Essential idea: Water is the medium of life. IB ASSESSMENT STATEMENT Draw and label a diagram showing the structure of water molecules to show their polarity.

Essential idea:

• Water is the medium of life

IB ASSESSMENT STATEMENT

• Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation.

Overview: The Molecule That Supports All of Life

• Water is the biological medium on Earth

• All living organisms require water more than any other substance

• Most cells are surrounded by water, and cells themselves are about 70-95% water

• The abundance of water is the main reason the Earth is habitable

The polarity of water molecules results in hydrogen bonding

• The water molecule is a polar molecule: The opposite ends have opposite charges

• Polarity allows water molecules to form hydrogen bonds with each other

Animation: Water Structure

OHH

_

++

The polarity of water molecules results in hydrogen bonding

• This image of water show the covalent bonds between oxygen and two hydrogen atoms.

• The nuclei of oxygen is significantly larger and greater charge (+8) than the hydrogen nuclei (+1).

• Consequently the electron pair in the covalent bond is found 'closer' to the oxygen than the hydrogen nuclei.

• This creates a polar molecule in which the oxygen carries and additional small negative dipole and each hydrogen a small positive dipole.

Hydrogenbonds

Consequences of Polarity

• Due to the polarity of the water molecules, the small negative charge on each oxygen atoms is attracted the hydrogen atoms of other water molecules.

• These attractions are called HYDROGEN BONDS.

• A hydrogen bond in water has a typical strength of 5.0 kcal/mol but these vary in strength in solutions.

• Hydrogen bonds can occur between water molecules and with other types of polar molecule

IB ASSESSMENT STATEMENT

• Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation.

IB Assessment Statements

• Outline the thermal, cohesive and solvent properties of water.

Four emergent properties of water contribute to Earth’s fitness for life

• Four of water’s properties that facilitate an environment for life:

1. Cohesive behavior

2. Ability to moderate temperature

3. Expansion upon freezing

4. Versatility as a solvent

Cohesion

• Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion

• Cohesion helps the transport of water against gravity in plants

• Adhesion of water to plant cell walls also helps to counter gravity

Animation: Water Transport

Water-conducting cells

100 µm

Xylem Cells Leaves

Cohesion and Adhesion

•Water molecules are weakly attracted to each other by hydrogen bonds (Cohesion). This action extends down the xylem creating a 'suction' effect.

•There is also adhesion between water molecules and the xylem vessels

Xylem Cells Leaves

Capillary Action

•The cohesion and adhesion act together to maintain the water column all the way up from the root to the stomata

•The tendency of water to rise in a thin tube due to adhesion & cohesion is called capillary action

• Surface tension is a measure of how hard it is to break the surface of a liquid

• Surface tension is related to cohesion

Surface Tension is a function of cohesive properties of water

Water is a kinetic energy/ heat energy sponge

– Liquid Water can absorb a lot of heat energy without changing temperature.

Moderation of Temperature

Water has four properties that allow it to moderate the temperature of living organisms and the temperature on earth

1. High Specific Heat Capacity

2. High Latent heat of Vaporisation

3. High Latent heat of fusion

4. Ability to Insulate of Bodies of Water by Floating Ice

Moderation of Temperature (specific heat capacity)

• Liquid water can absorb or release a large amount of heat with only a slight change in its own temperature

• The heat energy required to raise a substances temperature is called specific heat capacity.

• Liquid Water has a high specific heat capacity.

• Thus Liquid water can absorb a lot of heat without changing temperature.

Heat Capacity of Liquid Water

Water is a kinetic energy/ heat energy sponge

– Liquid Water can absorb a lot of heat energy without changing temperature.

Latent heat of vaporization

Latent heat of fusion

Water is a kinetic energy/ heat energy sponge

– Latent HEAT of FUSION – heat energy that can be released before something will start melting/ becoming a liquid. Note that heat is released but the temperature does not change

– Latent Heat of Vaporization – heat energy that can be absorbed before something starts to boil and become a gas. Note that heat is absorbed but the temperature does not change

Latent heat of vaporization

Latent heat of fusion

Water’s High Specific Heat (specific heat)

• Water’s high specific heat minimizes temperature fluctuations to within limits that permit life

– Heat is absorbed when hydrogen bonds break

– Heat is released when hydrogen bonds form

Evaporative Cooling (Latent Heat of vaporization)

• Evaporation is transformation of a substance from liquid to gas

• Latent Heat of vaporization is the heat a liquid must absorb for 1 gram to be converted to gas

• As a liquid evaporates, its remaining surface cools, a process called evaporative cooling

• Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water

Latent Heat of Fusion

• Latent Heat of Fusion -- The amount of heat required to be removed in order for a substance to become a solid.

• The latent heat of fusion is very high for water.

• Thus when water becomes ice it absorbs an incredible amount of heat.

Insulation of Bodies of Water by Floating Ice

• Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense

• If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

The Solvent of Life

• A solvent is the dissolving liquid

• The solute is the substance that is dissolved

• Water is a versatile solvent due to its polarity

• Water is an effective solvent because it readily forms hydrogen bonds

• When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules, a hydration shell

LE 3-6

Na+

Na+

Cl–Cl–

+

+

+

+

+

+

++

–

–

–

–

–

–

–

–

–

–

–

Hydrophilic and Hydrophobic Substances

• A hydrophilic substance is one that has an affinity (attraction) for water

• A hydrophobic substance is one that does not have an affinity (attraction) for water

• Water can also dissolve compounds made of nonionic polar molecules (sugars and alcohols)

• Likes dissolves likes

IB ASSESSMENT STATEMENTS

• Explain the relationship between the properties of water and its uses in living organisms as a coolant, medium for metabolic reactions and transport medium

Thermal Properties• Water absorbs a lot of heat before there is a

change in temperature

• Therefore water is a useful substance for living things both physiologically and ecologically.

Thermal Properties

Physiology: Cooling effects

•The heat generated by the body needs to be removed to prevent a denaturation of enzyme systems.

•Water absorbed a great deal of energy before entering the vapour phase. This makes it an effective agent for the removal of heat and the maintenance of body temperature.

•Blood which is mainly water can absorb and carry heat away from hot parts of the body to the cooler parts.

Ecological effects:

•Oceans, lakes and ponds have fairly stable temperature (since it can absorb a lot of heat without a temperature change) which means that poikilothermic organisms need not waste energy on thermoregulation. Surrounding air temperatures may show marked changes but water water in the same vicinity will remain relatively stable

Medium for metabolic reactions:

• Most biochemical reactions take place in solution where water is the solvent.

– The basis of this ability of water molecules to act as a solvent in covered in section 3.1.5.

– Remember that the cytoplasm is largely water and all cells exist in a medium which is water based e.g. tissue fluid, blood, pond.

Transport Medium:

• Transport systems require something to do the 'transporting', carrying of material from one place to another.

– Blood carries nutrient, gases and waste all of which are dissolved in the water of blood

– Once more this is achieved by taking advantage of the solubility of water.

HELP!!!!

Need more help check out the Basic Biochemistry Tutorial link below:

•http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html

Applications and skills:

•Application: Comparison of the thermal properties of water with those of methane.

•Application: Use of water as a coolant in sweat.

•Application: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water.