Topic 2: Biochemical
molecules
Subtopic: A1.1 Water
Syllabus statements:
A1.1.1—Water as the medium for life
A1.1.2—Hydrogen bonds as a consequence of the polar covalent
bonds within water molecules
A1.1.3—Cohesion of water molecules due to hydrogen bonding
and consequences for organisms
A1.1.4—Adhesion of water to materials that are polar or
charged and impacts for organisms
Syllabus statements:
A1.1.5—Solvent properties of water linked to its role as a
medium for metabolism and for transport in plants and
animals
A1.1.6—Physical properties of water and the consequences for
animals in aquatic habitats
A1.1.7—Extraplanetary origin of water on Earth and reasons for
its retention
A1.1.8—Relationship between the search for extraterrestrial life
and the presence of water
Water is needed for life
As we saw in the origins of life
subtopic, Life probably originated in
water.
Water is required for life as it is:
● The medium for metabolic
reactions.
● A transport medium.
● Involved in many chemical reactions
(e.g. hydrolysis reactions and
photosynthesis.)
Water molecules
● Water is a polar covalent molecule.
Two hydrogen atoms form covalent
bonds with an oxygen atom.
● The bonding produces a ‘bent’
molecule with an angle of 104.5
o
Water molecules
● The oxygen atom has a more
positive nucleus than the hydrogen
atoms, and has a greater
attraction for (the negatively
charged) electrons.
● The electrons are shared unequally
between the oxygen and hydrogen
atoms in water molecules.
Water molecules
● Oxygen therefore has a slight
negative charge ( δ-)
● Hydrogen has a slight positive
charge ( δ+)
● As opposite charges attract, there
is an attraction between the
oxygens and the hydrogens of
adjacent molecules.
Water molecules
● Water is a polar covalent molecule
with weak charges at the poles.
● Water is attracted to other polar
molecules forming hydrogen bonds.
● The hydrogen bonds that form
between water molecules explain
many of water’s properties.
Water molecules
Cohesion
● Cohesion is attraction between the
same kind of molecules.
● Water molecules are attracted to
each other. The negatively charged
oxygen of one molecule is
attracted to the positively charged
hydrogens of another, forming a
hydrogen bond.
Cohesion
● A relatively strong surface tension
forms at the surface of water due
to the cohesive nature of water
molecules.
● Surface tension allows some
animals like pond skaters to walk
on water, and the surface of
water becomes their habitat.
Cohesion
● Xylem vessels are thin narrow tubes
which transport water and dissolved
minerals from the roots to the
leaves of plants.
● The cohesive nature of polar water
molecules allows water to form a
continuous column of water
molecules moving up the xylem of
plants.
Adhesion
● In adhesion, polar water molecules
are attracted to other polar
molecules, forming hydrogen bonds.
● This is used to help in the
transpiration stream, as the water
molecules ‘stick’ to the polar
molecules lining the xylem vessels.
Water the solvent
● Water is often called the universal
solvent, as it allows a wide variety
of hydrophilic substances to
dissolve in it, because of its
polarity.
● Most metabolic reactions will only
occur when the reactants are
dissolved in water.
Water the solvent
● Water is the transport medium in
plants and animals, as many polar
molecules and ions dissolve in
water.
● Polar (charged) organic particles
dissolve in polar water, due to
hydrogen bonding (eg glucose).
Water the solvent
● Ions dissolve in water because of the
polar water molecules surrounding the
charged ions.
● Ionic minerals are dissolved in water as
it is transported up the xylem.
● Glucose and amino acids dissolve in
water, and are transported through the
phloem.
Water the transport medium
Humans transport many substances
around their bodies through the blood
including:
● Nutrients
● Oxygen
● Carbon dioxide
● Hormones
● Waste products
● Antibodies
Water the transport medium
Water the metabolism
medium
● Metabolism is the sum of all of the
enzyme controlled chemical
reactions that occur in the body.
● Most of these reactions occur in
the aqueous environment of the
cell with the substrates and
enzymes dissolved in the watery
environment.
Not all molecules dissolve
● Hydrophobic substances are not
charged and do not readily mix
with water.
Examples:
Lipids (fats and oils)
Steroid hormones (e.g. oestrogen,
progesterone, testosterone)
Physical properties buoyancy
● Buoyancy is the upward force
exerted by a fluid (liquid or gas)
on an object immersed in the fluid.
● Liquid water is a denser fluid than
air, providing greater buoyancy for
aquatic animals and allowing them
to float or swim more easily.
Physical properties buoyancy
● This helps many aquatic animals
conserve energy and allows them
to stay afloat without expending
a lot of effort.
● The ringed seal has a layer of
blubber that provides buoyancy
and helps it float on the surface
of the water.
Physical properties viscosity
● Viscosity is the measure of a
fluid's resistance to flow. The
higher the viscosity, the more
difficult it is for animals to move
through the fluid.
● Water has a higher viscosity than
air, and many aquatic animals, such
as the black-throated loon, have a
streamlined body shape which
allows them to smoothly move
Physical properties - thermal
conductivity
● Thermal conductivity is a
measure of a material's ability to
conduct heat.
● Water has a much higher thermal
conductivity than air. Aquatic
animals are more likely to lose
heat to the environment, and
must be adapted to reduce heat
loss.
Physical properties - thermal
conductivity
● Ringed seals have a layer of
insulating blubber to prevent heat
loss. Seals will huddle together out
of the water (on land), decreasing
overall exposed surface area and
reducing heat loss.
● Black-throated loons are insulated
by feathers which are coated with a
hydrophobic oil to keep the feathers
dry, which reduces heat loss.
Physical properties - specific
heat capacity
● Specific heat capacity is the
energy required to raise the
temperature of 1g of a
substance by 1 K (or 1°C).
● Water has a very high specific
heat capacity (4.186 J/g°C), as
energy is required to break the
hydrogen bonds between water
molecules. The specific heat
capacity of air is much lower
than water.
Physical properties - specific
heat capacity
● The temperature of large bodies
of water is much more stable
than air temperature, as the
water can absorb or release
much more heat energy without
experiencing significant
temperature change.
Physical properties - thermal
conductivity
● Animals cells (which are primarily
composed of water) are also
resistant to temperature change,
which helps maintain constant
body temperature in endotherms
such as birds and mammals.
Consequences of the physical
properties
● Explain the consequences of the
physical properties of water for
the ringed seal (buoyancy,
viscosity, thermal conductivity and
specific heat capacity).
Consequences of the physical
properties - Ringed seal
Ringed seals ( Pusa hispida ) live in the arctic seas.
● Buoyancy: The ringed seal has a layer of blubber which allows it to remain
buoyant while in the sea, reducing the energy required to swim in the sea.
● Viscosity: The streamlined body shape of the seals allow them to efficiently
move through water. The flippers use drag to facilitate movement.
Consequences of the physical
properties - Ringed seal
● Thermal conductivity: The seal’s layer of blubber insulates the animal when in
water. Seals tend to huddle, reducing exposed surface area and heat loss
when on land.
● Specific heat capacity: Ringed seals are endotherms and are adapted to
maintain a constant body temperature. The high specific heat capacity of
the water in their bodies helps maintain a stable body temperature.
Consequences of the physical
properties - Loon
Black-throated loons are migratory aquatic birds found in the Northern
Hemisphere.
● Buoyancy: The black-throated loon is able to adjust its density by changing
the volume of air in its air sacs. This allows the bird to control its depth
when swimming in water. The black-throated loon is able to increase its
buoyancy in air by increasing its surface area by spreading its wings. The
bones of birds are solid but light-weight, further reducing their density and
increasing the buoyancy of the black-throated loon, allowing it to fly.
Consequences of the physical
properties - Loon
● Viscosity: The streamlined shape of black-throated loons allow them to
efficiently move through air and water. The webbed feet help the birds to
move through water.
● Thermal conductivity: The black-throated loon is an endotherm that maintains
a constant body temperature. The bird’s feathers trap air and provide
insulation. The feathers are covered in a hydrophobic oil which keeps the
feathers dry.
● Specific heat capacity: the high specific heat capacity helps the
black-throated loon to regulate its body temperature, which is especially
important when it is diving into cold water.
Where did Earth’s water
come from?
● Earth and the other rocky planets
of the inner solar system formed
by the clumping together of solid
particles.
● Water, in the form of ice, was not
present, as the temperatures
were too high for water to exist
as a solid (ice). Any gaseous
water would have been moved
away by the solar winds.
Where did Earth’s water
come from?
● Water formed solid particles in the
outer solar system as a result of
low temperatures, due to the
distance from the sun.
● The ice formed objects such as
comets and asteroids. Earth's
water probably arrived from the
asteroids.
● The water probably arrived during
the Late Heavy Bombardment, four
billion years ago.
Where did Earth’s water
come from?
● Planet Earth’s gravity retains the
liquid water and water vapor on the
planet.
● The Earth is in a habitable zone of
the solar system where water can
exist as liquid water.
● The temperature of Earth is
suitable for the water cycle, as
water vapor condenses to liquid
water, and falls back to the Earth as
precipitation.
The search for life
● Scientists believe that liquid water
is required for life to exist on
planets.
● Scientists are searching for
exoplanets with liquid water which
may support life.
● Planets are in the habitable zone
(Goldilocks zone), when their orbit
around their star allows liquid
water.
● The planet is not too hot and not