Lesson 2 of 7 / Biomolecules: structure explains function
Starch stores; cellulose supports
How do glycosidic bonds change the chain?
I can explain starch and cellulose structure, bond formation and hydrolysis in relation to their roles.
I can explain formation and hydrolysis of glycosidic, ester and peptide bonds.Syllabus
Syllabus 8876, Core 1(f). Explain formation and hydrolysis of glycosidic, ester and peptide bonds.
I can relate starch, cellulose, triglyceride and phospholipid structures to roles.Syllabus
Syllabus 8876, Core 1(g). Relate starch, cellulose, triglyceride and phospholipid structures to roles.
About 6 min
Make a guess. You are not marked.
When a glycosidic bond is broken by hydrolysis, is water released or used up?
Show the answer
Used up, added across the bond.
Hydrolysis means splitting with water. Condensation is the reverse: it forms the bond and releases water.
The key idea
Starch has alpha-glucose chains suited to storage. Cellulose has beta-glucose chains that form strong fibres through many hydrogen bonds.
Explore the idea
Change the bonds, change the shape
alpha-1,4 glycosidic bonds. Unbranched alpha-glucose chain, coiled into a compact form. Insolubility helps make it a storage molecule.
Original chain model, not an atom-by-atom structure. Condensation forms glycosidic bonds and releases water; hydrolysis uses water to break them.
Explanation
A condensation reaction joins glucose units by a glycosidic bond and releases water. Hydrolysis uses water to break that bond. The numerical bond description identifies the carbons involved; it is not a count of the glucose units.
Starch contains amylose and amylopectin. Amylose is an unbranched chain of alpha-glucose joined mainly by alpha-1,4 bonds and coils into a compact shape. Amylopectin has alpha-1,4 chains with alpha-1,6 branch points, creating more ends from which enzymes can release units.
Starch is large and insoluble, so it forms a compact store without contributing many dissolved particles to cell water potential. It can be hydrolysed when glucose is required. Insoluble does not mean impossible to digest: appropriate enzymes break its bonds.
Cellulose has beta-1,4 glycosidic bonds, with alternate glucose units inverted, producing long unbranched chains. Many hydrogen bonds between neighbouring chains hold them together in microfibrils. Their tensile strength helps plant cell walls resist stretching when water enters.
Step by step
- 1
Name the glucose form and bonds
Alpha-1,4 and branch alpha-1,6 in starch; beta-1,4 in cellulose.
- 2
Trace the resulting structure
Coiled or branched storage molecules contrast with extended aligned cellulose chains.
- 3
Explain the useful property
Connect insolubility to storage and multiple interchain bonds to tensile strength.
Worked example
A strong wall from weak bonds
A hydrogen bond is individually weak. Why can cellulose still provide strong support?
One way to explain it
Many hydrogen bonds form between numerous aligned cellulose chains. Their combined effect holds chains together in microfibrils that resist pulling forces. Strength arises from the organised network, not from one hydrogen bond alone.
Why this answer works
- Distinguish individual bond strength from many bonds acting together.
- Link chain alignment to the wall's tensile strength.
Watch out for this
A student says: "Hydrolysis releases water when it breaks a glycosidic bond." What is wrong with this?
Show the answer
Hydrolysis uses water. Condensation forms the bond and releases water.
Check your understanding
Which feature helps cellulose perform a structural role?
Show the answer
Many hydrogen bonds hold aligned chains together.
The microfibrils resist tension in plant cell walls.
Quick recall
How do glycosidic bonds change the chain?