This summary concentrates on the 2016 Syllabus from Biology HL in the IB. It concentrates on chapter 2 and 7, which is the Chemicals of Life and Molecular biology of the Gene
Topic 2: molecular biology
Topic 2.2 Water
Water has two main characteristics
(a)Water Molecules are POLAR
The positioning of the 2 hydrogen atoms in relation to the single oxygen atom
allows the water molecule to have one side that is slightly positive, and another side
that is slightly negative…: water is a di- pole
(b) Water molecules form HYDROGEN
BONDS between each other…
Refers to the electromagnetic attraction force that
exists between the hydrogen atoms of one water
molecule and the oxygen atom of another
water molecule… this allows water to form
drops.
Properties of water
(1) Thermal
Water has a High heat capacity and it
does not change phase easily. This is the amount of heat required to
raise the temperature of 1 gram of water by one degree Celsius. It means that
water can absorb or lose a relatively large amount of heat without undergoing a
large change in temperature. The high number of hydrogen bonds in and around
water molecules ensures that they have to gain/lose a significant amount of energy
in order to change phase and/or temperature.
How does this relate to a function in living organisms?
- Adult humans have approximately 60% water in their body…
- Therefore, body temperature does not vary too much… as water gains and
loses heat slowly
- This allows enzymes and proteins to function without risk of
denaturing/breakdown
- 34 = hypothermia
- 39-42 = hyperthermia
Thermal properties of water VS Methane
Water (H₂O)
, Molecular mass = 18.02 g/mol
Has hydrogen bonds holding individual molecules together
Methane (CH₄)
Molecular mass = 16.04 g/mol
Has weak ‘van der Waal’ molecular forces holding individual molecules together –
because the hydrogens are equally distributed around the carbon molecule… so
here is no proper positive or negative side in a methane molecule
Property Water Methane Explanation
Melting point (solid 0 degrees -182 degrees Ice melts at a much higher temperature…
to liquid) due to hydrogen bonds being stronger than
vdWaals forces
Boiling point (liquid 100 degrees -160 degrees Water boils at a much higher temperature…
to gas) due to hydrogen bonds being stronger than
vdWaals forces… an less keen to separate
from each other
Heat capacity 4.2J/g/˚C 2.2J/g/˚C The amount of energy needed to raise the
(Heat “holding”) raise the temperature of 1g by 1˚C… again
the strength of the hydrogen bonds restrict
movement
Heat to vaporize 2257 J/g 760J/g Water needs a lot more energy input to
(liquid to gas) evaporate 1g compared to methane… the
hydrogen bonds hold each other much
more tightly than the vdWaals forces
(2)Cohesive and adhesive forces
Cohesive forces:
- Water is attracted to water
- Is when the polarity that exists between water molecules
allows them to attract each other to form droplets and
streams
Adhesive forces:
- Water is attracted to other molecules
- Is when the polar nature of water molecules allows them to
be attracted to other polar molecules
How does it relate to a function in living organisms?
- Cohesive forces: Water forms drops, which forms a “solid” stream
without air bubbles in our blood, and surface tension for animals walking
on water
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