Carbohydrates: Sugars, Starch and Fibre

Carbohydrates supply most of the energy in most people’s diets. They’re in bread and rice, fruit and milk, beans and vegetables, and in the sugar in a fizzy drink. They’re also the most abundant organic molecules on Earth, because the cellulose in plant cell walls is a carbohydrate. This guide explains what carbohydrates are at the molecular level, how they’re classified, and what happens to them in your body.

What makes a molecule a carbohydrate?

Carbohydrates are made of carbon, hydrogen and oxygen. Many have the general formula Cₙ(H₂O)ₙ, which is where the name comes from: “carbo-hydrate”, as if they were carbon combined with water. Glucose, for example, is C₆H₁₂O₆, or C₆(H₂O)₆.

Of course, there’s no actual water inside a sugar molecule. Chemically, carbohydrates are polyhydroxy aldehydes or ketones: carbon chains carrying several –OH (hydroxyl) groups plus one C=O (carbonyl) group, and the larger molecules built from them. All those –OH groups make simple sugars very polar and able to form many hydrogen bonds, which is why they dissolve so easily in water and taste sweet.

Three sizes of carbohydrate

Carbohydrates are classified by how many sugar units they contain.

Monosaccharides: single sugar units

The simplest carbohydrates, which can’t be hydrolysed into anything smaller. The most important are the six-carbon sugars (hexoses):

• Glucose: the main fuel for cells, and “blood sugar”.
• Fructose: “fruit sugar”, found in fruit and honey; the sweetest natural sugar.
• Galactose: part of the milk sugar lactose.

Five-carbon sugars (pentoses) such as ribose and deoxyribose form the backbones of RNA and DNA. Details are in monosaccharides.

Disaccharides: two units joined

Two monosaccharides joined by a glycosidic bond in a condensation reaction that releases water:

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

See disaccharides.

Polysaccharides: long chains

Hundreds to thousands of monosaccharide units linked together. They’re not sweet and most don’t dissolve well in water.

• Starch: the energy store of plants (potatoes, rice, wheat, maize).
• Glycogen: the energy store of animals, kept in the liver and muscles.
• Cellulose: the structural fibre of plant cell walls.

All three are made entirely of glucose. The differences between them, and why we can digest starch but not cellulose, come down to how the glucose units are linked (see starch vs glycogen vs cellulose).

What carbohydrates do in the body

Energy

Glucose is broken down in cellular respiration:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

The energy released is captured as ATP. Carbohydrates provide about 17 kJ per gram (roughly 4 kcal per gram). The brain is a particularly heavy user of glucose, and red blood cells rely on it almost completely.

Storage

Excess glucose is stored as glycogen in the liver and muscles, a reserve that can last roughly a day. Beyond that, surplus energy is converted into fat.

Structure and other roles

Cellulose gives plants their strength. In animals, carbohydrate chains attached to proteins and lipids on cell surfaces help cells recognise each other. Ribose and deoxyribose are part of nucleic acids.

From plate to cell: digestion

1. Mouth: the enzyme salivary amylase begins breaking starch into shorter chains and maltose. (Chew a plain cracker for a minute and it starts to taste slightly sweet.)
2. Stomach: amylase is inactivated by the acid, so little carbohydrate digestion happens here (see stomach acid chemistry).
3. Small intestine: pancreatic amylase continues breaking down starch. Enzymes on the intestinal wall (maltase, sucrase and lactase) split disaccharides into monosaccharides.
4. Absorption: glucose, fructose and galactose are absorbed into the blood and carried to the liver.
5. Regulation: the hormones insulin and glucagon keep blood glucose within a narrow range. Insulin helps cells take up glucose after a meal; glucagon releases glucose from glycogen between meals.

Digestion is a series of hydrolysis reactions: water is added across each glycosidic bond to split it.

Lactose intolerance happens when the body makes too little lactase. Undigested lactose reaches the large intestine, where bacteria ferment it, producing gas and discomfort.

Fibre: the carbohydrate we can’t digest

Humans don’t make enzymes that can break the glycosidic bonds in cellulose and several other plant polysaccharides. These pass through the small intestine undigested and are called dietary fibre. Far from being useless, fibre:

• adds bulk and helps food move through the gut;
• feeds beneficial bacteria in the large intestine, which ferment soluble fibre into useful short-chain fatty acids;
• slows the absorption of sugar, smoothing blood glucose rises;
• is linked with a lower risk of heart disease, type 2 diabetes and bowel cancer.

Wholegrains, beans, lentils, fruit and vegetables are good sources.

Simple vs complex carbohydrates, and glycaemic index

Nutrition advice often divides carbohydrates into simple (sugars) and complex (starches and fibre). A more useful idea is the glycaemic index (GI): how quickly a food raises blood glucose compared with pure glucose.

• High-GI foods (white bread, many breakfast cereals, sweets) are digested and absorbed fast.
• Low-GI foods (lentils, oats, most fruit, wholegrain bread) release glucose more slowly.

GI depends on more than sugar vs starch. Fibre, fat, protein, cooking and even ripeness change it. Some starches (like those in white bread) behave almost like glucose, while fructose-rich fruits have a fairly low GI.

Carbohydrates beyond the dinner plate

Carbohydrates matter far outside nutrition. Cotton, paper and wood are mostly cellulose, making it one of the most important raw materials in industry. Starch is used to make adhesives, to stiffen fabrics and paper, and as the feedstock for fermentation into ethanol fuel and biodegradable plastics such as polylactic acid. Glucose syrups made by enzymatically breaking down maize starch are major ingredients in processed foods. Chitin, a carbohydrate from crustacean shells, is being developed into wound dressings and water-treatment materials. Even the sugars on the surface of your cells have practical importance: the carbohydrate tags that define the ABO blood groups decide which blood transfusions are safe. Carbohydrate chemistry is everyday chemistry on a very large scale.

Common myths

• “Carbohydrates are fattening.” Any excess energy, from carbohydrate, fat or protein, can be stored as fat. Per gram, carbohydrates have less than half the energy of fat.
• “Natural sugar is chemically different from added sugar.” The sucrose in a beet, a cane or a bag of sugar is the same molecule. The difference in fruit is the fibre, water and nutrients that come with it.
• “Brown sugar is much healthier than white.” It’s mostly sucrose with a small amount of molasses; the nutritional difference is tiny.
• “All carbohydrates are sugars.” Starch and fibre are carbohydrates but aren’t sweet and behave very differently.

Key takeaways

• Carbohydrates are compounds of C, H and O, chemically polyhydroxy aldehydes or ketones and their polymers.
• They come in three sizes: monosaccharides (glucose, fructose, galactose), disaccharides (sucrose, lactose, maltose) and polysaccharides (starch, glycogen, cellulose).
• They provide energy (~17 kJ g⁻¹), energy storage and structure.
• Digestion hydrolyses starch and disaccharides into monosaccharides; humans can’t digest cellulose, which acts as fibre.
• See how carbohydrates fit alongside other biomolecules in the four major biomolecules.