Movement through Membranes Cells Continued Movement through Membranes

Passive Mechanisms - Diffusion Diffusion – process by which molecules or ions scatter or spread spontaneously from regions where they are in higher concentrations to regions of lower concentrations Sugar (solute) in water (solvent) Equilibrium – uniform distribution of a solute in a solvent

Diffusion cont. Diffusion of a substance into or out of a cell can occur only if: The cell membrane is permeable to that substance A concentration gradient is at higher concentration inside or outside the cell Carbon dioxide diffuses out while oxygen diffuses into cells

Passive– Facilitated Diffusion Facilitated diffusion – membrane proteins help substances that are unable to pass through the lipid bilayer (higher conc.  lower conc.) Protein carrier molecules – glucose is too large to pass through membrane, so it bonds to carrier molecule which changes carrier molecules shape and passes through and then is released Insulin (hormone) promotes this type of facilitated diffusion

Passive - Osmosis Osmosis – occurs whenever molecules diffuse from a region of higher water conc. to lower conc. across a selectively permeable membrane In solutions, a higher conc. of solute means a lower conc. of water (and vice versa)

Osmosis cont. Isotonic – any solution that has the same osmotic pressure as body fluids Ex. Intracellular and extracellular fluids Hypertonic – solutions with a higher osmotic pressure than body fluids (more solute than water) If cells are put into a hypertonic solution, there would be a net movement of water out of the cell  cells would shrink

Osmosis cont. Hypotonic – solution has a lower osmotic pressure than body fluids (less solute than water) Cells put into a hypotonic solution would intake water and swell Fig. 3.15

Passive - Filtration Filtration – hydrostatic pressure forces molecules through membranes Used to separate solids from water Blood pressure forces smaller molecules through tiny openings in capillary walls, while larger molecules like blood protein molecules are left inside

Active Mechanism – Active Transport Active transport – process that moves particles through membranes from lower conc.  higher conc. Specific carrier molecules are used that require energy from ATP (proteins with binding sites) Often called pumps Ex. Some sugars, amino acids, Na+, K+, Ca2+, H+

Active – Endocytosis Endocytosis – molecules that are too large to enter a cell via other methods are conveyed within a vesicle formed from a section of the cell membrane Pinocytosis (“cell drinking”) – membranes engulfs droplets of liquid from their surroundings Phagocytosis (“cell eating”) – membrane engulfs particles from surroundings Phagocytes – white blood cells that can take in bacteria and cellular debris

Active - Exocytosis Exocytosis – reverse process from endocytosis in which the cell secretes a substance stored in a vesicle Nerve cells release neurotransmitter chemicals in this manner to signal other cells