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