Homeostasis & Cell Transport Modern Biology©2009 Holt, Rinehart, & Winston Chapter 5

Passive Transport No input of energy Depends on concentration gradient Substances always flow from higher to lower conc. Diffusion: simplest form of passive transport Substances flow until equilibrium is reached Osmosis Flow of water across semi-permeable membrane Hypotonic: Low particle, high water concentration outside Results in cytolysis (burst) if extreme, but hypotonic is the norm. Maintains turgor pressure in plants Isotonic: same concentration on both sides of membrane Hypertonic: high particle, low water concentration outside Results in plasmolysis (shrink/wilt)

Facilitated Diffusion Carrier proteins assist in diffusion- still passive Are specific for molecules they transport Shields when it changes shape Helps when particles are too large or charged Ion channels Specific for 1 type of ion May be always open or gated Stretching, electrical, or chemical stimuli

Active Transport Requires input of energy Plants’ contractile vacuoles pump out water Cell membrane pumps Sodium-potassium pump 3 Na+ out for every 2 K+ in Creates higher positive charge outside the membrane Requires dephosphorylation of ATP PMCA: Plasma Membrane Calcium-ATPase pump Moves Ca2+ against its concentration gradient Essential for platelet function, 2nd messengers, etc…

Active Transport, II Movement of vesicles Endocytosis (ingest ) Transport large macromolecules, nutrients, etc… Endocytosis (ingest ) Substances merge with membrane & pinch off  vesicles Pinocytosis – solutes or fluids Phagocytosis – large particles or whole cells Phagocytes allow lysosomes to fuse, killing bacteria/viruses Exocytosis (expel) Vesicle merge with plasma membrane to spew contents (wastes, metabolites, hormones) outside