3.2 Carbohydrates

Carbohydrates are the go-to molecules for energy in living organisms, and they also play major structural roles. From simple sugars like glucose to massive polymers like cellulose, the shape and bonding of each carbohydrate directly determines what it does in a cell.

Understanding carbohydrate structure and function helps you see how organisms obtain, store, and use energy. It also explains why plant cell walls are rigid, why arthropods have tough exoskeletons, and why you can digest a potato but not a piece of wood.

Carbohydrate Structure and Function

Functions of carbohydrates in organisms

Carbohydrates fill two major roles: energy and structure.

On the energy side, glucose is the main fuel that cells break down to produce ATP. When an organism has more glucose than it needs right away, it links glucose molecules together into storage polysaccharides. Animals store glucose as glycogen (mostly in liver and muscle cells), while plants store it as starch (in roots, seeds, and tubers). When energy demand rises, these stores get broken back down into glucose.

On the structural side, cellulose forms the rigid walls around plant cells, giving stems and trunks their strength. Chitin serves a similar purpose in arthropod exoskeletons (think crab shells) and in fungal cell walls. Both cellulose and chitin are extremely tough because of how their glucose-based monomers are bonded together.

Types of carbohydrates

Monosaccharides are the simplest carbohydrates. They can't be broken (hydrolyzed) into smaller sugars. The three you'll see most often are glucose, fructose, and galactose. All three share the molecular formula $C_6H_{12}O_6$, but their atoms are arranged differently, making them structural isomers. These differences matter: fructose tastes sweeter than glucose, and galactose behaves differently in metabolism. Monosaccharides also serve as the building blocks for all larger carbohydrates.

Disaccharides form when two monosaccharides join through a condensation (dehydration synthesis) reaction, which releases a water molecule and creates a glycosidic bond. Common examples:

• Sucrose (table sugar) = glucose + fructose
• Lactose (milk sugar) = glucose + galactose
• Maltose (malt sugar) = glucose + glucose

Each disaccharide can be split back into its monosaccharides by hydrolysis (adding water to break the glycosidic bond). People who are lactose intolerant lack sufficient lactase enzyme to hydrolyze lactose.

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Their function depends on which monomers they contain and how those monomers are bonded:

• Starch (plants) and glycogen (animals) store energy
• Cellulose (plants) and chitin (arthropods, fungi) provide structural support

Structure vs function of carbohydrates

This is where the details really pay off on exams. The type of glycosidic bond between monomers is what separates an energy-storage carbohydrate from a structural one.

Monosaccharide structure is defined by the number of carbon atoms (typically 3, 5, or 6), the positions of hydroxyl ($-OH$) groups, and the location of the carbonyl ($C=O$) group. These features determine whether a sugar is glucose vs. fructose vs. galactose, and they affect how easily the molecule is metabolized.

Disaccharide structure depends on which two monosaccharides are joined and the specific glycosidic linkage between them. For example, sucrose's bond links the anomeric carbons of both glucose and fructose, making it a non-reducing sugar (more chemically stable). The type of bond also determines which enzyme can break it.

Polysaccharide structure is where you see the biggest functional differences:

• Starch and glycogen use $\alpha$(1,4) glycosidic bonds along the main chain, with $\alpha$(1,6) bonds at branch points. The $\alpha$ linkage causes the chain to coil into a compact, helical shape that's easy to pack for storage. Glycogen is more heavily branched than starch, which allows animals to mobilize glucose faster.
• Cellulose uses $\beta$(1,4) glycosidic bonds between glucose monomers. The $\beta$ linkage flips every other glucose unit, producing straight, rigid chains. These chains line up side by side and form hydrogen bonds with each other, creating strong fibers. Most animals can't digest cellulose because they lack enzymes that break $\beta$ linkages.
• Chitin also uses $\beta$(1,4) glycosidic bonds, but its monomer is N-acetylglucosamine (a modified glucose). This gives chitin even greater strength and chemical resistance, which is why it works well as an exoskeleton material.

Carbohydrate Metabolism and Synthesis

Two reactions build and break down carbohydrates:

1. Dehydration synthesis (condensation) joins monosaccharides together by removing a water molecule and forming a glycosidic bond. This is how disaccharides and polysaccharides are assembled.
2. Hydrolysis does the reverse: it adds a water molecule across the glycosidic bond, splitting a larger carbohydrate into smaller units.

On the metabolic side, glycolysis is the primary pathway cells use to break down glucose into pyruvate, generating a small amount of ATP. This happens in the cytoplasm of virtually every living cell.

Photosynthesis works in the opposite direction. Plants and other photosynthetic organisms capture sunlight energy and use it to build glucose from $CO_2$ and $H_2O$. The glucose produced can then be used immediately for energy or polymerized into starch or cellulose.