Human Physiology – Carbohydrates Digestion and Absorption

Human Physiology – Carbohydrates Digestion and Absorption

January 7, 2020 4 By Ewald Bahringer


>>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.