Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Giant Molecules (Sugars, Starch)
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Giant Molecules (Sugars, Starch)
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Subject: Chemistry
Class: Senior Secondary 3
Term: 1st Term
Week: 6
Theme: Chemistry Of Life
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Mention some sources of sugar Classify sugar as;a. mono, di, poly saccharidesb. reducing and non-reducing sugars Give examples of the different types of sugar. Prepare glucose from starch and sucrose Test for starch and glucose using Fehling's solution State the uses of starch and sugars
The main structural component of plant cell walls. A polymer of β-glucose units. Indigestible by humans.
Glycogen: The primary energy storage polysaccharide in animals (stored in liver and muscles). Highly branched, similar to amylopectin. b.
Classification by Reducing Properties: This classification is based on the presence of a free aldehyde (-CHO) or ketone (>C=O) group that can be oxidised, thereby reducing other substances.
Reducing Sugars: Definition: Sugars that possess a free anomeric carbon with a free aldehyde or ketone group, allowing them to donate electrons and reduce oxidising agents like Fehling's solution (Cu2+ to Cu+) or Benedict's solution.
Test: When heated with Fehling's or Benedict's solution (which are blue), they cause a colour change from blue through green, yellow, orange, to a brick-red precipitate (due to the formation of copper(I) oxide, Cu2O).
Examples: All monosaccharides (glucose, fructose, galactose) and some disaccharides (maltose, lactose).
Note: Fructose, a ketose, rearranges to an aldose in alkaline solutions, allowing it to act as a reducing sugar.
Non-reducing Sugars: Definition: Sugars that do not have a free aldehyde or ketone group capable of reducing oxidising agents. Their anomeric carbons are involved in the glycosidic bond.
Test: They do not give a positive result with Fehling's or Benedict's solution directly.
However, upon hydrolysis with dilute acid, they break down into monosaccharides (which are reducing sugars), and will then test positive.
Example: Sucrose. Its glucose and fructose units are linked through their anomeric carbons, making both aldehyde and ketone groups unavailable. 2.
4. Preparation of Glucose Glucose can be prepared by the hydrolysis of larger carbohydrates.
From Starch (Acid Hydrolysis): Starch is a polysaccharide composed of many glucose units. Boiling starch with dilute mineral acid (e.g., HCl or H2SO4) for an extended period breaks down the glycosidic bonds, yielding glucose. (C6H10O5)n + nH2O --(Dil. H2SO4/Heat)--> nC6H12O6 (Starch) + (Water) ----> (Glucose)
Procedure (for demonstration/practical):
1. Mix 10g of soluble starch with 100cm3 of water to form a paste.
2. Add 5cm3 of dilute hydrochloric acid (1M HCl) or sulfuric acid (1M H2SO4).
3. Boil the mixture gently for about 30-60 minutes.
4. Periodically test a small sample with iodine solution to check for starch (until it no longer turns blue-black).
5. Once starch is fully hydrolysed, cool the solution and neutralise the acid with sodium carbonate solution (Na2CO3) until effervescence stops.
6. The resulting solution contains glucose.
From Sucrose (Acid Hydrolysis): Sucrose is a disaccharide that yields glucose and fructose upon hydrolysis. C12H22O11 + H2O --(Dil. HCl/Heat)--> C6H12O6 (Glucose) + C6H12O6 (Fructose) (Sucrose) + (Water) ----> (Glucose) + (Fructose)
Procedure (for demonstration/practical):
1. Dissolve 10g of sucrose in 100cm3 of water.
2. Add 2cm3 of dilute hydrochloric acid (1M HCl).
3. Boil the solution gently for about 10-15 minutes.
4. Cool the solution and neutralise the acid with sodium carbonate solution.
5. The resulting solution contains a mixture of glucose and fructose (known as invert sugar). 2.
5. Testing for Starch and Glucose (Reducing Sugars)
Test for Starch (Iodine Test): Reagent: Iodine solution (iodine in potassium iodide). Initially yellowish-brown.
Procedure: Add a few drops of iodine solution to the sample.
Observation: A positive test for starch produces a characteristic blue-black colouration.
Explanation: Iodine molecules fit into the helical structure of the amylose component of starch, forming a complex that absorbs light and appears blue-black. Amylopectin gives a reddish-brown colour.
Note: Heating destroys the helix and fades the colour; cooling restores it. Test for Glucose (and other Reducing Sugars) using Fehling's Solution: Reagents: Fehling's solution is prepared by mixing two solutions just before use: Fehling's A (copper(II) sulphate solution) and Fehling's B (potassium sodium tartrate and strong alkali, typically NaOH). The mixed solution is deep blue.
Procedure:
1. Add equal volumes of Fehling's A and Fehling's B to a test tube containing the sample solution.
2. Heat the mixture gently in a water bath or over a Bunsen flame. * Observation: A positive test indicates a colour change from blue to green, then yellow, orange, and finally a brick-red precipitate. The intensity of the Fehling's solution is prepared by mixing two solutions just before use: Fehling's A (copper(II) sulphate solution) and Fehling's B (potassium sodium tartrate and strong alkali, typically NaOH). The mixed solution is deep blue.
Procedure:
1. Add equal volumes of Fehling's A and Fehling's B to a test tube containing the sample solution.
2. Heat the mixture gently in a water bath or over a Bunsen flame.
Observation: A positive test indicates a colour change from blue to green, then yellow, orange, and finally a brick-red precipitate. The intensity of the red precipitate indicates the concentration of the reducing sugar.
Explanation: The aldehyde group of the reducing sugar is oxidised to a carboxylic acid group, while the copper(II) ions (Cu2+) in Fehling's solution are reduced to copper(I) ions (Cu+), forming insoluble copper(I) oxide (Cu2O), which is the brick-red precipitate.
Equation: R-CHO + 2Cu2+(blue) + 5OH− → R-COO− + Cu2O(brick-red ppt) + 3H2O (Aldehyde) + (Copper(II) ions) ----> (Carboxylate) + (Copper(I) oxide)
Note: Sucrose (a non-reducing sugar) will not give a positive result directly. After hydrolysis with dilute acid, it will then give a positive Fehling's test due to the formation of glucose and fructose. 2.
6. Uses of Starch and Sugars Uses of Starch: Food: Major energy source. Used in making fufu, garri, tuwo, starch, bread, pastries. Thickener in soups and sauces.
Industrial Applications: Adhesives: Used in glues and pastes.
Textile Industry: Sizing agent to stiffen fabrics and improve weaving.
Paper Industry: Filler and binder to improve paper strength and printing quality.
Laundry: To stiffen clothes.
Pharmaceuticals: As binders, fillers, and disintegrants in tablets and capsules.
Ethanol Production: Fermented to produce ethanol (biofuel). Uses of Sugars (Glucose, Sucrose, etc.): Food: Primary source of quick energy. Sweetener in drinks, confectionery, baked goods, traditional Nigerian snacks (e.g., akara, puff-puff).
Production of Alcoholic Beverages: Fermentation of glucose/fructose by yeast produces ethanol and carbon dioxide (e.g., beer brewing, local palm wine production).
Pharmaceuticals: Used as sweeteners in medicines (syrups), energy supplements (e.g., oral rehydration solutions), and in intravenous drips.
Chemical Synthesis: Raw material for producing various organic chemicals.
Preservation: High sugar concentration acts as a preservative (e.g., jams, marmalades). This section provides the essential content knowledge for the teacher to deliver the lesson comprehensively. 2.
1. Introduction to Carbohydrates Carbohydrates are organic compounds with the general formula Cx(H2O)γ. They are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. They are broadly classified into sugars (saccharides) and non-sugars (polysaccharides like starch and cellulose). 2.
2. Sources of Sugars and Starch Sources of Sugars: Sugar Cane: A primary commercial source (e.g., cultivated in parts of Nigeria).
Sugar Beet: Another major commercial source, though less common in Nigeria.
Fruits: Many fruits are rich in simple sugars (glucose, fructose) – e.g., mangoes, oranges, pineapples, bananas.
Honey: A natural sweetener produced by bees, primarily a mixture of glucose and fructose.
Milk: Contains lactose (milk sugar).
Palm Wine: A local Nigerian alcoholic beverage, initially sweet due to fermentable sugars from the palm sap.
Sorghum: Used to make local sweeteners.
Sources of Starch: Cereals: Rice, maize (corn), millet, sorghum. These are staple grains across Nigeria.
Tubers: Yam, cassava, potato, cocoyam. These are fundamental components of the Nigerian diet.
Legumes: Small amounts found in beans.
Plantains: A common Nigerian food. 2.
3. Classification of Sugars Sugars are primarily classified based on: a. The number of saccharide units. b. Their reducing properties. a. Classification by Number of Saccharide Units: Monosaccharides (Simple Sugars): Definition: These are the simplest carbohydrates and cannot be hydrolysed into smaller sugar units. They typically have 3-7 carbon atoms.
General Formula: (CH2O)n, where n is usually 5 or 6 (e.g., C6H12O6).
Structure: They contain either an aldehyde group (-CHO) (aldoses) or a ketone group (>C=O) (ketoses), and multiple hydroxyl (-OH) groups. They exist in both open-chain and cyclic forms, with the cyclic form being more prevalent in solution.
Examples: Glucose: An aldohexose (6 carbons, aldehyde group). It is the most common monosaccharide, often called blood sugar. Abundant in fruits, honey.
Fructose: A ketohexose (6 carbons, ketone group). Often called fruit sugar. Found in fruits and honey. It is the sweetest natural sugar.
Galactose: An aldohexose. Not usually found free in nature but is a component of lactose (milk sugar).
Ribose: A pentose (5 carbons) found in RN
A. Disaccharides: Definition: Composed of two monosaccharide units joined together by a glycosidic bond (formed by a condensation reaction, with the elimination of a water molecule).
General Formula: C12H22O11 (derived from 2 C6H12O6 - H2O).
Hydrolysis: Can be hydrolysed by dilute acids or enzymes into their constituent monosaccharides.
Examples: Sucrose (Table Sugar): Glucose + Fructose. Found in sugar cane, sugar beet. It is a non-reducing sugar.
Maltose (Malt Sugar): Glucose + Glucose. Formed during the digestion of starch (e.g., in malting barley for beer). It is a reducing sugar.
Lactose (Milk Sugar): Glucose + Galactose. Found in milk. It is a reducing sugar.
Polysaccharides: Definition: Complex carbohydrates composed of many (hundreds to thousands) monosaccharide units (often glucose) linked together by glycosidic bonds. They are not sweet and are generally insoluble in water.
General Formula: (C6H10O5)n, where 'n' is a large number.
Hydrolysis: Can be hydrolysed into disaccharides and eventually monosaccharides by prolonged boiling with dilute acids or by specific enzymes.
Examples: Starch: The primary energy storage polysaccharide in plants. It is a polymer of α-glucose units.
It has two components: Amylose: Linear, unbranched chains of glucose units (α-1,4 glycosidic bonds). Soluble in hot water.
Amylopectin: Branched chains of glucose units (α-1,4 and α-1,6 glycosidic bonds). Insoluble in water.
Cellulose: The main structural component of plant cell walls. A polymer of β-glucose units. Indigestible by humans.
Glycogen: The primary energy storage polysaccharide in animals (stored in liver and muscles). Highly branched, similar to amylopectin. b.
Classification by Reducing Properties: This classification is based on the presence of a free aldehyde (-CHO) or ketone (>C=O) group that can be oxidised, thereby reducing other substances.
Reducing Sugars: Definition: Sugars that possess a free anomeric carbon with a free aldehyde or ketone group, allowing them to donate electrons and reduce oxidising agents This section outlines practical, classroom-friendly strategies for delivering the lesson. 3.
1. Introduction (5-10 minutes)
Teacher Activity: Begin by prompting students to identify common food items they eat daily (e.g., garri, yam, rice, bread, fruits). Ask them what common nutrient these foods provide. Introduce the concept of carbohydrates, specifically focusing on sugars and starch as giant molecules.
Student Activity: Students brainstorm and list common food items, identifying them as energy-giving foods. 3.
2. Exploring Sources of Sugars and Starch (15-20 minutes)
Teacher Activity: Facilitate a discussion on the various sources of sugars and starch, drawing extensively on local Nigerian examples. Guide students to differentiate between natural sources (fruits, honey, sugar cane) and processed sources. Display visual aids (pictures of sugar cane, cassava, yam, maize).
Student Activity: Students contribute local examples of sugar and starch sources. They can work in small groups to compile lists, fostering peer learning. 3.
3. Classification of Sugars (20-25 minutes)
Teacher Activity: Explain the classification of sugars into monosaccharides, disaccharides, and polysaccharides, using simple analogies to describe the 'building block' concept. Write the general formulas and provide key examples for each. Draw simplified open-chain structures for glucose and fructose on the board or use molecular models if available. Introduce the concept of reducing and non-reducing sugars, explaining the underlying chemical reason (presence of free aldehyde/ketone group). Emphasise which common sugars fall into each category.
Student Activity: Students take notes, ask clarifying questions, and identify examples for each classification type. They attempt to draw simplified structures if prompted. 3.
4. Practical Demonstration: Preparation of Glucose from Starch/Sucrose (20-30 minutes)
Teacher Activity: Conduct a live demonstration or guide students through a small-group practical for the hydrolysis of starch or sucrose (as outlined in section 2.4).
Safety Precaution: Emphasise careful handling of hot materials and dilute acids. Use safety goggles.
Focus: Explain each step clearly, highlighting the role of acid and heat. For starch hydrolysis, perform periodic iodine tests to show the disappearance of starch. For sucrose, perform Fehling's test before and after hydrolysis to show the formation of reducing sugars.
Student Activity: Students observe the demonstration, record observations, and answer questions regarding the purpose of each step and the expected outcomes. If conducting a practical, students follow instructions and record results. 3.
5. Practical Demonstration: Testing for Starch and Glucose (Reducing Sugars) (15-20 minutes)
Teacher Activity: Iodine Test for Starch: Demonstrate the iodine test using various samples (e.g., starch solution, garri flour, sugar solution, water as control). Explain the colour change and its significance. Fehling's Test for Glucose/Reducing Sugars: Demonstrate Fehling's test using glucose solution, sucrose solution, and the hydrolysed sucrose/starch solution from the previous practical, alongside a water control. Emphasise the need for heating and the distinct colour changes. Explain the chemical principle.
Safety Precaution: Remind students about handling hot test tubes and the chemical reagents.
Student Activity: Students observe the tests, record their observations, and interpret the results. They should be able to identify which samples contain starch or reducing sugars. 3.
6. Uses of Starch and Sugars (10-15 minutes)
Teacher Activity: Lead a discussion on the diverse uses of starch and sugars, linking them to food, industries, and everyday life in Nigeria. Prompt students to think beyond food consumption.
Student Activity: Students list and discuss various uses, relating them to local industries (e.g., breweries, local food processing) and personal experiences. 3.
7. Conclusion and Recap (5 minutes)
Teacher Activity: Summarise the key learning points, reiterating the classifications, preparation methods, tests, and uses of sugars and starch. Address any lingering questions.
Student Activity: Students ask final questions and review their notes.
Food Security and Nutrition in Nigeria: Sugars (especially glucose) and starch are primary sources of energy in the Nigerian diet. Staple foods like garri, fufu, rice, yam, and maize are rich in starch. Understanding their digestion (hydrolysis into glucose) is key to appreciating their nutritional value. This knowledge helps in making informed dietary choices for health and combating malnutrition. For example, knowing that garri is largely starch explains why it provides sustained energy.
Local Industries and Entrepreneurship: Brewing Industry: Fermentation of sugars is fundamental to the production of alcoholic beverages like beer and locally, palm wine. The conversion of starches (e.g., from maize or sorghum) into fermentable sugars (maltose, glucose) is a critical step. This knowledge can inspire students to understand the chemistry behind local industries and potentially venture into agro-processing.
Textile and Paper Industries: Starch, often derived from local crops, is used as a sizing agent in textile manufacturing to stiffen fabrics and as a binder and filler in paper production. This demonstrates the broader industrial utility of these compounds beyond food.
Pharmaceuticals: Sugars are used as sweetening agents in many oral medications, especially syrups for children, and as energy sources in intravenous infusions. Starch is a common excipient (binder, disintegrant) in tablet formulations. This links chemistry to healthcare and local drug manufacturing.
Food Processing and Preservation: The hydrolysis of starch in cassava processing (e.g., fermentation for garri) changes its properties. Also, high sugar concentration is used in local food preservation methods like making jams or candied fruits, inhibiting microbial growth. This connects chemical principles to traditional and modern food processing techniques commonly employed in Nigerian households and small businesses.