Carbohydrates: Structure, Classification & Functions Guide

Carbohydrates are organic compounds of carbon, hydrogen, and oxygen, roughly in the ratio Cn(H2O)n, and the body's preferred fuel. They range from single sugars like glucose to giant polymers like starch and cellulose. Every cell can burn glucose, and the brain depends on it almost exclusively.

They matter because they are the cheapest, most abundant energy source in human diets - rice, wheat, and potatoes feed billions - and because their chemistry underpins diabetes, digestion, and food science. Telling a starch from a sugar, or glucose from fructose, is the first step to understanding blood sugar, fibre, and nutrition labels.

Key Concepts

• Monosaccharides: single sugars - glucose, fructose, and galactose (all C6H12O6 but arranged differently).
• Disaccharides: two sugars joined - sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose).
• Polysaccharides: many sugars joined - starch and glycogen for storage, cellulose and chitin for structure.
• Aldoses vs ketoses: sugars with an aldehyde group (glucose) vs a ketone group (fructose).
• Glycosidic bond: the link between sugar units, formed by losing water; broken by hydrolysis during digestion.
• Alpha vs beta linkages: starch's alpha links are digestible by humans; cellulose's beta links are not, making it fibre.
• Energy value: carbohydrates provide 4 kcal per gram.

How It Works

Digestion dismantles carbohydrates top-down: salivary and pancreatic amylase chop starch into maltose, brush-border enzymes split disaccharides into monosaccharides, and only glucose, fructose, and galactose are absorbed. Glucose enters blood directly; fructose and galactose are converted to glucose in the liver. Rising blood glucose triggers insulin, which drives glucose into muscle and fat and stores the surplus as glycogen - and when glycogen is full, as fat. In fasting, glucagon reverses the flow.

Worked Example

A meal contains 50 g of digestible carbohydrate. Energy = 50 x 4 = 200 kcal. For starch hydrolysis: 16.2 g of starch (as glucose units of 162 g/mol) plus water gives 16.2 x (180 / 162) = 18 g of glucose, because each added water molecule adds mass. This is why complete digestion of starch yields more glucose by weight than the starch you started with - a favourite exam calculation.

Common Misconceptions

• All carbohydrates are bad sugar. Starch, fibre, and table sugar behave very differently: fibre slows absorption and feeds gut bacteria, while free sugars spike blood glucose.
• Humans can digest cellulose. We cannot - we lack cellulase, so cellulose passes through as fibre, which is exactly why it aids digestion.
• Glucose and fructose are identical because both are C6H12O6. Fructose is metabolised almost entirely in the liver and does not trigger insulin the same way, which is why excess fructose is linked to fatty liver.

Real-World Applications

• Diabetes care is built on carbohydrate counting and the glycaemic index.
• Oral rehydration solution uses glucose to drag sodium and water into the gut via co-transport.
• Sports drinks supply fast glucose to working muscle.
• Lactose intolerance is diagnosed and managed through carbohydrate digestion knowledge.

Carbohydrates are the body's standard fuel, built from simple sugars into disaccharides and polysaccharides by glycosidic bonds. Digestible alpha-linked starches feed us; indigestible beta-linked cellulose feeds our gut - and the difference between glucose and fructose shows why formula alone never tells the whole story.