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EK 2B2 Growth and dynamic homeostasis are

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Presentation on theme: "EK 2B2 Growth and dynamic homeostasis are"— Presentation transcript:

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



4 Passive Transport 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


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 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 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


10 Carrier proteins 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


12 Osmosis 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
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 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


18 Maintaining osmotic balance
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 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 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
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


23 Coupled transport 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


25 Bulk Transport Exocytosis 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


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 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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