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

2. 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)

3. 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

4. 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…

5. 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