The primary function of carbohydrates is for short-term energy storage (sugars are for Energy). A secondary function is intermediate-term energy storage (as in starch for plants and glycogen for animals).
Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds of thousands. Both are polymers (hence "polysaccharides"); that is, each is built from repeating units, monomers, much as a chain is built from its links.
Carbohydrates often attach to the external surface of integral proteins. These carbohydrates may hold adjoining cells together or act as sites where viruses or chemical messengers such as hormones can attach
Proteins-
A protein, a polymer consists of many subunits (monomers) known as amino acids The structure of a protein is determined by :
a) the types of amino acids
b) length of amino acids and
c)the sequence of amino acids.
These 3 points will determine the tertiary structure of the protein, as the amino acids will effect both the types of bond that will be formed for example the hydrogen bonds,di-sulphide bridges and ionic bonds, in addition to the further folding of the primary and secondary structure.
The general amino acid is shown above. An amino acid consists of 4 groups, the final group being the “R” group.This “R” group will determine the identity of the amino acid.Hence, every different amino acid will have a different molecule as the “R” group.
Proteins are macromolecules. They are constructed from one or more unbranched chains of amino acids; that is, they are polymers. A typical protein contains 200–300 amino acids but some are much smaller (the smallest are often called peptides) and some much larger (the largest to date is titin a protein found in skeletal and cardiac muscle; it contains 26,926 amino acids in a single chain).
Every function in the living cell depends on proteins.
- Motion and locomotion of cells and organisms depends on contractile proteins. [Examples: Muscles]
- The catalysis of all biochemical reactions is done by enzymes, which contain protein.
- The structure of cells, and the extracellular matrix in which they are embedded, is largely made of protein. [Examples: Collagens] (Plants and many microbes depend more on carbohydrates, e.g., cellulose, for support, but these are synthesized by enzymes.)
- The transport of materials in body fluids depends of proteins.
- The receptors for hormones and other signalling molecules are proteins.
- Proteins are an essential nutrient for heterotrophs.
- The transcription factors that turn genes on and off to guide the differentiation of the cell and its later responsiveness to signals reaching it are proteins.
- allow for communication with individual cells
- transport numerous substances through cells and the blood.
- and many more — proteins are truly the physical basis of life.
Lipids-
Lipids are fats. Their primary purpose in the body is energy storage- a very small mass of lipids can store a very large amount of energy,
Describe using examples the laboratory techniques which enable the above biological molecules (a-e) to be identified and characterised
Carbohydrate tests
If a chemist is given an unknown sample, he can use a number of chemicals tests to determine if the sample contains a carbohydrate. If it does contain a carbohydrate, further tests can be performed to classify and possibly to identify it. Some of the available chemical tests are listed below.
- The Molisch Test
- The Iodine/Potassium Iodide Test
- Benedict's Test
- Barfoed's Test
- Seliwanoff's Test
- Bial's Test
The molishch test shows positive test for all carbohydrates, the test reagent dehydrates pentoses to form furfural (top reaction) and dehydrates hexoses to form 5-hydroxymethyl furfural (bottom reaction). The furfurals further react with -naphthol present in the test reagent to produce a purple product (reaction not shown).
How the test is performed
Two ml of a sample solution is placed in a test tube. Two drops of the Molisch reagent (a solution of -napthol in 95% ethanol) is added. The solution is then poured slowly into a tube containing two ml of concentrated sulphuric acid so that two layers form. A positive test is indicated by the formation of a purple product at the interface of the two layers.
Below shows a negative test (left) and a positive test (right)
Lipids
The body stores lipids as reserve energy. Lipids are hydrophobic (“water-hating”) and thus much harder to break down for energy than carbohydrates. Lipids, however, contain more energy per unit weight then carbohydrates. Therefore it is more efficient for the body to use lipids as stored energy. The body will use its carbohydrate source for initial fuel, but if the “fast fuel” runs out, the body will turn to breaking down lipids for a rich energy source. Lipids are fat molecules and there are many different kinds.
Below are two tests to identify the main biologically important chemical compounds. For both Protein an lipid test a small amount of the substance is taken to test, a test tube is filled up with water, then the substance is placed in it and given a good shake. The substances are shaken up inside the test-tube to break up the cells and release the cell contents.
Lipids (emulsion test). Lipids do not dissolve in water, but do dissolve in ethanol. This characteristic is used in the emulsion test. So therefore this sample is not dissolved in water, but instead some of the test sample is shaken with about 4 cm³ of ethanol. The sample of liquid is poured into a test tube of water, leaving any undissolved substances behind. If there are lipids dissolved in the ethanol, they will precipitate in the water, forming a cloudy white emulsion.
Below shows a picture of the 2 boiling tubes used in this version of the emulsion test. Only a small amount of test substance is needed (1 drop of cooking oil, in this instance), and after shaking this with a few ml of ethanol (purity not necessary - so industrial methylated spirits (IMS) can be used it is left to settle.
In the picture below, note the clarity of both the ethanoic solution and the water which almost fills the second tube.
In the picture below, note the white emulsion (opaque suspension of oil in water) which forms when the contents of the other tube are gently poured into the tube of water.
Protein tests
Proteins are the most complex and functionally diverse molecules of living organisms. Proteins compose enzymes; blood cells and muscle tissue just to name a few and are therefore associated with meat products. Proteins are created by RNA during DNA Transcription and Translation, a process you will learn about in a later lab. The base elements of proteins are C, H, O and N. The monomers of proteins are 20 different amino acids. The amino acids are bonded together in unique combinations to create a polypeptide chain, the protein polymer. This chain is then folded into a unique, functional protein.
Protein (biruet test). To about 2 cm³ of test solution, an equal amount of volume of biuret solution is added down the side of the test tube. A blue ring forms at the surface of the solution, which disappears on shaking, and the solution turns lilac-purple, indicating protein. The colour is due to a complex between nitrogen atoms in the peptide chain and Cu2+ ions, so this is really a test for peptide bonds.
