PUFAs lower transition temperature (and melting point). Cis-double bonds cause a bend in the fatty acid (trans double bonds do not occur naturally) . This prevents PUFAs from stacking as well as saturated fatty acids so they don't pack tightly ("crystalize") until the temperature gets colder. Because saturated fatty acids stack very easily they crystalize at warmer temps. Therefore, a membrane with a high proportion of PUFAs would have a lower transition temperature than a membrane with a low proportion of PUFAs.

5. A vitamin C transporter was recently cloned and expressed in a cell type that doesn't usually transport vit C.

a. What mechanism of transport might be involved based on the following observations: When the concentration of [Na⁺] on the outside of the cells is normal (148mM) vit C is transported into the cell. But when [Na⁺] is lowered to below normal concentrations (and below intracellular [Na⁺]) vit C is transported out of the cell. (5 pts)

If transport is Na+ dependent, which it is, a carrier protein likely ferries vit C and Na+ across the membrane from high [Na+] to low [Na+]. So, I'd call this carrier mediated transport. If I could show that uptake of Vit C into cells was saturable at high [Vit C], it would lend further support of carrier mediated transport.

b. Based on the observations above, would vit C transport depend on ATP? Why or why not? (5 pts)

Yes, Vit C transport would depend on ATP, and technically be called seconday active transport. This transport depends on ATP because Vit C transport depends on a Na+ gradient; i.e., Vit C will not be carried into the cell unless [Na+] is higher on the outside of the cell. Hydrolysis of ATP is coupled to Na+/K+ ATPase which pumps Na+ against its electrochemical gradient so that high [Na+] is maintained on the outside of the cell. Therefore Vit C transport is indirectly dependent on ATP and is called secondary active transport.

6. Lectin is a carbohydrate binding protein that binds with very high affinity and specificity. Would you expect it to bind to the inside or outside of a cell membrane? (5 pts)

Lectin would bind on the outside of the membrane because this is where carbohydrate is found in the form of glycoproteins, and glycolipids. (pg. 215).

CM is negatively charged (carboxy) and will attract positively charged ions. So, calculate [NH2]/[NH3+] at pH=6 for His (pk_R=6) and Lys (pk_R=10.5).

pH - pK= ln[NH2]/[NH3+] (pg. 84);

• If pH = pK, then [NH2]/[NH3+] =1 (i.e., only half of the His molecules are charged).

• If pH<<pK, then [NH₃⁺]>>[NH2] (i.e., all of the Lys molecules are charged).

So, His elutes before Lys, because a smaller portion of His molecules are charged at pH=6.

Starch (and glycogen, "animal starch") contain glucose residues, linked by alpha(1-4) glycosidic bonds (pg. 206). These polymers tend to form alpha helices and like springs, pack well. Hence the alpha helical structure facilitates storage of these molecules.

Cellulose contains glucose residues, linked by beta (1-4) glycosidic bonds (pg. 205). These polymers form beta-pleated sheets. These sheets stack, H-bond with each other and have fewer atoms available for H-bonding with water. The structure of cellulose gives rise to strong water insoluble fibers (the same kind of fibers we might eat to maintain a high fiber diet). Also, mammals do not have enzymes capable of hydrolyzing beta (1-4) glycosidic bonds so we can not metabolize cellulose.

Condroitin sulfates are glycosaminoglycans which lubricate joints. The sulfate groups bind water to help hydrate the joint with further contributes to lubricant effect. (pg. 208-209)

If the side chain is an acid, take the average of the 2 LOWEST pks.

If the side chain is a base, take the average of the 2 LARGEST pks. (pg. 84)

In this case the side group was an acid so: PI=(2+4)/2

(note, enzymes are made of L-amino acids. Enzymes are chiral).

See page 248

See page 328

See page 165-168

See pages 80-81