3.1 Carbohydrates: structure and function

Carbohydrates are one of the four biological macromolecules, and they show up everywhere in living systems. They fuel cellular respiration, form the rigid walls of plant cells, and even help cells recognize each other. Understanding their structure explains why they can do such different jobs.

Carbohydrate Structure and Building Blocks

Building blocks of carbohydrates

All carbohydrates are built from carbon, hydrogen, and oxygen in a roughly 1:2:1 ratio, giving them the general formula $(CH_2O)_n$. The simplest carbohydrates are monosaccharides, single sugar units that serve as building blocks for everything larger.

• Common monosaccharides include glucose, fructose, and galactose
• They're classified by carbon count: trioses (3C), tetroses (4C), pentoses (5C), and hexoses (6C)
  • Glucose is a hexose (6 carbons) and the most important fuel molecule in cells
  • Ribose is a pentose (5 carbons) found in RNA
• In solution, monosaccharides shift between a linear (open-chain) form and a ring (cyclic) form. The ring form dominates in cells and determines how sugars link together.

When two monosaccharides join, you get a disaccharide. Three common ones to know:

• Sucrose = glucose + fructose (table sugar)
• Lactose = glucose + galactose (milk sugar)
• Maltose = glucose + glucose (produced during starch digestion)

Polysaccharides are long chains of monosaccharides, sometimes thousands of units long. The three you'll see most often are starch, glycogen, and cellulose. They're all made of glucose, yet they behave very differently because of how those glucose units are bonded together.

Role of glycosidic bonds

Glycosidic bonds are the covalent bonds that link monosaccharides into disaccharides and polysaccharides. Here's how they form:

1. The hydroxyl group ($-OH$) on one monosaccharide reacts with the hydroxyl group on another.
2. A water molecule is released (this is a dehydration synthesis or condensation reaction).
3. A covalent bond now connects the two sugars through an oxygen bridge.

To break a glycosidic bond, the reverse happens: a water molecule is added back in (hydrolysis).

The type of glycosidic bond matters enormously. It depends on which carbon atoms are involved and the orientation of the $-OH$ group on the anomeric carbon (carbon 1):

• Alpha (α) glycosidic bonds: the $-OH$ on the anomeric carbon points downward relative to the ring. Starch and glycogen use α-linkages, which makes them easy for enzymes to break down.
• Beta (β) glycosidic bonds: the $-OH$ on the anomeric carbon points upward. Cellulose uses β-linkages, which produce straight, rigid chains that pack tightly together. Most animals lack the enzymes to digest β-linkages, which is why you can't get calories from wood.

This single difference in bond orientation is what separates an energy-storage molecule (starch) from a structural molecule (cellulose), even though both are made entirely of glucose.

Carbohydrate Functions and Properties

Functions of carbohydrates

Energy storage is the most familiar role. Glucose is the primary fuel for cellular respiration, which generates ATP. Organisms don't keep free glucose floating around, though. Plants package excess glucose into starch (amylose and amylopectin), while animals store it as glycogen, a highly branched polymer found mainly in liver and muscle cells. Glycogen's heavy branching means enzymes can access many ends at once, allowing rapid glucose release when energy demand spikes.

Structural support comes from polysaccharides with β-linkages. Cellulose forms microfibrils in plant cell walls, making them rigid enough to resist turgor pressure. Chitin, a modified polysaccharide with nitrogen-containing groups, serves a similar structural role in arthropod exoskeletons and fungal cell walls.

Carbohydrates also fill other roles you might not expect:

• Ribose and deoxyribose (pentose sugars) form part of the backbone of RNA and DNA, respectively
• Glycoproteins (proteins with attached sugar chains) and glycolipids (lipids with attached sugars) sit on cell surfaces, where they function in cell-to-cell recognition, signaling, and immune responses

Simple vs. complex carbohydrates

Simple carbohydrates are monosaccharides and disaccharides. Because they're small molecules, the body digests and absorbs them quickly. They tend to be sweet-tasting and dissolve readily in water. Examples: glucose, fructose, sucrose, lactose.

Complex carbohydrates are polysaccharides, long chains that take more time to break down enzymatically. They're generally not sweet and can be insoluble in water. Examples: starch, glycogen, cellulose.

The key structural difference driving these properties is chain length and bonding. Longer chains require more hydrolysis reactions before absorption, which slows digestion. And as noted above, the type of glycosidic bond determines whether an organism can digest the polysaccharide at all. Cellulose passes through the human digestive tract largely intact, functioning as dietary fiber rather than an energy source.