• See Dr. K-I-A for questions about Exam 1 before Spring Break
• Please place your one-page commentaries into one of the envelopes up front
• I have unclaimed examinations

• J = Rate of diffusion in moles per unit time
• D = diffusion coefficient of the membrane
• A = area of the membrane
• Dc = concentration difference across the membrane
• Dx = thickness of the membrane

• Down DC and DE
• Increase entropy
• No energy requirement
• Works both directions
• Diffusion through membrane
• No V_max or K_a
• No selectivity; no competitive Inhibition

• Rate determined by

- Oil/water partition coefficient or solubility in lipid
- Inversely proportional to soluteís molecular radius
• No T_max and no saturation kinetics
• Solutes dissolve in and out of lipid bilayer

• Selectively permeable membrane
• Water is ~ freely permeable

- Aquaporins
• p depends upon DC
• In fact, p is a measure of DC

• Hypertonic

- Higher impermeable [S] than cytoplasm
- Cell shrinks when placed into it as water diffuses out
• Hypotonic

- Lower impermeable [S] than cytoplasm
- Cell swells as water enters to dilute solute
• Isotonic

• Carrier Proteins
• Change shape
• Channel Proteins
• Selects by charge and size of solute

• Down DC and DE
• Increase entropy
• No energy requirement
• Works both directions
• Carrier mediated
• Michaelis-Menton Kinetics
• Selective; Competitive Inhibition

• Change solute into different molecule

Glucose --> glucose - phosphate
• Bind solute to protein
• Transport into organelle or vesicle

• Independent of DC and DE
• Can concentrate solute
• May decrease entropy
• Energy requirement
• Works in one direction for each solute
• Carrier mediated
• Michaelis-Menton Kinetics
• Competitive Inhibition

• Direct

- Results from energy changing shape of carrier protein
- Also changes affinity of carrier for solute causing it to be released
 
• Indirect

- Uses gradient established by other transport

• Uniport

- One solute, one direction
 
• Symport

- Two solutes, same direction
 
• Antiport

- Two solutes, opposite directions

• Type P

- Phosphorylated intermediate
- Na⁺, K⁺, Ca⁺⁺, and H⁺
 
• Type V -Vesicle

 - Pumps protons (H⁺) into vesicles
 
• Type F - ATP synthases

 
• Type ABC - ATP binding cassette

- Various solutes in prokaryotes & eukaryotes

Na⁺ - K⁺ Ratios in mammalian cells

• [Na⁺]_inside : [Na⁺]_outside = 0.08 : 1 [K⁺]_inside : [K⁺]_outside = 35 : 1

• Use Dc for energy in symport

- E.g., glucose and Na⁺
- 1 glucose = 36 ATP
 
• Create Dc to set up ion gradients necessary to create DE across membrane

 - Cell membrane potential for signaling

• Sensor
• Reference + Comparator
• Effector for output

• Temperature
• R
• F
• Log₁₀ = (ln/2.303)

E_Ion (in mV) = 61.5 log₁₀[Ion]₁
                            z             [Ion]₂
 

_{At 20 ^o C,}

E_Ion (in mV) = 58 log₁₀[Ion]₁
                          z          [Ion]₂
 

What does E_Ion mean?
 
 

Use the permeable ion in the Nernst

If it doesnít diffuse across the membrane, it doesnít contribute to Vm

• When ions (charges) move through membrane channels, current flows!

- Amperes (A)
• The flow of ions (I) is impeded by R
• We use 1/R or conductance (g) to express the permeability of a membrane to an ion

• If G_Na+ increases, Vm willÖÖ?
• If G_K+ decreases, Vm willÖÖ.?
• If G_Na+ decreases, Vm willÖÖ?
• If G_K+ increases, Vm willÖÖ.?
• If [K⁺]_out increases, Vm willÖ..?
• If [K⁺]_in increases, Vm willÖ..?
• If [K⁺]_out decreases, Vm willÖ..?
• If [Na⁺]_out increases, Vm willÖ..?
• If [Na⁺]_out decreases, Vm willÖ..?