Complex Carbohydrates

Monosaccharides link together via glycosidic bonds to form disaccharides and polysaccharides — the complex carbohydrates that make up much of the body's carbohydrate structure and energy storage.

Key Takeaways

• Disaccharides form via a glycosidic bond with water as a byproduct: 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 its bond ties up both anomeric carbons; lactose and maltose remain reducing because one anomeric carbon stays free.

• Cellulose (β-1,4 glucose) is structural and indigestible to humans; starch (α-1,4/α-1,6 glucose, as amylose and amylopectin) is plant storage and digestible via alpha-amylase and beta-amylase; glycogen (α-1,4/α-1,6 glucose, more densely branched than amylopectin) is animal storage.

• Glycogen phosphorylase releases glucose-1-phosphate from glycogen's nonreducing ends; a debranching enzyme handles the α-1,6 branch points, releasing free glucose at that one step.

Disaccharides

A disaccharide forms when two monosaccharide units join via a glycosidic bond, releasing a water molecule in the process. When two glucose molecules link together, for example, they form maltose — the glycosidic bond forms between the anomeric carbon of one glucose and a hydroxyl group of the other, with water as a byproduct.

There are three disaccharides to know: their component sugars, their bond type, and their linkage position.

Sucrose

Sucrose is composed of glucose and fructose, joined by an alpha-1,2 linkage — the alpha-anomeric carbon of glucose (C1) links to carbon 2 of fructose. This bond ties up the anomeric carbons of both sugars, which is why sucrose is a non-reducing sugar: neither ring can reopen.

Lactose

Lactose is composed of galactose and glucose, joined by a beta-1,4 glycosidic bond — the beta-anomeric carbon of galactose (C1) connects to carbon 4 of glucose. Unlike sucrose, only one anomeric carbon is involved in the bond, so lactose is a reducing sugar.

Maltose

Maltose is composed of two glucose molecules, linked by an alpha-1,4 glycosidic bond. Like lactose, maltose is a reducing sugar, since one of its anomeric carbons remains free.

Polysaccharides: Homo- vs. Heteropolysaccharides

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. If a chain is made of only one type of sugar, it's a homopolysaccharide; if it's made of a mix of sugars, it's a heteropolysaccharide. In biological systems, the main sugar used to build polysaccharides is D-glucose.

Cellulose

Cellulose is made of β-D-glucose units connected by β-1,4 glycosidic bonds. These beta-linkages let cellulose form long, linear chains that pack tightly together with strong hydrogen bonds. Because of the beta-linkages, humans can't digest cellulose — we lack the enzymes needed to break those bonds.

Starch: Amylose and Amylopectin

Starch is the main storage form of glucose in plants, made of α-D-glucose. It comes in two forms:

• Amylose — a linear glucose polymer linked by α-1,4 linkages.

• Amylopectin — a branched polymer, with an α-1,4 backbone and branches formed by α-1,6 linkages.

Because starch's linkages are alpha, they're digestible by humans, using two enzymes:

• Alpha-amylase cleaves randomly along the chain, yielding shorter polysaccharide chains, maltose, and glucose.

• Beta-amylase cleaves amylose at the nonreducing end of the polymer, yielding maltose.

Glycogen

Glycogen is the storage form of glucose in animals. It's structurally similar to amylopectin — an α-1,4 backbone with α-1,6 branch points — but with significantly more branching. Glycogen branches roughly every 8-12 glucose residues, compared to amylopectin's roughly every 24-30 residues. This higher branch density increases glycogen's water solubility and lets glucose be mobilized much faster when the body needs it.

Glycogen phosphorylase cleaves glucose units from glycogen's nonreducing ends, releasing them as glucose-1-phosphate so they can enter glycolysis. Phosphorylase stops a few residues short of each α-1,6 branch point; a separate debranching enzyme then relocates the remaining residues and cleaves the branch point itself — the one step in this process that releases free glucose rather than glucose-1-phosphate.

Common MCAT Mistakes

• Forgetting which disaccharide is non-reducing. Sucrose is the only one of the three (sucrose, lactose, maltose) that's non-reducing, because its glycosidic bond ties up the anomeric carbons of both component sugars. Lactose and maltose each leave one anomeric carbon free.

• Mixing up sucrose's linkage direction. Sucrose's bond is written α-1,2 specifically because glucose's C1 (alpha) links to fructose's C2 — not a generic "1,2 bond" between arbitrary carbons.

• Assuming starch and glycogen are chemically identical. Both use α-1,4 backbones with α-1,6 branches, but glycogen branches roughly every 8-12 residues versus amylopectin's roughly every 24-30 — the denser branching is what makes glycogen more water-soluble and faster to mobilize.

• Thinking glycogen phosphorylase alone fully breaks down glycogen. Phosphorylase stops a few residues short of each α-1,6 branch point; a separate debranching enzyme is needed to clear the branch, and that step is the only one that releases free glucose instead of glucose-1-phosphate.

MCAT-Style Concept Check

Question: A researcher treats a polysaccharide sample with an enzyme that hydrolyzes β-1,4 glycosidic bonds. The sample is fully degraded into glucose monomers. Which polysaccharide was most likely used, and why can't the human digestive system perform this same reaction?

• A) Glycogen, because humans lack glycogen phosphorylase

• B) Amylopectin, because humans lack an enzyme for α-1,6 branch points

• C) Cellulose, because humans lack enzymes that cleave β-1,4 glycosidic bonds

• D) Starch, because amylase only cleaves α-1,6 bonds

Answer: C

Explanation: Cellulose is a homopolysaccharide of β-D-glucose units linked exclusively by β-1,4 glycosidic bonds, so an enzyme that hydrolyzes β-1,4 bonds would fully degrade it to glucose. Humans lack the enzymes needed to cleave these beta-linkages, which is exactly why cellulose passes through the human digestive system undigested. Option A misapplies glycogen phosphorylase, which does act on glycogen but cleaves α-1,4 bonds, not β-1,4. Option B misdescribes amylopectin's branch points, which humans can digest. Option D is wrong because amylase targets α-linkages, not β-1,4 bonds, and starch isn't the polysaccharide described here.

FAQ

Why is sucrose a non-reducing sugar while lactose and maltose are reducing sugars?

Sucrose's α-1,2 glycosidic bond forms between the anomeric carbons of both glucose and fructose, locking both rings closed. Lactose (β-1,4) and maltose (α-1,4) each form their bond using only one sugar's anomeric carbon, leaving the other sugar's anomeric carbon free to reopen and react.

What's the structural difference between starch and glycogen?

Both are α-D-glucose polymers with an α-1,4 backbone and α-1,6 branch points, but glycogen is far more densely branched — roughly every 8-12 residues, compared to amylopectin's roughly every 24-30. That denser branching gives glycogen more nonreducing ends for enzymes to act on, allowing faster glucose mobilization.

Why can't humans digest cellulose?

Cellulose is built from β-D-glucose units joined by β-1,4 glycosidic bonds. Humans don't produce enzymes capable of hydrolyzing beta-linkages, so cellulose passes through the digestive system undigested, unlike starch and glycogen, which use digestible alpha-linkages.

How does glycogen phosphorylase release glucose from glycogen?

Glycogen phosphorylase cleaves glucose units from glycogen's nonreducing ends, releasing them as glucose-1-phosphate rather than free glucose. It stops a few residues short of each α-1,6 branch point, at which point a debranching enzyme relocates the remaining residues and cleaves the branch, releasing the one free glucose molecule in the process.