4.1 Biological molecules

Table of Contents

Table of Contents

1. List the chemical elements that make up:

(a) carbohydrates

(b) lipids(fats and oils)

2. State that large molecules are made from smaller molecules, limited to:

3. Describe and be able to do chemical tests for:

(a) starch (iodine solution)

(b) glucose and maltose(Benedict’s solution)

(c) protein (biuret test)

(d) lipids(ethanol emulsion test)

1. List the chemical elements that make up:

(a) carbohydrates

carbon, oxygen and hydrogen
Long chains of simple sugars.
Glucose is a simple sugar (a monosaccharide).
When 2 glucose molecules join, maltose is formed (a disaccharide). Sucrose is also a disaccharide
When lots of glucose molecules join together, starch, glycogen or cellulose can form (a polysaccharide)

(b) lipids(fats and oils)

carbon, oxygen [less than carbohydrates]and hydrogen
Lipids are divided into fats(solids at room temperature)and oils(liquids at room temperature)

(c) proteins

all contain carbon, oxygen, hydrogen and nitrogen, and some contain small amounts of other elements such as sulphur and phosphorus
Long chains of amino acids.
There are about 20 different amino acids.
They all contain the same basic structure, but the ’ R’ group differs for each.
When amino acids are joined together, protein is formed.
The amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins

(d) DNA

all contain carbon, oxygen, hydrogen and nitrogen, phosphorus, and sometimes sulphur
Made up of long chains of nucleotides formed into two strands
A nucleotide is made up of a 5-carbon sugar molecule joined to a phosphate group and an organic base

2. State that large molecules are made from smaller molecules, limited to:

starch, cellulose, and glycogen from glucose;
proteins from amino acids;
lipids from fatty acids and glycerol;
DNA from nucleotides

3. Describe and be able to do chemical tests for:

(a) starch (iodine solution)

Add drops of iodine solution to the food sample.
A positive test will show a colour change from orange–brown to blue-black

(b) glucose and maltose(Benedict’s solution)

Add Benedict’s solution to a sample solution in the test tube.
Heat at 60 – 70 ° C in a water bath for 5 minutes.
Take the test tube out of the water bath and observe the colour.
A positive test will show a colour change from blue to orange or brick red

(c) protein (biuret test)

Add drops of biuret solution to the food sample
A positive test will show a colour change from blue to violet/purple

(d) lipids(ethanol emulsion test)

Food samples are mixed with 2 cm^3 of ethanol and shaken
The ethanol is added to an equal volume of cold water.
A positive test will show a cloudy emulsion forming

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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