6.5 Carbohydrates

Short Introduction Carbohydrates are important biological molecules present in all parts of the cell. Carbohydrates are also called sugars or saccharides due to their sweet taste. The word saccharide is derived from the Greek word "Sakcharon" meaning sugar. The most common and simple carbohydrate, glucose, is naturally produced in green plants by the process of photosynthesis using CO₂ from air and H₂O from soil.

Main Idea

Carbohydrates are organic biomolecules composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio, with the general formula Cₓ(H₂O)ᵧ. They are the major source of energy for living organisms and also serve structural roles. Carbohydrates are classified into three groups based on the number of monomer units: monosaccharides, disaccharides, and polysaccharides. They are produced in plants through photosynthesis. generated by DeepSeek to PDF

Core Concept

Definition and Structure Carbohydrates are the important biological molecules present in all parts of the cell. They are also called sugars or saccharides due to their sweet taste. The word saccharide is derived from the Greek word "Sakcharon" meaning sugar. Carbohydrates are compounds of carbon, hydrogen, and oxygen with a ratio of 1:2:1 respectively. The literal meaning of the word carbohydrate is "hydrates of carbon". This name is given because the number of carbon atoms is equal to the number of water molecules in the chemical formula of carbohydrates. The generalized formula of carbohydrates is Cₓ(H₂O)ᵧ, where X is the number of carbon atoms and Y is the number of water molecules. For example, the formula of the most common sugar, glucose, is C₆(H₂O)₆ or C₆H₁₂O₆. Photosynthesis and Carbohydrates The most common and simple carbohydrate, glucose, is naturally produced in green plants by the process of photosynthesis using CO₂ from air and H₂O from soil. Equation: 6CO₂ + 6H₂O → (Light, Chlorophyll) → C₆H₁₂O₆ + 6O₂ Plants use sunlight energy for food synthesis. This energy is stored as chemical energy in the bonds of glucose molecules. 4 kcal per gram of energy is released when glucose is broken down. Functions of Carbohydrates 1. Carbohydrates are the major source of useable and stored energy in the cells of living organisms. 2. Carbohydrates are structural material for the cell wall of: Plants: Cellulose Fungi: Chitin Bacteria: Peptidoglycan 3. Cellulose present in the cell wall of plant fruit, vegetables, bran, and whole grains is not digestible; hence it acts as fiber or roughage in the human diet. It helps in the movement of food efficiently through the digestive tract and is effective in avoiding constipation. 4. Cellulose fiber absorbs away toxic chemicals which might be present in food. It is used in the paper industry. It also helps in lowering blood cholesterol and regulates blood pressure. 5. Sucrose is used as common table sugar and as a natural sweetener. 6. Extra glucose in plants is stored as starch in their root, stem, fruits, or seeds, used as animal and human food. 7. Extra blood glucose is stored in animal muscles and liver cells as glycogen, which serves as an energy bank. It can be converted back to glucose when needed for energy. 8. Chitin makes the exoskeleton of arthropods. Sources of Carbohydrates 1. Monosaccharides such as glucose, fructose, and galactose are obtained from fruits, vegetables, and cereals. They are also present in honey. 2. Disaccharides such as sucrose are obtained from sugarcane, sugar beet, and fruits. Maltose is found in... (Note: The text is cut off here. Based on standard biology, maltose is found in germinating grains and malt.) generated by DeepSeek to PDF

3. Cellulose is obtained from plants, e.g., cotton is pure cellulose. 4. Starch is present in cereals, wheat, barley, rice, maize, potato, sweet potato, etc. Classification of Carbohydrates Carbohydrates are classified on the basis of the number of monomers (saccharide units) in the molecule. They are generally classified into three groups: 1. Monosaccharides: The simplest form of carbohydrates, primarily produced during photosynthesis. White crystalline solids, soluble in water, sweet in taste. Number of carbon atoms ranges from 3 to 7. Named on the basis of the number of carbon atoms: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), etc. Most common are pentoses (e.g., ribose, C₅H₁₀O₅, found in nucleotides of nucleic acids) and hexoses (e.g., glucose, C₆H₁₂O₆). Monosaccharides are not further hydrolysed. In crystalline form, they are found as open-chain structures; when dissolved in water, they convert into ring structures. 2. Disaccharides: Carbohydrates which release two monosaccharides on hydrolysis are called disaccharides. Formed by bonding of two monosaccharides. White crystalline solids, comparatively less soluble in water, and less sweet in taste. Molecular formula: C₁₂H₂₂O₁₁. Examples: Sucrose, maltose, and lactose. Sucrose is the most familiar disaccharide, commonly used sweetener, known as cane sugar. It is formed by the condensation of glucose and fructose releasing a water molecule. Equation: C₆H₁₂O₆ + C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O (Glucose + Fructose → Sucrose + Water) Lactose is naturally found in milk and milk products. It is formed by the condensation of galactose and glucose. Equation: C₆H₁₂O₆ + C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O (Galactose + Glucose → Lactose + Water) (Note: The text also mentions maltose is called malt sugar, produced during brewing beer by alcoholic fermentation of malted barley. Lactose is the only common sugar obtained from animals; milk of all mammals is composed of 2–8% lactose.) 3. Polysaccharides: The largest group of carbohydrates. Complex carbohydrates and polymers of more than ten monosaccharides. Produced in the cells of plants and animals to store extra carbohydrate food and energy (e.g., starch, glycogen). They are storage molecules of carbohydrates. Polysaccharides are not sweet in taste; usually tasteless. They yield many monosaccharide units upon complete hydrolysis for energy use. Polysaccharides are also structural components of cell walls in the cells of plants (cellulose), fungi (chitin), and bacteria (peptidoglycan).

Detailed Explanation

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Carbohydrates are the most abundant class of organic biomolecules and are essential for life. 1. Structure: Carbohydrates are composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio, giving the general formula Cₓ(H₂O)ᵧ. For example, glucose is C₆H₁₂O₆. They are classified based on the number of sugar units they contain. 2. Monosaccharides: These are the simplest sugars and cannot be hydrolyzed further. They include trioses (3C), tetroses (4C), pentoses (5C), and hexoses (6C). Important examples are ribose (a pentose found in RNA and ATP) and glucose (a hexose used for energy). Monosaccharides exist as open-chain structures in crystalline form but form ring structures in solution. 3. Disaccharides: These are formed by the condensation of two monosaccharides. Important examples are: Sucrose (glucose + fructose) — common table sugar. Lactose (galactose + glucose) — milk sugar. Maltose (glucose + glucose) — malt sugar, produced during brewing. The molecular formula of disaccharides is C₁₂H₂₂O₁₁. 4. Polysaccharides: These are polymers of more than ten monosaccharides. They are storage molecules (starch in plants, glycogen in animals) and structural components (cellulose in plant cell walls, chitin in fungal cell walls and arthropod exoskeletons, peptidoglycan in bacterial cell walls). They are tasteless and not sweet. 5. Functions: Carbohydrates provide energy (4 kcal/g), store energy (starch, glycogen), provide structural support (cellulose, chitin, peptidoglycan), act as fiber/roughage (cellulose), and are used in industry (paper). They are also components of nucleic acids (ribose and deoxyribose) and cell surface markers. 6. Sources: Monosaccharides are found in fruits, vegetables, cereals, and honey. Disaccharides are found in sugarcane, sugar beet, fruits, and milk. Polysaccharides like starch are found in cereals, wheat, barley, rice, maize, potato, and sweet potato. Cellulose is found in plants (cotton is pure cellulose).

Intermediate Level

Carbohydrates are organic biomolecules composed of C, H, and O in a 1:2:1 ratio (general formula Cₓ(H₂O)ᵧ). They are also called sugars or saccharides. They are the major source of energy and also have structural roles. Carbohydrates are classified based on the number of monomer units: Monosaccharides (simplest; 3–7 carbons; e.g., ribose (5C), glucose (6C); not hydrolyzed further; sweet, soluble; form ring structures in solution). Disaccharides (two monosaccharides; formula C₁₂H₂₂O₁₁; e.g., sucrose (glucose + fructose), lactose (galactose + glucose), maltose; less sweet, less soluble). generated by DeepSeek to PDF

Polysaccharides (more than ten monosaccharides; e.g., starch, glycogen, cellulose, chitin, peptidoglycan; storage and structural roles; tasteless). Functions include: major energy source (4 kcal/g), structural material (cellulose in plants, chitin in fungi, peptidoglycan in bacteria), fiber/roughage (cellulose), energy storage (starch in plants, glycogen in animals), and component of exoskeletons (chitin). Sources include fruits, vegetables, cereals, honey (monosaccharides); sugarcane, sugar beet, fruits, milk (disaccharides); cereals, wheat, barley, rice, maize, potato, sweet potato (starch); cotton (pure cellulose).

Important Points

• 1. Carbohydrates are biological molecules present in all parts of the cell.
• 2. They are also called sugars or saccharides (Greek: Sakcharon = sugar).
• 3. Composed of C, H, O in a 1:2:1 ratio; general formula Cₓ(H₂O)ᵧ.
• 4. Literal meaning: "hydrates of carbon".
• 5. Glucose is produced in green plants by photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂).
• 6. One gram of carbohydrate provides 4 kcal of energy.
• 7. Functions:
• Major source of useable and stored energy.
• Structural material: cellulose (plants), chitin (fungi), peptidoglycan (bacteria).
• Cellulose = fiber/roughage; aids digestion; lowers cholesterol; regulates blood pressure; used
• in paper industry.
• Sucrose = table sugar/sweetener.
• Starch = storage in plants (roots, stems, fruits, seeds).
• Glycogen = storage in animal muscles and liver (energy bank).
• Chitin = exoskeleton of arthropods.
• 8. Classification based on monomers:
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• Monosaccharides: Simplest; 3–7 carbons; trioses, tetroses, pentoses, hexoses; e.g., ribose
• (C₅H₁₀O₅), glucose (C₆H₁₂O₆); not further hydrolyzed; sweet, soluble; ring structure in water.
• Disaccharides: Two monosaccharides; formula C₁₂H₂₂O₁₁; sucrose, maltose, lactose; less sweet,
• less soluble.
• Polysaccharides: More than ten monosaccharides; largest group; storage (starch, glycogen)
• and structural (cellulose, chitin, peptidoglycan); tasteless.
• 9. Sources:
• Monosaccharides: fruits, vegetables, cereals, honey.
• Disaccharides: sugarcane, sugar beet, fruits, milk (lactose).
• Cellulose: cotton (pure cellulose).
• Starch: cereals, wheat, barley, rice, maize, potato, sweet potato.

Remember

• Carbohydrates = sugars/saccharides (C, H, O in 1:2:1 ratio).
• Formula: Cₓ(H₂O)ᵧ; e.g., glucose = C₆H₁₂O₆.
• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Energy: 4 kcal/g.
• Functions: Energy, structure (cellulose/chitin/peptidoglycan), fiber, storage (starch/glycogen),
• exoskeleton (chitin).
• Monosaccharides: Simplest (3–7C); ribose (5C), glucose (6C).
• Disaccharides: Two monosaccharides; C₁₂H₂₂O₁₁; sucrose (glucose + fructose), lactose (galactose +
• glucose), maltose.
• Polysaccharides: >10 monosaccharides; starch, glycogen, cellulose, chitin, peptidoglycan.
• Sources: Fruits, vegetables, cereals, honey; sugarcane, sugar beet, milk; cereals, potato; cotton (cellulose).

Common Mistakes

• 1. Thinking carbohydrates are only sugars: Carbohydrates include sugars (monosaccharides,
• disaccharides) and polysaccharides (starch, cellulose, glycogen, chitin).
• 2. Forgetting the 1:2:1 ratio: Carbohydrates have C, H, O in a 1:2:1 ratio, giving the general formula
• Cₓ(H₂O)ᵧ.
• 3. Confusing monosaccharides and disaccharides: Monosaccharides are single sugar units (e.g., glucose);
• disaccharides are two sugar units (e.g., sucrose).
• 4. Thinking all polysaccharides are storage molecules: Some are structural (cellulose, chitin,
• peptidoglycan).
• 5. Forgetting that cellulose is not digestible by humans: Cellulose acts as dietary fiber/roughage.
• 6. Mixing up glycogen and starch: Starch is the storage carbohydrate in plants; glycogen is the storage
• carbohydrate in animals.
• 7. Thinking chitin is only in fungi: Chitin is also found in the exoskeleton of arthropods.

Quick Revision

• Carbohydrates are organic biomolecules composed of C, H, and O in a 1:2:1 ratio, with the general formula Cₓ(H₂O)ᵧ.
• They are also called sugars or saccharides. They are the major source of energy (4 kcal/g) and serve structural roles.
• They are classified into three groups:
• Monosaccharides (simplest; 3–7 carbons; e.g., ribose, glucose; not further hydrolyzed; sweet, soluble).
• Disaccharides (two monosaccharides; formula C₁₂H₂₂O₁₁; e.g., sucrose, lactose, maltose; less sweet, less
• soluble).
• Polysaccharides (more than ten monosaccharides; e.g., starch, glycogen, cellulose, chitin, peptidoglycan;
• storage and structural; tasteless).
• Functions include energy source, structural material (cellulose, chitin, peptidoglycan), fiber/roughage (cellulose),
• energy storage (starch, glycogen), and exoskeleton formation (chitin). Sources include fruits, vegetables, cereals,
• honey (monosaccharides); sugarcane, sugar beet, milk (disaccharides); cereals, potato (starch); cotton (cellulose).

60-Second Revision

• Carbohydrates = sugars/saccharides (C, H, O in 1:2:1 ratio).
• Formula: Cₓ(H₂O)ᵧ; e.g., glucose = C₆H₁₂O₆.
• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Energy: 4 kcal/g.
• Functions: Energy, structure (cellulose/chitin/peptidoglycan), fiber, storage (starch/glycogen),
• exoskeleton (chitin).
• Monosaccharides: Simplest (3–7C); ribose (5C), glucose (6C).
• Disaccharides: Two monosaccharides; C₁₂H₂₂O₁₁; sucrose (glucose + fructose), lactose (galactose +
• glucose), maltose.
• Polysaccharides: >10 monosaccharides; starch, glycogen, cellulose, chitin, peptidoglycan.
• Sources: Fruits, vegetables, cereals, honey; sugarcane, sugar beet, milk; cereals, potato; cotton (cellulose).
• Frequently Asked Questions (FAQs)
• Q1. What are carbohydrates? Answer: Carbohydrates are important biological molecules present in all parts of the
• cell. They are also called sugars or saccharides due to their sweet taste. They are compounds of carbon, hydrogen,
• and oxygen with a ratio of 1:2:1.
• Q2. What is the literal meaning of carbohydrate? Answer: The literal meaning of the word carbohydrate is
• "hydrates of carbon." This name is given because the number of carbon atoms is equal to the number of water
• molecules in the chemical formula of carbohydrates.
• Q3. How is glucose produced in plants? Answer: Glucose is naturally produced in green plants by the process of
• photosynthesis using CO₂ from air and H₂O from soil.
• Q4. What are the functions of carbohydrates? Answer: Carbohydrates have many functions:
• 1. They are the major source of useable and stored energy.
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• 2. They are structural material for cell walls (cellulose in plants, chitin in fungi, peptidoglycan in bacteria).
• 3. Cellulose acts as fiber/roughage in the human diet.
• 4. Cellulose absorbs toxic chemicals, is used in the paper industry, lowers blood cholesterol, and regulates
• blood pressure.
• 5. Sucrose is used as table sugar and natural sweetener.
• 6. Extra glucose in plants is stored as starch.
• 7. Extra blood glucose is stored as glycogen in animal muscles and liver cells.
• 8. Chitin makes the exoskeleton of arthropods.
• Q5. What are the three classes of carbohydrates? Answer: Carbohydrates are classified into three groups:
• monosaccharides, disaccharides, and polysaccharides.
• Q6. What are monosaccharides? Give examples. Answer: Monosaccharides are the simplest form of
• carbohydrates, primarily produced during photosynthesis. They are white crystalline solids, soluble in water, and
• sweet in taste. The number of carbon atoms ranges from 3 to 7. Examples include ribose (a pentose, C₅H₁₀O₅) and
• glucose (a hexose, C₆H₁₂O₆).
• Q7. What are disaccharides? Give examples. Answer: Disaccharides are carbohydrates that release two
• monosaccharides on hydrolysis. They are formed by the bonding of two monosaccharides. The molecular formula is
• C₁₂H₂₂O₁₁. Examples include sucrose, maltose, and lactose. Sucrose is formed by the condensation of glucose and
• fructose. Lactose is formed by the condensation of galactose and glucose.
• Q8. What are polysaccharides? Give examples. Answer: Polysaccharides are complex carbohydrates and polymers
• of more than ten monosaccharides. They are produced in cells to store extra carbohydrate food and energy (e.g.,
• starch, glycogen). They are also structural components of cell walls (cellulose in plants, chitin in fungi, peptidoglycan
• in bacteria). They are usually tasteless.
• Q9. What are the sources of carbohydrates? Answer: Sources of carbohydrates include:
• Monosaccharides: fruits, vegetables, cereals, honey.
• Disaccharides: sugarcane, sugar beet, fruits (sucrose); milk (lactose).
• Cellulose: cotton (pure cellulose).
• Starch: cereals, wheat, barley, rice, maize, potato, sweet potato.
• Q10. How much energy does one gram of carbohydrate provide? Answer: 4 kcal per gram of energy is released
• when glucose is broken down.
• Multiple Choice Questions (MCQs)
• Q1. Carbohydrates are composed of carbon, hydrogen, and oxygen in a ratio of: A. 1:1:1 B. 1:2:1 C. 2:1:2 D. 1:2:3
• Correct Answer: B Explanation: Carbohydrates have C, H, O in a 1:2:1 ratio.
• Q2. The general formula of carbohydrates is: A. CₓHᵧOᵧ B. Cₓ(H₂O)ᵧ C. Cₓ(H₂O)ₓ D. CₓH₂ₓOₓ Correct Answer: B
• Explanation: The general formula is Cₓ(H₂O)ᵧ.
• Q3. Which of the following is a monosaccharide? A. Sucrose B. Lactose C. Glucose D. Starch Correct Answer: C
• Explanation: Glucose is a monosaccharide; sucrose and lactose are disaccharides; starch is a polysaccharide.
• Q4. Sucrose is formed by the condensation of: A. Glucose and galactose B. Glucose and fructose C. Galactose and
• fructose D. Glucose and glucose Correct Answer: B Explanation: Sucrose is formed by the condensation of glucose
• and fructose.
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• Q5. Lactose is formed by the condensation of: A. Glucose and fructose B. Galactose and glucose C. Glucose and
• glucose D. Fructose and galactose Correct Answer: B Explanation: Lactose is formed by the condensation of
• galactose and glucose.
• Q6. The molecular formula of disaccharides is: A. C₆H₁₂O₆ B. C₁₂H₂₂O₁₁ C. C₅H₁₀O₅ D. C₆H₁₀O₅ Correct Answer: B
• Explanation: The molecular formula of disaccharides is C₁₂H₂₂O₁₁.
• Q7. Which polysaccharide makes the exoskeleton of arthropods? A. Cellulose B. Starch C. Glycogen D. Chitin Correct
• Answer: D Explanation: Chitin makes the exoskeleton of arthropods.
• Q8. Which carbohydrate acts as fiber/roughage in the human diet? A. Starch B. Glycogen C. Cellulose D. Sucrose
• Correct Answer: C Explanation: Cellulose is not digestible and acts as fiber/roughage.
• Q9. Extra glucose in animals is stored as: A. Starch B. Glycogen C. Cellulose D. Chitin Correct Answer: B Explanation:
• Extra blood glucose is stored in animal muscles and liver cells as glycogen.
• Q10. Which of the following is a pentose sugar found in nucleic acids? A. Glucose B. Ribose C. Sucrose D. Maltose
• Correct Answer: B Explanation: Ribose is a pentose (C₅H₁₀O₅) found in the nucleotides of nucleic acids.
• Subtopic 6.5 complete. Reply "next" to approve and I will produce Subtopic 6: 6.6 Nucleic Acids (Structure of
• Nucleotide, Structure of DNA, Function of DNA, Structure of RNA, Function of RNA).