241 Carbohydrates


Understanding chemistry is a really
important part of understanding how the body works. Of course we are all made up of all sorts of chemicals, all sorts of complex biological molecules and in this series
of videos I really want to focus on those complex biological molecules, a lot
of terminology and a little bit about their chemical structure. The first
category of large biological molecules we are going to talk about are the carbohydrates. Before we start getting into the details about the carbohydrates in
particular I want to make sure that we’re all on the same page about the
idea of polymers and monomers. Let’s take a look at the prefixes to start: the
prefix “mono” means “one” the prefix “poly” means “many”. So when we’re talking about large biological molecules we’re oftentimes talking about polymers and
polymers are molecules that are made up of multiple small repeating subunits and
the subunits are the monomers. So here I have drawn an image of kind of a
generic polymer, here’s the polymer, and you see how it’s made up of repeating
red and green reddish and greenish subunits and those red and green
subunits are different monomers. The monomers don’t have to be completely
identical, you could have a polymer that was made up of a whole bunch of copies
of a single monomer but you can also have big complex polymers that are made
up of a few chemically similar monomers all strung together. So in this
example we’re showing a single polymer that’s basically a string of multiple
copies of two different monomers. If you remember the prefixes “mono” and “poly” you should be able to keep straight which is which. Now this will be important when we’re discussing the
structures of different carbohydrates but it also comes up a lot when we
talk about the proteins and nucleic acids in particular. This slide shows you
the different categories of carbohydrates, you have mono-saccharides, disaccharides and polysaccharides. Again pay attention to your prefixes,
you’ve got “mono” one, “di” meaning “two” and “poly” meaning “many”. So your
monosaccharides, these are going to be monomers, just a single sugar molecule and the
disaccharides these are dimers made up of two monomers joined together, chemically bonded together right there.
Polysaccharides are polymers made up of multiple, can be hundreds of thousands of individual sugar molecules, hundreds of thousands of monomers. Sometimes the name of a molecule gives you a clue as to what type of molecule you’re talking
about. If you see the suffix “-ose” you’re talking about some kind of
carbohydrate. You can see it here in glucose, in the dimer sucrose, in the monomer
fructose. That doesn’t necessarily hold true for the polysaccharides, but for the
monosaccharides disaccharides you see that -ose suffix. Carbohydrates largely
serve as fuel for the body they are molecules that can be broken down by by
your cells to give energy and some of the polysaccharides, these guys, they’re
also structural molecules as well. There are five biologically important
monosaccharides, you have the hexose sugars and the pentose sugars.
The hexose sugars include glucose, fructose, and galactose and these are
called hexose sugars because they have six carbons. So if you look at glucose, there’s 123456 carbons. The pentose sugars are called pentose because they have only five carbons, so if you look at deoxyribose you have 12345 carbons. This is a way to categorize them. Glucose is a hexose sugar. Glucose is
the type of sugar that circulates in our blood so when we talk about blood
sugar levels we’re talking about blood glucose levels. Fructose is frequently found in fruits.
Galactose is usually found as part of a dimer. The pentose sugars are deoxyribose and ribose and we’ll come back to these when we talk about the nucleic acids.
These are sugars that form a part of the nucleic acids; deoxyribose forms a
part of DNA and ribose forms a part of RNA. There are three biologically important
disaccharides and that’s what we’re looking at here. We have sucrose, sucrose
you may be more familiar with as table sugar and sucrose is made up of one molecule of glucose joined to one molecule of fructose. They’re joined by a
process called dehydration synthesis and dehydration synthesis is called that
because basically you stick these two monomers together by removing a
water so glucose + fructose gives you sucrose and in the process you lose a water molecule, so it’s called dehydration synthesis. And that’s what happens when you are making lactose or maltose as well. Lactose is the type of sugar that is found
in milk products and that’s made up of a molecule of glucose joined to one molecule of galactose. So if you are lactose intolerant, or if you know someone who’s
lactose intolerant, it basically means that your body, your gut cannot break
down the lactose molecule. Maltose is a dimer, a disaccharide, that is found oftentimes in
fermented products and that’s one molecule of glucose joined to another
molecule of glucose. There are a few biologically important polysaccharides
and the biologically important polysaccharides are primarily polymers
of glucose so when you see each of these little hexagons those are all glucose molecules so in these polysaccharides you have hundreds of glucose molecules joined
together. Starch is an important polysaccharide it
is the main storage carbohydrate in plants and can be found in a couple of
different forms but when you think starch, think of a nice starchy potato or corn or rice, what you think of as dietary carbohydrates you’re usually consuming
starch. Glycogen is the main way to store carbohydrates in animals you have
glycogen in your liver and you have it in your muscle cells. Your muscle cells
actually store a little bit of glycogen for their use when you’re exercising
strongly. If you have ever carbo loaded to get ready for a race, that’s just
eating a diet high in carbohydrates for a few days before a marathon or
something, What you’re basically doing is you’re building up your glycogen reserves in your muscle cells so that you have deeper reserves
to draw on when you actually start running the race. You also find cellulose in
plants and cellulose is a structural polysaccharide it makes up part of
plant cells. Cellulose is a form of dietary fiber, it’s a type of polysaccharide that
your body actually cannot break down and dietary fiber is an important part of a
healthy diet. You also see carbohydrates on the surface of your cells. Here we’re looking at a diagram of the cell membrane. It’s mostly made up of phospholipids, that’s what these little pin-looking molecules are, but some of these lipids have polysaccharides
attached to them. There’s a polysaccharide attached to a
lipid. When you have a phospholipid with a polysaccharide attached to it that’s
referred to as a glycolipid. There are also proteins embedded in the
plasma membrane and some of these proteins have polysaccharides attached
to them and if you have if you have a protein with a polysaccharide attached
to it it’s referred to as a glycoprotein. So anytime you see that prefix “glyco-”
basically think that there is a carbohydrate attached. This carbohydrate
coating on cells is collectively called the glycocalyx. The sugar-coating basically. That’s what “glycocalyx” means. These polysaccharides serve a number of different functions. One main function is that they
can serve as ID tags allowing cells to recognize each other. They can also help
anchor your cells in place or help your cells move and there’s even some
evidence that they can help protect cells from damage. After studying this
video you should be able to answer these questions. Go ahead and give it a whirl
now.

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