>>Dr. Ketchum: In this lecture video we are going to focus on digestion and absorption
of carbohydrates. So we’re working with the gastrointestinal system, but focusing
on carbohydrates. And so, so far we’ve provided you an overview of the gastrointestinal system
and it’s processes, the functional anatomy, which we’ve talked about the major organs
and the accessory organs, and now what we’re going to do is discuss, well once that food
enters into your digestive system or your GI system, how is it broken down? And then how
are those nutrients absorbed into the blood? Okay, so the focus once again is going to
be on carbohydrates.
So in a typical diet, we actually consume about 250 to 800 grams of carbohydrates. Most
of these are consumed as disaccharides or polysaccharides. But, here’s the concern
or here’s the, the, the important part: Only monosaccharides can be absorbed. So be
sure you understand what we mean by “can be absorbed.” So that means that we’re going
to—can take them from the lumen of the gut, and we can transport those monosaccharides,
we can absorb them into the blood, from lumen to the blood.
So the disaccharides and polysaccharides that we typically consume are things like
sucrose, which would be table sugar, lactose, which would be milk sugar, maltose, starch,
glycogen and cellulose. So pause the video for a moment and determine which of these
are disaccharides and which of these are polysaccharides. So sucrose, lactose and maltose are all
considered disaccharides, whereas starch, glycogen, and cellulose are all considered polysaccharides.
So cellulose cannot be digested. So if you just ate a salad for example, the cellulose
in the lettuce cannot be digested. So why is it still important? Well, that’s providing
the dietary fiber that’s important for intestinal mobility. In other words, you have to
have contents passing through your intestines, otherwise you’ll get very ill,
and you can actually die from that if your contents of your gut don’t
pass through. So how then are we going to go about digesting these
disaccharides and these polysaccharides so that we can absorb the resulting monosaccharide?
And so let’s look at digestion of starch. So if you recall, starch is a polysaccharide that consists of glucose
monomers. So each one of these green structures is a glucose subunit or a glucose monomer.
And so since starch is a polysaccharide, we cannot digest it, or we cannot absorb it into the blood.
So we have to be able to digest it. So here we’ve ingested starch. Maybe, for example, you had some
pasta for dinner. Or you had crackers for dinner that have a lot of starch in them.
This carbohydrate, we’ve got to start breaking it down. So we have two enzymes that can assist with the break
down of starch: salivary amylase and pancreatic amylase. Salivary amylase is produced by your salivary
glands and pancreatic amylase is produced by the pancreas. And so what this enzyme
can do is break down starch into two different products. You can wind up with limit dextrins
when you break down starch. Which are basically short-branched polysaccharides, or you can
wind up with maltose, which is a disaccharide, which consists of two glucose monomers. Okay,
so that doesn’t do us much good either, because we cannot absorb disaccharides and
we certainly cannot absorb short polysaccharides either. So we have to break these down even
further. So how do we do that? Well that’s going to require additional enzymes.
So dextrinase will break down those limit dextrins. So dextrinase catalyzes the reaction
where the limit dextrins are further broken down into glucose subunits. If it’s maltose
that we’re concerned with, the enzyme maltase will break down our maltose into two glucose
subunits. Okay, because once you have the glucose, and glucose is a monosaccharide,
then the body can absorb that monosaccharide. Now as far as some of these other disaccharides
that we see like sucrose, sucrose is broken down by sucrase, the enzyme sucrase, into
its subunits which are fructose and glucose. And then lactose, that sugar that’s found
in milk, is broken down into its subunits galactose and glucose via the enzyme lactase.
And so lastly we have glucoamylase, which will take any polysaccharide and break that
polysaccharide down into glucose. So what we wind up with because of all of these enzymes are
all of these monosaccharides. So be sure you know which of these are monosaccharides, which
are di and which are polysaccharides.
Okay, now the locations for these enzymes are on the brush border of the small intestine.
And so if you recall if you looked at the lining of the small intestine, you would see villi.
And at the tip of the villi are the microvilli.
And we’ll put the microvilli in green.
And so located on the brush border are enzymes. So this enzyme here is called the brush border enzyme. And
many of the enzymes that we talk about, excluding salivary and pancreatic amylase are brush
border enzymes. So now once we have our carbohydrate broken down into a monosaccharide,
now we can absorb the monosaccharide. Here what we will do is focus on the absorption of
glucose and galactose. So here we have the lumen, we have the epithelial cells,
and here we have the blood. So the goal is to absorb glucose...and galactose
and even fructose, okay? All monosaccharides. So we’re going to start on the basolateral
membrane. And we’re going to start with a type of transport that I’m going to label
a one. So now inside of an epithelial cell you know the concentration for sodium and
potassium. So pause the video, write in your concentration for sodium and potassium inside
of your epithelial cell and then see if you can’t determine what type of transport is
occurring in number two as well as number three, and number four. So if you’ve
already paused the video and determined what type of transport’s occurring at 1, 2, 3,
and 4, let’s see if you were correct. So we’re going to start with the concentrations
for sodium and potassium inside the epithelial cell. So we noticed that sodium is being pumped from
a low concentration toward a high concentration. Potassium is being pumped from a low concentration
toward a high concentration, and this requires ATP. So number one must be primary active
transport. So let’s look at number two now up on the apical membrane. So on the apical membrane
we have this purple thing, right? That’s representing a carrier protein. Sodium is going from a high
concentration in the lumen toward a low concentration within the cytoplasm of the epithelial cell. So
high to low using a carrier. Then we notice glucose is going from a low concentration
in the lumen to a high concentration in the epithelial cell. So if one ion is going along
a gradient, that’s releasing the indirect energy to drive the second solute against
its gradient. So number two must be sodium glucose co-transport, which is a type of secondary
active transport. Okay, so now once we have glucose at a high concentration inside the
epithelial cell, here it’s going from a high concentration toward a lower concentration,
but it’s using a carrier protein. So number three must be facilitated diffusion. And then
finally we have number four. So in number four, you can see how potassium is moving from
a high concentration in the epithelial cell through a channel toward a low concentration.
So number four must also be facilitated diffusion. So that explains how glucose is absorbed,
the mechanism for glucose absorption in the small intestine. Now there’s
also galactose absorption, which is the exact same as glucose. So instead of writing glucose
on the diagram, scratch that out and you could put galactose and it would be exactly the
same as what we just discussed for glucose. What’s different is fructose.
So fructose is actually absorbed by facilitated diffusion across both of the membranes. So
by both of the membranes, we mean apical membrane as well as the basolateral membrane.
So now, how do the brush border enzymes play a role? So here we have a brush border enzyme
called maltase. So this would be the lumen of the small intestine. Remember that most
of your digestion and absorption takes place in the small intestine, specifically the duodenum.
So maltose is the brush border enzyme that’s going to take maltose and catalyze a reaction
to break that bond between those two glucose units into your monosaccharide glucose.
And then we can absorb glucose just like I described in the previous diagram. The only extra step
here is that you have a brush border enzyme called maltase to help break down that disaccharide.