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EK 2B2
Growth and dynamic homeostasis are
maintained by the constant movement of molecules across membranes
http://www.youtube.com/watch?v=y31DlJ6uGgE
http://www.youtube.com/watch?v=RPAZvs4hvGA
http://www.youtube.com/watch?v=eQsAzXr0UCU
http://www.youtube.com/watch?v=-Ul2oJ_TkNw
http://www.youtube.com/watch?v=GTHWig1vOnY
http://www.johnkyrk.com/cellmembrane.html
Passive Transport4
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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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
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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
Channel proteins8
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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Carrier proteins10
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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Osmosis12
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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Osmotic concentration15
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
Osmotic pressure16
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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Maintaining osmotic balance
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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
Active Transport19
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
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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
Sodium–potassium (Na+–K+) pump
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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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Coupled transport23
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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Bulk Transport25
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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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.
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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