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In this concept, we are going to talk about how the cell gets energy from macromolecules
other than glucose. We will talk about how cellular respiration is regulated and how
it connects to anabolic pathways. Let's start with the carbohydrates. Sugars
other than glucose can go into glycolysis. They just step in lower down the assembly
line, so to speak. That's why the cell can gain energy from fructose and other monosaccharides
as energy. Remember, polysaccharides can broken down into monosaccharides.
Let's go over here to proteins. Proteins, like polysaccharides, have to be broken down
into their monomers before they can be used for energy. The monomers of proteins are . . . that's
right, amino acids. Next, the amine group has to be clipped off. Then, depending on
what type of amino acid it is, it can go in as pyruvate, acetyl CoA, or the citric acid
cycle. The ammonia that results from clipping off the amine group is either recycled in
another molecule or is it is excreted. Fats back a lot of energy because they are
huge, hydrogen rich molecules. The fatty acids can be clipped into two carbon units and be
fed into the aerobic respiration as acetyl CoA. The glycerol can go into about halfway
through. So, this is why you can get energy from low
carb diets! No pathway can be left on continually. If
the cell has generated enough ATP, aerobic respiration needs to be shut off. ATP doesn't
last very long and the cell can't afford to waste if by having it "spoil" so to speak
while it sits around. The pathway is shut down by negative feedback regulation. As ATP
concentrations build up, in inhibits the enzymes found in glycolysis, shutting down glycolysis.
Once glycolysis is shut down, everything else shuts down. If citrate from the citric acid
cycle concentrations build up inhibits glycolysis enzymes as well.
So, when ATP isn't shutting down cellular respiration, what is it doing? It is providing
energy to make complex carbohydrates for structures within the cell or for energy storage. While
the food is plentiful, the cell likes to stock up for the lean times. ATP is used to make
lipids, which go on to repair cell membranes. Of course, lipids are also used for food storage.
ATP is used to fuel new proteins, which replace worn out proteins throughout the cell. ATP
is used when the cell is duplicating its DNA before it divides. ATP is used to make messenger
and transfer RNA as a part of protein synthesis. It provides the energy for all of the anabolic
processes of the cell.