Carbohydrates

Composition and Structure

Basic Structure of a Monosaccharide

• Monosaccharides are the simplest form of carbohydrates and are commonly referred to as single sugars.
• They are composed of carbon (C), hydrogen (H), and oxygen (O) atoms.
• Examples include glucose, fructose, and galactose.
• These sugars have a backbone of a single sugar unit, which can be in a linear chain or a ring structure.

Formation of Disaccharides and Polysaccharides

• Disaccharides are formed when two monosaccharides undergo a dehydration reaction, resulting in the formation of a glycosidic bond and the release of water.
• Common disaccharides include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (two glucose molecules).
• Polysaccharides are complex carbohydrates formed by the polymerization of multiple monosaccharide units. They are classified based on their structure and functional properties.
• Starch, cellulose, and glycogen are examples of polysaccharides, which serve various roles like energy storage (starch and glycogen) and structural functions (cellulose).

Classification

Monosaccharides

Single sugar units like glucose, fructose, and galactose.

Disaccharides

Two monosaccharides linked together, such as sucrose, lactose, and maltose.

Polysaccharides

Complex carbohydrates consisting of many sugar units. Examples include starch (energy storage in plants), cellulose (structural component in plant cell walls), and glycogen (energy storage in animals).

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Sources

Monosaccharide

• Glucose: Found in fruits, sweet vegetables, and honey.
• Fructose: Abundant in fruits, honey, and root vegetables.
• Galactose: Not commonly found in its free form but is a part of lactose in milk.

Disaccharides

• Sucrose: Common sugar (table sugar), found in sugar cane, sugar beets, and fruits.
• Lactose: Found in milk and dairy products.
• Maltose: Produced during the breakdown of starch in germinating seeds, like barley.

Polysaccharides

• Starch: Majorly found in foods like potatoes, grains (rice, wheat), and legumes.
• Cellulose: Present in the cell wall of plants, hence in all plant-based foods like vegetables, fruits, and whole grains.
• Pectin: Found in fruits such as apples, plums, and citrus fruits. Used as a gelling agent in jams and jellies.
• Non-Starch Polysaccharides (Dietary Fibre): Includes cellulose, hemicelluloses, pectins, and gums. Found in whole grains, fruits, vegetables, nuts, and seeds.

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Exam Focus: Questions and Answers

1. Describe the basic structure of a monosaccharide and give an example.

A monosaccharide has a basic structure of a single sugar unit, which can be in a linear chain or a ring form. It consists of carbon, hydrogen, and oxygen atoms. An example of a monosaccharide is glucose.

2. Explain the formation of a disaccharide.

A disaccharide is formed when two monosaccharides undergo a dehydration reaction. This involves the removal of a water molecule and the formation of a glycosidic bond between them. For instance, sucrose is formed from glucose and fructose.

3. List two sources of dietary starch and explain its importance in the diet.

Two sources of dietary starch are potatoes and rice. Starch is important as it provides a major source of energy. In the digestive system, starch is broken down into glucose, which is a primary energy source for the body's cells.

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Carbohydrates-Properties and Functions

Properties of Carbohydrates

Sweetness

• Carbohydrates, especially simple sugars like glucose and fructose, are known for their sweet taste.
• The degree of sweetness varies among different carbohydrates. Fructose is the sweetest, followed by sucrose and glucose.

Solubility

• Carbohydrates exhibit varying degrees of solubility in water.
• Monosaccharides and disaccharides are generally soluble in water, with solubility decreasing as the carbohydrate becomes more complex.
• Polysaccharides like starch have limited solubility in cold water but can absorb water and swell.

Gelatinisation of Starch

• Gelatinisation occurs when starch granules absorb water and swell upon heating.
• This process is critical in cooking and baking, as it thickens sauces and gives structure to baked goods.

Hydrolysis

• In hydrolysis, carbohydrates are broken down into simpler molecules by the addition of water.
• This includes the breakdown of disaccharides into monosaccharides (e.g., sucrose into glucose and fructose) and is a crucial step in digestion.

Inversion

• Inversion is the process of converting sucrose into glucose and fructose, especially in the making of sweets and syrups.

Crystallisation and Caramelisation

• Crystallisation refers to the formation of sugar crystals from a concentrated solution of sugar.
• Caramelisation is the browning of sugar, which occurs through the application of heat, producing a distinctive flavour and colour.

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Dextrinisation

• Dextrinisation involves the breakdown of starch into dextrins due to dry heat, resulting in colour and flavour changes, as seen in toasted bread.

Pectin Extraction and Gel Formation

• Pectin, found in fruits, is extracted and used as a gelling agent in jams and jellies.
• It forms a gel when heated with sugar and acid, essential in preserving fruits.

Effects of Heat on Carbohydrates

• Dry heat causes caramelisation and dextrinisation.
• Moist heat leads to gelatinisation of starch and hydrolysis of carbohydrates.

Effects of Enzymes on Carbohydrates

• Enzymes like amylase break down carbohydrates during digestion.
• They convert complex carbohydrates into simpler sugars, facilitating absorption.

Functions of Carbohydrates

Biological Functions

• Sugars: Quick source of energy, essential for brain function, and as a dietary energy source.
• Starch: Major energy reserve in plants, slowly releases glucose for sustained energy.
• Non-Starch Polysaccharides: Includes dietary fibres that aid in digestion, prevent constipation, and help in weight management.

Culinary Functions

• Sugars: Provide sweetness, contribute to the texture and colour of foods, act as preservatives.
• Starch: Thickening agent in sauces and gravies, provides structure in baking.
• Pectin: Used as a gelling agent in jams and jellies, stabilises foods.

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Exam Focus: Questions and Answers

1. Explain the process of gelatinisation of starch.

Gelatinisation of starch occurs when starch granules are heated in water. The granules absorb water, swell, and eventually burst, releasing the starch molecules. This results in a thickened mixture, which is essential in cooking and baking for texture and consistency.

2. What are the effects of enzymes on carbohydrates during digestion?

During digestion, enzymes like amylase break down complex carbohydrates into simpler sugars. This breakdown begins in the mouth and continues in the small intestine, converting polysaccharides into disaccharides and monosaccharides, which are then absorbed into the bloodstream.

3. Describe the culinary functions of pectin.

Pectin is primarily used as a gelling agent in the culinary world. It is extracted from fruits and used to make jams and jellies. When heated with sugar and acid, pectin forms a gel-like structure, giving these food items their characteristic texture.

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Carbohydrates-Energy Value, Digestion and Absorption

Energy Value of Carbohydrates

Contribution to Total Energy Value of the Average Diet

• Carbohydrates are the primary energy source in the average diet, contributing significantly to the body's energy needs.
• They provide 4 kcal per gram, which is used for various bodily functions such as muscle movement, brain function, and maintaining body temperature.
• The primary sources of carbohydrates in the diet include grains, fruits, vegetables, and dairy products.

Dietary Targets for Non-Starch Polysaccharide Intake

• Non-starch polysaccharides, also known as dietary fibre, are essential for maintaining healthy digestion.
• Dietary recommendations suggest a daily intake of approximately 25-30 grams of fibre.
• This can be achieved by including a variety of fruits, vegetables, whole grains, legumes, nuts, and seeds in the diet.

Digestion and Absorption of Carbohydrates

Hydrolysis of Carbohydrates

• Hydrolysis is the chemical breakdown of carbohydrates into simpler sugars, facilitated by enzymes and water.
• It involves the cleavage of glycosidic bonds in polysaccharides and disaccharides, converting them into monosaccharides.

Digestion Sequences

• Digestion of carbohydrates begins in the mouth with the enzyme salivary amylase, which starts breaking down starch.
• In the small intestine, pancreatic amylase continues the process, breaking down remaining starch into maltose.
• Further digestion is carried out by enzymes in the intestinal lining, which break down disaccharides into monosaccharides.

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Absorption Mechanism

• Monosaccharides like glucose, fructose, and galactose are absorbed directly into the bloodstream through the intestinal wall.
• They are then transported to the liver, where they are metabolised or distributed throughout the body as a source of energy.

Outline of Utilisation of Glucose

• Glucose is the primary energy source for cells.
• It is either used immediately for energy or stored as glycogen in the liver and muscles for later use.
• Excess glucose can be converted into fat and stored in adipose tissue.

Exam Focus: Questions and Answers

1. Explain how carbohydrates contribute to the energy value of the average diet.

Carbohydrates are a major source of energy, providing 4 kcal per gram. They are easily broken down into glucose, which is used by the body for various functions such as muscle activity and brain function. The main dietary sources of carbohydrates include grains, fruits, vegetables, and dairy products.

2. Describe the digestion and absorption of carbohydrates.

The digestion of carbohydrates starts in the mouth with salivary amylase breaking down starch. In the small intestine, pancreatic amylase continues this process. Enzymes in the intestinal lining further break down disaccharides into monosaccharides, which are then absorbed into the bloodstream through the intestinal wall. These sugars are then utilised by the body for energy or stored in the liver and muscles.

3. How can the dietary targets for non-starch polysaccharide intake be achieved?

Dietary targets for non-starch polysaccharide intake, around 25-30 grams per day, can be achieved by consuming a variety of fibre-rich foods. This includes fruits, vegetables, whole grains, legumes, nuts, and seeds. Incorporating these food items into the diet ensures adequate fibre intake for healthy digestion.

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Carbohydrates-Additional Points

Carbohydrates in Disease Prevention

Adequate carbohydrate intake, especially dietary fibre, is linked to reduced risk of chronic diseases such as diabetes and heart disease.

Carbohydrates and Physical Activity

They are crucial for physical activity, providing the necessary energy for muscle contraction and endurance.

Balancing Carbohydrate Intake

It's important to balance carbohydrate intake with physical activity to maintain a healthy weight and prevent lifestyle-related diseases.