Carbohydrates: Starch and Sugars

This lesson covers the main types of carbohydrate — starch and sugars — as required by AQA GCSE Food Preparation and Nutrition specification 8585 (section 3.2.1). You will learn the classification of carbohydrates into monosaccharides, disaccharides, and polysaccharides, understand their food sources, and know that carbohydrate is the body's primary (main) source of energy.

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What Are Carbohydrates?

Carbohydrates are a macronutrient made up of carbon, hydrogen, and oxygen atoms. They are the body's primary source of energy, providing 3.75 kcal per gram (approximately 16 kJ per gram).

Carbohydrates are classified according to the complexity of their molecular structure:

graph TD
    A["Carbohydrates"] --> B["Sugars — Simple"]
    A --> C["Starch — Complex"]
    A --> D["Dietary Fibre — NSP"]
    B --> E["Monosaccharides"]
    B --> F["Disaccharides"]
    E --> E1["Glucose"]
    E --> E2["Fructose"]
    E --> E3["Galactose"]
    F --> F1["Sucrose = glucose + fructose"]
    F --> F2["Lactose = glucose + galactose"]
    F --> F3["Maltose = glucose + glucose"]
    C --> C1["Polysaccharides"]
    C1 --> C2["Long chains of glucose molecules"]

    style A fill:#4a90d9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#8e44ad,color:#fff

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Monosaccharides (Simple Sugars)

Monosaccharides are the simplest carbohydrates, consisting of a single sugar unit. They are the building blocks of all carbohydrates. Monosaccharides are:

• Quickly absorbed into the blood
• Sweet tasting
• Soluble in water

Monosaccharide	Sources	Key Details
Glucose	Fruits, vegetables, honey, sports drinks	The body's preferred energy source; all other carbohydrates are ultimately converted to glucose. Blood sugar is glucose.
Fructose	Fruits, honey, some vegetables	The sweetest natural sugar. Found in "free sugars" when added to foods and in fruit juice.
Galactose	Rarely found free in food; part of lactose (milk sugar)	Converted to glucose in the liver for use as energy.

Exam Tip: Remember that glucose is the specific sugar the body uses for energy. During respiration, glucose is broken down to release energy. When the exam refers to "blood sugar," it means blood glucose.

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Disaccharides (Double Sugars)

Disaccharides are made up of two monosaccharide units joined together by a chemical bond. They must be broken down into monosaccharides during digestion before they can be absorbed.

Disaccharide	Made From	Sources	Enzyme That Digests It
Sucrose (table sugar)	Glucose + Fructose	Sugar cane, sugar beet, honey, cakes, biscuits, fizzy drinks	Sucrase
Lactose (milk sugar)	Glucose + Galactose	Milk, yoghurt, cheese, cream	Lactase
Maltose (malt sugar)	Glucose + Glucose	Malted barley, malt extract, germinating seeds, beer	Maltase

Lactose Intolerance

Some people do not produce enough of the enzyme lactase and are unable to digest lactose properly. This is called lactose intolerance. Symptoms include bloating, cramps, diarrhoea, and gas after consuming dairy products. It is more common in people of East Asian, African, and South American heritage.

People with lactose intolerance can:

• Choose lactose-free dairy products (where the lactose has been pre-digested)
• Use plant-based milk alternatives (soya milk, oat milk, almond milk)
• Take lactase enzyme supplements before eating dairy
• Eat hard cheeses and yoghurt (which have lower lactose levels due to fermentation)

Exam Tip: Lactose intolerance is NOT the same as a milk allergy. Lactose intolerance is the inability to digest the sugar lactose; a milk allergy is an immune reaction to the protein in milk (casein or whey). Do not confuse these in the exam.

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Polysaccharides (Complex Carbohydrates)

Polysaccharides are long chains of many monosaccharide (glucose) units joined together. They are complex carbohydrates that take longer to digest, providing sustained, slow-release energy.

Starch

Starch is the main storage polysaccharide in plants. It is made up of long chains of glucose molecules and is the primary complex carbohydrate in the human diet.

Food Source	Details
Bread	Made from wheat flour; wholemeal bread retains more fibre and nutrients
Rice	White rice has the bran removed; brown rice retains the bran layer
Pasta	Made from durum wheat; wholemeal varieties available
Potatoes	Naturally high in starch; also provide vitamin C and potassium
Cereals	Wheat, oats, maize, barley — staple foods worldwide
Yams and sweet potatoes	Starchy root vegetables important in many cuisines
Flour	The basis of bread, cakes, pastry, and many other products
Plantain	Starchy fruit used as a vegetable in Caribbean and African cooking

Why Starch Is Important

• Starch is digested slowly, providing a gradual release of energy throughout the day
• It is the body's main source of energy for daily activities
• Starchy foods are generally affordable and widely available
• Wholegrain starchy foods also provide B vitamins, iron, and dietary fibre
• The government recommends that starchy carbohydrates make up just over a third of the diet (as shown on the Eatwell Guide)

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Free Sugars vs Intrinsic Sugars

The AQA specification distinguishes between different types of sugar in the diet:

Type	Definition	Examples
Intrinsic sugars	Sugars naturally contained within the cell structure of food	Sugar in whole fruits, vegetables
Milk sugars	Lactose naturally present in milk and dairy products	Lactose in milk, yoghurt
Free sugars (also called extrinsic sugars or "added sugars")	Sugars that have been added to food, or sugars naturally present in honey, syrups, and fruit juices	Table sugar, honey, maple syrup, fruit juice, sugar in fizzy drinks, cakes, biscuits

Free sugars are the type we are advised to limit because they:

• Are rapidly absorbed, causing a spike in blood sugar levels
• Contribute to tooth decay (dental caries) — bacteria in the mouth feed on free sugars and produce acid that erodes tooth enamel
• Provide "empty calories" — energy without vitamins, minerals, or fibre
• Contribute to obesity when consumed in excess

Current UK Guidance on Free Sugars

Age Group	Maximum Recommended Free Sugar Intake
Adults and children over 11	No more than 30 g per day (approximately 7 sugar cubes)
Children aged 7–10	No more than 24 g per day
Children aged 4–6	No more than 19 g per day

Free sugars should make up no more than 5% of total energy intake.

Exam Tip: The sugar in a whole apple is intrinsic (still within the cell walls of the fruit and released slowly). The sugar in apple juice is free (the cell structure has been destroyed during juicing, so the sugar is released rapidly). This is why whole fruit is healthier than fruit juice, even though both contain the same type of sugar.

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How Carbohydrates Provide Energy

When carbohydrates are eaten:

1. Digestion — Starch is broken down into maltose by the enzyme amylase (in saliva and pancreatic juice), then maltose is broken down into glucose by maltase. Sugars are broken down by their specific enzymes (sucrase, lactase).
2. Absorption — Glucose is absorbed through the wall of the small intestine into the blood.
3. Transport — The blood carries glucose to cells throughout the body.
4. Respiration — Inside cells, glucose is broken down during aerobic respiration to release energy:

glucose + oxygen → carbon dioxide + water (+ energy)

5. Storage — Excess glucose is converted to glycogen and stored in the liver and muscles. When glycogen stores are full, further excess is converted to fat.

Exam Tip: Remember the order: glucose is first stored as glycogen (in the liver and muscles). Only when glycogen stores are full is excess glucose converted to fat. This two-step process is important for understanding both energy storage and the consequences of excess carbohydrate.

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The Glycaemic Index (GI)

The glycaemic index (GI) ranks carbohydrate foods by how quickly they raise blood glucose levels after eating: