Proteins

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Proteins

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Proteins are large organic molecules made up of carbon

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hydrogen

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and nitrogen atoms. Some proteins also contain the elements sulfur

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phosphorus and iron. In contrast

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remember that carbohydrates and lipids

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only contain carbon

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hydrogen and oxygen.

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Proteins are made up of long chains of amino acids

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which are the building

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blocks of proteins. All amino acids contain an amino group and a carboxyl

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group. There are twenty amino acids that are used to form the large number of

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proteins that are found in the human body. When two amino acids combine:

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a dipeptide is formed (see Figure 7). When three or more amino acids

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combine

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a polypeptide chain is formed. Proteins consist of a chain of at least

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50 amino acids. Examples of food that contain significant amounts of protein

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are fish

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meat

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(a)

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Importance of proteins

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Figure 8 Examples of foods that are rich in protein: (a) fish

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(b) lentils and (c) almonds

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tissue.

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(b)

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• Proteins are very important in living organisms for growth and repair of

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They form the body structure of organisms.

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All enzymes are proteins.

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lungs.

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(c)

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An example of an essential protein is haemoglobin

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the pigment

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contained in red blood cells that carries oxygen to and from the

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Factors that affect the structure of proteins

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Testing for protein

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Temperature and pH are factors that affect the structure of proteins.

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The pH refers to the degree of acidity or alkalinity of a solution. High

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temperatures and extremely acid or basic solutions can cause permanent

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changes to the bonds that hold the structure of a protein in shape. Once

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the molecular shape of a protein is changed

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the protein is said to be

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denatured. The process of denaturation can be seen when you cook

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an egg (see Figure 9). The heat changes the egg white - which contains

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the protein albumin - from a clear

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gel-like material to a stiff

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substance.

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Biuret reagent

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a blue solution

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Contains sodium hydroxide and copper(l1) sulfate. In the alkaline

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conditions provided by the sodium hydroxide

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the peptide bonds of a

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protein form a complex with the copper(1) ions

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changing the solution

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to a deep purple colour. The darker the solution

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the more protein

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is present.

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(a)

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(b

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chenica

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bond

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Figure 7 Stru

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polypeptide c

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New

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amino acide

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blocks of pr

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denaturatio

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the molecul

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function of

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result of exp

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temperature

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pH condition

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Figure 9 The effec

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egg white:

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(a) before exposure

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(b) after exposure

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Unit 3 Organic cc