9: Carbohydrates

Introduction to Carbohydrates

Carbohydrates, as their name suggests (from “carbon” and “hydrate”), are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O), typically in the ratio C_x(H₂O)_x. This general formula reflects the presence of one water molecule for every carbon atom, giving carbohydrates their distinctive name as “hydrates of carbon.” Chemically, they are classified as polyhydroxy aldehydes or ketones, or substances that yield these upon hydrolysis. Carbohydrates are among the most abundant and diverse biomolecules on Earth and are indispensable to life due to their broad range of functions. These include serving as a primary source of metabolic energy, acting as structural building blocks in cells and tissues, and participating in cell recognition and signaling processes. From the sweetness of cotton candy (pure sucrose) to the toughness of plant stems composed of cellulose, carbohydrates appear in nearly every aspect of life’s structure and function.

    As the body’s first and most accessible source of energy, carbohydrates—especially in the form of glucose—are fundamental to cell metabolism. Glucose, a six-carbon monosaccharide, is the key molecule that fuels nearly every cell. It is metabolized through the biochemical pathways of glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation to generate ATP, the universal energy currency of the cell. In glycolysis, glucose is broken down into pyruvate, releasing a modest amount of ATP and NADH. The pyruvate is then further metabolized in the mitochondria, where high-energy electron carriers drive the production of a much larger ATP yield via the electron transport chain. This makes glucose an efficient and readily available energy source, especially for high-demand tissues like the brain and muscles.

    Plants store excess glucose in the form of starch, a polysaccharide composed of α-1,4-linked glucose chains. Starch consists of two components: amylose, which is unbranched and helical, and amylopectin, which is highly branched and more readily digested. This energy reserve is stored in roots, seeds, and tubers—common in foods like potatoes, rice, and corn. When humans consume starchy foods, enzymes like salivary and pancreatic amylase break the starch down into glucose molecules, which are then absorbed into the bloodstream. In animals, including humans, glucose is stored as glycogen, a similar but more highly branched polysaccharide. Glycogen is primarily found in the liver, where it helps regulate blood glucose levels, and in muscle tissues, where it serves as an on-demand energy supply during physical activity. Because of its extensive branching (every 8–10 glucose units), glycogen can be rapidly broken down to release glucose when energy is urgently needed, such as during exercise or fasting.

 Carbohydrates are the body's first line of energy.

• Starch is a plant-based storage polysaccharide made of glucose. It is stored in roots, seeds, and tubers—think of potatoes or rice.
• Glycogen, the animal equivalent of starch, is stored in liver and muscle tissues. During fasting or exercise, it is broken down to release glucose.

Beyond their role in energy metabolism, carbohydrates also serve critical structural roles in cells and tissues. One of the most important structural carbohydrates is cellulose, a linear polysaccharide composed of β-1,4-linked glucose units. Unlike starch and glycogen, cellulose molecules align side by side and form strong fibers due to hydrogen bonding, providing rigidity and strength to plant cell walls. Humans, however, lack the enzyme cellulase needed to hydrolyze the β-1,4 linkages in cellulose. As a result, cellulose cannot be digested and functions as dietary fiber, promoting digestive health by stimulating bowel movement and supporting gut microbiota. However, many herbivores, such as cows and termites, can digest cellulose thanks to symbiotic microorganisms in their digestive systems that produce cellulase.

   A structurally similar but chemically distinct molecule is chitin, found in the exoskeletons of insects, crustaceans, and the cell walls of fungi. Like cellulose, chitin is composed of β-1,4-linked units, but instead of glucose, it consists of N-acetylglucosamine—a glucose derivative with an acetylated amino group. This gives chitin added strength and resilience, making it ideal for forming protective shells and rigid structural frameworks. Though also indigestible to humans, some organisms like fungi and certain marine bacteria can enzymatically degrade chitin and use it as a carbon source.

    Another essential structural carbohydrate is peptidoglycan, unique to bacterial cell walls. It is a mesh-like polymer composed of alternating sugar units—N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)—linked by β-1,4-glycosidic bonds. These sugar chains are further cross-linked by short peptides, creating a strong, resilient structure that maintains bacterial shape and protects against osmotic stress. Importantly, the integrity of peptidoglycan is a major target for antibiotics. Enzymes like lysozyme, found in tears and saliva, can break β-1,4 bonds between NAG and NAM, compromising bacterial cell walls. Similarly, antibiotics like penicillin and vancomycin disrupt the cross-linking of peptidoglycan, leading to bacterial cell death. Thus, structural carbohydrates not only support cellular frameworks but also play roles in host-pathogen interactions and the development of antimicrobial therapies.

Certain carbohydrates are critical for maintaining the shape and strength of cells.

• Cellulose, found in the cell walls of plants and algae, gives structural integrity.
• Chitin, similar to cellulose, forms the exoskeleton of insects and the cell walls of fungi. It contains N-acetylglucosamine units instead of glucose.
• Peptidoglycan, exclusive to bacteria, provides rigidity to bacterial cell walls. It is made of alternating sugars—N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

    In summary, carbohydrates are incredibly versatile biomolecules, playing indispensable roles as energy sources, structural materials, and signaling agents. Their diverse chemical structures—from simple sugars to complex polymers—allow them to participate in nearly every aspect of biology. Whether we are talking about the sugar in fruit, the starch in pasta, the cellulose in plant stems, or the glycogen in our liver, carbohydrates underpin many processes essential for life.

Monosaccharides vs Polysaccharides

    Monosaccharides are the most basic units of carbohydrates, often referred to as simple sugars. These molecules consist of a single sugar unit and cannot be broken down further by hydrolysis into smaller carbohydrate units. They typically contain three to seven carbon atoms, and their general chemical formula is (CH₂O)ₙ, where n can range from 3 to 7. Monosaccharides are foundational building blocks for more complex carbohydrates like disaccharides and polysaccharides. They are highly soluble in water and have sweet tastes, which is why many simple sugars are commonly found in fruits and sweets.

    Among the most important monosaccharides is glucose, a six-carbon sugar (hexose) and the primary source of energy for most cells. It is an aldose, meaning it contains an aldehyde group at carbon 1. Glucose plays a central role in metabolism and is the sugar measured in blood sugar tests. Another key aldose is galactose, which is structurally similar to glucose but differs in the orientation of one hydroxyl group. Galactose is commonly found as part of lactose, the disaccharide in milk, where it is linked to glucose. On the other hand, fructose is a ketose, containing a ketone group at carbon 2. It is found naturally in fruits and honey and is notably sweeter than glucose, which is why it is often used as a sweetener in processed foods and beverages. Despite these sugars having the same molecular formula (C₆H₁₂O₆), their structures and functions are distinct, a phenomenon known as isomerism.

Common examples of single sugar (monosaccharides):

• Aldose
  • Glucose – the primary cellular fuel.
  • Galactose – part of lactose (milk sugar).
• Ketose
  • Fructose – found in fruits; sweeter than glucose.

    Polysaccharides, in contrast, are complex carbohydrates composed of long chains of monosaccharide units linked by glycosidic bonds. These macromolecules serve either as energy storage materials or structural components in cells, depending on their structure and the types of linkages between the sugars. In storage polysaccharides, the glucose units are typically connected by α-glycosidic linkages, which allow the chains to coil and be more easily broken down when energy is needed.

    In plants, the primary storage polysaccharide is starch, which is made up of two components: amylose, a linear polymer of glucose units linked by α-1,4-glycosidic bonds, and amylopectin, which contains both α-1,4 linkages and branch points formed by α-1,6-glycosidic bonds every 20 to 25 glucose residues. Starch is stored in plastids and provides a reserve of energy that can be mobilized during seed germination or when photosynthesis is not possible. Common dietary sources of starch include potatoes, corn, and grains. In animals, the equivalent storage molecule is glycogen, which is structurally similar to amylopectin but more extensively branched, with α-1,6 branches occurring approximately every 8 to 12 glucose units. This high degree of branching allows glycogen to be rapidly broken down into glucose when energy is needed quickly, such as during physical exertion or fasting. Glycogen is stored mainly in the liver and skeletal muscles. 

    Polysaccharides also have structural roles in organisms. One prominent example is cellulose, a polymer of glucose linked by β-1,4-glycosidic bonds. These bonds result in straight, rigid chains that align parallel to each other, forming fibers held together by hydrogen bonds. This structural arrangement gives strength and rigidity to the cell walls of plants and algae. Unlike starch and glycogen, cellulose cannot be digested by humans because we lack the enzyme cellulase necessary to break β-1,4 linkages. As a result, cellulose serves as dietary fiber, aiding digestion by promoting bowel regularity.

Common examples of polysaccharides:

• Storage polysaccharides:
  • Starch (plants): composed of amylose (unbranched α-1,4-glucose) and amylopectin (branched α-1,4 and α-1,6-glucose).
  • Glycogen (animals): similar to amylopectin but more highly branched (~every 8–12 residues).
• Structural polysaccharides:
  • Cellulose: unbranched β-1,4-glucose; forms fibers.
  • Peptidoglycan: alternating NAG and NAM with β-1,4-linkages and peptide cross-links.

 Another structural polysaccharide is peptidoglycan, which is found in the cell walls of bacteria. It consists of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) joined by β-1,4-glycosidic bonds. These sugar chains are further cross-linked by short peptide chains, creating a strong and protective mesh that helps bacteria maintain their shape and resist osmotic pressure. Without this rigid support, bacterial cells would burst due to the influx of water from their surrounding environment, especially in hypotonic conditions. Peptidoglycan is also a target for many antibiotics: for example, penicillin inhibits the enzymes that cross-link the peptides, weakening the bacterial cell wall and leading to cell lysis.

    To defend against bacterial invasion, the human body produces natural antimicrobial agents, one of which is lysozyme. Lysozyme is an enzyme found in tears, saliva, mucus, and other secretions, where it serves as a first line of immune defense. This enzyme specifically targets the β-1,4-linkages between NAG and NAM in the peptidoglycan structure. By cleaving these bonds, lysozyme disrupts the integrity of the bacterial cell wall, making the bacteria vulnerable to lysis. This is particularly effective against Gram-positive bacteria, which have thick, exposed layers of peptidoglycan. In contrast, Gram-negative bacteria possess an additional outer membrane that partially shields their thinner peptidoglycan layer, rendering them more resistant to lysozyme.

    Beyond natural defenses, scientists have developed powerful antibiotics that exploit the unique structure of peptidoglycan. One well-known example is penicillin, a member of the β-lactam antibiotic family. Penicillin works by inhibiting the bacterial enzyme transpeptidase (also known as penicillin-binding protein, PBP), which is responsible for forming the peptide cross-links between NAM residues. By blocking this enzyme, penicillin prevents the final step in cell wall synthesis, leaving the bacterial wall weak and incomplete. As the bacterium grows and tries to divide, the compromised wall cannot hold the pressure, and the cell eventually ruptures and dies. Another potent antibiotic is vancomycin, often used as a last-resort treatment for multi-drug-resistant Gram-positive infections, such as MRSA (methicillin-resistant Staphylococcus aureus). Unlike penicillin, vancomycin does not inhibit an enzyme. Instead, it physically binds to the peptide side chains on the NAM residues, thereby blocking the access of transpeptidase to its substrate. This prevents the enzyme from cross-linking the peptidoglycan chains. The result is the same—an unstable cell wall that cannot support the bacterial cell under internal pressure.

Method to inhibit Peptidoglycan

• Lysozyme: an enzyme in human tears and saliva
• Antibiotics that target peptidoglycan synthesis
  • Penicillin blocks the transpeptidase enzyme needed for cross-linking.
  • Vancomycin binds to the peptide chains, preventing proper wall formation.

    The assembly and breakdown of carbohydrates involve two important types of chemical reactions: dehydration (condensation) and hydrolysis. A dehydration reaction is how monosaccharides are linked together to form disaccharides and polysaccharides. In this process, two sugar molecules are joined, and a molecule of water is removed. For instance, two glucose molecules can undergo dehydration to form maltose and release a water molecule (glucose + glucose → maltose + H₂O). This is how larger carbohydrate structures are synthesized in cells.

Conversely, a hydrolysis reaction is the process by which complex carbohydrates are broken down into their simpler sugar units. In hydrolysis, a water molecule is added back to cleave the glycosidic bond between sugar units. This is the mechanism by which starch and glycogen are digested in our bodies. Enzymes like amylase, produced in saliva and the pancreas, hydrolyze the glycosidic bonds in starch to release glucose, which can then be absorbed into the bloodstream.

    An easy way to visualize these two processes is to think of dehydration as building a candy necklace—each candy (monosaccharide) is added to the string with a bond, and water is removed to form that link. Hydrolysis, on the other hand, is like cutting that necklace apart—each time you snip a link, you need to add a drop of water to separate the beads (sugar units). Together, these two reactions regulate the formation and breakdown of carbohydrates, ensuring a steady supply of energy and structural materials when and where they are needed.     

• Dehydration (Condensation) Reaction: Two monosaccharides are joined together, and a water molecule is removed in the process.
  • glucose + glucose → maltose + H₂O.
• Hydrolysis Reaction: Water is added to split glycosidic bonds.
  • Starch is hydrolyzed to glucose in our digestive tract.

Cyclization and Stereochemistry of Sugars

    Monosaccharides, especially those with five or more carbon atoms, rarely exist in their open-chain (linear) form when in aqueous (water-based) environments. Instead, they undergo an internal chemical reaction to form cyclic (ring) structures, which are more stable in solution. For glucose, which is a six-carbon sugar (an aldohexose), this ring formation occurs when the aldehyde group at carbon 1 (C1) reacts with the hydroxyl group (-OH) on carbon 5 (C5). This intramolecular reaction forms a hemiacetal, resulting in either a six-membered ring called a pyranose (resembling the structure of pyran) or, less commonly, a five-membered ring called a furanose (like furan). In glucose, the six-membered pyranose form is by far the most common in nature. Once glucose forms a ring, something important happens: a new chiral center is created at carbon 1—the carbon that used to be the aldehyde. This carbon is now called the anomeric carbon. Because of its new tetrahedral configuration, it can orient the hydroxyl group (-OH) in two different ways in space, giving rise to two stereoisomers called anomers. These small differences may seem trivial, but they have a huge impact on how sugars behave biologically and chemically.

• In the α-anomer, the hydroxyl group attached to the anomeric carbon points down (on the opposite side of the ring relative to the CH₂OH group on C5).
• In the β-anomer, the hydroxyl group at the anomeric carbon points up (on the same side as the CH₂OH group on C5).

    The difference between α- and β-linkages determines whether humans can digest certain polysaccharides. For instance, starch (in plants) and glycogen (in animals) are storage polysaccharides made of glucose units connected by α-glycosidic bonds, mostly α-1,4 and α-1,6 linkages. These α-linkages cause the glucose chain to coil into a helical, flexible structure that can be easily broken down by human enzymes such as amylase. This is why we can eat bread, rice, or potatoes and derive energy from them. In contrast, cellulose (found in plant cell walls) is composed of glucose units linked by β-1,4-glycosidic bonds. These β-linkages produce a straight, rigid structure that allows individual chains to align side-by-side and form strong hydrogen bonds, resulting in fibers. However, humans lack the enzyme cellulase, which is required to break these β-bonds. So, although cellulose is made of the same glucose units, it is indigestible for us and passes through our gut as dietary fiber.

    Stereochemistry refers to the spatial arrangement of atoms in molecules that have the same molecular formula. Sugars like glucose have multiple chiral centers—carbons attached to four different groups—which means they can exist in many stereoisomeric forms. A specific type of stereoisomerism is seen in enantiomers, which are mirror images of each other (like your left and right hands). For monosaccharides, we categorize them as either D-sugars or L-sugars based on the orientation of the hydroxyl group on the chiral carbon furthest from the carbonyl group in the linear form (usually the second-to-last carbon). In nature, virtually all sugars used by organisms are D-isomers. For example, D-glucose is the form our enzymes recognize and metabolize. On the other hand, L-glucose is the mirror image of D-glucose and, despite looking similar, cannot be used by our enzymes, and therefore, it passes through the body unmetabolized. This is an excellent demonstration of how subtle differences in three-dimensional structure can determine whether a molecule is biologically active or completely inert.

• Starch and glycogen use α-linkages (flexible and digestible).
• Cellulose uses β-linkages (rigid and indigestible to humans).
• D-sugars are the ones actively used.
• L-sugars are typically not metabolized.

Comparison of Starch, Glycogen, and Cellulose

     Overall, understanding the structure of carbohydrates is fundamental to biochemistry because carbohydrates play essential roles not only as energy sources but also in modulating protein function, enzyme specificity, and cell signaling. Many proteins, especially those on the cell surface or secreted, are glycoproteins—proteins covalently bonded to carbohydrate chains—which influence protein folding, stability, solubility, and recognition by other molecules. Enzymes such as glycosidases and glycosyltransferases are highly specific for the stereochemistry and linkage types of carbohydrates (e.g., α-1,4 vs. β-1,4), meaning even small structural differences can dramatically affect catalytic activity or substrate binding. Carbohydrate structures also determine how immune cells distinguish self from non-self, how pathogens attach to host tissues, and how hormones like insulin regulate glucose uptake. In biotechnology and drug development, understanding carbohydrate structure is key for designing enzyme inhibitors, vaccines, and glycoengineered therapeutics. Therefore, mastering carbohydrate chemistry is not only about memorizing sugars—it's about unlocking how biological systems recognize, regulate, and respond to one another at the molecular level.