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