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CHAPTER 1:

MOLECULES OF LIFE

1.1 WATER

1.2 CARBOHYDRATES

1.3 LIPIDS

1.4 PROTEINS

1.5 NUCLEIC ACIDS

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Explain the structure of water

molecule. Describe the properties of water and

its importance.

OBJECTIVES

1.1 WATER

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ROLES OF WATER

solvent for polar molecules

medium for exchange of

respiratory gases

medium for enzymatic reactions

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ROLES OF WATER

external fertilization

external media for aquaticorganisms

secrete waste products

locomotion

foodresources

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wide V-shaped structure 1 oxygen atom

2 hydrogen atoms

covalent bonds

STRUCTUREOF WATER MOLECULE

bond angle: 104.5

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Polar molecule

opposite ends of water moleculehave opposite charges

oxygen: partial negative charge;

hydrogen: partial positive charge; +

Polarity allows water molecules to formhydrogen bonds with each other


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Formation of hydrogen bond

hydrogen atoms of one water molecule

by hydrogenbonds

attracted to

oxygen atoms ofnearby watermolecules


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but strong enough to hold watermolecules together

hydrogen bonds are weaker thancovalent bonds

Hydrogen bond

each molecule will form hydrogen

bond to a maximum of four watermolecules


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Has high specific heat

capacity

Has low viscosity

Has high latent heat ofvaporization

As universal solvent

Has high surface tension Has maximum density at 4C

PROPERTIES OF WATER

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due to its polarity: water asuniversal solvent for ions and polarmolecules e.g. Na+ Cl

1. AS UNIVERSAL SOLVENT

oxygen regions are attracted topositively charged sodium

hydrogen regions are attractedto negatively charged chloride

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surroundsodium andchloride

separate

and avoidthem fromeach other

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solvent for most solutes

BIOLOGICAL IMPORTANCE

provides an aqueous mediumfor biochemical reactions

serves as the bodys major

transport medium e.g. in bloodcapillaries and xylem

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2. HAS LOW VISCOSITY

due to hydrogen bonds betweenwater molecules continuouslyform, break and reform

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as lubricant


i.e. lubricates movement ofsubstances

Water molecules can moves easilyand flow with less friction throughnarrow blood vessel

e.g: peristaltic movement of foodbolus in esophagus

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3. HIGH SPECIFIC HEAT CAPACITY

DEFINITION

large amount of heat energy(in J @ cal)

that must be absorbed @ lost for1g of substance (water molecule)

to change its temperature by 1C

specific heat capacity for water

= 4200 J kg-1

C-1

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stabilize ocean temperature

provide constant environment

in the sea for aquatic organisms tolive

act as heat buffer

prevent large fluctuation inbody temperature of terrestrialorganisms

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4. HIGH LATENT HEAT OF VAPORIZATION

DEFINITION

large amount of heat energy mustbe absorbed by 1g of water molecule

to turn liquid water into water vapour

latent heat of vaporization for water

= 540 cal g-1

hydrogen bond between water moleculesmake it difficult for them to be separated

and vaporized

caused by breakdown of hydrogen bond

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evaporation of water in sweat on skin,

or in transpiration from green leaves

or panting in animals e.g. dog

causes the cooling effect

because the escapingmolecules take a lot of energy

with them

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COHESIVE AND ADHESIVE FORCE

cohesive force: force of attractionbetween the same molecule

e.g. between water molecules

adhesive force: force of attractionbetween water molecule with theother molecule

e.g. between water moleculesandwall of xylem vessels

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Upward movement of water from rootsthrough xylem vesselsadhesion and cohesion creates

capillary actionas water evaporates from leaves,cohesion of water within xylemvessel drawn upwards from roots

adhesion of water to wall of xylemvessel resist the downward pull ofgravity

PHYSIOLOGICAL ROLE #1

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5. HIGH SURFACE TENSION

Surface tension is related to cohesiveforces between water molecules

Surface tension is a measure of how

hard it is to break the surface of a liquid

PHYSIOLOGICAL ROLE #2

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arrangement of water moleculeson the surface bonded by

hydrogen bond with watermolecules underneath form a skinlike layer at the surface

p one molecule is

attracted by adjacentmolecules

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surface tension allows insects to walkon the water surface e.g. water skater


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6. MAXIMUM DENSITY AT 4C

at 0

C, liquid water transforms into ice (solid)

ice is less dense than liquid water at 4C

Ice floats in liquid water because hydrogenbonds in ice are more ordered, making ice

less dense

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ice floats; act as insulator

to prevent ponds, lakes, oceansfrom freezing

thus, allow life to exist underthe floating ice

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CONCLUSION

All living organisms require WATER morethan any other substance

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

QUESTION

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QUESTION

Give TWO significances of hydrogenbonding in water molecules.

QUESTION

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QUESTION

Why must water absorb relatively largequantity of heat in order to change itstemperature? [1 mark]

To break hydrogen bond

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