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08-10-2012, 02:56 PM
Biology question
So Hylonome is working on her Biology homework and is stuck.

She has to explain why glucose solution changes colour when Benedict's solution is added. She found this;

"Benedict's reagent contains blue copper(II) ions (Cu2+) which are reduced to copper(I) ions (Cu+). These are precipitated as red copper(I) oxide which is insoluble in water."

but needs a little more in depth info.

Can anyone elaborate on it for her.

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08-10-2012, 03:21 PM
RE: Biology question
(08-10-2012 02:56 PM)Hughsie Wrote:  So Hylonome is working on her Biology homework and is stuck.

She has to explain why glucose solution changes colour when Benedict's solution is added. She found this;

"Benedict's reagent contains blue copper(II) ions (Cu2+) which are reduced to copper(I) ions (Cu+). These are precipitated as red copper(I) oxide which is insoluble in water."

but needs a little more in depth info.

Can anyone elaborate on it for her.

Benedict Solution is light blue because it contains copper sulfate. When it is mixed and heated with a sugar, such as glucose, which has electrons available to donate, the copper will accept the electrons and become reduced, which turns it brownish-orange. During this process, the blue copper (II) ion is reduced to a red copper (I) ion. While the copper is being reduced, the glucose gives up an electron and is oxidized.
Reducing is accepting electrons.

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08-10-2012, 03:25 PM (This post was last modified: 08-10-2012 03:40 PM by Bucky Ball.)
RE: Biology question
(08-10-2012 02:56 PM)Hughsie Wrote:  So Hylonome is working on her Biology homework and is stuck.

She has to explain why glucose solution changes colour when Benedict's solution is added. She found this;

"Benedict's reagent contains blue copper(II) ions (Cu2+) which are reduced to copper(I) ions (Cu+). These are precipitated as red copper(I) oxide which is insoluble in water."

but needs a little more in depth info.

Can anyone elaborate on it for her.

I'd have to look it up, but the solubility of anything is dependent on the pH of the solution. It has to do with the pH. I would bet that the pH, (ie acidic solution) breaks the sugar's bonds. Did she "Google" it ? Benedict's is a test for sugar, so if the solution changes color, it means the sugar bonds are broken by the acid. For exactly why, see Chas' post.

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08-10-2012, 03:41 PM
RE: Biology question
Thanks very much for that guys - really helpful Smile
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08-10-2012, 04:40 PM
RE: Biology question
What level of biology are you doing?, we had to know about the tests for different Saccharides, but we never had to know the why they react the way they do when tested. That sounds more of a chemistry question than biology.

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09-10-2012, 09:07 AM (This post was last modified: 09-10-2012 09:10 AM by robotworld.)
RE: Biology question
I'll try Smile

Benedict's solution is a reagent used to detect the presence of reducing sugars. Examples of reducing sugars include monosaccharides (the simplest unit of carbohydrates) such as glucose, and even some disaccharides, such as maltose.

Reducing sugars have free anomeric carbons. This means that the anomeric carbon is NOT involved in the formation of the glycosidic bond.

Anomeric carbons, in the case of carbohydrates, are carbons that are bonded to two oxygen atoms. A free anomeric carbon allows isomerisation, as seen in maltose below.

Below are two examples of sugars. Try locating the anomeric carbon Big Grin

Here is maltose, a disaccharide with a free anomeric carbon.
[Image: 660px-Maltose_Gleichgewicht.svg.png]

Here is sucrose, a disaccharide with NO free anomeric carbons (Both anomeric carbons are involved in the formation of the glycosidic bond)
The glycosidic bond is the bond joining the two sugar subunits.
[Image: 345px-Saccharose2.svg.png]

Now, as you have mentioned,
Quote:"Benedict's reagent contains blue copper(II) ions (Cu2+) which are reduced to copper(I) ions (Cu+). These are precipitated as red copper(I) oxide which is insoluble in water."

To add on to Chas, reducing sugars have a reducing form. The reducing form of maltose can be seen in the diagram for maltose above, the right one is the reducing form. We can see that the anomeric carbon has become an aldehyde group(-OCH).

The aldehyde group acts as a reducing agent, to reduce the blue Cu2+ to colourless Cu+, the Cu+ then precipitates into Cu2O, the brick-red precipitate. The aldehyde group in return is being oxidised into a carboxylic acid group (-COOH).

Here's something extra: If you add HCl (hydrochloric acid) to a sucrose solution, then carry out Benedict's test on it, you get a positive result. This is because the HCl is able to hydrolyse the glycosidic bond, thus "freeing" the anomeric carbons.

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09-10-2012, 09:20 AM
RE: Biology question
Obviously because 'God' did it.

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09-10-2012, 10:16 AM
RE: Biology question
(08-10-2012 04:40 PM)FSM_scot Wrote:  What level of biology are you doing?, we had to know about the tests for different Saccharides, but we never had to know the why they react the way they do when tested. That sounds more of a chemistry question than biology.

It's just A-level biology but we're studying biochemistry and biological molecules at the moment.
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09-10-2012, 11:20 AM
RE: Biology question
Thanks so much robotworld, that straightened things out for me!! Smile
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