic respiration

10. 4.1 Plasma Membrane Structure and Function (5)
5 Membrane Protein Functions (1)
Figure 4.2a
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11. 5 Membrane Protein Functions (2)
Figure 4.2b-c
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12. 5 Membrane Protein Functions (3)
Figure 4.2d-e
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13. 4.2 Permeability of the Plasma Membrane (1)
The plasma membrane can regulate the passage of molecules into and out of the cell because it is selectively permeable.
Which molecules can freely cross the membrane and which may require carrier proteins and/or energy depends on:
Size
Nature of molecule – polarity, charge

14. 4.2 Permeability of the Plasma Membrane (2)
Small, uncharged molecules freely cross membrane
Examples: CO2, O2, glycerol, and alcohol
Slip in between the hydrophilic heads and pass through hydrophobic tails
Driven by the concentration gradient

15. 4.2 Permeability of the Plasma Membrane (3)
Concentration gradient
More of a substance on one side of the membrane
Going “down” a concentration gradient
From an area of higher to lower concentration
Going “up” a concentration gradient
From an area of lower to higher concentration
Requires input of energy

16. 4.2 Permeability of the Plasma Membrane (4)
Water which is polar would not be expected to readily cross the membrane.
Aquaporins are special channels that allow water to cross the membrane.
Aquaporins are present in the majority of cells.

17. 4.2 Permeability of the Plasma Membrane (5)
Large molecules, ions, and charged molecules are unable to freely cross the membrane, but can cross the membrane via:
Channel proteins forming a pore through the membrane
Carrier proteins that are specific for substance they transport
Vesicle formation in endocytosis or exocytosis

18. Figure 4.3
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19. Passage of Molecules Into and Out of the Cell
TABLE 4.1 Passage of Molecules Into and Out of the Cell
Name
Direction
Requirement
Examples
Diffusion
-Energy Not Required
Toward lower concentration
Concentration gradient
Lipid-soluble molecules and gases
Facilitated transport
Channels or carrier and concentration gradient
Some sugars and some amino acids
Active transport
-Energy Required
Toward higher concentration
Carrier plus energy
Sugars, amino acids, and ions
Exocytosis
Toward outside
Vesicle fuses with plasma membrane
Macromolecules
Endocytosis
Toward inside
Vesicle formation
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20. Diffusion and Osmosis (1)
Movement of molecules from an area of higher to lower concentration
Down a concentration gradient
Occurs until equilibrium is reached
For example, when a crystal of dye is placed in water the dye and water molecules move about until equilibrium occurs
Solution contains a solute (solid) and a solvent (liquid)

21. Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction.
Figure 4.4
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22. Gases can diffuse through a membrane
Oxygen and carbon dioxide enter and exit this way O2 O2 O2 O2 O2 O2
Figure 4.5
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23. Diffusion and Osmosis (2)
Several factors influence the rate of diffusion:
Temperature
As temperature increases, the rate of diffusion increases.
Pressure
Electrical currents
Molecular size

24. Osmosis (1)
Diffusion of water across a differentially permeable membrane
Diffusion always occurs from higher to lower concentration.
Osmotic pressure is the pressure that develops in a system due to osmosis.
The greater the possible osmotic pressure, the more likely it is that water will diffuse in that direction.

25. Membrane is not permeable to solute
Figure 4.6
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26. Osmosis (2)
Isotonic: the solute concentration is equal inside and outside of a cell
Hypotonic: a solution has a lower solute concentration than the inside of a cell
Hypertonic: a solution has a higher solute concentration than the inside of a cell

27. Isotonic: Hypotonic: Hypertonic: No net gain or loss of water
0.9% NaCl
Hypotonic:
Cell gains water
Cytolysis – hemolysis
Hypertonic:
Cell loses water
Crenation
Figure 4.7a
all photos: © David M. Phillips/Science Source
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28. Isotonic: Hypotonic: Hypertonic: No net gain or loss of water
Cell gains water
Turgor pressure keeps plant erect – cell wall
Hypertonic:
Cell loses water
Plasmolysis
Figure 4.7b
left: © Dwight Kuhn; middle: © Dwight Kuhn; right: © Ed Reschke/Getty Images
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29. Top: all photos: © David M
Top: all photos: © David M. Phillips/Science Source Bottom: left: © Dwight Kuhn; middle: © Dwight Kuhn; right: © Ed Reschke/Getty Images
Figure 4.7c
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30. Transport by Carrier Proteins (1)
The plasma membrane impedes the passage of all but a few substances.
Substances enter or exit cells because of carrier proteins.
Carrier proteins are specific:
Combine with a molecule or ion to be transported across the membrane
Change shape to move molecules across membranes

31. Transport by Carrier Proteins (2)
Carrier proteins are required for:
Facilitated transport
Active transport

32. Facilitated Transport (1)
Facilitated transport explains the passage of molecules such as glucose or amino acids.
Neither molecule is lipid-soluble
Reversible combination and transport occurs
Like diffusion, ATP is not required because molecules are transported down their concentration gradient

33. Facilitated Transport (2)
Small molecules that are not lipid-soluble
Molecules follow the concentration gradient
Energy is not required
Figure 4.8
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34. Active Transport (1) Active Transport:
Molecules or ions combine with carrier proteins
Often called pumps
Molecules move against the concentration gradient
Entering or leaving cell
Accumulate either inside or outside the cell
Energy and carrier proteins are required
Usually ATP is used

35. Active Transport (2)
Proteins in active transport are referred to as pumps.
Proteins use energy to move molecules against the concentration gradient.
Na+/K+ pump is especially important for nerve and muscle cells –it moves Na+ out and K+ into cells.
The carrier changes shape after phosphate attaches, and then again after it detaches.

36. The Sodium-potassium Pump (1)
Figure 4.9a
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37. The Sodium-potassium Pump (2)
Figure 4.9b
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38. The Sodium-potassium Pump (3)
Figure 4.9c
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39. The Sodium-potassium Pump (4)
Figure 4.9d
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40. The Sodium-potassium Pump (5)
Figure 4.9e
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41. The Sodium-potassium Pump (6)
Figure 4.9f
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42. Bulk Transport
Macromolecules are transported into or out of cells by vesicle formation.
Macromolecules are too large to be transported by carrier proteins.
Energy is required to form vesicles.
Vesicle formation is called membrane-assisted transport.
Exocytosis – exit from cell
Endocytosis – enter into cell

43. Exocytosis The vesicle fuses with plasma membrane as secretion occurs.
The vesicle membrane becomes part of the plasma membrane.
Cells of particular organs are specialized to produce and export molecules.
Pancreatic cells release insulin or enzymes.
Anterior pituitary cells release growth hormone.

44. Figure 4.10
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45. Endocytosis (1) Cells take in substances by vesicle formation.
Part of the plasma membrane invaginates to envelop the substance.
The membrane then pinches off to form an intracellular vesicle.
Three types of endocytosis:
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis

46. Endocytosis (2)
Phagocytosis: large, particulate matter such as “food” molecules, viruses or whole cells
Amoeba and macrophages
Pinocytosis: liquids and small particles dissolved in liquid
Certain blood cells or plant root cells
Receptor Mediated Endocytosis: a type of pinocytosis that involves a coated pit
Certain placental cells

47. Phagocytosis
amoeba ingesting paramecium: © Eric Grave/Phototake; vesicles: © Don W. Fawcett/Science Source; coated pit (both): © Mark Brestscher
Figure 4.11a
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48. Pinocytosis
amoeba ingesting paramecium: © Eric Grave/Phototake; vesicles: © Don W. Fawcett/Science Source; coated pit (both): © Mark Brestscher
Figure 4.11b
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49. Receptor-mediated Endocytosis
amoeba ingesting paramecium: © Eric Grave/Phototake; vesicles: © Don W. Fawcett/Science Source; coated pit (both): © Mark Brestscher
Figure 4.11c
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50. 4.3 Modifications of Cell Surfaces
Cells live and interact with external environment.
Extracellular environment is made of large molecules produced by nearby cells.
Materials are deposited by secretion.
Plants, prokaryotes, and fungi are surrounded by cell walls.
Animals have more varied extracellular environments that can change.

51. Cell Surfaces in Animals
Animal cells have two different types of cell surfaces:
Extracellular matrix outside of cells
Junctions that occur between cells
Both can associate with the cytoskeleton and contribute to cell-to-cell communication.

52. Extracellular Matrix (1)
A meshwork of proteins and polysaccharides closely associated with cells that produced them
Common structural proteins in ECM
Collagen resists stretching
Elastin provide resilience to ECM
Fibronectin is an adhesive protein that links integrin

53. Extracellular Matrix (2)
Polysaccharides made of amino sugars in ECM attach to proteins called proteoglycans.
Proteoglycans attach to a long, centrally placed polysaccharide
Resist compression of ECM
Assist cell signaling by regulating the passage of molecules through ECM to plasma membrane

54. Junctions Between Cells
Cell surfaces in certain tissues of animals
Junctions Between Cells
Adhesion Junctions
Intercellular filaments between cells
Tight Junctions
Form impermeable barriers between cells
Gap Junctions
Plasma membrane channels are joined (allows communication)

55. Plant Cell Walls (1) All plant cells have a cell wall.
It contains cellulose as the main component.
Pectins allow the walls to stretch as cells grow.
Noncellulose polysaccharides harden the wall as the cell matures.
Pectin is abundant in the middle lamella, a layer of adhesive substances that holds cells together.

56. Plant Cell Walls (2)
Plasmodesmata are narrow channels that penetrate the cell wall to connect adjacent cells.
Each channel contains a strand of cytoplasm.
Cytoplasm allows exchange of materials between cells.
Only water and small solutes pass freely.
Cytoplasm connects all the cells within a plant.