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RELEVANT LEARNING OUTCOME:
(b) Describe the formation and breakage of glycosidic bond.
Structure of Disaccharides
Structure of Disaccharides
Watch the video from 1:28 to 2:45 to learn more about the structure and some properties of disaccharides.
NOTE: Disaccharide is considered to be a SIMPLE carbohydrate, not a complex carbohydrate.
🖊️Check Your Understanding: Learning Goals Checkpoint
🖊️Check Your Understanding: Learning Goals Checkpoint
What are the monosaccharides found in different disaccharides?
Maltose is made of glucose and glucose.
Lactose is made of glucose and galactose.
Fructose is made of glucose and fructose.
💡Glycosidic bond: Formation and Breakage
💡Glycosidic bond: Formation and Breakage
Understanding the process of condensation and hydrolysis is important in describing the formation and breakage of glycosidic bond, a type of covalent bond.
Take note of the differences when drawing an alpha glycosidic bond and beta glycosidic bond.
There are two ways to draw a beta glycosidic bond:
Zig-zag: draw the bond in a zig- zag manner - no need to 'flip' the second molecule when drawing (see right - cellobiose).
Flipped: Flip the second molecule, such that the bond is above the ring.
To name the type of bonds, follow these steps:
Which two carbons are involved?
Is the bond above (beta) or below (alpha) the plane of the ring?
What type of biomolecule (and thus type of bond) is this?
Example: Two glucose units
Example: Two glucose units
Which two carbons are involved?
C1 of one glucose and C4 of one glucose
Is the bond above (beta) or below (alpha) the plane of the ring?
The hydroxyl group on C1 of both glucose are below the plane of the ring, thus both are α-glucose.
What type of biomolecule (and thus type of bond) is this?
As they are simple sugars, thus they will form glycosidic bonds.
Therefore, the name of the bond is α - (1, 4) - glycosidic bond.
🖊️Check Your Understanding: Learning Goals Checkpoint
🖊️Check Your Understanding: Learning Goals Checkpoint
How are glycosidic bonds formed? How are glycosidic bonds broken?
Through the process of condensation (formation of glycosidic bond) and hydrolysis (breakage of glycosidic bond).
Remember to explain the processes of condensation and hydrolysis when asked to 'Describe' or 'Explain' (See Condensation and Hydrolysis in 'Introduction to Biological Molecules).
Properties and Roles of Disaccharides
Properties and Roles of Disaccharides
Physical properties
Physical properties
Small, sweet, crystalline in appearance, and readily soluble in water (i.e. form favourable interactions with water)
Chemical properties
Chemical properties
All disaccharides are reducing sugars (except sucrose).
Disaccharides also have a crystalline appearance (looks like crystals).
💡Benedict's Test
💡Benedict's Test
Watch the video on how Benedict's Test is conducted to test for the presence of reducing sugars. You are required to carry out the experiment yourself [For H2 Paper 4 Practical].
Benedict's Test measures the amount of reducing sugar in a solution.
It is known as a semi-quantitative test because by making observations of the colour of the solution or precipitate, it is possible to estimate the concentration of the sugar.
🖊️Check Your Understanding: Learning Goals Checkpoint
🖊️Check Your Understanding: Learning Goals Checkpoint
Does Benedict’s test work for all sugars?
Benedict's Test will only show positive results for reducing sugars. This includes simple sugars (monosaccharides and disaccharides except sucrose), but not complex sugars (polysaccharides) like starch, glycogen and cellulose.
A negative result: blue colouration of Benedict's solution remains.
A positive result: green / yellow / orange / brick-red precipitate forms. The colour depends on the amount of reducing sugar present.
🕹️Simulation
🕹️Simulation
Check in with your teacher for the following simulation.
Labster: Benedict’s Test for Simple Carbohydrates
🖊️Check Your Understanding
🖊️Check Your Understanding
Attempt Qn 4-6 of the Carbohydrates Learning Goals.
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