Homeostasis Cell Transport Modern Biology 2009 Holt Rinehart

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)

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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

Passive versus Active Transport

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

Sodium Potassium ATPase Pump

Active Transport, II • Movement of vesicles – 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

Endocytosis/ Exocytosis