Biological Molecules

Monosaccharides are soluble substances that have a general formula (CH2O)n where n can be any number between three and seven. Examples of monosaccharides are 

• Glucose (alpha or beta)
• Galactose 
• Fructose

Test for Reducing Sugars: 

All monosaccharides are reducing sugars. A reducing sugar is a sugar that can donate electrons to another chemical. In the test, the chemical which receives the electrons is Benedict's reagent. The test is as follows:

• Add 2cm^3 of the food sample to a test tube
• Add an equal volume of Benedict's reagent 
• Heat the mixture in a gently boiling water bath. 
• A positive result means the blue reagent turns a red colour 

When two monosaccharides join together, it forms a disaccharide. In this reaction, a glycosidic bond is formed and a molecule of water is removed. This reaction is called a condensation reaction.

When water is added to a disaccharide under suitable conditions, the glycosidic bond is broken and the two constituent monosaccharides are released. This is called a hydrolysis reaction.

Test for Non-Reducing Sugars:

• First, you have to carry out the Benedict's test to prove that there are no reducing sugars (the solution will not change colour)
• Then , add another 2cm^3 of the food sample to 2cm^3 of dilute hydrochloric acid in a test tube and place in a gently boiling water bath. The HCl will hydrolyse any disaccharide
• Slowly add sodium hydrogencarbonate solution to neutralise the hydrochloric acid. 
• Re-test the resulting solution with the Benedict's test
• If a non-reducing sugar was present in the original solution, the reagent will turn orange-brown.

Polysaccharides are polymers formed by combining many monosachharide molecules. This is formed by condensation reactions, forming glycosidic bonds. 

As polysaccharides are very large molecules, they are insoluble. This feature makes them suitable for storage. Some polysaccharides, such as cellulose are not used for storage but give structural support to plant cells.

Test for Starch:

• Place 2cm^3 of the sample into a test tube
• Add two drops of iodine solution and shake
• The presence of starch is indicated by a blue-black colour

Starch is a polysaccharide found in many parts of a plant in the form of small grains. It is made up of chains of alpha-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions. The chains may be branched or unbranched (wound into a tight coil that makes it very compact). 

Starch has a main role of energy storage, its structure is especially suited for this because:

• insoluble - doesn't affect water potential, so water isn't drawn in by osmosis
• large/insoluble - doesn't diffuse out of cells
• compact - can be stored in a small space
• forms alpha-glucose when hydrolysed - easily transported and used in respiration
• branched form has many ends - glucose monomers released rapidly when enzymes act upon it

Starch is NEVER found in animal cells

Glycogen is very similar in structure to starch but has shorter chains and found mainly in the muscles and liver. It is found in animals and bacteria but NEVER in plant cells.

Its structure is suitable for storage because:

• insoluble - doesn't affect water potential and doesn't diffuse out of cells
• compact - can be stored in a small space
• more highly branched than starch - so more rapidly broken down by enzymes to form glucose monomers for respiration. This is important as animals have a higher metabolic/respiratory rate than plants because they are more active

Cellulose is made out of beta-glucose monomers which produces differences in the structure and function. It has straight, unbranched chains running parallel to each other.

This allows hydrogen bonds to form cross-linkages across adjacent chains and because there are so many of the bonds, it considerably strengthens the structure of cellulose.

Cellulose molecules are grouped together to form microfibrils --> which form fibres.

All of these features are useful in the structure of cellulose and also is suited to its function as a structural material. the features provide support and rigidity 

Roles of lipids:

• cell membranes - contribute to the felxibility of the membrane
• source of energy - when oxidised, lipids provide twice the energy as carbohydrates ad release valuable water
• waterproofing - insoluble in water so useful for waterproofing such as for a waxy cuticle on plants
• insulation - fats are slow conductora so they help reatin body heat beneath the body surface
• protection - fat is often stored around delicate organs e.g. the liver

Test for lipids:

• In a test tube, add 2cm^3 of the sample, then add 5cm^3 of ethanol
• shake the tube to dissolve any lipid
• add 5cm^3 of water and shake gently
• a cloudy-white colour indicates the presence of a lipid

Contain three fatty acids attached to a glycerol molecule. Each fatty acid forms an ester bond with glycerol in a condensation reaction. Therefore hydrolysis splits up a triglyceride molecule. 

The variation of a triglyceride comes from the fatty acids as the glycerol is the same in all molecules. If there are no carbon-carbon double bonds in the fatty acid chain, then it is descrived as saturated --> each carbon atom is linked to the maximum possible of hydrogen atoms.              A single carbon-carbon double bond is described as mono-unsaturated, more than one is described as polyunsaturated

Structure related to their properties:

• high ratio of energy-storing C-H bonds - excellent source of energy
• low mass to enery ratio - good storage molecules, especially in animals so they don't have to carry extra mass
• large, non-polar - so insoluble in water so doesn't affect water potential
• high ratio of H to O atoms - important source of water when oxidised

Similar to lipids except one of the fatty acid molecules is replaced by a phosphate molecule. It made up of two parts:

• a hydrophilic head - interacts with water as it is attracted to it
• a hydrophobic tail - orientates itself away from water

Structure related to their properties:

• polar molecules - form a bilayer within the cell-surface membrane 
• phosphate head - help to hold at the surface of the CSM
• form glycolipids - important in cell recognition

Amino acids are the monomers that make up a polymer called a polypeptide. Each amino acid is made up of different chemical groups:

amino group (NH2), carboxyl group (COOH), hydrogen atom, R group - each amino acid has a different R group, made out of different chemical groups

Amino acid monomers combine to form a dipeptide through a condensation reaction, forming a peptide bond. These bonds are formed throughout the protein structure:

• primary structure - the order of amino acids in a polypeptide chain
• secondary structure - hydrogen bonds form between the polypeptide chain, which form alpha-helices and beta-pleated sheets
• tertiary structure - additional folding of the chain which lead to more bonds such as disulfide bridges, ionic bonds and extra hydrogen bonds
• quaternary structure - the final 3-D structure of the protein with several polypeptide chains linked together 

Test for proteins --> biuret test --> sample + equal volume of NaOH solution --> add few drops of copper (II) sulfate solution --> purple colour formed indicates proteins are present

Enzymes act as biological catalysts as they lower the activation energy --> the minimum amount of energy required for the reaction to occur

They are globular proteins and have a specific 3-D shape. Within this shape is a specific region where it is functional, this is known as the active site. It is made up of a relatively small number of amino acids and this is where the enzyme attaches to the substrate molecule to form an enzyme-substrate complex.

Induced fit model --> the process of enzyme action:

• a specific enzyme interacts with the specific substrate molecule
• the enzyme's active site changes shape slightly as the substrate molecule is then able to bind with its active site
• as they bind the enzyme puts strain on particular bond(s) in the substrate which consequently lowers the activation energy needed to break the bond

You can measure enzyme-catalysed reactions through the formation of products or the disapperance of the reactants.

Temperature:

• increased temperature --> increased kinetic energy of molecules --> enzyme and substrate collide more frequently --> faster rate of reaction
• temperature too high --> causes the tertiary structure of enzyme to change shape --> no longer can bind to the substrate --> the reaction stops and the enzyme becomes denatured

pH: --> measure of its hydrogen ion concentration

• change in pH --> alters charges on amino acids making up the active site --> (can break bonds in the tertiary structure of the active site) --> enzyme-complexes can't be formed

Enzyme/substrate concentration:

• rate increases as concentration increases until there is enough active sites for the substate molecules, or if all the active sites are being used up with an excess of substrate

Competitive inhibitors --> bind to the active site of the enzyme:

• have a molecular shape similar to that of the substrate - similar tertiary structure
• they occupy the active site of the enzyme
• enzyme can no longer bind to the substrate so no enzyme-substrate complex is formed
• if substrate concentration increases, the effect of the enzyme is reduced because the inhibitor is NOT permanently bound to the active site

Non-competitive inibitors --> bind to the enzyme at a position other than the active site:

• attach on a different place on the enzyme - allosteric site
• they alter the tertiary structure of the enzyme and therefore the shape of the active site
• the substrate can no longer bind to the enzyme so no product will be made
• increasing the substrate concentration has no effect on the rate of reaction as the enzymes are no longer able to catalyse the reaction as the enzymes can't function