Edexcel International A Your notes
Level (IAL) Biology
Biological Molecules
Contents
The Importance of Water
Saccharides
Core Practical 1: Estimating the Concentration of Sugars & Starch
Condensation & Hydrolysis
Triglycerides & Ester Bonds
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,The Importance of Water
Your notes
The Importance of Water
Water is the medium in which all metabolic reactions take place in cells and in which all
substances are transported around the body
Water is composed of atoms of hydrogen and oxygen
One atom of oxygen combines with two atoms of hydrogen by sharing electrons;
this is covalent bonding
The sharing of the electrons is uneven between the oxygen and hydrogen atoms
The oxygen atom attracts the electrons more strongly than the hydrogen atoms,
resulting in a weak negatively charged region on the oxygen atom (δ-) and a weak
positively charged region on the hydrogen atoms (δ+)
This separation of charge due to the electrons in the covalent bonds being unevenly
shared is called a dipole
When a molecule has one end that is negatively charged and one end that is positively
charged it is said to be a polar molecule
Water is a polar molecule
The covalent bonds of water make it a polar molecule
Hydrogen bonds form between the positive and negatively charged regions of nearby
water molecules as a result of the polar nature of water
Hydrogen bonds are weak when they are few in number, so they are constantly
breaking and reforming; this means that water molecules flow past each other in a
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, liquid state
Hydrogen bonds contribute to the many properties water molecules have that make
them so important to living organisms Your notes
The polarity of water molecules allows hydrogen bonds to form between adjacent water
molecules
Water's dipole nature makes it good at transporting
substances
The polar nature of water gives water properties that make it good at transporting
substances
Water is cohesive
Water is a solvent
Cohesion and adhesion
Hydrogen bonds between water molecules allow for strong cohesion between water
molecules
Cohesion is the attraction of water molecules to each other
Water is also able to form hydrogen bonds with other molecules; this is known
as adhesion
The forces of cohesion and adhesion within a body of water and between water and its
surroundings mean that water flows easily
Water molecules pull other water molecules along due to cohesion
Water adheres to the sides of a vessel due to adhesion
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, Solvent
As water is a polar molecule many ions, e.g. sodium and chloride ions, and covalently Your notes
bonded polar substances, e.g. glucose, will dissolve in it
Water molecules surround charged particles; the positive parts of water are
attracted to negatively charged particles and the negative parts of water are
attracted to positively charged particles
The surrounded molecules break apart e.g. sodium chloride molecules break into
sodium ions and chloride ions
The ions surrounded by water molecules have dissolved
This allows chemical reactions to occur within cells as the dissolved solutes are more
chemically reactive when they are free to move about
Metabolites can be transported efficiently in a dissolved state
Water molecules surround charged particles due to forces of attraction between polar
water and the charged particles. The surrounded particles are said to have dissolved
Examiner Tips and Tricks
Examiners will often assess how structure relates to function so it is important you
know the structure of water and how its properties help it carry out its role, specifically
with regards to transport.
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Level (IAL) Biology
Biological Molecules
Contents
The Importance of Water
Saccharides
Core Practical 1: Estimating the Concentration of Sugars & Starch
Condensation & Hydrolysis
Triglycerides & Ester Bonds
© 2026 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
,The Importance of Water
Your notes
The Importance of Water
Water is the medium in which all metabolic reactions take place in cells and in which all
substances are transported around the body
Water is composed of atoms of hydrogen and oxygen
One atom of oxygen combines with two atoms of hydrogen by sharing electrons;
this is covalent bonding
The sharing of the electrons is uneven between the oxygen and hydrogen atoms
The oxygen atom attracts the electrons more strongly than the hydrogen atoms,
resulting in a weak negatively charged region on the oxygen atom (δ-) and a weak
positively charged region on the hydrogen atoms (δ+)
This separation of charge due to the electrons in the covalent bonds being unevenly
shared is called a dipole
When a molecule has one end that is negatively charged and one end that is positively
charged it is said to be a polar molecule
Water is a polar molecule
The covalent bonds of water make it a polar molecule
Hydrogen bonds form between the positive and negatively charged regions of nearby
water molecules as a result of the polar nature of water
Hydrogen bonds are weak when they are few in number, so they are constantly
breaking and reforming; this means that water molecules flow past each other in a
© 2026 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
, liquid state
Hydrogen bonds contribute to the many properties water molecules have that make
them so important to living organisms Your notes
The polarity of water molecules allows hydrogen bonds to form between adjacent water
molecules
Water's dipole nature makes it good at transporting
substances
The polar nature of water gives water properties that make it good at transporting
substances
Water is cohesive
Water is a solvent
Cohesion and adhesion
Hydrogen bonds between water molecules allow for strong cohesion between water
molecules
Cohesion is the attraction of water molecules to each other
Water is also able to form hydrogen bonds with other molecules; this is known
as adhesion
The forces of cohesion and adhesion within a body of water and between water and its
surroundings mean that water flows easily
Water molecules pull other water molecules along due to cohesion
Water adheres to the sides of a vessel due to adhesion
© 2026 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3
, Solvent
As water is a polar molecule many ions, e.g. sodium and chloride ions, and covalently Your notes
bonded polar substances, e.g. glucose, will dissolve in it
Water molecules surround charged particles; the positive parts of water are
attracted to negatively charged particles and the negative parts of water are
attracted to positively charged particles
The surrounded molecules break apart e.g. sodium chloride molecules break into
sodium ions and chloride ions
The ions surrounded by water molecules have dissolved
This allows chemical reactions to occur within cells as the dissolved solutes are more
chemically reactive when they are free to move about
Metabolites can be transported efficiently in a dissolved state
Water molecules surround charged particles due to forces of attraction between polar
water and the charged particles. The surrounded particles are said to have dissolved
Examiner Tips and Tricks
Examiners will often assess how structure relates to function so it is important you
know the structure of water and how its properties help it carry out its role, specifically
with regards to transport.
© 2026 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 4
