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CELL MEMBRANE STRUCTURE AND FUNCTION
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FLUID MOSAICS OF LIPIDS AND PROTEINS
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COMPOSITION
Phospholipids Give membrane hydrophilic and
hydrophobic properties Phosphate – hydrophilic, face aqueous exterior
or interior Fatty acid – hydrophobic, face each other inside
the membrane
Proteins Cholesterol, Glycolipids, and
Glycoproteins
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COMPOSITION
Phospholipids Proteins: many different roles
Hydrophilic Have charged and polar side groups
Hydrophobic Have nonpolar side groups
Cholesterol, Glycolipids, and Glycoproteins
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COMPOSITION
Phospholipids Proteins Cholesterol, Glycolipids, and
Glycoproteins Cholesterol: resists changes in membrane
fluidity caused by changes in temperature Glycolipids & Glycoproteins: have a
carbohydrate attached; cell-cell recognition
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SELECTIVE PERMEABILITY
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THE PURPOSE: TO SEPARATE THE INTERNAL FROM THE EXTERNAL ENVIRONMENT
Can Pass Through: Small, Uncharged polar Small Nonpolar
Can NOT Pass Through (without assistance): Hydrophilic substances Large Polar Ions Water
Note: Plant Cell Walls are made of cellulose and are external to the cell membrane. They are also found in Prokaryotes and Fungi.
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PASSIVE AND ACTIVE TRANSPORT
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PASSIVE TRANSPORT
Does not require the input of energy Net movement of molecules from high to
low concentration Used to import resources and export wastes Water moves across the membrane through
proteins called “aquaporins.” Osmosis: the diffusion of water across a
selectively permeable membrane. Tonicity: the ability of a solution to cause a
cell to gain or lose water
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PASSIVE TRANSPORT
Solutions outside a cell may be… Isotonic – same concentration of solutes as the interior of the cell; water moves across membrane at the same rate in both directions
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KEY: term – exterior solution; effect – direction of water flow
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PASSIVE TRANSPORT
Solutions outside a cell may be… Hypotonic – lower concentration of solute than the interior of the cell; water moves across membrane into the cell faster than it flows out, causing it to swell and lyse (burst)
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KEY: term – exterior solution; effect – direction of water flow
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PASSIVE TRANSPORT
Solutions outside a cell may be… Hypertonic - higher concentration of solute than the interior of the cell; water moves across membrane out of the cell faster than it flows in, causing it to shrivel and possibly die
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KEY: term – exterior solution; effect – direction of water flow
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ACTIVE TRANSPORT
Requires energy to move molecules from low to high concentration
Free energy often from ATP Uses membrane proteins
Example: Sodium-potassium pump
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EXOCYTOSIS AND ENDOCYTOSIS
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MOVING LARGE MOLECULES BETWEEN THE INTERNAL AND EXTERNAL ENVIRONMENT
Exocytosis – internal vesicles fuse with the plasma membrane to secrete macromolecules out of the cell
Endocytosis – the cell takes in macromolecules and other particles by forming vesicles from the plasma membrane
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