1. Water is the medium of life.
Topic 2.2
IB Biology
Miss Werba

2. TOPIC 2 – MOLECULAR BIOLOGY
2.1
MOLECULES TO METABOLISM
2.2
WATER
2.3
CARBOHYDRATES & LIPIDS
2.4
PROTEINS
2.5
ENZYMES
2.6 STRUCTURE OF DNA & RNA
2.7 DNA REPLICATION, TRANSCRIPTION & TRANSLATION
2.8 CELL RESPIRATION
2.9 PHOTOSYNTHESIS
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3. THINGS TO COVER U.1 U.2 U.3 A.1 A.2 A.3 IM.1 Statement Guidance
Water molecules are polar and hydrogen bonds form between them.
U.2
Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.
Should know at least one example of a benefit to living organisms of each property of water.
Transparency of water and maximum density at 4°C do not need to be included.
U.3
Substances can be hydrophilic or hydrophobic.
A.1
Comparison of the thermal properties of water with those of methane.
Comparison of the thermal properties of water and methane assists in the understanding of the significance of hydrogen bonding in water.
A.2
Use of water as a coolant in sweat.
A.3
Modes of transport of glucose, amino acids, cholesterol, fats,
oxygen and sodium chloride in blood in relation to their solubility in water.
IM.1
There are challenges for the increasing human population in sharing water resources equitably.
NOS 2.2
Use theories to explain natural phenomena
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4. STRUCTURE OF WATER Chemical formula: H20 Made up of hydrogen & oxygen
δ –
δ +
Chemical formula: H20
Made up of hydrogen & oxygen
Covalent bonds exist between the atoms
Oxygen nucleus is larger & more charged than the hydrogen nuclei. This pulls the electron pair in the covalent bond 'closer' to the oxygen
This means that water molecules are polar – as one part is more negatively charged than the other
This means that water molecules can attract each other
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5. STRUCTURE OF WATER
U.1
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6. STRUCTURE OF WATER Hydrogen bonds form between the molecules U.1
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7. THEORIES TO EXPLAIN NATURAL PHENOMENA
NOS 2.2
Theories can be used to explain natural phenomena.
The theory that hydrogen bonds form between water molecules explains the properties of water.
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8. PROPERTIES OF WATER Properties of water to be covered:
U.2
Properties of water to be covered:
Cohesive properties
Adhesive properties
Thermal properties
Solvent properties
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9. PROPERTIES OF WATER: Cohesion and adhesion
U.2
Water molecules are strongly cohesive – ie. they stick to each other
They also adhesive – ie. they stick to other molecules that are charged or polar
Polarity of water molecule allows it to form hydrogen bonds with substances
Creates surface tension & allows small insects to be able to walk on water
Allows water to be drawn up xylem vessels in plants
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10. PROPERTIES OF WATER: Thermal properties
U.2
Water has a high specific heat capacity - ie. it takes a lot of energy to raise the temp of water
Water has a high heat of vaporisation - ie. it takes a lot of energy to change it to a vapour
Water has a high heat of fusion - ie. it needs to lose a lot of energy to freeze
Extensive hydrogen bonding requires considerable thermal energy to break
This allows lakes to absorb lots of heat energy & buffer temperature changes for aquatic organisms.
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11. PROPERTIES OF WATER: Thermal properties
U.2
A.2
This is also used by animals and plants as a coolant.
Plants use heat from their leaves for evaporation, thus preventing over-heating.
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12. PROPERTIES OF WATER: Thermal properties
U.2
A.2
This is also true of animals, who use the evaporation of sweat to remove heat from the body.
The blood is also ~95% water. This means that cooler blood can be circulated to other parts in order to cool them down.
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13. THERMAL PROPERTIES: Water vs Methane
Formula
H2O
CH4
Molecular mass 18 16
Polarity
polar
non-polar
Specific heat capacity
4.2
2.2
Heat of vaporisation
2257
760
Melting point (°C)
-182
Boiling point (°C)
100
-160
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14. THERMAL PROPERTIES: Water vs Methane
The key difference between water and methane is their polarity.
Methane has no hydrogen bonds, only covalent.
As a result, little energy is needed to separate it’s molecules from each other.
This causes the differences in their physical properties, particularly their thermal properties.
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15. PROPERTIES OF WATER: Solvent properties
U.2
Water can dissolve many organic and inorganic substances that have charged or polar regions.
Polar attraction of large quantities of water can overcome intramolecular bonds:
positive ions will become surround by the negatively polar oxygen region of the water molecules
negative ions will become surround by the positively polar hydrogen region of the water molecules
Water is often called the universal solvent because of how well it can act to dissolve substances.
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16. PROPERTIES OF WATER: Solvent properties
U.2
Metabolic reactions happen readily in solutions of water.
Water in cells dissolve the reactants/substrates for these reactions.
Substances that are soluble can therefore be easily transported around an organism and be brought into/out of the cells by diffusion.
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17. HYDROPHOBIC vs HYDROPHILIC
U.3
All substances that dissolve in water are hydrophilic
eg. glucose
its positive and negative regions make it polar and soluble
All substances that do not dissolve in water are hydrophobic
eg. oil
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18. MODE OF TRANSPORT: Glucose
Polar molecule
Freely soluble
Carried by the blood plasma
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19. MODE OF TRANSPORT: Oxygen
Non-polar molecule
Soluble in water due to its small size
Solubility of oxygen decreases at higher temps
Carried by haemoglobin protein in red blood cells
adult haemoglobin has four oxygen binding sites
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20. MODE OF TRANSPORT: Cholesterol
Steroid (=lipid)
Hydrophobic molecules
Carried by lipoprotein complexes in the blood plasma
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21. MODE OF TRANSPORT: Fats
Large, non-polar
Hydrophobic molecules
Carried by lipoprotein complexes in the blood plasma
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22. MODE OF TRANSPORT: Sodium chloride (salt)
Ionic molecule (charged)
Freely soluble
Carried by the blood plasma as Na+ and Cl- ions
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23. WATER Q1. Which diagram represents the polarity of a water molecule?
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24. WATER
Q2. Which diagram best illustrates the interactions between water molecules?
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25. WATER
Q3. Describe the importance of water to living organisms. (5 marks)
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