Download Lecture 05 Notes: Diffusion, Osmosis and Membranes

Pierce College
Putman/Biol 160
Lecture 05 Notes: Diffusion, Osmosis and Membranes
1. Structures of cell (plasma) membrane. See class notes!
2. Phospholipids have hydrophilic (phosphate) heads and hydrophobic (hydrocarbon) tails.
When they are mixed with water, the phosphate heads orient themselves toward water, the
hydrocarbon tails orient themselves away from water. This creates a lipid bilayer, with heads
pointing outward on either side of the membrane, with the tails pointing toward one another.
3. A concentration gradient is where you have more of a target molecule in one area than in a
contiguous area.
a. Particles tend to spread out in a concentration gradient from high to low concentration,
until the concentration is equivalent throughout the system. Kinetic energy powers this
movement.
b. In passive diffusion, particles move from one side of a membrane to the other along a
concentration gradient.
4.
Since kinetic energy is heat, increasing heat will increase the rate of diffusion.
5. Molecules that can pass through the lipid bilayer by passive transport include
a. Lipid solubles: Steroids & fat-soluble vitamins
b. Small molecules: Water, urea, methanol, ethanol, O2, CO2
6. Osmosis is the passive transport of water across a membrane.
a. Isotonic solution: Equal solute concentration in & out of cell
1. No net water diffusion
2. Animal cell optimum
3. Plants flaccid
b. Hypotonic solution: Low solute (high water) concentration outside of cell
1. Net water diffusion into cell
a. Animal cells may explode!
b. Plants turgid; plant cell optimum
c. Hypertonic solution: High solute (low water) concentration outside cell
1. New water diffusion out of cell
a. Animal cells shrivel
b. Plant cell membranes shrink away from their cell wall
7. Osmotic strategies: Most take lots of ATP!
a. Contractile vacuoles
b. Pump Na+ in/out across cell membranes
1. Gills
2. Kidneys
3. Halophytic plants
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c. Live where osmotic stress isn’t a problem!
8. Facilitated diffusion (a type of passive diffusion) is the movement of particles with their
gradient, thus they use kinetic energy for movement; however, since these particles cannot go
across the lipid bilayer, the require their channel proteins or transport proteins to get across.
a. Channel proteins have pores through which ions can travel, from high to low
concentration.
b. Transport proteins bond to their target molecule then must change shape to move the
target molecule across the membrane. In facilitated diffusion, the kinetic energy of the
target molecule causes the shape change.
9. In active transport, transport proteins pump target particles against their concentration
gradients; thus, kinetic energy would actually work against active transport. An outside
energy source is needed to cause the necessary shape change; ATP usually does the job.
10. Mechanisms particles that cannot freely cross lipid bilayer use to get across:
a. Channel proteins: Ions
b. Transport proteins: Amino acids, glucose, nucleotides
11. Receptor proteins bind to specific messenger molecules (hormones). This results in a signal
transduction, which then causes something to happen inside the cell. For instance, it may
cause cells to start or stop growing, may cause cells to absorb or secrete glucose, etc. etc.
12. Since enzymes are generally found attached to membranes, cells that are metabolically active
have lots of internal membranes.
13. Endocytosis is the bringing into a vacuole a large amount of material for future use.
a. Phagocytosis. Cells use pseudopods to envacuolate large particles. Unselective.
b. Cell-Mediated: Cells line pits with receptors to bring in small, target molecules. Highly
selective.
c. Pinocytosis. Cells bring fluids into tiny vacuoles. Unselective.
14. Exocytosis is the reverse of endocytosis. It is the secretion or excretion of large amounts of
materials by moving a fusing a vacuole with the cell membrane and releasing its contents.
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