This here is my biology notebook from my freshman year
of high school.
I swear I'm not a hoarder.
So then why do I have an old high school notebook?
Because it's just like hiring my 14-year-old self
to be a script-writing intern.
Sure, sometimes you have to fact-check what she writes,
but she's just so excited and earnest and willing to help--
kidding.
So just like last week, there were some lectures
from high school that I thought were really
good explanations of the topics that we're trying to cover
and that still stick out in my mind.
March 9th of 2004, a Tour of the Digestive System--
so on the exam for this section, I
remember that we had to demonstrate that we understood
the digestive system by talking about how a hamburger and all
of its different parts would be broken down as it
moved through a human body.
That test has always stuck in my mind,
and so that's what we're going to do today.
Mechanical digestion begins as you
take a bite out of your burger.
Your teeth chew the food up and grind it
into smaller particles.
These smaller particles are mixed
with saliva, which contains an enzyme known
as salivary amylase.
The amylase breaks starches down into smaller sugars,
and so here's where we start to digest our hamburger bun.
Salivary amylase is actually the reason
why, if you take a Saltine cracker
and you put it in your mouth and you let it sit for a while,
the bland cracker will start to taste sweet.
This is because the amylase is breaking down
the starches in the cracker into smaller sugars, which
you perceive as sweet.
So back to the burger--
so now your teeth have begun mechanical digestion
of the food, and the salivary amylase
has started to break down the hamburger bun.
And now your tongue forms the food
into a wad called a bolus, which you swallow.
The food moves through your pharynx
and into your esophagus, where peristalsis--
rhythmic contractions of the esophagus--
move it down into your stomach.
Here's where the serious digestion business starts.
Your stomach has glands that produce
three main types of secretion.
Secretion number 1 is mucus, and it coats your stomach
and helps prevent it from getting digested, too,
which is important.
Secretion number 2 is hydrochloric acid or HCL.
HCL helps convert secretion number 3, pepsinogen,
to pepsin.
Pepsin helps break down proteins,
and what part of the burger is made out of protein,
but the burger itself--
also some of the cheese if you get a cheeseburger.
HCL also helps keep the pH of your stomach
around 2, which helps digest the food
and also helps kill small microorganisms.
Your food spends about two to three hours
in your stomach getting all churned up,
and then it is moved through the pyloric valve
into your small intestine.
At this point, the partially digested food
is known as chyme.
Your small intestine releases some additional peptidases,
which help break down proteins, as well some additional carb
and sugar-digesting enzymes.
Your pancreas also releases pancreatic fluid
into the small intestine.
This fluid contains amylases, like our saliva,
as well as proteases, which keep working
on the protein in the burger, and some lipases, which
will help break down the fats--
so things like cheese or mayo on your burger.
The pancreatic fluid also contains bicarbonate ions,
which help to neutralize some of that HCL from your stomach.
Your liver also introduces bile into your small intestine,
which helps to emulsify fats so that they can be
better absorbed by your body.
And speaking of absorption, your body
isn't just digesting food for the fun of it.
It's doing it so that it can absorb the broken
down molecules.
Your body does this very efficiently
in your small intestine, because its surface
is folded in on itself in structures called villi.
And on this villi are even smaller folds
known as microvilli.
Essentially, the surface of your small intestine
is folded on itself over and over
and over again so that it can fit a lot of surface
into a very small area and increase its absorption
efficiency.
So now your small intestine has done a lot of digestion
and a lot of absorption, so it will
move the food onto the large intestine, or the colon.
So the colon's pretty cool, because a lot
of the digestion that happens here
is actually due to a large population of bacteria
that lives in your gut.
These bacteria can help you break down things
that you can't break down on your own,
like the cellulose found in plants.
Here is where the lettuce and the pickle on your burger
would disappear.
While in the large intestine, your body
observes much of the remaining water from the food,
leaving behind solid waste, which can then be excreted.
14-year-old me didn't phrase that so politely.
So there you go, all the parts of your hamburger digested
via my high school biology notes.
Go forth, do science.
This is great.
There are a bunch of my old tests and lab reports in here.
This one I got 105% on.
I don't know where that extra 5 points came from.
Also this exam, which I got 100% on,
in which I memorized almost all of the important hormones
and where they come from and what their function is, and I
did it all perfectly when I was 15.
But I could not for the life of me
do this last year when I was studying for the GREs.
But 15-year-old me, man, she had no problem.