Water Biochemistry

This lesson is aimed at more able and post 16 students. In this lesson students are encouraged
to understand how the chemistry of water makes it so essential for life on earth, with specific
relevance to pond ecology.

Resources
• Starter Activity: Water Chemistry (see end of pdf)
Why does ice float? Encourages students to think about why ice is less dense than water
and introduces the concept of hydrogen bonding.
• Lesson Plan: Water Chemistry (see end of pdf)
A circus of practical activities and demonstrations that are linked to understanding a
range of concepts. Adaptations of freshwater invertebrates, behaviour and feeding
relationships, and the abiotic advantages of temperature are all covered.
• Teachers’ notes (see end of pdf)
Further advice, support and background information for teachers delivering this aspect of
the curriculum.
 Lesson Plan: The biochemistry of water and pond ecology
Water is one of the smallest and lightest of molecules with a unique collection of
properties.

Life in aquatic habitats has many advantages over life on land, but there are also a
range of problems that come with living in such habitats.

Starter Activity: Hydrogen bonds

Demonstration of Water and Ice cubes in a beaker

The ice should be clearly floating in the water for this demonstration.

The majority of students will be aware that solid water unlike other substances floats
rather than sinks as it is regularly demonstrated at KS3/KS4. However they are
unlikely to be able to explain why this is the case in detail.

This activity focuses on understanding the biochemistry of water and linking this to
ecological concepts

Question- If the particles in a solid are more densely packed than in a liquid,
why does ice float on water?

What are some of the likely responses? Ice is less dense

than it should be

Ionic, Covalent
bonding?

Hydrogen What does this tell you

bonding about the distance
between the molecules?

The atoms/molecules in
What are the types of ice are further apart than
bonds we know about? would be expected

How are these

molecules being held
Forces/bonds/energy further apart?
between the molecules
must be holding the
molecules further apart

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
 Lesson plan: Chemistry of water and its effects on pond ecology

Main activity: The properties of water

This activity focuses on the main properties of water, includes a range of demonstrations and asks students to
consider how the properties of water contribute to life in ponds.

It is useful for students if they are aware of hydrogen bonding before beginning this practical. The activities
outlined below will provide a more visual representation of hydrogen bonding and how this contributes to the
various properties of water.

Students will be aware of some aspects of these properties as they will have come across them throughout
their science education.

A circus of activities is set up that demonstrates each property.

The abstract nature of hydrogen bonding requires some discussion of the various properties of water.

Students should be encouraged to develop their own ideas to explain their observations.

Activity 1: Demonstrating surface tension

Hydrogen bonding, Van der Waals Forces, the impact of detergents on aquatic environments

Activity 2: Demonstrating capillary action

Cohesion, adhesion, meniscus

Activity 3: Water as a temperature buffer

Specific heat capacity, Latent heat of vaporisation, heat of fusion

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
Activity 1: Floating a needle on water
Requires: Petri dish filled with distilled water,
Small needle,
Detergent

Distilled water
Students should first attempt to float a needle on distilled water; this should be a relatively easy task although
they will need to think about how they manage this. There are a variety of methods but delicacy is the key!

Distilled water will have limited impurities, thus the hydrogen bonding and Van der Waals forces between
water molecules will be strong and the surface tension high. Small objects can float on this even if technically
they are denser than water.

Detergent

With the needle remaining floating a drop of detergent can be gently added to the water just behind the
needle. The needle will immediately speed off away from where the detergent has just been added

Adding detergent to the water breaks the hydrogen bonding, significantly reducing the surface tension, initially
this causes the needle to be pulled away by the remaining hydrogen bonds, eventually the majority of bonds
break and the needle sinks. Students will not be able to get the needle floating on the water surface again

This can be used to model the impact of pollutants on the properties of water

Discussion points:

Mobility
Organisms such as Pond Skaters use the surface tension to allow them to move across the surface of the
water. They are able to hunt for food (dead and dying animals) on the surface of the water.

Note: Pond skaters also have hairs on their feet and waxy cuticle making their legs hydrophobic as an
additional adaptation to allow them to walk on the water surface

Interesting facts:

The bigger the organism, the bigger the surface area required for them to be able to walk on water……… If
we were to be able to walk on water, our feet would need to be about 1.5 miles long.

Jet Propulsion? Some members of the Stenus species (Rove beetle) are able to secrete a chemical molecule
reducing surface tension locally and therefore creating asymmetrical forces that cause them to move forwards
with considerable greater speed as demonstrated by adding the detergent behind a floating needle

The molecule could be mimicked to spread on mosiquito infested waters (see below)

Respiration
A key adaptation to life in water is to find ways of breathing air by using snorkel like structures which mean
submerged organisms can access the air above. Organisms such as mosquito larvae rely on this surface
tension as a way of anchoring themselves to the underside of the water surface. This anchorage allows them
to access the air for gaseous exchange with minimum effort. Reducing the surface tension reduces the ability
of the organisms to remain at the surface and would cause suffocation

Additional resources for younger students:

http://www.stanford.edu/group/henrysplace/Activities/fall/2_Surface_Tension.doc

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
Activity 2: Capillary action as a result of Cohesion and Adhesion
Requires: Water (with food colouring added for visual impact if required),
Measuring cylinders of decreasing diameters,
Capillary tubes,
Mercury thermometer or barometer if available

Demonstrating the meniscus

All students will be familiar with the concept of the meniscus in a measuring cylinder where water forms a
concave surface. Comparing this to the mercury in a thermometer or barometer they will notice that mercury
forms a convex surface.

Students should compare the meniscus in decreasing measuring cylinders to determine if the radius of the
container has any effect.

Capillary action
Placing a clean capillary tube in water, students will notice that the water appears to rise in the tube. The
narrower the tube, the greater the height difference between the reservoir at the bottom and the height to
which the water rises is likely to be.

Straws can be used to demonstrate drinking and also transpiration in plants as water is very easily drawn up
through the straw.

A subjective assessment can be made quickly between straws of varying diameter.

Discussion points

Both phenomena can be explained by cohesion and adhesion.

Cohesion is a measure of the ability of molecules within a liquid to bond (via hydrogen bonds) to each other
where as adhesion refers to the bonding between the molecules in a liquid and an external substance (in this
case, the sides of the measuring cylinder/thermometer/capillary tube)

When the forces of adhesion are stronger than the cohesion within the liquid a concave meniscus is formed.
The water molecules are “sticking” more strongly to the surface of the container than they are to each other.
If the sides of the measuring cylinder are greased, this adhesion is reduced and it’s possible to see a much
reduced concave meniscus and even possible to observe a convex meniscus which is lower than that of the
surrounding water

The concave and convex meniscus

formed as a result of the varying
forces of cohesion and adhesion

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
A convex meniscus occurs because the forces of cohesion between the molecules within the liquid are
stronger than the adhesive forces of the liquid with the container sides

Cohesion and adhesion in ecology

Cohesion and adhesion help to explain why water moves up through the soil on dry land and contribute to the
flow of water through the xylem vessels in plants.

The majority of aquatic organisms need to ensure that they reduce the adhesive properties of any structures
which are in contact with the water, usually through production of waxy cuticles or production of oils to coat
skin, fur and feathers.

Note: Oil pollution should based on the theory above reduce the adhesive properties actually damages the
natural mechanism of such organisms in a number of ways thereby increasing the adhesion.

If you have access to feathers a good demonstration of the impact of oil on birds feathers can be found at
http://www.biologycorner.com/worksheets/oilspill.htm

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
Activity 3: Water as a temperature buffer

Requires: Water,
Alcohol,
Hot lamp,
Thermometer/temperature probes,
Warmed tile

Observing specific heat capacity

Specific heat capacity can be measured in any number of ways, a good example can be found at
http://timjoh.com/specific-heat-capacity-of-water-h2o/ should you wish to carry out a more detailed practical.

A simple and quick demonstration however that highlights the principle allows students to compare the
temperature rises in water, alcohol and air under a simple heat lamp.

Three test tubes filled with air, alcohol and water should be left under a hot lamp, their temperature recorded
at regular intervals

Care should be taken with the alcohol to monitor its temperature carefully and avoid accidental overheating

Heat of fusion

The practical above can also be carried out in reverse, with the water; alcohol and air cooled in beakers of ice
or kept in a freezer compartment.

Latent heat capacity

The latent heat capacity can also be demonstrated by placing a single drop of alcohol and water onto a
warmed tile. The alcohol will evaporate very quickly and the water much more slowly. On hot sunny days, a
warmed tile is not necessarily required.

Discussion points
The practical ideas here demonstrate the transfer of energy between ponds and the environment, comparing
water to the rates of transfer for air and alcohol.

It takes significantly more energy to heat water than other liquids or air. The Earth which already has a limited
temperature range due to the unique nature of the atmosphere has and due to the high specific heat capacity
of water an even more limited temperature range in its aquatic environments.

Reducing the effect of temperature rises

The high specific heat capacity means that the temperature in ponds on hot sunny days tends not to rise
significantly, an almost constant temperature is maintained. The advantage of this is that the effect of
temperature on respiratory, photosynthetic and gaseous exchange rates from air to water and water to air is
avoided, i.e rising water temperatures often reduces oxygen concentration in water causing respiratory
challenges for aquatic organisms.

Surviving the winter freeze

The heat of fusion in winter means that ponds are less likely to freeze. Due to the density of water when
freezing occurs, ice floats on the surface and aquatic organisms can survive under the surface over winter

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
Ponds can freeze over, although they do not freeze completely. Pond organisms are moiré likely to be
troubled by the reduced oxygen content of water during a winter freeze than the colder temperatures.

Surviving and adapting to summer droughts

The high latent heat of vaporisation means that the volume of water can remain reasonably constant and
ponds tend not to dry out in hot summers however some ponds can and do dry out and in some cases the
local ecology is highly dependant on this annual drying out. Temporary or seasonal ponds are an important
and also threatened habitat often containing rare species, especially amphibian and invertebrate species.
Such organisms prefer temporary ponds because the seasonal drying out kills off fish, their main predator.

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
 Chemistry and biochemistry of water and its effect on pond ecology
Teachers Notes

Water is one of the smallest and lightest of molecules with a unique collection of
properties.

Life in aquatic habitats has many advantages over life on land, but there are also a
range of problems that come with living in such habitats.

Water is essential to life on earth and much of the credit lies in the hydrogen bonding
that exists between water molecules.

Each hydrogen atom is covalently bound to the oxygen

through which electrons are shared, but the distribution
δ+ H of electrons throughout the molecule means that there is
a slight polarity with the hydrogen areas being more
H positive and the remaining electron rich oxygen being
O more negative.

δ- δ+ H
This polarity gives rise to the solvent properties of water
and also enables the hydrogen bonding which results in
the remaining properties of water.
H
O
These unique properties make liquid water the
δ - cornerstone of life on earth. Creating habitats for flora
and fauna to live in whilst contributing a minimum 60%
of all living biomass on the plant

Property Explanation Keywords

Molecules of water are attracted to each
other as a result of hydrogen bonding, in
the case of water this means that up to
15% of water molecules in a glass of water
Hydrogen
are hydrogen bonded to their neighbours at
bonding
any one time. This pulls the water
Sticky
molecules closer together than would other
Transpiration
wise be expected and makes them “sticky”
Formation of
droplets In cohesion, water molecules stick to each
other, where as in adhesion the water
Cohesion
molecules are sticking with other surfaces
and
adhesion
It causes rain drops to form and makes it
possible for humans to drink through
straws and in tall plants helps water move
through the xylem tissues through
transpiration
Hydrogen bonding infers that a water
molecule is pulled equally towards all its
Surface adjacent neighbours but, at the surface of
Pond skater
tension water, these molecules can only be pulled
left, right and down. The pulling of the
water molecules downwards means that a

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
 more solid surface is created, known as
surface tension. Some aquatic
invertebrates like the Pond Skater are able
to make use of this surface tension and
with additional physiological adaptations
“walk” on water.
The transfer of oxygen and carbon dioxide
in and out of all organisms as part of
respiration. The greatest challenge for
aquatic organisms is gaseous exchange.
Gases like oxygen and carbon dioxide will
dissolve in water meaning that some
animals do not need to breathe air in order
to respire but they must still be able to
Dissolved absorb oxygen and excrete carbon dioxide. Respiration
gases There is always less oxygen in water than Dissolved gases
there would be in air and this makes
animals very sensitive to factors that affect
the rate at which oxygen might be
dissolved in water. These include factors
such as temperature, and organic content.
Aquatic fauna have developed a wide range
of strategies to increase the oxygen they
can absorb
Pure water contains no nutrients and as
Solvent such would be a very poor habitat for
organisms. However water is an excellent
solvent and never occurs in its pure form in
the environment. The solvent properties of
water are due to the polarity of the Polarity
Inorganic
Hydrogen and oxygen molecules Photosynthesis
ions and
Inorganic ions dissolved in water mean that Pollutants
nutrients
phytoplankton can photosynthesise forming
the basis of an aquatic food web. Issues
can be caused however by the ease of
which such ions are dissolved in terms of
pollutants from farming and industrial
processes
Because living things are mostly made up
of water, in humans this is around 60-70%,
they float very easily in water. Many
Flotation aquatic organisms develop strategies to Air sac
allow them to alter their ability to float and
swim by having changeable air sacs within
body cavities
Unlike all other liquids, the molecules in
solid water are actually further apart than
they are in liquid water. This makes ice
less dense than liquid water and therefore
ice will float on water. In winter this
Ice means that ponds freeze from the top
Ice as insulation
formation down and ice even acts as an insulating
Density layer protecting the water beneath from
further freezing. Organisms can survive
over winter without freezing. Without this
feature there would be no life in water in
temperate and polar regions
Water is actually at its most dense at 40C,
Viscosity and therefore it’s most viscous. In winter, Streamlining
ponds are less likely to freeze but it does

©British Ecological Society. Chemistry of water and it’s effect on pond ecology
 cause problems for the animals that have
to swim through it. Most aquatic
organisms have developed streamlined
bodies to reduce the effect.
Specific heat capacity refers to the amount
of energy required to increase the
temperature of 1Kg of water by 10C. In
the case of water this is 4.2 Kj and is
significantly higher than would be expected
and higher than most other liquids. In
summer months this means that water
Specific heat Kj
must absorb a great deal of energy in the
capacity
form of heat from the sun in order for the
temperature to increase. Since most
bodies of water are large enough not to be
significantly effected by the heat from the
sun, water provides an almost constant
temperature for the plants and animals
Temperature living there
buffer In summer small water bodies like ponds
are at risk of drying out but again, the
Latent heat amount of energy required to vaporise or
of evaporate water is so high that the impact
vaporisation is less than would otherwise be expected.
Making it rare for water bodies to dry up
and so depriving organisms of their habitat.
As summer turns to winter, water that is
already colder than the surrounding land
should be at risk of freezing, however, the
Heat of energy water must now loose in order to
fusion freeze is so high that the water
temperature again remains roughly
constant, as winter deepens in fact it could
be higher than surrounding land.

©British Ecological Society. Chemistry of water and it’s effect on pond ecology