TOPIC 3:
CHEMISTRY of LIFE
3.1: Chemical Elements and Water
3.1.1: State that the most frequently occurring chemical elements in living
things are carbon, hydrogen, oxygen and nitrogen
3.1.2: State that a variety of other elements are needed by living organisms, including
sulfur, calcium, phosphorus, iron and sodium.
3.1.3: State one role for each the elements mentioned in 3.1.2
Trace Elements
Biological Functions
Sodium (Na)
nerve function (animal)
Calcium (Ca)
bone formation (animal)
Iron (Fe)
constituent of red blood cells (animal)
Phosphorus (P)
nucleic acid, ATP formation (animal, plant,
prokaryotes)
Sulfur (S)
stabilizes the structure of proteins (animal, plant,
prokaryote)
IB QUESTION: State four elements that are needed by living organisms, other than carbon,
hydrogen and oxygen, giving one role of each. [4]
IB QUESTION: State four elements that are needed by living organisms, other than carbon,
hydrogen and oxygen, giving one role of each. [4]
nitrogen – structure of organic molecules/proteins/nucleotides;
sulfur – amino acid / protein structure;
phosphorus – nucleic acids / energy carriers;
calcium – bone structure / trigger exocytosis (e.g. neurotransmitters);
iron – prosthetic groups / hemoglobin;
sodium – membrane potential; [4 max]
Accept other valid roles for those elements already listed.
Accept other valid examples of elements with their roles.
To award [4 max], responses need an appropriate role for each element stated.
3.1.4: Draw and label a diagram showing the structure of water molecules to
show their polarity and hydrogen bond formation.
Property of water
(High thermal heat capacity
High latent heat of vaporisation)
Relationship
between property
of water and its
use in living
organisms
IB Question: Outline the thermal, cohesive and solvent properties of water.
IB Question: Outline the thermal, cohesive and solvent properties of water.
Answer:
Thermal Properties:
water has a high specific heat capacity;
a large amount of heat causes a small increase in temperature;
water has a high latent heat of vaporization;
a large amount of heat energy is needed to vaporize/evaporate water;
Cohesive Properties:
hydrogen bonds between water molecules make them cohesive/stick together;
this gives water a high surface tension / explains how water rises up xylem;
Solvent Properties:
water molecules are polar;
this makes water a good solvent; [5 max]
Award [4 max] if thermal, cohesive and solvent properties are not all
mentioned.
Thermal properties Coolant:
hydrogen bonds between polar water molecules cause water to resist change
high heat of vaporization (energy required to change liquid water to vapor) because hydrogen
bonds must be broken
thus, evaporation of water from plant leaves (transpiration) or from human skin (sweat)
removes heat, acting as a coolant
IB Question: Explain how chemical bonding between water molecules makes water a valuable
coolant in living organisms. [2]
IB Question: Explain how chemical bonding between water molecules makes water a valuable
coolant in living organisms. [2]
hydrogen bonding between water molecules;
breaking (hydrogen bonds) needs/removes energy/heat;
hydrogen bonds must break when water evaporates/vaporizes;
Cohesive property  Transport medium
hydrogen bonds between polar water molecules cause them to cohere
as water is lost by transpiration/evaporation from plant leaves, hydrogen bonds between
adjacent water molecules pull water up columns of xylem
thus, plants move water against gravity, caarying with ot dissolved mineral, ions etc. by as much
as 100 meters
Solvent Medium for metabolic processes
cytoplasm is primarily water, providing a polar medium in which other polar or charged
molecules dissolve
many enzymes are globular proteins that are water soluble so they dissolve in cytoplasm where
they control metabolic reactions
IB Question: Blood plasma is about 90 % water. List two properties of water that enable blood
to perform its role effectively. [1]
IB Question: Blood plasma is about 90 % water. List two properties of water that enable blood
to perform its role effectively. [1]
Accept any two of the following. solvent / wide temperature range as liquid / high heat capacity
/ cohesion between molecules [1] Do not accept “thermal properties” as it is too vague.
IB Question: Explain how the properties of water are significant to living organisms. [9]
IB Question: Explain how the properties of water are significant to living organisms. [9]
water is transparent / light passes through water;
this allows organisms to live below the surface / plants to photosynthesize;
hydrogen bonds between water molecules make water cohesive;
this gives water a high surface tension allowing animals to live on the surface / maintains lung
structure (pleural membranes);
helps in water movement through plants/transpiration;
water has a high latent heat of vaporization / OWTTE;
evaporation/sweating/transpiration leads to cooling;
water has a high specific heat capacity / OWTTE;
this provides a stable environment for water organisms;
water is a universal solvent;
can transport materials around organisms/plants/animals;
can be a solvent for chemical reactions in organisms;
ice is less dense than water / water has a maximum density at ; 4 C
surface (pond/lake/ocean) freezes first, allowing organisms to survive in the water below; [9
max]
Accept hydrogen bonds between water and other substance makes water adhesive from AHL.
IB Question: Explain the properties of water that allow living organisms to use it as a habitat.
[3]
is transparent so it allows photosynthesis / capture of prey;
aquatic food chains can exist;
resists changes in temperature / has a high specific heat capacity;
provides a stable thermal environment;
water contains dissolved gases needed for life;
hydrogen bonding/surface tension enables water surface to be used as a habitat;
high boiling point means natural water habitats rarely boil;
when water cools/freezes it becomes less dense so ice forms at surface, providing
insulation to water below, in which living organisms can survive; [4 max]
IB Question: Outline the significance to organisms of the different properties of water. [5]
IB Question: Outline the significance to organisms of the different properties of water. [5]
water is transparent / allows light to pass through for photosynthesis;
cohesion of water molecules allow transport in plants;
solvent - chemical reactions take place in water;
many substances dissolve in water and can be transported;
high boiling point making liquid water available to organisms / water is liquid over
a range of temperatures;
water is most dense at 4°C so ice floats over water providing winter habitat;
high specific heat capacity so stable environment (internal/external);
high surface tension - supports (near) surface dwelling organisms;
coolant - absorbs heat when it evaporates / changes states; [5 max]