Biological Molecules.

  • Created by: Fflur Haf
  • Created on: 10-05-19 16:57


Elements - 

All the different kinds of matter which make up the universe, whether the matter is living or dead, comprise fundamental materials called elements. An element cannot be split up chemically into anything simpler. Gold, silver, carbon(solids), oxygen, nitrogen(gases) are elements, and over 100 such elements have been identified. It's useful in chemical formulae to represent the elements by symbols. All of the known elements are listed, with their symbols, in the periodic table.  Four elements, and their symbols, that you will meet frequently in Biology are:

Carbon - C                                                                                                                                           Hydrogen - H                                                                                                                                      Oxygen - O                                                                                                                                          Nitrogen - N 

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An atom is the smallest particle of an element that can take part in a chemical reaction. Each element consists of atoms, different from the atoms of any other element. For example, the element, the element oxygen is made up of lots of oxygen atoms. 

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A compound is a substance containing two or more elements, combined in such a way as to change their properties. An example of a compound would be salt, sodium chloride (NaCl). This is made up of the element sodium (Na) and the element chlorine (Cl). The properties of sodium chloride are quite different from those of the element sodium and the element chlorine.

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A molecule is the smallest part of an element or compound that can exist alone. We can use the symbols for the elements to represent individual atoms in molecules and, in so doing, construct formulae. A molecule of oxygen consists of two oxygen atoms, and its formula is O2. The subscript 2 denotes that two atoms of oxygen are involved. Atoms of different elements can combine to form molecules of compounds. A molecule of carbon dioxide (CO2) contains one atom of carbon and two of oxygen. Carbon monoxide has one atom of oxygen in its molecule. Its formula is, therefore, CO. C6H12O6 is the chemical formula representing one molecule of glucose, containing six atoms of carbon, twelve atoms of hydrogen and six atoms of oxygen. You will need to remember the above formulae.

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Water is vital for life. It forms two thirds of the human body weight and 90% of blood plasma. Chemical reactions within the cell (metabolism) occur within water, and it is also involved in the chemical changes which occur within the gut and within the cell. Most of the body’s secretions (mucus, saliva, semen) contain water. Water moves in and out of the cell by a process called osmosis. This is a special kind of diffusion, so first we will look at exactly what diffusion is.

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Chemical Nature of the Cell.

Unlike plants, humans must have a supply of organic food to provide the raw materials for growth and repair as well as supplying a source of stored energy.

The cell is built up from the nutrients it absorbs. There are three main types of nutrient involved in the structure and functioning of the cell. These nutrients are absorbed into the body from the food we eat:

  • Carbohydrates

  • Proteins

  • Fats

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Carbohydrates are the main source of energy in the cell. They contain carbon, hydrogen, and oxygen and are made up of sugars, one of the simplest being glucose. The sugars may be in single units, known as monosaccharides, for instance fructose (fruit sugar), or they may be in double units or disaccharides, where two monosaccharide sugars are joined together – examples include sucrose (cane sugar) and lactose (milk sugar).

Monosaccharide and disaccharide sugars are sweet and dissolve quite easily in water. The monosaccharides may also join together to form long chains, or polymers. These are known as polysaccharides; they are not sweet and do not dissolve easily in water. Most polysaccharides are simply made up of long chains of glucose molecules. Polysaccharides are also an important form of energy storage in plants (as starch) and in animals (as glycogen). Another important polysaccharide is cellulose.

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Test for Carbohydrates.

In each case pay close attention to your observations and make a note of them. The usual size test tube is 12cm long by 1cm wide. 10-15 drops of a liquid are approximately equal to 1cm3 (cubic centimetre) in volume.

Always carry out control tests. A control test for starch is carried out by testing a starch/water mixture with iodine solution, so that a colour is produced which can be used as a comparison for testing various foods.

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Test for Carbohydrates 2.

Test for Starch

You will need

  • White tiles or test tubes (or any other white surface)

  • An aqueous solution of iodine in potassium iodide

  • Starch solution

  • A selection of bread, meat, potato, cheese, onion, fish, bread rolls or slimming biscuits.

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Method - Step 1 + 2 (Carbs).

1) First, test pure starch, or foods containing starch (such as potatoes or flour) by adding a little water and mixing. Boil the mixture, leave it to cool, then add a few drops of the iodine solution.

The brown iodine solution stains the starch grains blue-black, according to its strength. This first test will give you a colour for comparison when testing various other foods (i.e. it is a control test).

2) Now you can test any other foods you wish, to see if they contain starch – try small pieces of bread, meat, potato, fish, cheese and onion; also try starch-reduced bread rolls and slimming biscuits.

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Method - Step 3 (Carbs).

It is possible to time the digestion of starch by your own saliva!

  • Put a small amount of crushed biscuit into a container.

  • Add about 10 ml of saliva and mix well.

  • Put a small sample of this mixture onto a white tile and add a few drops of the iodine solution.

  • Note any colour change.

  • Repeat the test, taking small samples at one-minute intervals, noting the colour changes each time.

  • Continue until no further colour change occurs.

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Results and Conclusion - (Carbs).

You will get a positive starch test at first, but the blue-black colour will be fainter on subsequent tests, until eventually the starch test is negative (i.e. it has been digested).

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Reducing Sugar - (Carbs).

Benedict's Solution for Reducing Sugar.

You will need -                                                                                                                                     Benedict's solution.                                                                                                                              Crushed food from the previous experiment.                 

Method - Mix a small amount of the crushed food with water and add the same volume or excess Benedict's solution to produce a faint blue colour.                                                                             Boil for about a minute in a water bath until a colour change is observed.                

Results - A yellow, orange or red colour indicates the presence of reducing sugar, such as glucose, maltose, or lactose. Ordinary sugar (sucrose) does not give the colouration as it is a nonreducing sugar.

Repeat with different types of food.

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Fats (lipids).

Like carbohydrates, fats contain carbon, hydrogen, and oxygen, but in different proportions. Fats also have different properties. Each fat molecule provides. Each fat molecule provides twice as much energy as a molecule of carbohydrate. The biological name for fats is lipids. Those lipids which are liquid at ordinary temperature are known as oils, those which are solid are known as fats. Fats can either be of either plant or animal origin, for instance, beef fat, fish oil, butter, coconut, linseed oil. Fats can be recognised by the spreading grease-mark they leave on paper or cloth.

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Storage of Fats.

Fats are an important storage compound in the human body, where they are stored in specialised adipose tissues. These not only act as energy reserves, but also insulate the body and cushion body organs such as kidneys, providing protection against injury. In natural, three molecules of fatty acids combine with one molecule of glycerol. The fatty acids which often make up many vegetables oils are often called unsaturated. This means that they have a double bond in their molecule. 

If there is more than a double bond affects the properties of the fats. There is some evidence that polyunsaturated fatty acids are less likely to contribute to heart and arterial disease than are saturated fatty acids, although this view is not held by all nutritionists or doctors.

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Test for Lipids.

You will need:

Test tubes.                                                                                                                                          Test tube rack.                                                                                                                                     2cm^3 pipettes.                                                                                                                                   Ethanol.                                                                                                                                              Distilled water.                                                                                                                   

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Method + Results (Fats).

  • Put a small sample of butter into a test tube.    
  • Add 2cm^3 of ethanol with a pipette.
  • Shake the test tube well and allow to settle in a test tube rack for two minutes.
  • Add 2cm^3 of distilled water with a pipette.
  • Observe the contents of the test tube.                    

Results - 

You will get a milky-white emulsion forming since butter contains a lot of fat.                                  Repeat with other foods to see if they contain fat. 

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Proteins are a particular importance as they are formed into new protoplasm - they are often referred to as body-building foods. When digested they provide the chemical substances needed to build cells and tissues, such as skin, muscle, blood, and bones. Neither carbohydrates nor fats can do this.  

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Amino Acids - Proteins.

Proteins are broken down into amino acids, their component parts, before being absorbed in the body. They contain carbon (C), hydrogen (H), nitrogen (N) and oxygen (O), and sometimes sulphur (S). Some proteins also contain minerals. Proteins may contain any number and mix of amino acids, giving thousands of possible combinations. The structure of a protein is often essential to its function; for instance, keratin is a very strong, fibrous protein which makes up the fingernails and hair. The amino acid chain may also be folded or coiled to form a globular protein - this includes enzymes, an important group of proteins.  

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Structure and Proteins.

When a protein molecule is heated above about 40 degrees Celsius or subjected to big changes in pH (i.e. acidity/alkalinity), there may be irreversible changes in structure. Consider what happens to an egg when you cook it. The egg albumen, which is protein, changes, resulting in solid egg white. When the structure of a protein is irreversibly changed by heat, chemicals or extremes of pH, it is known denaturation. Often the protein will change so much that it can no longer function. 

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Test for Proteins.

You will need:

Test tube.                                                                                                                                             Test tube rack.                                                                                                                                     Pipette.                                                                                                                                            NaOH (Sodium hydroxide).                                                                                                                 1% CuSO4 (Copper sulphate).                                                                                                            Distilled water.                                                                                                                                 Selection of foods such as bread, butter, milk, cheese, fruit and vegetables.

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Method + Results (Proteins).


  • Make a small solution of a bread item with the distilled water, in a test tube.
  • Add an equal amount of NaOH to this solution and mix carefully. 
  • Add a few drops of 1% Copper sulphate with a pipette: don't shake the mixture.

Results and Conclusions - 

If the solution turns from blue to violet(purple), proteins are present in the food item. This colour change is dependent upon the number of peptide bonds in the solution, so the more protein present, the more intense the colour will be.

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