Carbohydrates

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Welcome to class! 

In today’s class, we will be talking about carbohydrates. Enjoy the class!

Carbohydrates

Carbohydrates are naturally occurring organic compounds containing carbon, hydrogen and oxygen with oxygen in the ratio 1:2 as in water.  The general molecular formula of carbohydrates is C_x(H₂O)_y  or  C_xH_2yO_y.  Carbohydrates are generally polydroxyl aldehydes, ketones or any compound that on hydrolysis yields any of the polyhydroxides.  They are synthesized in green plants by photosynthesis.

Classifications of carbohydrate

Evaluation

1. Define carbohydrates.
2. State one example each of simple sugar and complex sugar.

Monosaccharides:

Monosaccharides are simple sugar with three to six carbon atoms per molecule.  The most common and important of these are the ones with six carbon atoms per molecule called the HEXOSES. They have the same molecular formula C₆H₁₂O₆ but a different spatial arrangement.  Examples include glucose, fructose, galactose and mannose.

• Glucose (C₆H₁₂O₆)

Glucose, commonly known as grape sugar or dextrose, is present in grapes, in honey and the sap of plants.

Glucose is synthesized by green plants during photosynthesis.

Preparation

Glucose can be prepared in the laboratory by the acid hydrolysis of starch.

The starch is heated with dilute hydrochloric acid or tetraoxosulphate VI acids.  The acid functioning as a hydrolyzing agent in the reaction.

(C₆H₁₀O₆)n      nH₂O             →         nC₆H₁₂O₆

Starch        Dil H­₂SO₄                      glucose

Physical properties

1. It is a white crystalline solid.
2. It is soluble in water.
3. It has a sweet taste.

Chemical properties

1. It dehydrates to a black residue of carbon when with conc. H₂SO₄

C₆H₁₂O₆             →                6C + 6H₂O

2. It is a strong reducing agent. This is due to the presence of the –CHO group in the molecule.
3. It is readily fermented to ethanol and carbon VI oxides by the enzyme (ZYMASE) in yeast.

Test:

Add a few drops of Fehling’s solution to 5cm³ of glucose solution in a test tube.  A brick-red precipitate is obtained on boiling.

Glucose structural formula

the functional groups in the molecule are:

-CHO and OH

But -CHO determines the chemical properties.

• Fructose (C₆H₁₂O₆)

An isomer of glucose is fructose.   Fructose is a non-reducing sugar (it contains the >CO group)

Fructose structural formula

The functional groups in the molecule are:   -C=O and –OH

Evaluation

1. State two (2) physical properties of starch.
2. Give the difference between glucose and starch.

Disaccharides:

Disaccharides are simple sugar containing two molecules of monosaccharide per molecules.  Their general molecular formula is C₁₂H₂₂O₁₁.  Some important disaccharides and the component monosaccharide are;

Disaccharide               Component monosaccharides

Sucrose (cane sugar)                   Glucose + fructose

Lactose (milk sugar)                      Glucose + galactose

Maltose (malt sugar)                     Glucose + glucose

• Sucrose:

This is the common granulated sugar used at home to sweeten food.  It occurs naturally in plant and fruits e.g. pineapple, carrots sorghum, sap and sugar cane.

Preparation

Sucrose is prepared industrially by;

1. Extraction of juice from sugar cane,
2. The concentration of the juice by evaporation,
3. Crystallization of the solid sugar from the concentrated juice and
4. Filtration of sugar crystals from the molasses. 

Physical properties

1. It is a white crystalline solid.
2. It dissolves readily in water.
3. It has a sweet taste.

Chemical properties

1. It chars by dehydration to a black residue of carbon on the addition of Conc. H₂SO₄.

C₁₂H₂₂O₁₁          →                12C  +  6H₂O

2. It undergoes hydrolysis to an equal quantity of glucose and fructose when boiled with dil. H₂SO₄ or HCL (aq)

3. It undergoes inversion under the influence of invertase in yeast

Sucrose              →               invertase       Glucose + Fructose

4. It is a non-reducing sugar.

Test:

Glucose gives a red precipitate with freshly prepared Seliwanoff’s reagent.

Uses

1. For sweetening foods and beverages.
2. For ethanol production by fermentation.

Polysaccharides:

Polysaccharides are a group of complex carbohydrates composed of very long chains of monosaccharide linked together by condensation polymerization.

Monosaccharide   ^condensation     →          polysaccharides  +   n/₂  water

^Hydrolysis

Some important examples of polysaccharides are starch, glycogen, and cellulose.  The general molecular formula of polysaccharides is (C₆H₁₀O₅)n

Evaluation

1. Name three disaccharides and the component monosaccharide.
2. Describe what would be observed on addition of Conc. H₂SO₄ to sucrose.

Protein

Proteins are made up of polypeptide chains.  Polypeptide chains are products of condensation polymerization of amino acid molecules.  The condensation polymerization of amino acid is the interaction between amino groups, -NH₂, and Carboxyl groups, -COOH, to form the polypeptide chain.

A polypeptide chain:

Occurrence

Proteins are found in living cells where they perform different functions.  Examples are haemoglobin,  collagen and insulin, ribonucleic.  The structure has cross-linkage between chains which are called disulphide bridges or disulphide cross-linkages.

Properties

1. They are denatured easily at a temperature above 40⁰C by variation in PH and by certain organic solvents and chemical reagents.
2. Proteins are hydrolyzed to give amino acids by boiling with dilute acids (e.g. HCI) and alkalis or using enzymes.

Uses

1. They are used for building up of tissues and repairing of worn-out tissues in the body.
2. They are necessary for growth, especially in infants.

Tests

1. Million’s test: A white ppt. is form when a few drops of Million’s reagent are added to some egg-white in a test-tube. The white ppt. turns brick red on heating (indicating the presence of proteins).
2. Biuret test: when few drops of dilute NaOH solution is added to some egg white solution in a test tube and 1% CUSO₄ solution is added drop by drop and the test solution is shaken vigorously after each drop.  Violet colour is seen indicating a positive test for proteins.
3. Proteins form an intense yellow colour with a few drops of conc. HNO₃ acid.

Evaluation

1. Explain a chemical test for proteins.
2. What is the product of hydrolysis of proteins?

Amino-acids

Amino acids are the basic structural units of proteins.

The functional groups in amino acids are an amino group (-NH₂) and a carboxyl group (-COOH)

General structure

Evaluation

1. Give the functional group of protein.
2. Draw the structural formula of a polypeptide.

α carbons are carbons in amino acids to which the functional groups are attached.

Examples – glycine, phenylalanine, alanine etc.  Amino acids are mainly in form of dipolar ions in neutral solution (zwitterions).

General evaluation

1. Name the functional groups present in amino acids molecule.
2. What are proteins?
3. State two properties of protein.

Weekend assignment

1. One of the following is not a monosaccharide A. glucose   B.  maltose    C.  fructose    D. maltose
2. The product obtained when sucrose is treated with dil. HCI are A. glucose and galatose B.  fructose and galatose    C.  glucose and fructose  D. glucose
3. Which of the following is a reducing sugar? A. fructose   B.  glucose    C.  sucrose           D. ribose
4. Which one of these substances is not a polysaccharide? A. lactose    B.  starch       C.  cellulose         D. mannose
5. Few drops of conc HNO₃ is added to a solution and an intense yellow colour is obtained, then the solution contained. A. proteins   B.  carbohydrates  C.  fat     D. oil

Theory

1. Explain a laboratory test for proteins.

2a.      What are carbohydrates?

b. Name the component monosaccaharide of

(i)        sucrose                   (ii)     maltose.

 

In our next class, we will be talking about Amines and Amides.  We hope you enjoyed the class.

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