Complex Carbohydrates

3 disaccharides to know: sucrose (glucose + fructose, α-1,2, non-reducing), lactose (galactose + glucose, β-1,4, reducing), maltose (glucose + glucose, α-1,4, reducing).
Sucrose is non-reducing because the bond ties up BOTH anomeric carbons; lactose and maltose keep one anomeric carbon free, so they remain reducing.
Homopolysaccharide = one sugar type; heteropolysaccharide = mixed sugars. Biological polysaccharides mainly use D-glucose.
Cellulose = β-D-glucose, β-1,4 linkages, no branching — linear, tightly H-bonded, structural, indigestible by humans (no enzyme breaks β-bonds).
Starch (plant storage) = α-D-glucose: amylose (linear, α-1,4) + amylopectin (branched, α-1,4 backbone + α-1,6 branches every ~24-30 residues). Digested by alpha-amylase (random internal cuts) and beta-amylase (nonreducing-end cuts, yields maltose).
Glycogen (animal storage) = same backbone as amylopectin but branches every ~8-12 residues (denser) → more water-soluble, faster glucose mobilization.
Glycogen phosphorylase releases glucose-1-phosphate from nonreducing ends; stops near α-1,6 branch points, where a debranching enzyme takes over — the only step releasing free glucose.

Glycosidic bond: Bond linking 2 monosaccharides, formed with loss of water; named by anomeric configuration + carbon positions (e.g. α-1,4).
Non-reducing sugar: Both anomeric carbons are tied up in bonds (e.g. sucrose) — can't reduce Cu²⁺/Ag⁺.
Homopolysaccharide / heteropolysaccharide: Polysaccharide of one sugar type / a mix of sugar types.
Amylose / amylopectin: Linear (α-1,4) / branched (α-1,4 + α-1,6) forms of starch.
Debranching enzyme: Relocates residues near an α-1,6 branch point and cleaves it, releasing free glucose.

Disaccharide	Components / Bond / Reducing?
Sucrose	Glucose + fructose, α-1,2, non-reducing
Lactose	Galactose + glucose, β-1,4, reducing
Maltose	Glucose + glucose, α-1,4, reducing

Polysaccharide	Bonds / Branching / Digestible?
Cellulose	β-D-glucose, β-1,4, no branching — NOT digestible by humans
Starch (amylose/amylopectin)	α-D-glucose, α-1,4 (+ α-1,6 branches ~every 24-30 residues in amylopectin) — digestible
Glycogen	α-D-glucose, α-1,4 + α-1,6 branches ~every 8-12 residues (denser than amylopectin) — digestible

1Glycogen phosphorylase cleaves α-1,4 bonds from the nonreducing ends, releasing glucose-1-phosphate.
2Phosphorylase halts a few residues short of each α-1,6 branch point.
3A debranching enzyme relocates the remaining residues to a nearby chain.
4The debranching enzyme hydrolyzes the α-1,6 branch bond itself, releasing free glucose (the only non-phosphorylated product in this pathway).

Cellulose and starch/glycogen use the SAME monomer (D-glucose) — the only difference is α vs. β linkage, which is entirely why one is digestible and the other isn't.
Sucrose being non-reducing is because its bond involves BOTH anomeric carbons, not because of which sugars are involved — lactose/maltose keep 1 anomeric carbon free and ARE reducing.
Don't mix up branch density: glycogen branches MORE often (~8-12 residues) than amylopectin (~24-30) — more branching = faster mobilization, tested as a direct comparison.

Why is sucrose a non-reducing sugar?

What linkage makes cellulose indigestible by humans?

What structural feature lets glycogen mobilize glucose faster than amylopectin?

What does glycogen phosphorylase release, and what handles the branch points?

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