Carbohydrates Essay, Research Paper

INTRODUCTION

A theme that we have seen and studied in this class is that a particular

form or

structure of a molecule plays a significant role in the function of that

molecule. Since we

are interested in the function of molecules. it helps to study their

structure. One of the

major classes of organic compounds found in cells are carbohydrates. These

carbohydrate

are made of carbon, hydrogen, and oxygen in a ratio of 1:2:1 respectively

with a general

formula of X(CH2O)n. When the carbohydrates consists of one unit of sugar,

X=1, it is

called monosaccharide. If it consists of 2 units, X=2, the carbohydrate is

called

disaccharide. Carbohydrates made up of more than two units, X>2, are called

polysaccharides. Carbohydrates can also be branched or unbranched depending

on the

type of linkage. Those with alpha 1:4 linkages are linear or unbranched,

while those with

alpha 1:6 linkages are branched. Carbohydrates are necessary biomolecules

because they

play a role in energy metabolism as a source of potential chemical energy,

also they are

important building blocks for other biomolecules.

The word carbohydrate is very general, so in order to understand these

molecules

more precisely, we need to be able to identify more specific classifications.

Our

experiments try to accomplish this using three common bioassay tests. The

first, the

Benedict test, will test various compound for reducing sugars. All

six-carbon hexose

sugars are reducing carbohydrates, as are most disaccharide. Sucrose is the

exception.

Most polsaccharides are not reducing. Secondly, we have the Barfoed test

which is

designed to test for monosaccharides. The third and final Iodine test is used

to test for

polysaccharides that are either branched or unbranched. By combining these

tests we

were able to make accurate predictions about the carbohydrate contents of a

given sample.

Now, let’s take a closer look at how these bioassays do work. The Benedict

and

the Barfoed tests are based on the reaction of cupric ions with aldehyde or

ketone groups.

In the presence of a reactive group, the blue cupric ions are reduced to red

cuprous ions.

The Benedict test is a basic solution and upon heating turns green, yellow,

orange or brick

red which indicates a positive reaction. The final color is dependent on the

number of

reactive sites available; green indicates few sites, yellow more, and red

denotes many sites.

The Barfoed solution is acidic and only free aldehyde or ketone groups of

monosaccharides can reduce the blue ions to red ions. The color change to

red will occur

immediately. The lack of a change indicates only that the solution is not a

monosaccharide. The iodine test is used for polysaccharides. Iodine

combines with any

existing alpha helices. The more coiled the sample the darker the iodine

will turn. The

color change can range from deep black-blue with a sample of many coils to a

rust red

violet with fewer coils and more branchings. When there are no coils, there

is no color

change. Mono and disaccharides give negative results.

In summary, this lab attempts to investigate several different samples by

means of

series of tests, and based on the combined results of all three tests we can

attempt to

understand the carbohydrate composition of unknown samples. We hope to be

able to

predict the results of three bioassays for an unknown solution if given its

saccharide type

and reducing property. We should also be able to predict the saccharide type

and reducing

capability of an unknown solution if given the results of the three

bioassays.

MATERIALS AND METHODS*

Like any other experiment, this experiment needs some specific materials

including, beaker, graduated cylinder, hot plate, 11 test tubes, test tube

holder, wax pencil,

liquid soap, and test tube brush. Also, we used the Barfoed reagent,

Benedict reagent,

and iodine reagent.

Our eleven samples were distilled water (control), glucose, fructose,

maltose,

lactose, sucrose, glycogen, starch, potato soup, and dilute honey.

First, we marked our test tubes with the wax pencil to keep track on the

subtances,

then we place the eleven samples in the corresponding tubes. The first test

that we

performed was Benedict, followed by Barfoed, ending with iodine test. When

needed the

samples were heated and our results were immediately recorded in the

following tables. In

all three cases distilled water was used as a control.

*The details of the materials and the methods can be obtained from the lab

manual:

Experiments in Biology, From chemistry to sex by Linda Van Thiel, page 13.

RESULTS

The actual results of the Benedict test are as follows: distilled water

remained

blue, glucose turned a dark green, fructose blue-green, galactose was red,

maltose was

slightly red, lactose blue-green on the top of the test tube and red on the

bottom, sucrose,

glycogen, starch, and potato soup were all negative(blue). Finally, the

dilute honey sample

was dark orange.

The actual results of the Barfoed test are as follows: distilled water

formed no

precipitate, glucose, fructose and galactose did form red precipitate,

maltose, lactose,

sucrose, glycogen, starch, and potato soup did not form a precipitate, dilute

honey did

form a precipitate.

The actual results of the iodine test are as follows: distilled water,

glucose,

fructose, maltose, lactose, and sucrose all remained yellow or negative.

Glycogen turned

a rust color, starch was black-blue, potato soup was rust colored, and final

sample dilute

honey remained yellow.

DISCUSSION

Combining the three tests we have the over all results as follows: for our

control

distilled water we can conclude that it is non-reducing, non-monosaccharide,

and non-

polysaccharide; glucose, fructose, and galactose were all reducing,

monosaccharides,

non-polysaccharides. Maltose and lactose were both reducing,

non-monosaccharides,

non-polysaccharides. Sucrose was non-reducing, non-monosaccharide, non-

polysaccharide. Glycogen was a non-reducing, non-monosaccharide, and a

branched

polysaccharide. Starch was a non-reducing, non-monosaccharide, and a

unbranched

coiled polysaccharide. Potato soup was non-reducing, non-monosaccharide, and

a

branched polysaccharide. Dilute honey was reducing, monosaccharide, and a

non-

polysaccharide.

Let’s continue the discussion of this lab with a closer look at our

monosaccharides.

Based on our results we can conclude that glucose, fructose, galactose and

dilute honey

are the monosaccharides since they all formed a precipitate in the Barfoed

test. The

sample of dilute honey was of greatest interest to me since we did not know

prior to the

test whether it were a monosaccharide or not. I suspected that it was

reducing since the

honey was diluted. A non-reducing carbohydrate I do not believe we could

dilute since it

will not dissolve. Based on the precipitate formation of dilute honey in the

Barfoed, it can

be concluded that it is comprised of monosaccharides.

Looking at our results I can reasonably conclude that the disaccharide

samples are

maltose, lactose, and sucrose since they all were negative for both the

Barfoed and iodine

tests. If we also look at the probable disaccharides, we see that none of

our tests used

were designed to specifically test positively for them. Since we know that

disaccharides

are comprised of two monosaccharides by way of dehydration reaction, we could

test for

disaccharides by adding water to the possible disaccharide samples and may be

heat them

so they will undergo a hydrolysis process, then run them through the Barfoed

test again.

If the sample which before adding water was negative in the Barfoed test, but

was positive

after adding water then we could conclude that the original sample was a

disaccharide.

Our tested samples that we believe to be polysaccharides are glycogen,

starch, and

potato soup since they all had some color change in the presence of Lugol’s

iodine.

Polysaccharides can be further classified by their overall structure, in

particular, whether

they are highly branched, highly coiled and unbranched, or both slightly

coiled and

branched. We learned that the starches can be coiled profusely or coiled

with no branches.

The iodine test will result in a different degree of color change based on

the amount of

coiling present. Namely, a highly coiled carbohydrate will turn a dark

blue-black color.

The particular highly coiled polysaccharide that we discussed in the class is

amylose which

is an unbranched storage starch found in plants. Since our starch sample

turned black, it

may be compromised of amylose starch. The potato soup sample did not turn as

dark, a

color indicating to me that the starch in this sample probably consisted of

smaller starch

units called dextrin. Dextrin have very short terminal ends that coil only

sightly so the

color change would not be so dramatic as in the presence of highly coiled

starch like

amylose. The potato soup was made from dehydrated buds. This dehydration

process of

the fresh potatoes does cause structural change in the starches. A fresh

potato sample I

predict to turn a dark black since its starches would be intact. Glycogen

turned a rust

color as we should expect since we know that glycogen is a slightly coiled

polysaccahride.

I did predict prior to the experiment that the color change in the presence

of the iodine

would be different for starch and glycogen since they have different coiling

characteristics.

The data, in my opinion, did not conflict with our expected results. These

tests

when used together allow us to make predictions about unknown samples with

confidence. I believe that the data provide sufficient information to better

understand

carbohydrates and how we can more precisely describe carbohydrates.