Biological Molecules

Food Tests

Benedicts Test-(Reducing Sugars) Biuret Test-(Protein) Iodine test-(Starch) Emulsion test(Lipids) Non-reducing sugars test

Benedict’s Test

Add 2cm3 of the sugar solution to a test-tube, and then an equal volume of Benedict’s solution. Bring to the boil in a water-bath.

Result: Turns green, then yellowish, then may form brick-red precipitate.

Basis: The Cu++ ions in the copper sulphate in Benedict’s are reduced by the sugar to insoluble Cu+ which precipitates out

Biuret Test

Add equal quantities of test solution and potassium hydroxide and mix. Add two drops of copper sulphate, and mix

Result: Mauve or purple colour slowly develops

Basis: Nitrogen atoms in the peptide chain form a purple complex with the Cu++ ions

Iodine Test

Add a few drops of potassium iodide solution to the test solution and mix

A blue/black colouration develops

A polyiodide complex forms with the starch molecules

Emulsion test

Add equal quantities of the suspected lipid and absolute ethanol in a test tube. Shake vigorously to dissolve. Add an equal quantity of cold water.

Result: A cloudy white suspension

Lipids are insoluble in water, so adding water to a solution of the lipid in alcohol results in an emulsion of tiny lipid droplets in the water which look white as they reflect light.

Non Reducing Sugars

Add 2cm3 sucrose solution to a test-tube. Add 1cm3 dilute sulphuric acid. Boil for one minute, and then carefully neutralize with sodium bicarbonate. (take care, as it will effervesce). Carry out Benedict’s test

The acid hydrolyses the sucrose (breaks the glycosidic bond) making two monomer molecules (glucose and fructose), which are reducing sugars

Monomer – building blocks of biological molecules

Polymer - a chemical compound consisting of repeating structural units (monomers)

Macromolecules – another name for a polymer

Carbohydrates

Made of C, H and O

Three types:

1. Monosaccharide

2. Disaccharide

3. Polysaccharide

Monosaccharides

Simplest carbohydratesGlucose, fructose, galactose

Glucose

Two types:

Glycosidic Bonds

Condensation

Hydrolysis

Polysaccharides

Starch

Storage polysaccharide in plants Made of two substances:

1. Amylose

2. Amylopectin

Amylose

α glucose molecules with 1-4 links

Coils into a spiral Held together with

hydrogen bonds

Amylopectin

α glucose with 1-4 links and 1-6 links

Causes branching chains

Glycogen

Storage in animals and fungi

α glucose molecules with 1-4 links and 1-6 links

Forms a branching chain

Held together with hydrogen bonds

Cellulose β glucose with 1-4 links Adjacent molecules in the chain are flipped 180

degrees Hydrogen bonds form between different chains

forming a bundle of microfibrils

Proteins

Primary Structure

Monomer – amino acid

Amino acids joined by peptide bonds Condensation reaction

Secondary Structure

Two possible structures: α helix β pleated sheet

Held together by hydrogen bonds

Tertiary Structure

Secondary structure folded and bonds form between the R chains

Different types of bonds: Hydrogen bonds Ionic bonds Disulphide bridges Hydrophobic interactions

Tertiary Structure Bonds

Quaternary structure

More than one tertiary structure joined together.

Haemoglobin

Globular protein 4 tertiary structures joined together Two alpha chains (141 aa), and two beta

chains (146 aa) Each chain has a haem (containing an iron

atom) group attached This is used to bond the oxygen

Some amino acids have hydrophobic side chains (repelling water), and some have hydrophilic side chains (attracting water)

The “R” groups on amino acids are sometimes referred to as side chains.

Haemoglobin

Haemoglobin

Haem

On the four polypeptide chains that make up the haemoglobin, amino acids with hydrophobic side chains point inwards, helping to hold the molecule together

Amino acids with hydrophilic side chains point outwards, making the haemoglobin molecule soluble.

Haemoglobin

Collagen

Fibrous protein Three strands plaited together Very strong

Lipids

Made of two parts: Glycerol Three fatty acid chains

High energy due to many hydrogen atoms

Saturated and unsaturated

Saturated fats have no carbon to carbon double bonds, they are solid at room temperature

Unsaturated fats have one or more carbon to carbon double bonds. These form kinks in the fatty acid chains and so they are liquids at room temperature.

Phospholipids

Polar region is the phosphate group and it allow it to be soluble in water – hydrophillic

The non-polar fatty acid chains are insoluble in water - hydrophobic

Structure and function

Their insulating properties keep mammals warm.

They contain twice the stored energy of carbohydrates, gram for gram.

They are used in the formation of cell-surface membranes.

In aquatic mammals, the fat is less dense than water, so it acts as a buoyancy aid.

Water

Properties of water

High specific heat capacity High heat of vaporisation

both of which are a result of the extensive hydrogen bonding between its molecules.

These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature.

at approximately 4 °C pure water reaches its maximum density  Protects aquatic environments

Universal solvent Water molecules stay close to each other

(cohesion), due to the collective action of hydrogen bonds between water molecules.  Leading to high surface tension.

Water also has high adhesion properties because of its polar nature. 

Essay

a) Describe the characteristics of water.

b) How do these characteristics enable living organisms to survive.