Polysaccharides: Starch, Cellulose & Glycogen

CHECarbohydrates

Polysaccharides are glucose polymers that differ mainly in linkage type and branching, which dictates their function as storage or structural material. This subtopic tests amylose/amylopectin, the starch-iodine test, and starch-vs-cellulose-vs-glycogen comparisons.

Polysaccharide formula

(C_6H_{10}O_5)_n

Repeating dehydrated glucose (

C_6H_{12}O_6

) unit;

n

is large for starch, cellulose and glycogen.

Linkage summary

\text{amylose: } \alpha\text{-}(1\rightarrow4) \quad \text{amylopectin/glycogen: } \alpha\text{-}(1\rightarrow4)+\alpha\text{-}(1\rightarrow6) \quad \text{cellulose: } \beta\text{-}(1\rightarrow4)

\alpha

-links are digestible by humans;

\beta\text{-}1,4

of cellulose is not.

Starch = amylose (linear, $\alpha\text{-}(1\rightarrow4)$ chains, water-soluble fraction, about 20%) + amylopectin (branched, $\alpha\text{-}(1\rightarrow4)$ chains with $\alpha\text{-}(1\rightarrow6)$ branch points, water-insoluble fraction, about 80%).
Starch gives a blue colour with iodine because $I_2$ molecules sit inside the helical amylose coil; on heating the helix uncoils and releases $I_2$ , so the colour disappears (and returns on cooling).
Cellulose is a linear polymer of glucose joined by $\beta\text{-}(1\rightarrow4)$ linkages, giving straight chains with strong inter-chain H-bonding — a structural (plant cell wall) material that humans cannot digest.
Starch (storage in plants) uses $\alpha\text{-}(1\rightarrow4)$ and $\alpha\text{-}(1\rightarrow6)$ links and is digestible; the linkage type ( $\alpha$ vs $\beta$ ) is the key structural difference from cellulose.
Glycogen ('animal starch') is the storage polysaccharide in animals (liver, muscle); like amylopectin it is $\alpha\text{-}(1\rightarrow4)$ with $\alpha\text{-}(1\rightarrow6)$ branches but is much more highly branched (branch points every 8–12 units vs every 24–30 in amylopectin).
Glycogen differs from starch by being more highly branched and stored in animals rather than plants, allowing rapid glucose mobilisation.
All three are condensation polymers of D-glucose with general formula $(C_6H_{10}O_5)_n$ ; on complete hydrolysis each gives only D-glucose.
Amylose is the linear, water-soluble fraction giving the intense blue iodine colour; amylopectin is the branched, less soluble fraction giving a red-violet colour.
Cellulose is the most abundant organic polymer on Earth; its straight $\beta$ -chains pack via hydrogen bonding into rigid fibres, suiting it to a structural role.
Function follows form: $\alpha$ -links allow coiling/branching for compact, mobilisable storage (starch, glycogen); $\beta$ -links give extended, strong fibres for support (cellulose).
Glycogen's heavy branching provides many non-reducing chain ends, enabling rapid simultaneous release of glucose when energy is needed.
Partial hydrolysis of starch gives dextrins then maltose, before final hydrolysis to glucose; this stepwise breakdown underlies the starch-to-sugar conversion in digestion.

Where the marks go

Saying cellulose has $\alpha\text{-}1,4$ bonds — cellulose is $\beta\text{-}1,4$ ; the $\alpha$ vs $\beta$ difference is exactly why humans digest starch but not cellulose.
Explaining the disappearing iodine colour as a chemical reaction destroying iodine — it is physical: heat uncoils the amylose helix and releases trapped $I_2$ , and the colour returns on cooling.
Claiming amylose is branched — amylose is linear; amylopectin (and glycogen) carry the $\alpha\text{-}1,6$ branch points.
Stating glycogen is a plant product — it is the animal storage polysaccharide (liver and muscle), the analogue of plant starch.
Forgetting that all three hydrolyse to a single monosaccharide (D-glucose) — students sometimes write 'glucose + fructose' by analogy with sucrose.

How the board asks it

Give reasonsstarch-iodine helix; iodine trapped in the amylose coil
Account for the following: Starch gives a blue colour with iodine which disappears on heating and reappears on cooling.
Distinguish $\alpha$ vs $\beta$ glycosidic linkage and digestibility
How will you distinguish between starch and cellulose? State the type of glycosidic linkage present in each and account for why humans can digest starch but not cellulose.
Structure / namingamylose/amylopectin composition; glycogen as animal starch
Name the two components of starch and state which one is responsible for the intense blue colour with iodine. What is glycogen and where is it stored in the body?
Give reasonsbranching and $\alpha$ / $\beta$ links; form follows function
Give reasons: Glycogen is much more highly branched than amylopectin, whereas cellulose forms long straight chains. Relate each of these structural features to its biological function.
Predict the productstepwise hydrolysis of starch to glucose
Starch has the general formula $(C_6H_{10}O_5)_n$ and on complete hydrolysis gives only D-glucose. Name the intermediate products formed during its partial hydrolysis and the final monosaccharide obtained.

Practise this topic

PreviousGlucose: Structure, Evidence & Tests NextAmino Acids & Proteins: Structure & Terms

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.