Monosaccharides and Disaccharides

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Section 1
Relative solubility of Glucose and Galactose

Result of the experiment:

| |Weight of the |The weight of |Water added (g)|Weight of the |The weight of the|Amount of the |The saturation | | |beaker (g) |the beaker | |magnetic bar |beaker and |sugar added (mg)|points of the | | | |together with | |(g) |magnetic bar with| |sugars (mg/ml) | | | |the water (g) | | |saturated | | | | | | | | |solution (g) | | | |Glucose |38 |47.82 |9.82 |2.97 |58.3 |58.3 |7510 | |Galactose |35.04 |44.25 |9.21 |2.99 |49.66 |2420 |262.76 |

Interpretation of the result above:

The saturation point of the sugar depends on bonding between sugar and water molecules. As we adding the sugar into water and stirred it, the sugar dissolved quickly. It is because the water molecules can interact with the sugar molecule easily by intramolecular hydrogen bond. When keep adding the sugar, they dissolved slowly and some of them are undissolved. It can be explained that the water molecules are not enough to form intermolecular H-bond with sugar molecules. That’s why we need to stir the solution. It can help the water molecules to form intramolecular H-bond with sugar molecules easily. The sugar can dissolve finally and there no more sugar can be dissolved when saturation point is reached.

The saturation point of the glucose is higher than that of the galactose. Both glucose and galactose are monosaccharides. However, the difference of them is the –OH group inside the molecules. By considering carbon no. 4 of them, the –OH group of glucose which is pointing downward while that of the galactose is pointing upwards. Besides, the carbon no.3 of them, both –OH groups are pointing upwards.

The structure of glucose and galactose are shown (FROM WIKI): Galactose Glucose

[pic] [pic]
More importantly, because of the different spatial arrangement of –H and –OH groups of carbon no. 4, thus the bonding can be different. It affects the intramolecular hydrogen bond The difference of the van der waal’s of them is negligible because they have similar mass.

For glucose,

Therefore, the –OH group of carbon no.1 and 2 can form intramolecular H-bond

For galactose.

There are two –OH groups in galactose can form intramolecular H-bond (that on carbon no.1,2,3&4). As a result, the extensiveness of the H-bond in glucose is higher than that of the galactose. This effect cause glucose more soluble in water and has higher saturation point.


The difference of saturation points can be related to the use of these sugars in humans. First of all, all cell action need energy to maintain their work. During respiration, energy can be provided by using glucose molecules. The higher the solubility of sugar the easier be used in our body. It is because it can transport through blood circulation. As a result, the energy can be provided in a short time and non-stop. In the contrast, if the sugar has lower saturation points, our body need time to digest and it is hard to be used. For example, our brain and muscle need large amount of energy to work, we must absorb the sugar which has higher saturation points to supply them in a short time. Otherwise, their efficiency may decrease.

Section 2
Relative sweetness and taste threshold

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