BIOLOGICAL MOLECULES

CHAPTER 2 AS LEVEL CAMBRIDGE

 Lesson Objectives
Describing Describing the Describing and Explaining some
how large structure of carrying out key properties of
biological carbohydrates, biochemical water that make
molecules are lipids and proteins tests to identify life possible
made from and how their carbohydrate,
smaller structure relates lipids and
molecules to their functions proteins
 MONOMERS, POLYMERS AND
MACROMOLECULES
Polymer: a giant molecule made from many similar repeating
subunit (smaller and simple) joined together in a chain.

Monomer: a relatively simple molecule which is used as basic

building block for the synthesis of a polymer.

Condensation reaction: a chemical reaction involving the joining

together of two molecules by removal of a water molecule.

Hydrolisis: a chemical reaction in which a chemical bond is broken

by the addition of a water molecule
2.4 CARBOHYDRATES

Elements: carbon, hydrogen, and oxygen

General formula: Cx(H2O)y
Carbohydrates are divided into three main groups:
monosaccharide, disaccharide and
polysaccharide.
"saccharide" means sweet.
MONOSACCHARIDE

Monosaccharides consist of a single sugar

molecule ( "mono" means one, "saccharide" means
sugar.
General formula: (CH2O)n
The main types of monosaccharides:
- trioses (3C)
- pentoses (5C)
- hexoses (6C) --> glucose, fructose, galactose.
Molecular Structure and Formulae of
monosaccharide

Glucose formulae : C6H12O6

-OH is known as a hydroxyl
group
When glucose forms such a
ring, carbon atom number 1
joins to the oxygen on carbon
atom number 5.
 ctu
tru r
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R S
ing
Functions of monosaccharide in living
organisms

as source of energy as building block for larger molecules

 diSACCHARIDE
Monosaccharides are formed by two
monosaccharides joining together ( "di" means two)
Three most common disaccharides:
- maltose (glucose + glucose)
- sucrose (glucose + fructose)
- lactose (glucose + galactose)
The process of joining two monosaccharides is an
example of a condensation reaction.
The reverse process is an example of a hydrolysis
reaction.
 types of sugar
REDUCING SUGAR
Reducing sugars can carry out a type of chemical
reaction known as reduction.
The reducing sugars include all monosaccharides and
some disaccharides. The only common nonreducing
sugar is sucrose.
This is the basis of Benedict’s test for the presence
of sugar.
 types of sugar
NON-REDUCING SUGAR
e.g.: sucrose
The disaccharide is first broken down into its two
monosaccharide constituents.
The chemical reaction is hydrolysis and can be
brought about by adding hydrochloric acid.
The constituent monosaccharides will be reducing
sugars and their presence can be tested for using
Benedict’s test after the acid has been neutralised.
 polysaccharides
Polysaccharides are polymers made by
joining many monosaccharide molecules
If you continue joining together
by condensation.
monosaccharides with glycosidic bonds
The most important polysaccharides you can make very long chains of
are starch, glycogen and cellulose, all of sugars.
which are polymers of glucose.
Polysaccharides are not sugars.
 STARCH
The storage polysaccharide in plants is
starch.
Starch grains are commonly found in
chloroplasts and in storage organs, such as
potato tubers and the seeds of cereals and
legumes
Starch is a mixture of two substances –
Starch in raw potato cells amylose and amylopectin.
 starch - aMYLOSE
Amylose is made by condensations between α-
glucose molecules.
Amylose is a long, unbranching chain of several
thousand 1,4 linked glucose molecules is built up.
The chains are curved and coil up into helical
structures like springs, so the final molecule is
compact.

amylose
 starch - aMYLOPECTIN
Amylopectin is branched
polymer of α-glucose made up
from a combination of 1,4-
glycosidic bonds and 1,6-
glycosidic bonds.
This branching changes the
properties of the molecule.
The side chains can be easily
broken off when energy is
required
 GLYCOGEN
This is polysaccharide as the energy store for
animals (liver and muscle) and fungi.
Made of many α-glucose molecules and
combination of 1,4-glycosidic bonds and 1,6-
glycosidicbonds.
Basically the same as amylopectin but with
even more branching!
Glycogen
 celluLOSE
Cellulose is a polysaccharide made from β-
glucose subunits.
It used as a strengthening material in plant cell
walls
Two β-glucose molecules lined up to form a 1,4
link. Note that one glucose molecule must be
rotated 180° relative to the other
Cellulose is a very strong substance used for making cell walls.
In order to achieve this strongness, It has a very specific structure.
The strands can form cross-links with other strands.
These cross links are made from hydrogen bonds and they give cellulose
its great strength.
 2.5 LIPIDS
Lipids are all organic molecules which are
insoluble in water.
Most lipids are formed by fatty acids combining
with an alcohol (glycerol) joined by ester bond.
Elements: CHO
The most familiar lipids are fats and oils.
Fats are solid at room temperature and oils are
liquid at room temperature, but chemically they
are very similar.
 fatty acids
Fatty acids contain the carboxyl group
(-COOH) forms the ‘head’ and long
hydrocarbon tails attached to the
carboxyl group.
Types of fatty acid:
1. Saturated fatty acids have the
maximum number of hydrogen atoms
possible and no double bonds. e.g. lard
2. Unsaturated fatty acids have one or more
double bonds. e.g. plant oils, fish oil
 triglycerides
Triglyceride is a type of lipid formed when three
fatty acid molecules combine with glycerol, an
alcohol with three hydroxyl (−OH) groups.
Triglycerides are insoluble in water but are
soluble in certain organic solvents such as
ethanol.
This is because the hydrocarbon tails are non-
polar: they have no uneven distribution of
electrical charge. Consequently, they are
hydrophobic
 Functions of triglycerides

Energy stores (higher than An unusual role for triglycerides is as

carbohydrates) a metabolic source of water
 phospholipids
Phospholipids consist of
a hydrophilic head
containing phosphate
group (polar) and two
hydrophobic fatty acid
tails (non-polar).
This allows
phospholipids to form a
membrane around a cell.
 2.6 proteins
Elements: CHON
Monomer: amino acids joined by peptide bond.
Proteins have many important functions. They are
used to make:
enzymes
cell membranes
hormones
antibodies
haemoglobin
hair, nails, etc.
 amino acids
There are 20 types of amino acid.
All amino acids have a central carbon
atom which is bonded to:
- an amino group (–NH2)
- a carboxylic acid group (–COOH)
- a hydrogen atom (H)
- an R group
Two amino acids can join together. The link
is called a peptide bond.
 peptide bond
peptide bond: the covalent bond joining neighbouring amino acids together in proteins; it is a C–N link
between two amino acid molecules, formed by a condensation reaction.
The new molecule is made up of two linked amino acids and is called a dipeptide. A molecule made up of
many amino acids linked together by peptide bonds is called a polypeptide.
 primary structure
Primary structure is the particular amino
acids contained in the chain, and the
sequence in which they are joined.
A change in a single amino acid in a chain
made up of thousands may completely
alter the properties of the polypeptide or
protein.
 secondary structure
Secondary structure is the structure of a
protein molecule resulting from the regular
coiling or folding of the chain of amino acids (an
α-helix or β-pleated sheet)
α-helix: a helical (coil, cylinder) structure
formed by a polypeptide chain, held in place by
hydrogen bonds;
β-pleated sheet: a loose, sheet-like (arrow)
structure formed by hydrogen bonding
between parallel polypeptide chains;
 tertiary structure
Tertiary structure is the compact structure of
a protein molecule resulting from the
three-dimensional coiling of the chain of amino
acids.
The shape of the molecules is very precise, and
the molecules are held in these exact shapes by
bonds between amino acids in different parts
of the chain.
the four types of bond that help to keep folded proteins in their precise shapes
 quartenary structure
Quaternary structure is the three-dimensional
arrangement of two or more polypeptides, or
of a polypeptide and a non-protein component
such as haem, in a protein molecule.
e.g. haemoglobin
The polypeptide chains in quaternary
structures are held together by the same four
types of bond as in tertiary structures.
 globular protein
Globular protein is a protein whose molecules
are folded into a relatively spherical shape,
often has physiological roles and is often
water-soluble and metabolically active,
e.g. insulin, haemoglobin and enzymes
 haemoglobin - a globular protein

Haemoglobin is the red pigment found in red blood

cells, whose molecules contain four iron atoms
within a globular protein made up of four
polypeptides; it combines reversibly with oxygen.
Each haemoglobin molecule contains four
polypeptide chains. The two α chains and the two β
chains. Each polypeptide chain contains a haem
group.
Each haem group contains an iron atom. One
oxygen molecule (O2 ) can bind with each iron atom.
 sickle cell anaemia

Sickle cell anaemia is a

genetic disease caused
by a faulty gene coding
for haemoglobin, in
which haemoglobin
tends to precipitate
when oxygen
concentrations are
low.
 fibrous protein
Fibrous protein is a protein whose molecules
have a relatively long, thin structure that is
generally insoluble and metabolically inactive,
and whose function is usually structural,
e.g. keratin and collagen
 collagen - a fibrous protein
Collagen is the main structural protein of
animals; known as ‘white fibres’,
A collagen molecule consists of three helical
polypeptide chains wound around each other,
forming a ‘triple helix’ with high tensile
strength.
Collagen is found in the connective tissue of
animals including cartilage, bones, teeth, skin,
and blood vessels
 2.7 water

Water is the most important

biochemical of all.
Water is a major component of cells.
Water provides an environment for
those organisms that live in water.
Water molecules joined by Hydrogen
bond.
properties Water has a high specific heat
capacity, which makes liquid water

of water relatively resistant to changes in

temperature.
Water acts as a solvent for ions
and polar molecules, and causes
non-polar molecules to group
together.
Water has a relatively high latent
heat of vaporisation, meaning that
evaporation has a strong cooling
effect.
Experiment Time!
Testing for Biological Molecules
 benedict's test

Benedict’s test is a test for the presence Benedict’s reagent is copper (II)
of reducing sugars; sulfate in an alkaline solution and has
The unknown substance is heated with a distinctive blue colour.
Benedict’s reagent, and a change from a Reducing sugars reduce the soluble
clear blue solution to the production of a blue copper sulfate to insoluble
yellow, red or brown precipitate indicates brick-red copper oxide, containing
the presence of reducing sugars such as copper(I). The copper oxide is seen
glucose as a brick-red precipitate.
benedict's test
 iodine test

The Iodine test is used for testing the

presence of starch.
Procedure:
Iodine solution is orange-brown. Add a
drop of iodine solution to the solid or
liquid substance to be tested. A blue-
black colour is quickly produced if starch
is present.
 emulsion test

The emulsion test is used for testing the presence of

lipid.
Procedure
The substance that is thought to contain lipids is shaken
vigorously with some absolute ethanol (ethanol with little
or no water in it). This allows any lipids in the substance
to dissolve in the ethanol. The ethanol is then poured into
a tube containing water. If lipid is present, a cloudy white
suspension is formed.
 biuret test

Biuret test is a test for the presence of amine groups

and thus for the presence of protein;
it uses biuret reagent (potassium hydroxide or sodium
hydroxide, and a dilute solution of copper(II) sulphate).
Procedure:
The biuret reagent is added to the solution to be tested.
No heating is required. A purple colour indicates that
protein is present. The colour develops slowly over
several minutes.
thank you!