What Are Carbohydrates?
Carbohydrates are one of the four major classes of biological macromolecules, alongside lipids, proteins, and nucleic acids. The name reflects their empirical formula: (CH₂O)ₙ — literally "hydrates of carbon." However, this formula is only approximate; many carbohydrates deviate from it.
Carbohydrates serve three fundamental biological roles: 1. Energy storage: glucose is the primary fuel for cellular respiration; starch and glycogen store energy in plants and animals, respectively. 2. Structural support: cellulose forms plant cell walls; chitin forms insect exoskeletons and fungal cell walls. 3. Signaling and recognition: glycoproteins and glycolipids on cell surfaces mediate cell-cell communication, immune response, and pathogen recognition.
Monosaccharides: Simple Sugars
Monosaccharides are the simplest carbohydrates — they cannot be hydrolyzed into smaller sugar units. They are classified by the number of carbon atoms:
| Carbons | Name | Example |
|---|---|---|
| 3 | Triose | Glyceraldehyde |
| 4 | Tetrose | Erythrose |
| 5 | Pentose | Ribose, deoxyribose |
| 6 | Hexose | Glucose, fructose, galactose |
| 7 | Heptose | Sedoheptulose |
The most important monosaccharide is glucose (C₆H₁₂O₆), the central metabolic fuel. Glucose contains both an aldehyde group (making it an aldose) and multiple hydroxyl groups. Fructose is a ketone-bearing sugar (ketose).
Fischer Projections
Monosaccharides contain multiple stereocenters. D- and L- designations are determined by the orientation of the hydroxyl group on the carbon farthest from the carbonyl: - D-sugars: –OH on the right in Fischer projection (most natural sugars are D-form) - L-sugars: –OH on the left
Glucose has four stereocenters, giving 16 possible stereoisomers (8 D-forms, 8 L-forms). These are called aldohexoses and include glucose, galactose, mannose, and others — each a different diastereomer.
Cyclic Forms: Haworth Structures
In aqueous solution, monosaccharides predominantly exist in cyclic form. The aldehyde group reacts with a hydroxyl group (intramolecular hemiacetal formation):
For glucose: the C-1 aldehyde reacts with the C-5 hydroxyl → a six-membered ring called a pyranose (resembling pyran).
This creates a new stereocenter at C-1 called the anomeric carbon, giving two anomers: - α-D-glucose: –OH at C-1 is axial (below the ring plane in Haworth) - β-D-glucose: –OH at C-1 is equatorial (above the ring plane in Haworth)
In aqueous solution, α- and β-glucose interconvert through the open-chain form — a process called mutarotation. At equilibrium, the mixture is ~64% β (equatorial OH preferred) and ~36% α.
Fructose forms a five-membered ring called a furanose (C-2 ketone + C-5 OH cyclize).
Reducing Sugars
A reducing sugar has a free anomeric –OH (hemiacetal or hemiketal) that can open to reveal the aldehyde/ketone, allowing it to reduce metal ions. Glucose, maltose, and lactose are reducing sugars. Sucrose is not a reducing sugar — its anomeric carbons are both locked in the glycosidic bond.
Benedict's test uses Cu²⁺ in alkaline solution — reducing sugars produce a brick-red precipitate of Cu₂O. Used clinically to detect glucose in urine.
Disaccharides
Disaccharides consist of two monosaccharides joined by a glycosidic bond — an acetal linkage formed between the anomeric carbon of one sugar and a hydroxyl of another, with loss of water.
Key disaccharides:
-
Sucrose (table sugar): glucose + fructose, α-1,β-2 linkage. Found in sugar cane and sugar beet. Non-reducing. Hydrolysis ("inversion") by sucrase gives invert sugar (used in candy and preserves).
-
Lactose (milk sugar): galactose + glucose, β-1,4 linkage. Present in mammalian milk (~5% in cow's milk, ~7% in human milk). Lactose intolerance results from insufficient lactase enzyme — lactose reaches the colon and is fermented by bacteria, causing bloating and discomfort.
-
Maltose (malt sugar): glucose + glucose, α-1,4 linkage. A reducing disaccharide. Formed during starch digestion by amylase; abundant in germinating barley (malt). Used in brewing and baking.
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Cellobiose: glucose + glucose, β-1,4 linkage. The repeating structural unit of cellulose. Humans lack the enzyme to hydrolyze β-1,4 bonds.
Polysaccharides
Polysaccharides are long-chain carbohydrate polymers of monosaccharide units. They can be linear or branched.
Starch
Starch is the main energy storage carbohydrate in plants. It consists of: - Amylose (~20–30%): linear chains of glucose in α-1,4 linkages. Adopts a helical conformation that gives the characteristic blue-black color with iodine (iodine is trapped in the helix). - Amylopectin (~70–80%): branched chains with α-1,4 linkages in the chain and α-1,6 linkages at branch points (approximately every 24–30 glucose units).
Starch is digested by amylases in saliva and the pancreas. Potatoes, rice, wheat, and corn are major sources.
Resistant starch escapes digestion in the small intestine and reaches the colon, acting as a prebiotic. Cooling cooked starch (retrograde starch) increases resistant starch content.
Glycogen
Glycogen is the animal equivalent of starch — the primary glucose storage form in liver and muscle. It is more highly branched than amylopectin (branch point every 8–12 glucose units), allowing very rapid glucose mobilization when energy demand increases.
During exercise, glycogen phosphorylase cleaves glucose units from branch ends (many ends available simultaneously due to branching), releasing glucose-1-phosphate rapidly.
Cellulose
Cellulose is the most abundant organic compound on Earth — the structural polymer of plant cell walls. It consists of glucose units in β-1,4 linkages.
The key difference from starch: the β linkage causes each glucose to be flipped 180° relative to its neighbor, creating straight, rigid chains that hydrogen-bond with adjacent chains to form microfibrils with exceptional tensile strength. Cellulose cannot be digested by humans (we lack β-glucosidase), but termites, cows, and other ruminants harbor microbes that produce cellulase.
Industrial uses: paper (cellulose pulp), cotton and linen textiles, cellulose derivatives (nitrocellulose in explosives; cellulose acetate in photographic film and textiles).
Chitin
Chitin is a structural polysaccharide made of N-acetylglucosamine (glucose with an acetylamine group replacing the C-2 OH) in β-1,4 linkages. It forms the exoskeletons of insects, crustaceans (shrimp, crabs, lobsters), and the cell walls of fungi. The second most abundant natural polymer after cellulose.
Applications: medical sutures (biodegradable), wound dressings, water treatment (chitosan, deacetylated chitin, flocculates colloidal impurities).
Blood Group Sugars
The ABO blood group system is determined by oligosaccharides on red blood cell surfaces: - Type A: N-acetylgalactosamine added to the H antigen - Type B: Galactose added to the H antigen - Type AB: Both sugars present - Type O: Only the H antigen (neither modification)
This explains why incompatible blood transfusions trigger an immune response — the antibodies recognize the unfamiliar sugar patterns on foreign red blood cells.