1. Membrane Structure and Cellular Transportation Ch. 7
AP Biology
Ms. Haut

2. Membrane Structure Made of phospholipid bilayer
Polar (hydrophilic) heads of phospholipids oriented towards protein layers
amphipathic
Nonpolar (hydrophobic) tails of phospholipids are oriented between polar heads

3. Membrane Structure Proteins are individually embedded in the bilayer
Hydrophilic portions exposed to water
Hydrophobic portions in nonaqueous environment inside the bilayer
Phospholipid bilayer

4. Fluid-Mosaic Model
Membranes held together by weak hydrophobic interactions
Lipids and some proteins can drift laterally within the membrane

5. Mosaic of Different Molecules
Integral proteins—transmembrane proteins that span the hydrophobic interior of the membrane
Transport proteins

6. Mosaic of Different Molecules
Integral proteins—transmembrane proteins that span the hydrophobic interior of the membrane
Peripheral proteins—attached t the membrane’s surface
Transport proteins
Carbohydrates—function in cell-to-cell recognition (cell markers)
Glycolipids, glycoproteins

9. Traffic across Membranes
Membrane’s molecular organization results in selective permeability
Permits exchange of nutrients, waste products, oxygen, and inorganic ions.
Allows some substances to cross more easily than others:
Hydrophobic molecules—hydrocarbons, CO2, and O2 dissolve in and cross membrane
Very small polar molecules, including H2O can cross easily

10. Passive Transport: Diffusion
Diffusion—movement of a substance down its concentration gradient due to random thermal motion
Spontaneous process that decreases free energy and increases entropy
⁂ NO ENERGY EXPENDED
Diffusion of a gas➞

11. Free energy
Less stable
Free energy
More stable
A substance will diffuse from where it is more concentrated to where it is less concentrated

12. ←Water
Selectively Permeable
Membrane
Solute Molecules

13. Passive Transport: Osmosis
Osmosis—diffusion of water across a selectively permeable membrane
Water diffuses down its own concentration gradient (from hypotonic solution to hypertonic solution)
Hypotonic—lower concentration of solutes
Hypertonic—higher concentration of solutes
Isotonic—equal solute concentration

14. AP LAB: EXERCISE 1A: Diffusion
H2O/Lugol’s Solution (IKI)
15% glucose/
1% starch
Initial contents
Initial Color
Bag
15%glucose and 1% starch
clear
Let stand 30 minutes
Beaker
H2O + IKI
yellow

15. AP LAB: EXERCISE 1A: Diffusion
H2O/Lugol’s Solution (IKI)
15% glucose/
1% starch
yellow
H2O + IKI
Beaker
clear
15%glucose and 1% starch
Bag
Initial Color
Initial contents
blue
Final Color - +
Initial Glucose
Final Glucose

16. Alternative Methods of Cellular Transportation Ch. 8
AP Biology

17. Facilitated Diffusion
Diffusion of solutes across a membrane, with the help of transport proteins
Is passive transport because solute is transported down its concentration gradient
Aides transport of many polar molecules and ions that are inhibited by phospholipid bilayer

18. Facilitated Diffusion
Transport proteins share similar properties with enzymes:
They are specific for the solutes they transport
They can be saturated with solute—maximum rate occurs when all binding sites are occupied
They can be inhibited by molecules that resemble the solute (similar to competitive inhibition)

19. Active Transport
Energy-requiring process during which a transport protein pumps a molecule across a membrane, against its concentration gradient
Is energetically uphill (+G) and requires the cell to expend energy
Helps cells maintain steep ionic gradients across cell membrane (e.g., Na+, K+, Mg 2+, Ca 2+ , and Cl-)
Transport proteins involved get energy from ATP to pump molecules against their concentration gradients

20. Sodium/Potassium Pump

21. Membrane Potential Gererated by some ion pumps
Membrane potential—voltage across membranes
Ranges from –50 to –200 mv (the inside of the cell is negatively charged relative to outside
Affects traffic of charged substances across membrane
Favors diffusion of cations into cell; anions out of cell (due to electrostatic attractions—cytoplasm is negatively charged)

23. Electrochemical Gradient
Two forces drive the diffusion of ions across a membrane
Chemical force—concentration gradient
Electrical force—effect of membrane potential

24. Electrochemical Gradient
Electrogenic pump—transport protein that generates voltage across membranes
Example is Na+/K+ Pump
3 Na+ ions out/ 2 K+ ions in equals a net transfer of one positive charge from the cytoplasm to the extracellular fluid (a net loss of one positive charge), a process that stores energy in the form of voltage
Stored energy can be trapped for cellular work

26. Cotransport
Process where a single ATP-powered pump actively transport one solute and indirectly drives the transport of other solutes against their concentration gradient
ATP-powered pump actively transports one solute and creates potential energy in the gradient it creates
Another transport protein couples the solute’s downhill diffusion as it leaks back across the membrane with a second solute’s uphill transport against its concentration gradient

28. Endocytosis Exocytosis
Process of importing macromolecules into a cell by forming vesicles derived from the cell membrane
Process of exporting macromolecules from a cell by fusion of vesicles with the cell membrane
Endocytosis
Exocytosis
Vesicle forms from a localized region of cell membrane that sinks inward; pinches off into cytoplasm
Vesicle usually budded from the ER or Golgi and migrates to cell membrane
Used by cells to incorporate extracellular substances
Used by secretory cells to export products (insulin in pancreas; neurotransmitter from neuron)

31. Phagocytosis—endocytosis of solid particles
Forms food vacuoles that fuse with lysozome to be digested
Pinocytosis—endocytosis of fluid droplets
Takes in solutes dissolved in the droplet
Receptor-mediated endocytosis—process of importing specific macromolecules into the cell by inward budding of vesicles formed from coated pits
Occurs in response to binding specific ligands to receptors on cell’s surface