6.1 Monosaccharides, disaccharides, and polysaccharides

Carbohydrates range from single sugar units to massive polymeric chains, and their structure directly determines their biological function. Understanding how monosaccharides link together into disaccharides and polysaccharides is essential for grasping energy metabolism, cell signaling, and structural biology.

Simple Sugars

Monosaccharides

Monosaccharides are the simplest carbohydrates and the monomers from which all larger carbohydrates are built. Each one contains a single polyhydroxy aldehyde (an aldose) or a polyhydroxy ketone (a ketose).

• The general formula is $(CH_2O)_n$, where $n$ is typically 3 (triose), 5 (pentose), or 6 (hexose)
• In aqueous solution, monosaccharides with five or more carbons predominantly cyclize to form ring structures (furanose or pyranose rings) rather than remaining as open chains
• Common examples: glucose, fructose, and galactose

The classification system matters because it tells you two things at once: the number of carbons and whether the carbonyl is an aldehyde or ketone. Glucose, for instance, is an aldohexose (6-carbon aldose), while fructose is a ketohexose (6-carbon ketose).

Glucose

Glucose is a 6-carbon aldohexose and the most metabolically central carbohydrate in biology.

• It exists in both an open-chain (acyclic) form and a ring (cyclic) form
• In aqueous solution, >99% of glucose molecules are in the cyclic pyranose form
• When glucose cyclizes, the anomeric carbon (C-1) can adopt two configurations: $\alpha$-glucose (hydroxyl on C-1 points axial/down in a Haworth projection) and $\beta$-glucose (hydroxyl on C-1 points equatorial/up)
• This $\alpha$ vs. $\beta$ distinction at the anomeric carbon has huge consequences for polysaccharide structure and digestibility
• Beyond energy, glucose serves as a precursor for synthesizing amino acids, fatty acids, nucleotides, and vitamin C

Fructose and Galactose

Fructose is a 6-carbon ketohexose commonly found in fruits and honey. Its carbonyl group is at C-2 (rather than C-1 as in glucose), and it cyclizes into a five-membered furanose ring. Fructose tastes sweeter than glucose, which is why high-fructose corn syrup is used as a sweetener.

Galactose is a 6-carbon aldohexose that differs from glucose only in the stereochemistry at C-4. This single configurational change makes galactose and glucose C-4 epimers. Galactose is a key component of lactose (milk sugar) and of glycolipids and glycoproteins important in cell recognition.

Both fructose and galactose are converted to glucose (or glucose metabolic intermediates) in the liver, funneling them into glycolysis for energy production.

Compound Sugars

Disaccharides

Disaccharides form when two monosaccharides undergo a condensation (dehydration) reaction. Here's how it works:

1. The hydroxyl group on the anomeric carbon of one monosaccharide reacts with a hydroxyl group on the other monosaccharide.
2. A water molecule is released.
3. A covalent glycosidic bond forms between the two sugar residues.

The bond is named by specifying the anomeric configuration ($\alpha$ or $\beta$) and the carbon numbers involved. For example, an $\alpha$-1,4 glycosidic bond connects C-1 of one sugar (in the $\alpha$ configuration) to C-4 of the other.

Disaccharides retain many monosaccharide-like properties: they're water-soluble, sweet, and relatively small.

Sucrose, Lactose, and Maltose

Sucrose (table sugar) is glucose + fructose joined by an $\alpha$-1,$\beta$-2 glycosidic linkage. Because both anomeric carbons are involved in the bond, sucrose is a non-reducing sugar. It cannot open to expose a free anomeric carbon. Plants synthesize sucrose as their primary transport sugar.

Lactose (milk sugar) is galactose + glucose joined by a $\beta$-1,4 glycosidic linkage. It's a reducing sugar because the glucose residue retains a free anomeric carbon. People who lack sufficient lactase enzyme cannot hydrolyze this bond, leading to lactose intolerance.

Maltose (malt sugar) is glucose + glucose joined by an $\alpha$-1,4 glycosidic linkage. It's produced during starch digestion and during grain germination (especially barley, which is why it's associated with brewing). Maltose is also a reducing sugar.

Complex Carbohydrates

Polysaccharides

Polysaccharides are polymers of monosaccharide units joined by glycosidic bonds. Three structural features determine a polysaccharide's properties and biological role:

• Monomer identity (glucose, N-acetylglucosamine, etc.)
• Glycosidic linkage type ($\alpha$ vs. $\beta$, and which carbons are connected)
• Degree of branching (linear vs. branched)

These polymers fall into two broad functional categories: storage polysaccharides (starch, glycogen) and structural polysaccharides (cellulose, chitin).

Starch and Glycogen

Starch is the primary energy storage polysaccharide in plants. It's actually a mixture of two polymers:

• Amylose (~20–30% of starch): unbranched chains of glucose linked by $\alpha$-1,4 glycosidic bonds. Amylose adopts a helical shape.
• Amylopectin (~70–80% of starch): branched chains with $\alpha$-1,4 bonds along the backbone and $\alpha$-1,6 bonds at branch points (roughly every 24–30 glucose residues).

Glycogen is the storage polysaccharide in animals, structurally similar to amylopectin but much more extensively branched (branch points every 8–12 glucose residues). This heavy branching creates many non-reducing ends, which allows rapid simultaneous enzymatic degradation when energy is needed quickly. Glycogen is stored primarily in the liver (for blood glucose regulation) and skeletal muscle (for local energy demands).

Cellulose and Chitin

Cellulose is a structural polysaccharide in plant cell walls and the most abundant organic molecule on Earth. It consists of linear chains of glucose connected by $\beta$-1,4 glycosidic bonds.

The $\beta$-linkage flips every other glucose residue, producing a straight, extended chain rather than a helix. These linear chains pack tightly together via extensive hydrogen bonding, forming strong, rigid microfibrils. Most animals cannot digest cellulose because they lack a cellulase enzyme capable of cleaving $\beta$-1,4 bonds. (Ruminants like cows rely on symbiotic gut bacteria that produce cellulase.)

Chitin is a structural polysaccharide found in arthropod exoskeletons (insects, crustaceans) and fungal cell walls. Its monomer is N-acetylglucosamine (a glucose derivative with an acetylated amino group at C-2), linked by $\beta$-1,4 glycosidic bonds. Like cellulose, chitin forms rigid, hydrogen-bonded fibrils that provide mechanical strength.