Carbohydrates

Table of Contents
1. What are Carbohydrates?
2. Classification of Carbohydrates
3. Functions of Carbohydrates
4. Sources of Carbohydrates
5. Carbohydrate Foods and Dietary Considerations
6. Examples of Carbohydrates
7. Derivatives of Monosaccharides
8. Oligosaccharides - Details and Types
9. Polysaccharides
10. Mucopolysaccharides (Glycosaminoglycans)
11. Special Points
12. Frequently Asked Questions
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What are Carbohydrates?

Carbohydrates are macronutrients composed of carbon, hydrogen and oxygen atoms. They are one of the principal sources by which living organisms obtain chemical energy for cellular activities. The name carbohydrate derives from the French phrase hydrate de carbone meaning "hydrate of carbon". A general empirical formula often used for simple carbohydrates is C_n(H₂O)_n.

Carbohydrates include simple sugars, oligosaccharides, starches and dietary fibres. They occur widely in plants and animals and are abundant in grains, vegetables, fruits, milk and dairy products. During digestion many dietary carbohydrates are converted to glucose, which is transported in blood and used by cells to produce energy in the form of ATP. Excess glucose is stored mainly in the liver and muscles as glycogen for later use.

Classification of Carbohydrates

Carbohydrates are classified on the basis of their chemical structure and degree of polymerisation into two broad groups: simple carbohydrates and complex carbohydrates.

Simple Carbohydrates (Monosaccharides, Disaccharides and Oligosaccharides)

Simple carbohydrates are sugars made of one or few monosaccharide units. They are rapidly digested and absorbed, causing a quicker rise in blood glucose compared with complex carbohydrates. Common dietary sources include fruits, milk, honey and refined sugars.

Plants synthesise glucose (C₆H₁₂O₆) from carbon dioxide and water using sunlight through the process of photosynthesis. This stored chemical energy is then transferred through food chains when consumers feed on plants.

1. Monosaccharides

Monosaccharides are the simplest sugars and cannot be hydrolysed to smaller carbohydrates. Examples are glucose, fructose, galactose and mannose. Their structures may be represented in open-chain or ring forms depending on the number of carbon atoms and ring formation.

Monosaccharides are also classified by the number of carbon atoms:

• Trioses (C₃H₆O₃) - e.g. glyceraldehyde.
• Tetroses (C₄H₈O₄) - e.g. erythrose.
• Pentoses - e.g. ribose, deoxyribose.
• Hexoses - e.g. glucose, fructose, galactose.
• Heptoses - less common, occur in specialised metabolic pathways.

2. Disaccharides

Disaccharides form when two monosaccharide units join by a glycosidic bond with elimination of one molecule of water. Examples include sucrose (glucose + fructose), lactose (glucose + galactose) and maltose (glucose + glucose).

3. Oligosaccharides

Oligosaccharides are carbohydrates made of 2-9 monosaccharide units. They are linked by glycosidic bonds and include disaccharides as the simplest oligosaccharides. Common oligosaccharides occur in plant phloem and in some legumes; some oligosaccharides are not fully digested in the human small intestine and function as prebiotics in the gut.

Complex Carbohydrates (Polysaccharides)

Complex carbohydrates, or polysaccharides, are polymers of many monosaccharide units. They are digested and absorbed more slowly than simple sugars. Common dietary sources include pulses, whole grains, starchy vegetables (potato, corn) and cereals.

Polysaccharides can be structural (e.g. cellulose) or storage (e.g. starch, glycogen). Many are homopolymers of glucose (made of only glucose units) while others are heteropolymers made of different monosaccharides.

• Starch consists of two components: amylose (mostly linear chains) and amylopectin (branched chains).
• Glycogen is the principal storage polysaccharide in animals and is highly branched.
• Cellulose is a linear polymer and the main structural component of plant cell walls; it is fibrous and provides tensile strength.

Functions of Carbohydrates

The primary function of carbohydrates is to provide energy. Glucose oxidation yields ATP which powers cellular processes. Carbohydrates also:

• act as a protein-sparing nutrient preventing excessive protein breakdown for energy;
• participate in fat metabolism and prevent ketosis by providing an adequate carbohydrate supply;
• serve structural roles (e.g. cellulose in plant cell walls, chitin in arthropod exoskeletons);
• form components of glycoproteins and glycolipids that are important in cell recognition and signalling (introductory level).

The enzyme amylase (salivary and pancreatic) initiates the hydrolysis of starch into smaller sugars which are further broken down to glucose for absorption and metabolism.

Sources of Carbohydrates

• Fructose is abundant in many fruits.
• Galactose is a constituent of lactose and thus present in dairy products.
• Lactose is the principal sugar in milk.
• Maltose occurs during starch digestion and is present in cereals and germinating seeds.
• Sucrose is the common transport sugar in many plants and is extracted commercially from sugarcane and sugar beet.

Whole foods such as whole grains, fruits and vegetables provide carbohydrates along with fibre, vitamins and minerals. Refined foods like polished white rice, white bread and table sugar often lack fibre and micronutrients and are referred to as "refined" or "enriched" foods.

Carbohydrate Foods and Dietary Considerations

A balanced intake of carbohydrates is important for health. Excessive consumption of refined sugars contributes to excess caloric intake and can lead to obesity and metabolic disorders. Complex carbohydrates with dietary fibre are considered "good" carbohydrates because they release glucose more slowly and support digestive health.

Examples of Carbohydrates

• Glucose
• Galactose
• Maltose
• Fructose
• Sucrose
• Lactose
• Starch
• Cellulose
• Chitin

Derivatives of Monosaccharides

Monosaccharides undergo chemical modifications to form biologically important derivatives:

• Amino sugars - where an -OH group (commonly at C-2) is replaced by an -NH₂ or an N-acetyl derivative, e.g. glucosamine and galactosamine.
• Sugar alcohols - the aldehyde group (-CHO) of an aldose is reduced to a primary alcohol (-CH₂OH); examples are sorbitol (from glucose) and mannitol (from mannose).
• Sugar acids - oxidation of terminal -CHO or -CH₂OH groups yields carboxylic acids such as glucuronic acid and galacturonic acid.
• Glycosides - formed when the anomeric hydroxyl of a sugar reacts with an alcohol or another compound to give an acetal (glycosidic) linkage; many natural products are glycosides. Glycosidic bonds can be cleaved by acids or specific enzymes to release the sugar and the aglycone.

Oligosaccharides - Details and Types

Oligosaccharides yield 2-10 monosaccharide units on hydrolysis. Monosaccharide units are joined by glycosidic bonds, which form by reaction between an aldehyde or ketone group of one monosaccharide and a hydroxyl group of another with elimination of water (dehydration synthesis). Common glycosidic linkages are described by the carbon numbers involved, e.g. 1→4 or 1→6 bonds, and by the anomeric configuration (α or β).

Types of Oligosaccharides

(i) Disaccharides - e.g. maltose, sucrose, lactose, trehalose. All disaccharides are generally water-soluble and sweet.

Maltose (malt sugar) is an intermediate product of starch digestion and has an α(1→4) linkage between two α-D-glucose units.

Lactose (milk sugar) contains a β(1→4) linkage between galactose and glucose and is the least sweet of common disaccharides. Human milk has the highest percentage of lactose among common milks (≈7%).

Sucrose (cane or table sugar) consists of glucose and fructose and is the principal form for long-distance sugar transport in many plants.

Trehalose occurs in the haemolymph of insects and has a glycosidic bond between the anomeric carbons of two glucose units.

(ii) Trisaccharides - e.g. raffinose (galactose + glucose + fructose).

(iii) Tetrasaccharides - e.g. stachyose (galactose + galactose + glucose + fructose).

(iv) Pentasaccharides - e.g. verbascose or other larger oligosaccharides occurring in seeds and phloem.

Raffinose and stachyose occur in plant phloem and are involved in carbohydrate translocation.

Polysaccharides

1. Polysaccharides are polymers made of many monosaccharide units.
2. The suffix "-an" is often added and such polymers are referred to as glycans.
3. Pentose polysaccharides are sometimes called pentosans (e.g. xylan, arabinoxylan).
4. Examples include araban (from L-arabinose) and xylan (from D-xylose), which are components of plant cell walls.
5. Hexose polysaccharides (often informally called "hexans") include mannan (from mannose), cellulose and starch.
6. Polysaccharides are generally insoluble in water and are not sweet to the taste.
7. Most structural polysaccharides are non-reducing in nature.
8. Functionally they can be classified as nutritive (storage) or structural.
9. Structurally they may be homopolysaccharides (one type of monomer) or heteropolysaccharides (different monomers).

(I) Homopolysaccharides

These are composed of repeating units of the same monosaccharide.

(a) Cellulose - a linear polymer of β-D-glucose units (thousands of residues). Monomers are joined by β(1→4) linkages. Partial hydrolysis of cellulose yields cellobiose (a disaccharide). Cellulose is the main component of plant cell walls and is the most abundant organic polymer on earth. Cotton is mostly cellulose (~90%), wood contains about 50% cellulose.

Some animals (e.g. tunicates) produce cellulose-like materials sometimes referred to as tunicine. Cellulose is also chemically used to make regenerated fibres such as rayon.

(b) Starch - the principal storage polysaccharide in plants, composed of α-D-glucose units and consisting of two components:

• Amylose - largely unbranched chains joined by α(1→4) linkages; typically 250-300 glucose residues.
• Amylopectin - branched chains with α(1→4) linkages in chains and α(1→6) linkages at branch points; branch points occur every ~24-30 residues.

• Amylose gives a characteristic blue colour with iodine.
• Amylopectin gives a reddish-brown colour with iodine.
• Typical potato starch contains approximately 20% amylose and 80% amylopectin.

(c) Glycogen - the storage form of carbohydrate in animals and fungi. Glycogen is highly branched and is stored mainly in liver and muscle (liver stores regulate blood glucose; muscle glycogen supplies local energy). Its chains contain α(1→4) linkages and branch points with α(1→6) linkages. Glycogen produces a reddish colour with iodine.

(d) Chitin - a linear polymer of N-acetyl-D-glucosamine units joined by β(1→4) linkages. N-acetyl-D-glucosamine is an amino-sugar derivative of glucose (contains -NH-CO-CH₃). Chitin is a major component of arthropod exoskeletons and fungal cell walls and is the second most abundant natural polysaccharide after cellulose.

(e) Inulin - a water-soluble storage polysaccharide composed mainly of fructose units linked primarily by β(2→1) bonds; found in roots of plants such as dahlia and Jerusalem artichoke. Inulin is used clinically to estimate glomerular filtration rate because it is freely filtered and neither metabolised nor secreted by renal tubules.

(f) Dextrin - intermediate products of starch and glycogen breakdown. Enzymatic or acid hydrolysis of dextrins yields maltose and glucose. Dextrins are also produced by heating starch and are used industrially and in food processing.

(II) Heteropolysaccharides

Heteropolysaccharides contain two or more different kinds of monosaccharide residues and often play structural and functional roles in extracellular matrices and connective tissues.

(a) Hyaluronic acid - a long, unbranched heteropolysaccharide composed of alternating units of D-glucuronic acid and N-acetyl-D-glucosamine. It is abundant in the vitreous humour of the eye, in synovial fluid of joints (acting as a lubricant) and in the extracellular matrix of connective tissues where it contributes to hydration and structural integrity.

• It functions as a lubricant and a space-filling, water-binding polymer in many tissues.

(b) Chondroitin sulphate - composed of D-glucuronic acid and N-acetylgalactosamine (often sulphated). It occurs in cartilage, tendons and bone and contributes to the tensile strength and resilience of these tissues.

(c) Heparin - a highly sulphated glycosaminoglycan that acts as an anticoagulant. It consists of alternating uronic acid and sulfated amino-sugar residues and is widely used clinically to prevent clot formation.

(d) Pectins - polymers rich in galacturonic acid with various neutral sugars (galactose, arabinose). Pectins bind cellulose microfibrils in plant cell walls and calcium and magnesium pectates form the middle lamella that glues plant cells together; they are often termed "plant cement".

(e) Hemicelluloses - a heterogeneous group of branched polysaccharides (e.g. xylans, mannans) that associate with cellulose in the cell wall and differ from cellulose in sugar composition and linkages.

• Some plant sources such as certain palms yield hard, compact hemicelluloses used historically for objects like billiard balls (artificial ivory) and other materials.

Mucopolysaccharides (Glycosaminoglycans)

Mucopolysaccharides, more commonly called glycosaminoglycans (GAGs), are long, negatively charged polysaccharides that bind large amounts of water and often link to proteins to form proteoglycans. They are slimy, viscous substances that provide lubrication and structural integrity to connective tissues. Examples include hyaluronic acid, chondroitin sulphate and heparin. In plants, mucilage is a common mucopolysaccharide composed mainly of galactose and mannose units.

Special Points

• Peptidoglycan - the major structural polymer of bacterial cell walls. It consists of alternating N-acetylglucosamine and N-acetylmuramic acid residues cross-linked by short peptide chains (typically 4-5 amino acids).
• Agar-agar - a gelatinous polysaccharide obtained from certain red algae such as Gracilaria, Gelidium and Chondrus. Agar is composed of alternating units of D-galactose and L-galactose and is frequently sulphated; it is used as a gelling agent in microbiology and food industries.

Frequently Asked Questions

Q1. What are carbohydrates?
Ans: Carbohydrates are biomolecules comprising carbon, hydrogen and oxygen atoms. They are an important source of energy. They are sugars, starch and fibres found in fruits and vegetables.

Q2. How are carbohydrates classified?
Ans: Carbohydrates are classified into the following:

• Simple carbohydrates
• Complex carbohydrates

Q3. How are carbohydrates important to our body?
Ans: Carbohydrates provide energy to the body. It breaks down into glucose and enters our bloodstream. The body cells utilize glucose to produce ATP.

Q4. Name a few sources of carbohydrates.
Ans: Carbohydrates are obtained from a variety of sources such as bread, milk, potatoes, cookies, corn, etc.

Q5. How are the carbohydrates digested?
Ans: Carbohydrates start being digested in the mouth by the action of salivary amylase. They are not completely broken down in the stomach but in the intestine.

Q6. What are simple carbohydrates? Give examples.
Ans: Simple carbohydrates are the ones that are quickly broken down by the body to be converted into energy. Fruits, milk and milk products are the main sources of simple carbohydrates.

Q7. How are complex carbohydrates different from simple carbohydrates?
Ans: Complex carbohydrates are the ones in which the sugar molecules are strung in long, complex chains. Peas, beans, vegetables and grains are important sources of carbohydrates.

Q8. What are the three types of simple carbohydrates?
Ans: Three types of simple carbohydrates include:

• Monosaccharides
• Disaccharides
• Polysaccharides

Q9. Name some bad carbohydrates that are harmful to the body.
Ans: The bad carbs include:

• White bread
• Sugary drinks
• Pastries
• Candies and chocolates