2.2 · Biological Molecules
Molecular Bonding
By the end of this topic you should be able to demonstrate and apply your knowledge of
monomers and polymers and the importance of condensation and hydrolysis reactions,
how hydrogen bonding occurs between water molecules and relates to the roles of water
for living organisms, and the chemical elements that make up biological molecules.
✦ OCR A SPEC POINTS
• the concept of monomers and polymers and the importance of condensation and hydrolysis reactions
in a range of biological molecules
• how hydrogen bonding occurs between water molecules and relate this and other properties of water,
to the roles of water for living organisms
• the chemical elements that make up biological molecules
✦ KEY DEFINITIONS
Condensation reaction Polymer
A reaction that occurs when two molecules are joined A large molecule made from many smaller molecules
together with the removal of water. called monomers.
Hydrolysis reaction Hydrogen bond
A reaction that occurs when a molecule is split into A weak interaction that can occur wherever
two smaller molecules with the addition of water. molecules contain a slightly negatively charged atom
bonded to a slightly positively charged atom.
Monomer
A small molecule which binds to many other identical
molecules to form a polymer.
✦ COVALENT BONDS
Atoms are most stable when their outermost electron shell is full. By sharing electrons with other atoms, atoms can
"fill" their outer shell — this shared pair of electrons forms a strong covalent bond, drawn as a single line.
ELEMENT HYDROGEN (H) OXYGEN (O) NITROGEN (N) CARBON (C)
Covalent bonds formed 1 2 3 4
H
H
H
H C H C
H separate atoms
H
methane (CH4) — 4 covalent bonds
Learning tip: when drawing biological molecules, hydrogen only ever has a
single bond, oxygen only ever has two, nitrogen only ever has three, and carbon
always has four. A double bond counts as two of these bonds.
Molecular Bonding · OCR A Biology · page 1 of 6
, ✦ CONDENSATION & HYDROLYSIS
A condensation reaction occurs when two molecules are joined together with the removal of water. Almost all
condensation reactions happen the same way — a –OH group on one molecule reacts with a –OH group on
another and a molecule of water (H₂O) is released as a new covalent bond forms between them.
A hydrolysis reaction is essentially the reverse — literally "water-splitting". A molecule is split into two smaller
molecules by the addition of water, breaking the covalent bond that joined them.
Molecule 1 -OH HO- Molecule 2
condensation hydrolysis
(H2O removed) (H2O added)
Molecule 3 (a dimer / bigger molecule)
✦ MONOMERS & POLYMERS
Condensation and hydrolysis reactions are responsible for linking and splitting apart biological molecules in living
things. The units that are joined together are called monomers. When two monomers join together they form a
dimer. When many monomers join together they form a polymer.
✦ TABLE 1 — MONOMERS & POLYMERS OF BIOLOGICAL MOLECULES
TYPE OF ELEMENTS
MONOMER POLYMER
MOLECULE PRESENT
Carbohydrates C, H, O Monosaccharides (e.g. glucose) Polysaccharides (e.g. starch)
Proteins C, H, O, N, S Amino acids Polypeptides and proteins
Nucleic acids C, H, O, N, P Nucleotides DNA and RNA
Lipids C, H, O Do not form polymers like the others — but are still built from smaller molecules
(fatty acids & glycerol).
Element recall: all four biological molecule groups contain carbon, hydrogen
and oxygen. Proteins additionally contain nitrogen and sulfur; nucleic acids
additionally contain nitrogen and phosphorus.
✦ WORKED EXAMPLE
Two glucose monomers (monosaccharides) undergo a condensation reaction. A bond forms between the two
glucose molecules, and a molecule of water is released. The product is a disaccharide called maltose. To break
maltose back down into its two glucose monomers, a hydrolysis reaction would be needed — adding a molecule
of water back in to break the bond.
Examiner tip: hydrolysis in cells is catalysed by enzymes (e.g. amylase
hydrolyses starch) — never just "water" alone.
Molecular Bonding · OCR A Biology · page 2 of 6
Molecular Bonding
By the end of this topic you should be able to demonstrate and apply your knowledge of
monomers and polymers and the importance of condensation and hydrolysis reactions,
how hydrogen bonding occurs between water molecules and relates to the roles of water
for living organisms, and the chemical elements that make up biological molecules.
✦ OCR A SPEC POINTS
• the concept of monomers and polymers and the importance of condensation and hydrolysis reactions
in a range of biological molecules
• how hydrogen bonding occurs between water molecules and relate this and other properties of water,
to the roles of water for living organisms
• the chemical elements that make up biological molecules
✦ KEY DEFINITIONS
Condensation reaction Polymer
A reaction that occurs when two molecules are joined A large molecule made from many smaller molecules
together with the removal of water. called monomers.
Hydrolysis reaction Hydrogen bond
A reaction that occurs when a molecule is split into A weak interaction that can occur wherever
two smaller molecules with the addition of water. molecules contain a slightly negatively charged atom
bonded to a slightly positively charged atom.
Monomer
A small molecule which binds to many other identical
molecules to form a polymer.
✦ COVALENT BONDS
Atoms are most stable when their outermost electron shell is full. By sharing electrons with other atoms, atoms can
"fill" their outer shell — this shared pair of electrons forms a strong covalent bond, drawn as a single line.
ELEMENT HYDROGEN (H) OXYGEN (O) NITROGEN (N) CARBON (C)
Covalent bonds formed 1 2 3 4
H
H
H
H C H C
H separate atoms
H
methane (CH4) — 4 covalent bonds
Learning tip: when drawing biological molecules, hydrogen only ever has a
single bond, oxygen only ever has two, nitrogen only ever has three, and carbon
always has four. A double bond counts as two of these bonds.
Molecular Bonding · OCR A Biology · page 1 of 6
, ✦ CONDENSATION & HYDROLYSIS
A condensation reaction occurs when two molecules are joined together with the removal of water. Almost all
condensation reactions happen the same way — a –OH group on one molecule reacts with a –OH group on
another and a molecule of water (H₂O) is released as a new covalent bond forms between them.
A hydrolysis reaction is essentially the reverse — literally "water-splitting". A molecule is split into two smaller
molecules by the addition of water, breaking the covalent bond that joined them.
Molecule 1 -OH HO- Molecule 2
condensation hydrolysis
(H2O removed) (H2O added)
Molecule 3 (a dimer / bigger molecule)
✦ MONOMERS & POLYMERS
Condensation and hydrolysis reactions are responsible for linking and splitting apart biological molecules in living
things. The units that are joined together are called monomers. When two monomers join together they form a
dimer. When many monomers join together they form a polymer.
✦ TABLE 1 — MONOMERS & POLYMERS OF BIOLOGICAL MOLECULES
TYPE OF ELEMENTS
MONOMER POLYMER
MOLECULE PRESENT
Carbohydrates C, H, O Monosaccharides (e.g. glucose) Polysaccharides (e.g. starch)
Proteins C, H, O, N, S Amino acids Polypeptides and proteins
Nucleic acids C, H, O, N, P Nucleotides DNA and RNA
Lipids C, H, O Do not form polymers like the others — but are still built from smaller molecules
(fatty acids & glycerol).
Element recall: all four biological molecule groups contain carbon, hydrogen
and oxygen. Proteins additionally contain nitrogen and sulfur; nucleic acids
additionally contain nitrogen and phosphorus.
✦ WORKED EXAMPLE
Two glucose monomers (monosaccharides) undergo a condensation reaction. A bond forms between the two
glucose molecules, and a molecule of water is released. The product is a disaccharide called maltose. To break
maltose back down into its two glucose monomers, a hydrolysis reaction would be needed — adding a molecule
of water back in to break the bond.
Examiner tip: hydrolysis in cells is catalysed by enzymes (e.g. amylase
hydrolyses starch) — never just "water" alone.
Molecular Bonding · OCR A Biology · page 2 of 6
