It's professor Dave, let's talk about metabolism and ATP.
One of the most incredible things about your body is the way it takes the food you
eat and turns it into the energy that allows you to think, and move your body.
This is an overwhelmingly complex set of processes which can be collectively
referred to as metabolism, which is simply when enzymes transform certain
molecules into other molecules through multi-step pathways. So let's learn how
this works. First, metabolic pathways can be of two varieties. They can be
catabolic, where larger molecules are broken into smaller pieces, releasing
energy in the process, or they can be anabolic, where small molecules are
assembled into a larger one, which requires energy to occur. We can call
these ones biosynthetic pathways, because these are the ways that nature
synthesizes molecules. The most incredible thing to consider is that
while it is tempting to view these pathways as though the enzymes are tiny
factory workers, performing their tasks with complete sentience,
it is actually the case that every single reaction in the pathway occurs
simply because it is thermodynamically favorable. Enzymes recognize their
substrate because of electrostatic attractions in the active site and the
reactions they catalyze are fundamentally no different than the
simple reactions between small molecules that we learned in organic chemistry.
There's a nucleophile and an electrophile in proximity, so a reaction occurs.
This means that everything we will discuss is just a consequence of
the electromagnetic force operating on biomolecules, nothing more than plus and minus.
Although these sets of metabolic pathways occur spontaneously in every
cell in your body, some of the reactions involved are endothermic, and require
energy input to happen. The cellular currency of energy that will be used
to facilitate such reactions is called adenosine triphosphate, or ATP. ATP is
essentially just an RNA nucleotide. Notice the adenine base, the ribose sugar
and phosphate groups.
The only difference is that in a nucleic acid like RNA, each nucleotide has one
phosphate, but here there are three, and it is these phosphate groups that give
ATP its energy-storing properties. There is an abundance of negative charge on
these phosphates, and we know that like charges repel one another, so there is a
lot of potential energy stored in this part of ATP, kind of like a compressed
spring. Just the way a compressed spring wants to expand, releasing the stored
potential energy and converting it into kinetic energy, these phosphates want to
come apart, so when ATP undergoes hydrolysis and transfers one phosphate
group to something else during an enzymatic reaction, thus becoming
adenosine diphosphate, or ADP, it releases some of that potential energy which can
then be converted into the energy needed to promote a reaction, or pump ions
across a membrane against the concentration gradient, or any number of other things.
ADP can then be phosphorylated to become ATP again, so this back and forth
between the two forms is the method that nature has discovered to provide energy
for cellular processes.
Next we will look at the specific reactions that have to occur in order to
generate energy for our bodies to use.
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