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EK 2B2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.

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Presentation on theme: "EK 2B2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes."— Presentation transcript:

1 EK 2B2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes

2     

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4 Passive Transport 4  Passive transport is movement of molecules through the membrane in which  No energy is required  Molecules move in response to a concentration gradient  Diffusion is movement of molecules from high concentration to low concentration  Will continue until the concentration is the same in all regions

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6 6  Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules  Nonpolar molecules will move until the concentration is equal on both sides  Limited permeability to small polar molecules  Very limited permeability to larger polar molecules and ions

7 7  Facilitated diffusion  Molecules that cannot cross membrane easily may move through proteins  Move from higher to lower concentration  Channel proteins Hydrophilic channel when open  Carrier proteins Bind specifically to molecules they assist  Membrane is selectively permeable

8 Channel proteins 8  Ion channels  Allow the passage of ions  Gated channels – open or close in response to stimulus (chemical or electrical)  3 conditions determine direction Relative concentration on either side of membrane Voltage differences across membrane Gated channels – channel open or closed

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10 Carrier proteins 10  Can help transport both ions and other solutes, such as some sugars and amino acids  Requires a concentration difference across the membrane  Must bind to the molecule they transport  Saturation – rate of transport limited by number of transporters

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12 Osmosis 12  Cytoplasm of the cell is an aqueous solution  Water is solvent  Dissolved substances are solutes  Osmosis – net diffusion of water across a membrane toward a higher solute concentration

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15 Osmotic concentration 15  When 2 solutions have different osmotic concentrations  Hypertonic solution has a higher solute concentration  Hypotonic solution has a lower solute concentration  When two solutions have the same osmotic concentration, the solutions are isotonic  Aquaporins facilitate osmosis

16 Osmotic pressure 16  Force needed to stop osmotic flow  Cell in a hypotonic solution gains water causing cell to swell – creates pressure  If membrane strong enough, cell reaches counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out  Cell wall of prokaryotes, fungi, plants, protists  If membrane is not strong, may burst  Animal cells must be in isotonic environments

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18 Maintaining osmotic balance 18  Some cells use extrusion in which water is ejected through contractile vacuoles  Isosmotic regulation involves keeping cells isotonic with their environment  Marine organisms adjust internal concentration to match sea water  Terrestrial animals circulate isotonic fluid  Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid

19 Active Transport 19  Requires energy – ATP is used directly or indirectly to fuel active transport  Moves substances from low to high concentration  Requires the use of highly selective carrier proteins

20 20  Carrier proteins used in active transport include  Uniporters – move one molecule at a time  Symporters – move two molecules in the same direction  Antiporters – move two molecules in opposite directions  Terms can also be used to describe facilitated diffusion carriers

21 Sodium–potassium (Na + –K + ) pump 21  Direct use of ATP for active transport  Uses an antiporter to move 3 Na + out of the cell and 2 K + into the cell  Against their concentration gradient  ATP energy is used to change the conformation of the carrier protein  Affinity of the carrier protein for either Na + or K + changes so the ions can be carried across the membrane

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23 Coupled transport 23  Uses ATP indirectly  Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule  Symporter is used  Glucose–Na + symporter captures the energy from Na + diffusion to move glucose against a concentration gradient

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25 Bulk Transport 25  Endocytosis  Movement of substances into the cell  Phagosytosis – cell takes in particulate matter  Pinocytosis – cell takes in only fluid  Receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor  Exocytosis  Movement of substances out of cell  Requires energy

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27 27  In the human genetic disease familial hypercholesterolemia, the LDL receptors lack tails, so they are never fastened in the clathrin-coated pits and as a result, do not trigger vesicle formation. The cholesterol stays in the bloodstream of affected individuals, accumulating as plaques inside arteries and leading to heart attacks.

28 28  Exocytosis  Movement of materials out of the cell  Used in plants to export cell wall material  Used in animals to secrete hormones, neurotransmitters, digestive enzymes


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