7.2 Carbohydrates

Carbohydrate Structure and Function

Carbohydrates are one of the four major classes of biological macromolecules. They serve two broad purposes: storing energy and providing structural support. In microbiology, understanding carbohydrate structure matters because these molecules make up bacterial cell walls, form biofilm matrices, and fuel microbial metabolism. The differences between carbohydrates come down to how their sugar units are arranged and bonded together.

Common Monosaccharides and Functions

Monosaccharides are the simplest carbohydrates. They can't be broken (hydrolyzed) into smaller sugars. Their general formula follows a 1:2:1 ratio of carbon to hydrogen to oxygen. For six-carbon sugars (hexoses), that's $C_6H_{12}O_6$.

Monosaccharides are classified by the position of their carbonyl group ($C=O$):

• Aldoses have the carbonyl group at the end of the carbon chain (e.g., glucose, galactose)
• Ketoses have the carbonyl group in the middle of the chain (e.g., fructose)

The most important monosaccharides to know:

• Glucose is the most abundant monosaccharide and the primary energy source for cells. During cellular respiration, glucose is oxidized to produce $ATP$ (adenosine triphosphate). It also serves as a precursor for synthesizing amino acids, nucleotides, and other molecules.
• Fructose is found in fruits and honey. It's sweeter than glucose, which is why high-fructose corn syrup is widely used as a sweetener in processed foods.
• Galactose combines with glucose to form the disaccharide lactose, the sugar found in milk.
• Ribose is a five-carbon sugar (pentose) that forms the backbone of $RNA$. It's also a component of $ATP$, $NADH$, and other coenzymes critical to metabolism.

Formation of Complex Carbohydrates

Larger carbohydrates are built by linking monosaccharides together through glycosidic bonds. These bonds form via dehydration synthesis (also called condensation), a reaction where a water molecule is removed as two sugar units join. Breaking these bonds back apart requires hydrolysis, which adds water back in.

Disaccharides consist of two monosaccharides joined by a single glycosidic bond. The three you should know:

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

Oligosaccharides are short chains of about 3–10 monosaccharide units. These often appear attached to proteins or lipids on cell surfaces, where they play roles in cell recognition and signaling.

Polysaccharides are long chains of many monosaccharide units joined by repeated glycosidic bonds. They can be linear or branched. The specific monosaccharides used and the types of glycosidic bonds determine the polysaccharide's properties and biological function.

Starch vs. Glycogen vs. Cellulose

Starch, glycogen, and cellulose are all polysaccharides made entirely of glucose. What makes them different is the type of glycosidic bond and the degree of branching. These structural differences lead to completely different biological roles.

Starch is the storage polysaccharide of plants (found in potatoes, grains, rice). It has two components:

1. Amylose is a linear chain of glucose units connected by $\alpha(1 \rightarrow 4)$ glycosidic bonds. It coils into a helix.
2. Amylopectin is a branched chain with $\alpha(1 \rightarrow 4)$ bonds along the main chain and $\alpha(1 \rightarrow 6)$ bonds at branch points.

Plants break down starch into glucose when they need energy. Humans and many microbes digest starch easily using amylase enzymes.

Glycogen is the storage polysaccharide of animals, stored primarily in liver and muscle cells. It's structurally similar to amylopectin but much more highly branched. The extensive branching allows rapid release of glucose during exercise or fasting, since enzymes can work on many branch ends simultaneously.

Cellulose is a structural polysaccharide that forms plant cell walls (think wood, cotton, paper). It uses $\beta(1 \rightarrow 4)$ glycosidic bonds instead of alpha bonds. This single difference flips alternating glucose units, producing straight, rigid chains that pack tightly together through hydrogen bonding. Most animals, including humans, can't digest cellulose because they lack the enzyme (cellulase) needed to break $\beta(1 \rightarrow 4)$ bonds. Cellulose passes through the human gut as dietary fiber. Some microorganisms, however, do produce cellulase, which is why certain bacteria in ruminant stomachs can break down plant material.

Chitin is another structural polysaccharide worth knowing. It's found in fungal cell walls and arthropod exoskeletons. Like cellulose, it uses $\beta(1 \rightarrow 4)$ linkages, but its monomer is $N$-acetylglucosamine rather than glucose.

Carbohydrate Isomers and Modifications

Anomers are a specific type of stereoisomer that arises when a monosaccharide cyclizes (forms a ring). The hydroxyl group ($-OH$) on the anomeric carbon (carbon 1 in aldoses) can end up in two orientations:

• Alpha ($\alpha$) anomer: the $-OH$ on the anomeric carbon points down (opposite the $CH_2OH$ group in a Haworth projection)
• Beta ($\beta$) anomer: the $-OH$ points up (same side as the $CH_2OH$ group)

This distinction matters because it determines the type of glycosidic bond that forms. Alpha linkages produce digestible storage polysaccharides (starch, glycogen), while beta linkages produce rigid structural polysaccharides (cellulose, chitin).

Glycosylation is the process of attaching carbohydrate groups to other molecules, typically proteins or lipids. The resulting glycoproteins and glycolipids are found on cell surfaces, where they function in cell-to-cell recognition, immune responses, and pathogen interactions. In microbiology, glycosylation of surface proteins is one way bacteria and viruses interact with host cells.