Lesson 1 of 7 / Biological molecules
Glucose isomers and glycosidic bonds
How can the same molecular formula produce different polymers?
I can describe alpha/beta glucose and condensation or hydrolysis of glycosidic bonds.
I can describe glucose isomers, glycerol, fatty acids and amino acids.Syllabus
Syllabus 9477, Core 1(g). Describe glucose isomers, glycerol, fatty acids and amino acids.
I can explain condensation and hydrolysis of glycosidic, ester and peptide bonds.Syllabus
Syllabus 9477, Core 1(h). Explain condensation and hydrolysis of glycosidic, ester and peptide bonds.
About 6 min
Make a guess. You are not marked.
Alpha-glucose and beta-glucose have the same formula, C6H12O6. What is different about them?
Show the answer
The arrangement of groups on carbon 1.
They are isomers. Flipping the groups on carbon 1 changes how the units can join in a polymer.
The key idea
Alpha and beta glucose differ at carbon 1; the orientation of their glycosidic bonds changes the polymer built.
Explore the idea
Locate the carbon-1 hydroxyl group
Alpha: the carbon-1 hydroxyl group and CH2OH lie on opposite sides of the ring. Both have molecular formula C6H12O6.
Simplified Haworth-style D-glucose model. Numbers mark ring carbons; most hydrogen atoms are omitted. Changing the page orientation does not change the relative arrangement.
Keep the bond and water accounting straight
A glycosidic bond is a covalent join. Forming one join by condensation releases one water molecule; breaking it by hydrolysis uses water. A linear chain of n glucose units contains n - 1 joins. Hydrogen bonds between neighbouring chains are a separate type of interaction.
Explanation
Alpha-glucose and beta-glucose have the same molecular formula, C6H12O6, but a different arrangement around carbon 1 in their ring forms. In the usual Haworth drawings of D-glucose, the carbon-1 OH is opposite the CH2OH group in alpha-glucose and on the same side in beta-glucose.
The many hydroxyl groups make glucose polar and soluble in water. It can be transported in solution and used in respiration. Joining many glucose units reduces the number of dissolved particles per stored glucose unit, helping avoid the osmotic effect of storing equivalent amounts of free glucose.
A condensation reaction joins monosaccharides by a covalent glycosidic bond with removal of water. The positions identify the bond: a 1,4 bond connects carbon 1 of one unit to carbon 4 of another. A 1,6 bond creates a branch in suitable alpha-glucose polymers.
Hydrolysis uses water to break a glycosidic bond, yielding smaller carbohydrates. Condensation and hydrolysis are not the same as forming and breaking hydrogen bonds between neighbouring chains; distinguish the covalent backbone from the interactions stabilising its arrangement.
Step by step
- 1
Locate carbon 1
Compare the OH orientation relative to CH2OH.
- 2
Name the link
State the linked carbon positions.
- 3
Account for water
Water is removed in condensation and added in hydrolysis.
Worked example
Work through the evidence
A linear carbohydrate contains 8 glucose units. How many joining bonds and water molecules are involved in forming it from the units?
One way to explain it
Seven glycosidic bonds form and seven water molecules are released, assuming one linear chain with no additional rings or cross-links.
Why this answer works
- A chain of n units has n - 1 joins.
- Each condensation join releases one water molecule.
Watch out for this
A student says: "Alpha and beta glucose differ in the number of carbon atoms." What is wrong with this?
Show the answer
They have the same composition; the arrangement around carbon 1 differs.
Check your understanding
Which change breaks a glycosidic bond by hydrolysis?
Show the answer
Adding water across the glycosidic bond
Hydrolysis splits the covalent link using water.
Quick recall
How can the same molecular formula produce different polymers?