Chemical Elements
and Water
Presentation Modified from J. Naftzinger
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Atoms and Ions

An atom is a single particle of a chemical element
 If an atom either gains or loses electrons it becomes an ion
 Atoms are uncharged particles and ions are charged – they
have either positive or negative charges
 For example, if a sodium atom (Na) loses an electron it
becomes a sodium ion (Na+)
 This is important for when discussing solubility
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Elements Found in Living Organisms
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Living organisms contain
many chemical elements,
some in large quantities
and some in very small
amounts
The 4 most frequently
occurring chemical
elements of life are
carbon, hydrogen, oxygen
and Nitrogen (CHON)
They are part of all the
main organic compounds
in living organisms
Organic compounds:

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contain carbon
found in living organisms
• carbohydrates, lipids,
amino acids are all organic
molecules
Other elements needed by living things:
1. Calcium (bones)
2. Phosphorus (Nucleic acids)
3. Iron (blood)
4. Sodium (Na+/K+ pumps)
5. Sulfur (amino acids)
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Examples of Chemical Elements
and Their Roles
Element
Role in Plants or Animals
Sulfur
Part of the amine groups of amino
acids and therefore proteins
Calcium
Needed to make the mineral that
strengthens bones and teeth
Phosphorus
Part of the phosphate groups in ATP
and DNA molecules
Iron
Needed to make hemoglobin and
thus to carry oxygen in blood
Sodium
Used in neurons (nerve cells) for
the transmission of nerve impulses
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The Significance of Water to Living
Organisms

It is used by them in many
different ways
 These uses can be explained by
referring to the properties of
water
 Water is used a coolant

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Water is used as a transport
medium and medium for
metabolic reactions


Refer to thermal properties
Refer to cohesion, solvent
properties, and thermal properties
Water is used as a habitat

Hydrogen bonding, refer to
cohesion, transparency, and thermal
properties
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
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Polarity and Hydrogen Bonding
in
Water
Water molecules consist of 2
hydrogen atoms bonded to an
oxygen atom
The hydrogen atoms have a slight
positive (+) charge and the oxygen
atom has a slight negative (-)
charge
So, water molecules have 2 poles
(they are dipoles) – a positive
hydrogen pole and a negative
oxygen pole
This feature of a molecule is called
polarity
A bond can form between the
positive pole of one water
molecule and the negative pole of
another
This is called a hydrogen bond
In liquid water, many of these
bonds form
Hydrogen bonds provide water
with much of its basic properties
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Water molecules form Hydrogen
bonds
slightly positive
charge
hydrogen bond
between (+) and (-)
areas of different
water molecules
slightly negative
charge
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Thermal Properties:
Heat Capacity
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Water has a high heat capacity
This means that large amounts of
energy are needed to raise its temp
The energy is needed to break one
of the hydrogen bonds
This heat energy is given out again
when the water is cooled
The temperature of water tends to
remain quite stable
This is useful for organisms such
as fish that use water as a habitat
Blood, which is mainly composed
of water, can carry heat from
warmer parts of the body to cooler
parts
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The Importance of Ice Floating

If ice sank to the bottom
of a body of water as it
cooled…
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Ponds and lakes would
freeze up from the bottom
Fish and other organisms
would be trapped in the
small amount of water
that is left
There would be no
access to the nutrients at
the muddy bottom
Ice atop a body of
water, insulates the
water below

Life persists under the
frozen surface
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Thermal Properties:
Boiling and freezing points

The boiling point of water is relatively
high because to change it from a
liquid to a gas all of the hydrogen
bonds between the water molecules
have to be broken
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In natural habitats on Earth, water rarely
boils
Living organisms could not survive if the
water inside them boiled
Water also freezes at a relatively high
temperature but because it becomes
less dense as it cools to freezing
point, ice forms at the surface first

The ice that forms on the surface of lakes
or seas insulates the water underneath,
so living organisms can survive there
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Thermal Properties:
The Cooling Effect of Evaporation

Water can evaporate at temps
below boiling point
 Hydrogen bonds have to be
broken to do this
 The heat energy needed to
break the bonds is taken from
the liquid water, cooling it
down
 Evaporation of water from
plant leaves (transpiration) and
from human skin (sweat) has
useful cooling effects
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Cohesion
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Water
molecules stick
to each other
because of the
hydrogen
bonds that
form between
them
Strong pulling
forces can be
exerted to suck
columns of
water up to the
tops of the
tallest trees in
their transport
systems

These columns
of water rarely
break
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Trees have specialized structures to transport water:
xylem and phloem “plumbing”
Water molecules are “dragged” from the roots to the
top of the tree by capillary action (adhesion) and
cohesion: hydrogen bonds help water molecules
hydrogen bond to each other
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Adhesion
 Adhesion
refers to
attraction to other
substances.

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Water is adhesive
to any substance
with which it can
form hydrogen
bonds.
Think water
climbing up
capillary tubes…
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Cohesion

At a surface, the cohesion of
water molecules can make it
difficult for small objects to
break through

surface tension
• a measure of the force necessary
to stretch or break the surface of a
liquid
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Some animals such as
mosquito larvae use the
surface of water as a habitat
Though they are denser than
water they remain on the
surface and do not sink
because of the high surface
tension of water caused by
cohesion
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Solvent Properties
 Many
different
substances dissolve
in water because of
its polarity
 Inorganic particles
such as sodium
ions and organic
substances such as
Water’s polarity “pulls” apart the ions of a
glucose can
particular substance (above NaCl). The positive
dissolve
hydrogen ends are attracted to the anions of a
substance and the negative oxygen ends are
attracted to the cations of a substance
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Like dissolves like:
water can interact with other polar molecules
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 Water
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transports molecules dissolved in it
Blood, a water-based solution, transports
molecules of nutrients and wastes within
organisms
Nutrients dissolved in water get transported
through plants
Unicellular organisms that live in water absorb
needed dissolved substances
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Transparency
 The
fact that water is clear allows light
to pass through it
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Aquatic plants can receive sunlight
Light can pass through the eyeball to
receptor cells in the back
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Some acids and bases (HCl and NaOH) are strong acids
or bases.
 These molecules dissociate completely in water.

Other acids and bases (NH3) are weak acids or bases.
 For these molecules, the binding and release of
hydrogen ions are reversible.

Carbonic acid (H2CO3) is a weak acid:
H2CO3 <=> HCO3- + H+
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pH scale
• the environment needs to
be at a specific pH for many
organisms to survive
•Ex: most cells have an
internal pH of 7 (neutral)
•If the pH changes, even
slightly, this can disrupt
the chemical reactions
inside the cell
•Blood pH =7.4, a person
will die if pH reaches 7.8
or 7 for even a little
while!
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
The chemical processes in the cell can be disrupted
by changes to the H+ and OH- concentrations away
from their normal values near pH 7.
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To maintain cellular pH values at a constant level,
biological fluids have buffers.

Buffers resist changes to the pH of a solution when H+
or OH- is added to the solution.
 Buffers accept hydrogen ions from the solution
when they are in excess and donate hydrogen ions
when they have been depleted.
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