Bio III Questions

1) How does diffusion (e.g. through a cell membrane) compare to what occurs at the glomerulus (why can glucose easily pass through even though particle is charged)? Can urea passively pass through by diffusion for both a cell membrane and the glomerulus? Is filtration purely based on charge?
Diffusion will only allow the passage of small, nonpolar, uncharged molecules (besides water) through the cell membrane.  Filtration in the kidney, however, allows through those particles PLUS some biomolecules (amino acids, glucose, vitamins being the noteworthy ones), a number of ions (Na+ and Cl- being the main ones), and some other waste products.  This is because the glomerulus has much larger pores, which permit through these larger molecules.  Urea requires a carrier protein, but no energy, to pass through a cell membrane as well as the collecting duct and ascending loop of Henle.  It doesn't really show up significantly in the glomerulus.  And filtration isn't based on charge at all; it's based on size.

2) Hb sat curve shift: why isn’t the Hb saturation curve the same directional shift as for a person on a mountain (mountain dweller or mountain goat) as it is for a fetus (in both cases less O₂ present)? If this is the case, why would a mountain dweller like the case for muscles, why would O₂ need less binding while exercising? Hypothetically, what would happen if no shift, how would muscles be impacted?
Let's think about this numerically.  Let's say you're standing in air with 50 mmHg of oxygen (half the normal amount).  Let's say that there's no shift.  Well, looking at the curve, you'll have about 80% saturation.  Then, in the muscles (around 20 mmHg), you'll have about 20% saturation.  So, in the lungs, the blood is carrying 50 mmHg * 80% = 40 mmHg of oxygen.  Then, in the muscles, the blood is carrying 20 mmHg * 20% = 4 mmHg.  How much went into the muscles?  Well, the difference:  40 mmHg - 4 mmHg = 36 mmHg.

With the shift, let's say we accomplish 100% saturation at the 50 mmHg of oxygen, and have 30% saturation in the muscles.  We then have 50 mmHg * 100% = 50 mmHg in the lungs, and 20 mmHg * 30% = 6 mmHg on the blood in the muscles.  Thus, we've given the difference of 50 mmHg - 6 mmHg = 44 mmHg to the muscles.  This may not seem like a huge difference to the muscles, but it's still an improvement!

3) What does the MCAT use as their definition of sense and anti-sense strands (since the definition found in the foundation review contradicts the one given in the review notes)?
The "sense" strand is the strand of DNA NOT paired with mRNA during transcription (although, because of base-pairing, it will have the identical sequence of nucleotides to the mRNA but with T switched for U).  The "antisense" strand is the one base-pairing with the mRNA.

4) For differentiating amides and steroids, we stated in class the need for a carrier protein to carry steroids through the blood stream given its lipophilic character. When the steroid passes approaches the cell membrane is it released from the carrier, and the after enter the cell does it once again pick up another carrier since the cytosol/cytoplasm is polar environment? How are T3/T4 able to wiggle past through the cell membrane when both are amides and why do they not require a carrier protein if they have the ability to pass through the lipid membrane? Finally, typically, when referring to thyroid hormone, is T4 the prevalent form in the blood stream?

I have to admit that I'm not sure about the carriers inside the cell for steroid hormones; my guess would be that they exist, but I'm not sure.
T3 and T4 are usually grouped as amino-acid derived hormones, which is true; however, since they're derived from tyrosine and each features two benzene rings and are quite small, they can get through the cell membrane.  On the other hand, they do have a polar amide on one end and a number of iodine atoms, which help them dissolve in the aqueous environment of the blood.  The generic term "thyroid hormone" would refer to both T3 and T4 as a group, not just one of them.