Carbohydrates: Structure, Types and Functions
Welcome to one of the most essential topics in Food Science and Technology! Whether you are enjoying a bowl of rice, biting into a crisp apple, or sipping a warm drink, carbohydrates are at the centre of your daily meals. In this chapter, we will break down the chemistry, classifications, and bodily functions of carbohydrates in a straightforward, step-by-step way so you can master every question in HKDSE Paper 1.
Note: Related topics such as the mechanical breakdown of nutrients during digestion, specific diet-related illnesses like diabetes, and food science reactions like gelatinisation will be explored in detail in their dedicated chapters. Here, our focus is firmly on chemical structure, types, and primary functions.
---1. Chemical Composition of Carbohydrates
Don't let the word "chemical" intimidate you! Carbohydrates are simply organic compounds made up of three fundamental chemical elements:
• Carbon (\(C\))
• Hydrogen (\(H\))
• Oxygen (\(O\))
Where does the name come from?
The name "carbohydrate" literally means "hydrate of carbon". In most simple carbohydrates, the hydrogen and oxygen atoms exist in a \(2 : 1\) ratio, just like in water (\(H_2O\)).
The Fundamental Molecular Formula:
The basic building block unit for common single sugars (hexoses) is represented by the formula:
\(C_6H_{12}O_6\)
This means one molecule contains exactly 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.
Analogy to remember: Think of a single sugar unit as an individual plastic toy building brick. You can use one brick alone, snap two together, or link hundreds together to make a giant tower.
---2. Classification and Types of Carbohydrates
Carbohydrates are classified according to the number of simple sugar units (saccharide units) joined together in their structure. Let us examine the four main groups from smallest to largest.
A. Monosaccharides (Single Sugar Units)
"Mono" means one, and "saccharide" means sugar. These are simple sugars that cannot be broken down into simpler carbohydrate units by hydrolysis.
• They all share the molecular formula \(C_6H_{12}O_6\).
• They are sweet, crystalline, and dissolve easily in water.
• Common examples include:
- Glucose: The primary form of sugar circulating in the blood; found naturally in fruits and onions.
- Fructose: The sweetest of all natural sugars, found abundantly in ripe fruits and honey.
- Galactose: A component of the sugar found in milk.
B. Disaccharides (Double Sugar Units)
"Di" means two. When two monosaccharide units bond chemically, a disaccharide is formed.
• They have the general molecular formula \(C_{12}H_{22}O_{11}\).
• Like monosaccharides, they are water-soluble and taste sweet.
• Common examples include:
- Sucrose: Formed by glucose + fructose (ordinary granulated table sugar extracted from sugar cane or sugar beet).
- Lactose: Formed by glucose + galactose (the natural sugar found in mammalian milk).
- Maltose: Formed by glucose + glucose (malt sugar produced during the malting of barley and the breakdown of starch).
C. Oligosaccharides
"Oligo" means a few. Oligosaccharides are short carbohydrate chains consisting of roughly 3 to 10 monosaccharide units linked together.
• They are found naturally in legumes (beans, peas, lentils), onions, and certain whole grains.
• Many oligosaccharides are not fully broken down by human digestive enzymes in the small intestine, passing instead to the large intestine where beneficial gut bacteria ferment them.
D. Polysaccharides (Complex Carbohydrates)
"Poly" means many. Polysaccharides are complex molecules formed by linking dozens, hundreds, or even thousands of monosaccharide units together.
Unlike simple sugars, polysaccharides are generally insoluble in cold water and do not taste sweet. The official syllabus highlights five critical polysaccharides:
1. Starch:
The main storage form of carbohydrate in plants. It is composed of long chains of glucose units. Starch is the predominant carbohydrate in staple foods such as rice, wheat flour, potatoes, and maize.
2. Dextrin:
Intermediate, shorter-chain polysaccharides produced when starch is partially broken down by dry heat, acid, or enzymes. For example, when bread is toasted, the surface starch breaks down into dextrins, contributing to a golden colour and slightly sweet, toasted flavour.
3. Cellulose:
A structural polysaccharide that forms the rigid cell walls of plants. It is made of tightly packed glucose chains. Humans lack the digestive enzymes required to split these specific bonds, meaning cellulose cannot be digested for energy; it acts as insoluble dietary fibre.
4. Pectin:
A complex structural polysaccharide found in the cell walls and intercellular spaces of fruits and vegetables (abundant in apples, citrus rinds, and plums). It forms a gel in the presence of sugar and acid, functioning both as soluble dietary fibre and as a setting agent in jams.
5. Glycogen:
Known as "animal starch", glycogen is the storage form of carbohydrate in animals and humans. It is made of highly branched chains of glucose and is stored primarily in the liver and skeletal muscles to provide a ready reserve of energy.
Quick Review Box: Summary of Carbohydrate Types
• Monosaccharides (1 unit): Glucose, Fructose, Galactose (\(C_6H_{12}O_6\))
• Disaccharides (2 units): Sucrose, Lactose, Maltose
• Oligosaccharides (3–10 units): Short chains found in beans, seeds, and grains
• Polysaccharides (many units): Starch, Dextrin, Cellulose, Pectin, Glycogen
3. Functions of Carbohydrates
In human nutrition, carbohydrates serve distinct bodily and dietary functions depending on their form: simple sugars, complex starches, or non-digestible dietary fibre.
A. Functions of Sugar
• Immediate Energy Release: Simple sugars are rapidly digested and absorbed into the bloodstream, delivering quick fuel for high-intensity physical activity and brain function.
• Palatability and Flavour: Adds natural sweetness to foods, making meals enjoyable and encouraging food intake.
• Protein-Sparing Action: Providing sufficient sugar (and starch) ensures the body does not have to break down dietary protein or muscle tissue to generate glucose.
B. Functions of Starch
• Primary Source of Dietary Energy: Starch serves as the main, staple source of daily energy in human diets worldwide. Because it is digested more gradually than simple sugars, it provides sustained energy release.
• Satiety: Starch-rich foods add bulk to a meal, producing a feeling of fullness (satiety) after eating.
• Carrier of Micronutrients: Unrefined starchy staples (such as brown rice, oats, and wholemeal grains) carry vital B vitamins and minerals needed for cellular energy metabolism.
C. Functions of Dietary Fibre (Cellulose, Pectin, etc.)
Dietary fibre includes non-starch plant polysaccharides that resist digestion in the human small intestine.
• Stimulating Peristalsis: Insoluble fibre (e.g., cellulose) absorbs water, adding soft bulk to the stool. This stimulates the muscular contractions of the intestinal walls (peristalsis) and promotes regular bowel evacuation.
• Preventing Digestive Sluggishness: By shortening intestinal transit time, dietary fibre helps keep the digestive tract healthy and clean.
• Regulating Blood Sugar and Cholesterol: Soluble fibres (such as pectin) dissolve in water to form a viscous gel. This slows down the rate of carbohydrate digestion and glucose absorption, moderating blood glucose spikes, and binds bile acids to assist in controlling blood cholesterol levels.
• Weight Control and Satiety: Fibre adds volume without extra kilocalories, increasing the sensation of fullness and helping to moderate total energy consumption.
4. Common Exam Traps & Misconceptions
Make sure you do not lose straightforward marks on these common HKDSE pitfalls:
• Confusing Glycogen with Glucagon: Glycogen is a polysaccharide (storage carbohydrate). Glucagon is a hormone produced by the pancreas. Always double-check your spelling!
• Assuming All Carbohydrates Are Sweet: Starch and cellulose are carbohydrates, but they have no sweet taste because their large polymer molecules do not bind to taste-bud sweetness receptors.
• Calling Cellulose an "Energy Source": While cellulose is made of glucose units, human digestive enzymes cannot break its bonds. It contributes virtually zero usable dietary energy to humans, functioning instead as dietary fibre.
• Misidentifying the Basic Formula: Remember that \(C_6H_{12}O_6\) belongs specifically to single hexose sugars like glucose, not to large polysaccharides like starch, which are long condensation polymers.
5. Key Takeaways
1. Carbohydrates consist of Carbon (\(C\)), Hydrogen (\(H\)), and Oxygen (\(O\)), with hexose monosaccharides represented by \(C_6H_{12}O_6\).
2. Monosaccharides (glucose, fructose, galactose) and disaccharides (sucrose, lactose, maltose) are simple sugars.
3. Polysaccharides are complex chains: starch (plant energy reserve), dextrin (toasted/degraded starch), cellulose (insoluble plant structural fibre), pectin (soluble gelling fruit fibre), and glycogen (animal carbohydrate reserve in liver and muscle).
4. In the body, sugars and starches supply essential metabolic energy and spare protein, while dietary fibre maintains healthy bowel peristalsis, satiety, and metabolic regulation.