Terms in this set (1842)

Whenever you see a question on the MCA"f® that asks about metabolic
pathways, always start by considering the source. Is the source of energy
the intestines or the liver? If the question refers to someone who has just
eaten a meal, or says that only one or two hours have passed since eating,
the source is the intestines. In this case, you are in luck! All organs carry out
glycolysis, the citric acid cycle, and the electron transport chain. Carbohydrates
and lipids are stored as glycogen and triglycerides, respectively.
If more than two or three hours have passed since a meal, the source is
the liver. Metabolism is more complex when this is the case. First, think
about how the liver creates glucose for other organs -then you know that
the liver is carrying out glycogenolysis and gluconeogenesis. Second, think
aboutflxed demand. Ask yourself what parts of the body REALLY need the
glucose- then you know that the red blood cells and the brain are carrying
out glycolysis, the citric acid cycle, and the electron transport chain. Third,
think about variable demand. Ask yourself how all of the other organs in the
body get their energy- you know they obtain it from fatty acids, not glucose.
Thus all other organs use beta-oxidation! There are only two exceptions to
this rule that you must know. One is that muscles have their own supply of
glycogen to use internally. The second is that many hours after eating a meal
(approximately eight to twelve hours), the liver also makes ketone bodies,
which can be used by all parts of the body except red blood cells.
Think about source, flxed demand, and variable demand when you answer any
metabolism question on the MCA"f®. The MCA"f® will often give you one piece of
information and require you to flll in another. Try two examples:
1) If the liver is undergoing gluconeogenesis, what is happening in an adipocyte?
In this scenario, the liver is behaving as a source of glucose. The flxed
demand (the red blood cells and the brain) have priority for this supply.
All other tissues use beta-oxidation. Thus adipocytes must be carrying
out fatty acid breakdown. They can also use some fatty acids for betaoxidation
themselves.
2) If the liver is undergoing glycolysis, what is happening in the muscle?
In this scenario, the source must be the intestines. Thus all organs are
undergoing glycolysis, including muscle. Since muscles also form their own
glycogen stores, they are carrying out glycogenesis as well.
In any half-life problem, there are 4 , and only 4 , possible variables. They are:
1) the initial amount of substance, 2) the final amount of substance, 3) the
length of the half-life, and 4) the number of half-lives (often given as a time
period, in which case you simply divide by the length of a half-life). Any MCA"f®
half-life question will provide you with three of these variables in some form,
and will ask you to find the fourth. This type of question should be a fast, free
point. To answer a half-life question, count the number of half-lives on your
fingers. For instance, if 12.5% of a substance remains after 5 years, what is
the half-life? The initial amount is, of course, 100%. The final amount is
12.5%. The number of half-lives is found by dividing the initial amount
by 2 until you arrive at the final amount. Keep track on your fingers
of the number of times that you divide by 2 : 50% is once, 25% is
twice, 12.5% is three times. That's three half-lives. In another
example: how long will it take for 500 grams of a substance with
a half-life of 2 years to decay to 62 grams? The initial amount
equals 500, the final amount equals 62, and the half-life is 2
years. Divide the initial amount by 2 until you arrive at the final
amount. Keep track of the number of half-lives on your fingers.
250 is one, 125 is two, 62.5 is three. Rounding off numbers is the
rule on the MCA"f®, so the answer is 3 half-lives or 6 years. Whatever
the combination, look for the 3 variables and solve for the fourth by
counting half-lives on your fingers.
Knowing all of the information presented in this lecture is only helpful if you
are able to apply it. When you see a reaction, whether or not you are familiar
with it, use the following steps to reason your way through the reaction.
Note that most organic chemical and biochemical reactions on the MCA"f®
will involve oxygen-containing compounds, as discussed in the next lecture.
1. Focus on the part of the reaction that will answer the question- the
functional groups. You don't need to understand every step of the
reaction.
2. Examine the reaction closely- what changed'?
a. What are the new or missing bonds'?
b. Are there any new or missing functional groups? What are they?
3. Now look at the individual reactants. To determine which molecule
attacks which and where, ask yourself:
a. Does the molecule have a net dipole'? If so, find the partially negative
and partially positive side of the molecule. The more polar
molecule is likely to be more reactive and will attack the other
reactant. The most negative or nucleophilic region usually attacks
the most positive or electrophilic region.
b. Where is the center of negative charge on each molecule (i.e. where
do the electrons hang out)'? Look especially for oxygen and nitrogen
as centers of negative charge. Also look for carbons bonded to
oxygen - the carbon often carries a partial positive charge.
c. Do the active functional groups tend to donate or receive
electrons'? When you see a familiar functional group, think about
how it tends to behave in reactions you have seen before.