Biology: Unit F212: Molecules, Biodiversity, Food and Health
Biological Molecules
o Monosaccharides are the simplest carbohydrates. They are the monomers from
which large carbohydrates are made.
o Glucose is a monosaccharide. It has 6 carbon atoms and is therefore a hexose
sugar. It exists in 2 forms – alpha and beta glucose. Alpha glucose has the H
above the carbon-1. Beta glucose has the H below the carbon-1.
o Disaccharides are formed when 2 monosaccharide molecules link together, by a
1-4 glycosidic covalent bond, formed in a condensation reaction.
o In a condensation reaction, a new covalent bond is formed, a water molecule is
released and a large molecule is formed.
α-glucose
+
α-glucose

maltose + water
o A disaccharide can be broken down into two monomers with an
enzyme in a hydrolysis reaction. In a hydrolysis reaction, a
covalent glycosidic bond is broken, a water molecule is used
and smaller molecules are produced.
o Polysaccharides form when lots of monomers join together.
1. Amylose is a plant storage polysaccharide. When plants form sugars in
photosynthesis, they need to be stored in a suitable form. Amylose is formed
when thousands of alpha glucose monomers are joined by C1-C4 glycosidic
bonds in condensation reactions. It curls up into a compact helix, making it an
insoluble dense store of energy.
2. Amylopectin is the second part of starch, along with amylase. Amylopectin is
made from alpha glucose monomers joined by glycosidic bonds in a
condensation reaction. Amylopectin contains both C1-C4 and C1-C6
glycosidic bonds.
Enzymes can only work at the end of polymers.
This means that amylopectin, with a branched
structure, can be broken down easily and quickly
to release glucose.
Amylose + Amylopectin = Starch. The amylose helix wraps
around the amylopectin branches to create the final starch
molecule. Starch is stored in chloroplasts and in membrane
bound starch grains. When glucose is required to release
energy, enzymes break down the starch.
3. Glycogen is the storage polysaccharide in animals. It has a very similar
structure to amylopectin. It is made of alpha glucose monomers joined by C1C4 and C1-C6 glycosidic bonds in a condensation reaction. The only
difference is that glycogen has more C1-C6 branches. Glucose is stored a
glycogen in the liver and in muscles. It is insoluble and its branched structure
makes it easy for enzymes to break glucose from it when required.
4. Cellulose is a structural polysaccharide made from Beta Glucose monomers,
joined by B1-B4 glycosidic bonds in condensation reactions.
Cellulose chains are long and straight. Ever other β glucose monomer is inverted by 180 degrees so that bonds can form.
6
CH2OH
H
4
HO
5
H
OH
3
H
O
H
O
1
OH
GLUCOSE
H
OH
H
5
2
CH2OH
2
3
4
6
OH
H
O
6
CH2 OH
GLUCOSE
1
5
1
O
4
H
OH
3
H
OH
H
O
H
3
O
1
4
OH
H
5
2
OH
GLUCOSE
4 glycosidic bonds
2
H
O
6
CH2 OH
GLUCOSE
1
Cellulose fibres are arranged in a very specific way to form plant cell
walls. The beta glucose monomers contain many OH groups, allowing
the formation of hydrogen bonds between cellulose chains. 60-70
cellulose molecules become cross linked by H bonds forming
microfibrils; microfibrils are held together by more H bonds to form
macrofibrils; macrofibrils comprise a cellulose fibre.
o Lipids are made entirely of hydrogen, oxygen and carbon. They have several
functions; phospholipids make up membrane; insulation; energy storage;
hormones. They are insoluble in water, but are soluble in organic solvents like
ethanol.
o Triglycerides are fats and oils; animal triglycerides are saturated and solid fat;
oils are plant triglycerides which are unsaturated and liquid.
o A triglyceride is formed from a glycerol molecule and a fatty acid.
In a condensation reaction, water is released and a covalent ester bond is
formed between the fatty acid and the glycerol molecule.
o Phospholipids are diglycerides; they have a
glycerol molecule with 2 fatty acid ‘tails’ and a
phosphate ‘head’. The fatty acid tails are
hydrophobic; the phosphate head is hydrophilic.
The bipolar nature of the phospholipids means that
they can form bilayers in cell membranes.
o Cholesterol is a lipid belonging to the steroid subgroup. Cholesterol molecules
are located in between the Phospholipid tails to stabilise
the membrane. All steroids are built around a 4 ringed
skeleton. Steroids have a role in membrane stabilisation
and the synthesis of hormones such as testosterone,
progesterone and oestrogen.
o Proteins are biological molecules which have many functions; growth and repair,
metabolism – enzymes; communication – hormones; immune system – white
blood cells and antibodies.
o Amino acids are the monomers in the protein polymer. All amino acids have the
same general structure, with a COOH group, and NH2 group and a variable R
group. There are 20 different amino acids defined by the different R groups.
o In order to form a polypeptide, amino acids must be joined
together. This happens in a condensation reaction with an
enzyme. A water molecule is released, when the ‘OH’ from
the carboxyl group of one amino acid combines with an ‘H’
atom from the amino group of a second amino acid, and a
peptide bond is formed.
o Primary Structure of a protein is the sequence of specific amino acids along the
polypeptide chain. There are peptide bonds between the individual amino acid
monomers.
o Secondary Structure is the simple folding of the polypeptide chain into either the
Alpha Helix or the Beta Pleated Sheet due to hydrogen bonding.
o Tertiary Structure is the complex 3D folding of the polypeptide chain due to 4
types of bonding between the R groups of the amino acids.
o Disulphide Bonds form between two amino acids
which contain sulfur in their R groups (cysteine and
methionine amino acids) - STRONGEST
o Ionic bonds form between maino acids with positively
and negatively charged R groups. – 2nd STRONGEST
o Hydrophobic/Hydrophillic Interactions – some amino
acids are hydrophobic and so arrange themselves away
from water. This causes inwards folding in globular
proteins, so that hydrophibic amino acids are in the
centre.
o Hydrogen Bonds form between δ+ and δ- groups
o Tertiary structure is important to enzyme function:
enzymes are able to react with substrates because they
have an active site with a complementary shape to the
substrate. The enzyme’s shape is formed by the bonds of
the tertiary structure. The
ionic/disulphide/hydrophobic/hydrogen bonds allow the
enzyme to be the particular shape necessary to be complementary to the
substrate.
o Denaturation occurs when a protein is heated. Under high temperatures, the
protein has more kinetic energy. The molecule vibrates and some of the bonds
are broken (most are weak as they are not covalent). The tertiary structure
breaks down, the protein loses its shape, and is said to have become denatured.
The enzyme will cease to function, because the shape of its active site is no
longer complementary to the substrate. Even after cooling, the bonds do not
reform – the shape of the active site has permanently changed shape.
o Quaternary Structure is the highest level of protein
structure where a protein is made from more than 1
polypeptide chain, or where there is a non-protein
prosthetic group included in the polypeptide. Insulin is an
example of this; 6 polypeptides joined by a zinc ion.
o Globular Proteins tend to be a compact globe shape. They
are usually soluble in water and play metabolic roles
within organisms – eg enzymes and receptors
o Haemoglobin is a globular protein. It has a complex 3D shape
and contains 4 haem prosthetic groups. The haem groups bind
reversibly to oxygen to allow this globular protein to transport
gases around the body. It is made from 2 alpha chains (each
141 amino acids long) and 2 beta chains (each 146 amino acids
long).
o Fibrous Proteins are long fibres.
They tend to be insoluble in
water, metabolically unreactive
and play structural roles – eg.
Collagen, keratin and elastin
o Collagen is a structural, fibrous
polypeptide. It consists of three
helical polypeptides wrapped
around each other to form a
‘triple helix rope’. Every third
amino acid is glycine, the smallest amino acid. This allows the three strands to lie
close to each other to form a tight coil. The three strands are held together by
hydrogen bonds. Bonds form between collagen molecules running parallel to
each other – between R groups of lysines. These cross links hold many collagen
molecules together side by side to form fibrils. Hydrogen bonds between fibrils
form fibres.
Collagen has 4 key properties; Muscles are attached to bones with strong,
inelastic tendons which are made from
 High tensile strength
collagen. Artery walls are strengthened by
 Straight
collagen. Bones are formed from collagen.
 Insoluble in water
Cartilage and connective tissues are made
from collagen.
o Water is a unique biological molecule.
 Liquid at room temperature – hydrogen bonding means that water is
a liquid at room temperature (whereas similar molecules like H2S are
gases due to absence of hydrogen bonding). This means that it is a
liquid medium for the chemistry of life.
 Specific Heat Capacity – Lots of energy is required to make water
molecules vibrate because of hydrogen bonds holding molecules
firmly together. Lots of energy must be added or removed to change
the temperature of water. This means that aquatic environments are
slow to change temperature giving a stable environment in which to
live.
 High latent heat of evaporation – Lots of energy is required to break
hydrogen bonds and turn water from liquid to gas. Organisms use this
as a cooling mechanism – the evaporation of sweat or transpiration
from plants causes a marked cooling; a loss of a little sweat takes lots
of energy with it.
 High latent heat of fusion – Lots of energy must be removed before
freezing occurs – aquatic environments are slow to freeze in cold
temperatures.
 Ice is less dense than water – Ice forms on the surface of water which
insulates the water below. This means that the water below remains
liquid so aquatic life can survive.
 Excellent solvent – Water dissolves polar molecules – most organic
molecules like carbohydrates, proteins and minerals carry charge so
will dissolve. This is useful in blood – all of these chemicals can be
carried in solution.
 Surface tension – Water molecules at the surface with air orientate so
that hydrogen bonds face inwards. The water molecules are pulled
downwards, creating the surface tension. This is used by a pond
skater to move across the surface.
 Low viscosity - moves easily through capillaries with ease.
 Strong Adhesive Properties – Allow for capillarity – water can move
through very narrow spaces such as between soil particles and in cell
walls.