1. Movement through Membranes
Cells Continued
Movement through Membranes

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. 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

9. 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+

10. 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

11. 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