B3.
1 – Biological Molecules
1. List the chemical elements that make up:
Carbohydrates
Fats
Proteins
All three biomolecules contain Carbon, Hydrogen and Oxygen.
Proteins also contain Nitrogen. Proteins sometimes contain Sulfur and Phosphorus too.
2. State that large molecules are made from smaller molecules, limited to:
Starch and glycogen from glucose
Proteins from amino acids
Fats and oils from fatty acids and glycerol
Polymers are large molecules
made from small, similar molecules (often referred to as subunits).
In the case of carbohydrates, starch and glycogen are large polymer molecules made of
glucose.
Note: the simplest sugar is a glucose molecule (C6H12O6).
In the case of proteins, amino acids are the monomers.
Different amino acids bond together, to form chains known as peptides. The bonds between
these amino acids are known as peptide bonds. These peptides come together to form proteins
such as enzymes.
The most common type of fats and
oils are triglycerides. A triglyceride is made up of three fatty acid molecules bonded to a
glycerol molecule. The bond between a fatty acid and a glycerol molecule is known as an
ester bond. Because the subunits in fats and oils aren’t all similar, they aren’t known as
polymers – they are not made of similar subunits (fatty acids and glycerol are rather different,
actually).
Fats are triglycerides that are solid at rtp (room temperature and pressure). Remember, rtp is a
temperature of 20oC and a pressure of 1 atm.
Oils are liquid at rtp.
3. Describe the use of:
Iodine solution to test for starch
This test is super easy! All that needs to be done is adding a few drops of iodine to the test
solution/ test material. If it contains starch, the solution will turn blue-black, if not, it’ll
remain an orangey/ brown colour (the colour of iodine).
Benedict’s solution to test for reducing sugars
To a known volume of test solution, you add the equal volume of Benedict’s reagent/
solution. Give it a stir and look for any colour changes. If none, try heating it in a warm water
bath (about 80oC), and look for any colour changes. If there are no changes, there are no
reducing sugars present in the solution.
Benedict’s reagent is blue in colour. If there are any sugars present, it’ll change from blue to
green, to yellow, to orange, to red (the fire colours). Green means that there are only traces of
reducing sugars and red means that the solution has a high concentration of reducing sugars.
Note: sucrose is not a reducing sugar.
Biuret test for proteins
Biuret reagent is a mix of two chemicals – copper sulfate (CuSO4) mixed with either sodium
hydroxide (NaOH) or potassium hydroxide (KOH).
The biuret test is done to show the presence of peptide bonds. Remember, peptide bonds are
the bonds that link the amino acids together in a protein. If peptide bonds are present, the blue
biuret reagent will turn mauve or purple.
To perform the test, simply
add the biuret reagent to the test solution. (Note, if the test material is solid and not liquid,
crush it and mix it with distilled water, to form a solution). The volume of biuret reagent you
add should be the same as the volume of the test solution you add it to.
In some cases, you won’t get a ready mixed biuret reagent solution. In this case,
Measure out a known volume of test solution into a test tube. About 1cm3 should be enough.
Add the same volume of NaOH (or KOH) to the test tube and stir.
Add a few drops of CuSO4 solution, shaking after each drop.
After you have completed the test, observe the colour of the solution. If it’s mauve or purple,
there are peptide bonds in the solution, so there are probably proteins in the solution.
Ethanol emulsion test for fats and oils
This test is pretty simple too! You add the test sample to a concentrated ethanol solution. You
put the resulting mixture into a test tube of distilled water, close it, and shake it around. If a
cloudy emulsion forms, fats are present; if not, there are no fats.
4. State that water is important as a solvent
Water is extremely important as a solvent. All reactions in our body occur in water, our cells
are 70% water on average, substances are transported around our body after being dissolved
in water, etc.
So water is essential for life.
