Carbs: Edexcel Unit 1 (A Level Biology)


Uploaded by freeeschool on 13.10.2012

Transcript:
A Level Biology: Edexcel Unit 1 - Carbs
Hi! Welcome to our third video following the A Level Edexcel Biology specification. Today,
we are going to be looking at Carbohydrates and hopefully by the end this presentation,
you should be able to understand in great detail how monosaccharides join to form disaccharides
and polysaccharides; the importance of glycosidic bonds; and how that is made and how that also
splits, as well.
First things first, we need to understand that carbohydrates are used as a source of
energy for lots and lots of different organisms, but they are kind of also be found in structural
materials in many organisms as well. It says on the slide here that it can also be found
membranes, in cell walls and also in exoskeletons for many arthropods.
All carbohydrates contain the elements carbon, hydrogen and oxygen. The general formula for
carbohydrate is and what we got here is (CH2O)n. Glucose which is one that everyone should
be familiar with is C6H12O6. The formula for Sucrose which is a disaccharide is C12H22O11;
so it all follows a very general formula.
We mentioned early that some exoskeletons contain things like carbohydrates and chitin
would be an example of a carbohydrate which is found in exoskeletons. The reason why that
is because there is an amino group present which causes there to be even more hydrogen
bonding between the chains. Therefore, it is extremely resilient and tough.
When we are classifying carbohydrates, we can split them into two separate subgroups.
The sugars or the saccharides and we have the polysaccharides. The sugars themselves,
these are your reducing sugars contain the monosaccharides and the disaccharides. Mono,
obviously meaning single units and disaccharides are double sugar units. We need to be very
careful when we use the word “sugar” because sugar in everyday use is actually the disaccharide
sucrose, not glucose as most people think.
As we got here, we got monosaccharides which include things like glucose, fructose and
galactose; disaccharides which are two sugar units joined together and include things like
sucrose, maltose and lactose. But we also have the non-reducing sugars here as well.
These are the polysaccharides and they could be storage, things like glycogen and starch;
or they could be structural, things like cellulose which is found in cell walls and chitin, as
mentioned earlier is found within exoskeletons.
The ratio of atoms within a monosaccharide and between carbon, hydrogen and oxygen is
1:2:1. The most common monosaccharide can be split up into subgroups and you got the
Trioses, the Pentoses and the Hexoses as well. These just tell you the number of carbon atoms
that these types of monosaccharides contain. Glyceraldehyde contains three carbons. Ribose
and deoxyribose, which is found within DNA and mRNA and contain five and hexoses contain
things like glucose, which contains 6 carbon atoms.
Glucose can exist in both a straight chain form and also as a wing structure, as well.
The only difference is this occurs when it’s dissolved in water. If we look at the structure
of glucose more closely, we can see that the carbon atoms are actually numbered. If so,
we start with carbon 1 which is very important and 2-6. When we look at bonding, where the
actual glycosidic bond forms will usually be denoted by a number. For example: a 1-4
glycosidic bond will mean that two saccharides are joined from carbon 1 to carbon 4.
Not to make things even more confusing, glucose can actually exist in two forms. It can exist
in an alpha form and in the beta form. This is known as stereoisomerism. You got two isomers
of glucose. The only difference between these two forms of glucose is the position of the
OH group. As you can see on the alpha glucose, the OH group is resting towards the bottom
of the Carbon 1 while on the beta glucose, it is resting above the Carbon 1.
In the formation of disaccharide, for example maltose, what we got here are two alpha glucose
molecules. As you can, at carbon 1, you got the hydroxide group there and on carbon 4,
you got the hydroxide group in as well. In this reaction, water actually gets formed.
So what you got is the hydroxide group reacting with the hydrogen over here to form water
which is removed. What you are left with is glycosidic bond between Carbon 1 and Carbon
4. It is known as a 1 to 4 glycosidic bond.
It is very important that you understand that when water is removed from a reaction, it
is known as a condensation reaction. Now, the opposite to a condensation reaction is
known as a hydrolysis reaction. For that to occur, it even needs to add heats and an acid.
For example, hydrochloric acid or you use an enzyme, so something like amylase.
That concludes the short presentation. I hoped you enjoyed it. Don’t forget to keep a lookout
for some more videos coming out soon.
[end of audio – 06:51] A Level Biology: Edexcel Unit 1 - Carbs
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