WATER

Water and Lifewater:the most abundant substance in living systems

of earth’s surface

Human body = 65% Plants = 80%Fruits and Vegetables = 70-80%

70%

The Chemical Basis of Life

Living organisms are composed of

about 25 chemical

elements

make up the bulk of living

matter

2.4 Atoms consist of  protons, neutrons,andelectrons•Atom: The smallest particle of matter that stillretains the properties of an element

                                                                                                                                       

Atoms are made up of:

Protons: positive charge +1

Electrons: negative charge –1

Neutrons: zero charge

2.4 Atoms consist of  protons, neutrons,andelectrons•Atom: The smallest particle of matter that stillretains the properties of an element

                                                                                                                                       

Atoms are electrically neutral: protons = electrons

Atoms of each element are distinguished by a

specific number of protons

Electron arrangement determines the chemical properties of an atom

Electrons in an atom are arranged in shells, which may contain different numbers

of electrons

Electrons in an atom are arranged in shells, which may contain different numbers

of electrons

Molecules are groups of atoms held together in a stable association.

Compounds are molecules containing more than one type of element.

C6H12O6

Atoms are held together in molecules or compounds by chemical bonds.

Atoms whose shells are not full, tend to interact with other atoms and:

1. Gain 2. Lose 3. Share electrons

7e- in outer shell 1e- in outer shell 4e- in outer shell

Types of chemical bonds:1.Ionic bond2.Covalent bond3.Hydrogen bond

NaSodium atom(an uncharged

atom)

ClChlorine atom(an uncharged

atom)

Na+

Sodium ion(a cation)

Cl–

Chlorine ion(an anion)

Sodium chloride (NaCl)

Ionic bonds are attractions between ions of opposite charge• When atoms gain or lose electrons

– Charged atoms called ions are created

Ionic bonds are formed by the electrical attraction of positive and negative ions.

Ionic bond in sodium chloride

A single crystal of table salt (NaCl)

shown microscopically.

• Covalent bonds join atoms into molecules through electron sharing

• In covalent bonds, two atoms share one or more pairs of outer shell electrons, forming molecules

Differences in electronegativity dictate how electrons are distributed in covalent bonds.

Sharing of electrons can be:

Equal Unequal

Electronegativity is an atom’s affinity for electrons.

Sharing of electrons is:

e.g. in a hydrogen molecule   

polar covalent bonds

nonpolar covalent bonds

Sharing of electrons is equal in:

Oxygen (O2)

Name(molecularformula)

Electron-shell

diagram

Structuralformula

Space-fillingmodel

OVERVIEW WATER THE KINDS OF MOLECULES THAT

CHARACTERISE ORGANISMS CARBOHYDRATES LIPIDS PROTEINS NUCLEIC ACIDS IMPORTANCE OF CARBON IN

BIOLOGICAL MOLECULES

H2O

Protons: 1Electrons: 1

Protons: 8Electrons: 8

is more electronegative

than

Thus shared electrons in a water molecule are far more likely to be

found near the oxygen nucleus than the hydrogen nuclei.

So oxygen attracts the electrons more strongly.

The oxygen end of the water molecule has a slight negative charge

(delta -).

The hydrogen end of the water molecule has a slight positive

charge (delta +).

Water is considered a polar molecule.– It has a positive and negative end.

Due to the presence of a pair of electric charges equal in magnitude but opposite polarity,

separated by some distance

The water molecule said to be dipolar

All living organisms are dependent on water.

The structure of water is the basis for its unique properties.

The most important property of water is the ability to form hydrogen bonds.

Hydrogen Bonds• Polar covalent

compounds, like water, can form hydrogen bonds.

• A hydrogen bond occurs when two compounds that contain charged areas attract each other.

Hydrogen bonding between water molecules

Each hydrogen bond is individually very weak, however, the cumulative effects of large numbers

of these bonds can be enormous

Water has unusual properties e.g. ice floats,is an excellent solvent, is both cohesive (sticking with itself) and

adhesive (sticking to things)

What gives water these unusual properties?1.shape of the water

molecule2.polar nature3.capacity to form

hydrogen bonds

H-bonds occur between:

Biological significance of water

1. Solvent PropertiesWater is known as the universal solvent.because being polar, it can dissolve many different solutes.

WATER: is an excellent solvent for: 

A substance in solution is more reactive than in a solid as its

particles are free.

Polar substances e.g. glucose

Ionic substances e.g. salt

1. a full charge (ion) e.g. NaCl

2. a charge separation (polar)

e.g. glucose

Water molecules gather closely around any substance that bears an electrical charge, whether that substance carries:

  Water organises nonpolar molecules

Water causes hydrophobic molecules to aggregate or assume

specific shapes.

hydrophilic: “water-loving”

hydrophobic: “water-hating”

Nonpolar molecules do not dissolve

Oil

Hydrophobic interactions are important in maintaining stability of various substances such as :-

membranesmany protein moleculesnucleic acids

Phospholipid bilayer in a membrane

WATER: has a role in transport in plants & animals

1°C

2. High Specific Heat CapacityThe heat needed to raise the temperature

of 1kg of water by 1°C is termed the specific heat capacity.

1kg of water

Water can absorb large amounts of heat energy before its temperature increases by a significant amount.

Water has a large specific heat capacity. What does this mean?

Why is this?A lot of heat energy is

needed to overcome the hydrogen bonds.

Heat must break these bonds to make the water molecules free to move

As water does not quickly heat up or

cool down as air does.

A high heat capacity ensures a stable environmental temperature. Why?

Water acts as a thermal buffer in:

Large bodies of water: e.g. oceans & lakes

Terrestrial organisms: prevents rapid changes in body temperature

3. High heat of vaporisationWhile changing state, a substance will:

This is termed Latent Heat

1. take in heat energy

2. expel heat energy

without a change in temperature: OR

Water has a large latent heat of vaporisation. What does this mean?

It absorbs a large amount of heat energy while changing from water to vapour.

Add 540 calories/g

Remove 540 calories/g

has a cooling effect as in sweating, panting and transpiration

The importance of water having a high heat of vaporisation is that water:

A lot of heat energy is needed to overcome the

H-bonding

Another feature of water: has a high boiling point.

4. High Heat of Fusion• a relatively large amount of heat, called the

latent heat of fusion, is required to convert a liquid to a solid or vice versa

Ice needs a large amount of heat to melt

Water must lose a large amount of heat to freeze

Cell contents & their environments are less likely to freeze

Water is less dense as a solid!

WHY?

5. Density and Freezing Point

Hydrogen bonds are stable in ice – each molecule of water is bound to four of its

neighbours.

Liquid

water molecules are constantly bonding

and rebonding – space is always

changing

Solid

water molecules are bonded together –

space between fixed

Unlike other substances, water expands as it freezes.

H H

O

H H

OH H

OH H

O

H H

O

H H

O

H H

O

H H

O

H H

O

H H

O

H H

O

Water molecules in ice form a rigid structure: there is more space between

them, this is seen as expansion as a whole.

Liquid Ice

Water molecules are able to approach one another quite closely.

• As water expands when it freezes, its density (mass per unit volume) will decrease.

• This means that ice: is less dense than water will therefore float on top of it.

The graph shows the relationship between water density and temperature.

The high relative density of water explains why: many organisms readily float in water

helps the swimming of motile gametes and in the dispersal of fruit, seeds and spores

Ice floats on the surface of the water, acting as an insulator.

What is the significance of bodies of water freezing from the top down?

Life goes on in the liquid portions!!

Water below 4C rises and circulates resulting in nutrient cycling

Turnover in a pond Top layer is coldWater warms up = more denseWater SINKS

Turnover in a pond Top layer is warmWater cools = more denseWater SINKS

Turnover in a pond

Water has many special properties which support life on Earth. The graph shows the relationship between water density and temperature.

Question:

a) Describe the relationship shown by the graph.Relative density decreases below and above 4 C.

b) Explain how the change in water density protects aquatic animals when temperatures fall below 4C.

Ice floats and acts as an insulator.

6. High Surface Tension, Cohesion & Adhesion

The polarity of water causes it to be cohesive and adhesive.

cohesion: water molecules stick to other water molecules by hydrogen bonding

adhesion: water molecules stick to other polar molecules by hydrogen bonding

Adhesion

– Helps plants transport water up their stems

Water moves up a narrow glass tube by capillary action.

The narrower the tube, the greater the surface area available for adhesion for a given volume of water, and the higher the water rises in the tube.

Adhesion & cohesion of water molecules can be seen in the

formation of a drop.

Cohesion:- is the attraction between

molecules of water

- causes water to form beads or droplets

- creates the effect of surface tension

Surface tension causes the surface of water to behave like an elastic sheet

Cohesive forces at the surface can support small

organisms.

Water strider

Jesus Lizard

Explain the picture.

Surface tension holds water molecules

together

Question: [SEP, 2008]NASA’s Phoenix Lander has been exploring the arctic plain of the planet Mars since 25th May 2008 and has already provided definitive proof that ice and water exist on Mars. This has been interpreted, in some quarters, as suggesting that Martian life is a possibility. On the other hand, another craft, the Spirit Rover, recently discovered extensive salt deposits on Mars, suggesting that any Martian water would have been very saline

1 Why should the presence of water encourage speculation about life on Mars?

2 Why is very saline water not conducive to life?

Very high salinities lead to protein aggregation and

enzyme inactivation

Reduction in dissolved oxygen content with increasing salinity

of water.

Cells dehydrate and cannot

function properly.

2 Why is very saline water not conducive to life?

Salt in Dead Sea

Density problems for aquatic organisms which tend to float on the surface of highly saline water.

Is water really necessary for life? Discuss.

[MAY, 2008]

Essay Title

OVERVIEW WATER THE KINDS OF MOLECULES THAT

CHARACTERISE ORGANISMS CARBOHYDRATES LIPIDS PROTEINS NUCLEIC ACIDS IMPORTANCE OF CARBON IN

BIOLOGICAL MOLECULES

• are large, complex assemblies• biological macromolecules are grouped into

FOUR major categories:

MACROMOLECULE SUBUNITProteins Amino acids

Carbohydrates Monosaccharides

Nucleic acids Nucleotides

Lipids form large structures from a limited set of smaller molecules

Polymers

Not Polymers

Polymer : a long molecule made of subunits called monomers

Functional groups give specific properties to biological molecules

Functional groups give specific properties to biological molecules

Each functional group• has specific chemical properties • when it is attached to a larger molecule, it

confers those properties on the larger molecule

• one of these properties is polarity e.g. OH group

• as macromolecules are so large, they contain many different functional groups

e.g. a single protein molecule may contain: hydrophobic polar charged functional groups

each gives different specific properties to local

sites on the macromolecule

These diverse groups and their properties help:

• determine the shapes of macromolecules• as well as how they interact with other

molecules

Polymers are formed in condensation reactions.

Monomers are joined by covalent bonds.

A water molecule is removed.

Condensation and Hydrolysis

condensation  or

dehydration synthesis) 

Polymers are broken down into monomers in hydrolysis reactions.

(hydro, “water”; lysis, “break”)

Question: SEP, 2010 (End-of-Year Exam)

Use your knowledge of biology to describe the significance of the following.

Hydrolyses and condensation reactions are important in formation of biological molecules. (5 marks)

Question: May, 2011 (End-of-Year Exam)

Use your knowledge to discuss the biological significance of the following:

H-bonds are present between water molecules. (5 marks)

Definition and explanation of the hydrogen bond as forming from the attraction of the oppositely charged sides of the polar water molecule.

[1] Significance: any FOUR of the following [1 mark per point; maximum 4 marks]• Water adhesion/ cohesion properties and implications (e.g. insects

waking on water, surface tension, capillarity, etc.),• Solvent properties of water re both organic substances and inorganic

ions,• High specific heat capacity of water due to larger amounts of energy

required to break the bonds (useful as water will take longer to cool down and provide a more stable environment),

• Water becoming less dense when freezing (therefore enabling aquatic organisms to survive in cold conditions),

• High heat of vaporization of water (useful for cooling down surfaces e.g. leaves, skin when sweating etc.),

END OF SECTION