1

The Significance of Water to Living Organisms

• Water is of immense importance to all 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– Refer to thermal properties

• Water is used as a transport medium– Refer to cohesion, solvent

properties, and thermal properties• Water is used as a habitat

– Hydrogen bonding, refer to cohesion, transparency, and thermal properties

2

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

3

slightly positive charge

slightly negative charge

hydrogen bond between (+) and (-) areas of different water molecules

Water molecules form Hydrogen bonds

4

Thermal Properties:Heat Capacity

• 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

5

The Importance of Ice Floating• If ice sank to the bottom of

a body of water as it cooled…– 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 are the muddy bottom

• Ice atop a body of water, insulates the water below – Life persists under the frozen

surface

6

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– 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

7

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

8

Cohesion• 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

9

• 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

10

• Adhesion refers to attraction to other substances.– Water is adhesive to

any substance with which it can form hydrogen bonds.

– Think water climbing up capillary tubes…

Adhesion

11

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

• 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

12

Solvent Properties

• Many different substances dissolve in water because of its polarity

• Inorganic particles such as sodium ions and organic substances such as glucose can dissolve Water’s polarity “pulls” apart the ions of a

particular substance (above NaCl). The positivehydrogen ends are attracted to the anions of a substance and the negative oxygen ends are attracted to the cations of a substance

13

Like dissolves like:water can interact with other polar molecules

14

• Water 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

15

Transparency

• The fact that water is clear allows light to pass through it– Aquatic plants can receive sunlight– Light can pass through the eyeball to receptor

cells in the back