Topic 3 of SL Biology. IB students testing out of SL Biology need to know these key points from the syllabus.

1. Topic 3
The Chemistry of Life

2. Polarity of Water
• Hydrogen pole is positive
• Oxygen pole is negative
• This makes water molecules polar

3. Hydrogen Bonding in Water
• Hydrogen bond: bond that can form
between the positive pole of one water
molecule and the negative pole of another
• Many bonds form in liquid water which
makes it useful for living organisms

4. The Properties of Water
Property name Outline Use in living organisms
Cohesion Hydrogen bonds let water
molecules stick to each other
Used as a transport
medium in plants
Solvent properties Water’s polarity allows it to
dissolve many different
substances
Water is the medium for
metabolic reactions
Heat capacity Large amounts of energy are
needed to raise water’s
temperature
Water in blood can
transfer heat
Boiling point Water has a high boiling point;
large amounts of energy are
needed to break hydrogen bonds
to turn water into a gas
Water is the medium for
metabolic reaction when it
is a liquid
Cooling effect of
evaporation
Water can evaporate at temps
below boiling point; heat energy is
taken from breaking H bonds
Water can act as a coolant
(i.e. sweat)

5. Elements in Living Organisms
• Four most common elements:
1. Carbon
2. Hydrogen
3. Oxygen
4. Nitrogen
• Other important elements:
– Sulfur, Calcium, Phosphorus, Iron, Sodium

6. Organic and Inorganic Compounds
• Organic compounds: containing carbon and
found in living organisms
• All compounds that do not contain carbon
are inorganic
• Few compounds that do contain carbon are
inorganic (Example: Carbon dioxide)
• Living organisms generally have three types
of organic compounds:
– Carbohydrates
– Lipids
– Proteins

7. Subunits of Organic
Macromolecules
• Many organic compounds are made up of
large molecules called macromolecules
• They are built up using subunits

8. Condensation Reactions
• Two molecules are joined together and
water is also formed in the reaction
• Two amino acids in a condensation
reaction create a peptide bond
• Two monosaccharides in a condensation
reaction create a disaccharide
• Fatty acids link to glyceral in a
condesnation reaction and creates
gylcerides

9. Hydrolysis Reactions
• Hydrolysis: breaks down large molecules
into smaller molecules with water
• The reverse of condensation reactions

10. Examples of Carbohydrates
Name Example In animals In plants
Monosaccharide •Glucose
•Fructose
•Galactose
Glucose is carried
by blood to
transport energy
Fructose makes
fruit sweet to
attract animals to
disperse seeds
Disaccharide •Lactose
•Maltose
•Sucrose
Lactose is the
sugar in milk that
provides energy
Sucrose
transports energy
through the
phloem
Polysaccharide •Starch
•Gycogen
•Cullulose
Glycogen is used
for short-term
energy storage in
the liver/muscles
Cellulose makes
strong fiber to
construct the plant
cell wall

11. Functions of Lipids
• Energy storage
– Create fat in humans
– Create oil in plants
• Heat insulation
– Layer of fat under skin reduces heat loss
• Buoyancy
– Lipids are less dense than water so they can
help animals float

12. Carbohydrates and Lipids in
Energy Storage
Advantages of lipids Advantages of carbohydrates
Contain more energy per gram than
carbohydrates, making stores lighter
More easily digested than lipids,
making energy release faster
Insoluble in water and so do not effect
osmosis in cells
Soluble in water and so are easier to
transport
Usually used for long term energy
storage
Usually used for short term energy
storage

13. Nucleotide Subunits of DNA
• DNA is made of nucleotides
• Each nucleotide contains a sugar,
phosphate group, and a base
• There are four different bases found in
nucleotides:
– Adenine
– Thymine
– Guanine
– Cytosine

14. Building DNA Molecules
• Two DNA nucleotides can be linked
together by a covalent bond
– Bond forms between the sugar of one
nucleotide and the phosphate of another
• DNA consists of two strands of nucleotides
in a double helix

15. Complementary Base Pairing
• Certain nucleotide bases will only bond
with one other kind of nucleotide base
– Adenine bonds with Guanine
– Thymine bonds with Cytosine
• This is called complementary base pairing

16. DNA Replication
• Semi-conservative: each molecule formed by
replication contains one strand of the original
DNA and one new strand
• Stages of replication:
1. DNA uncoils and hydrogen bonds are broken by
the enzyme Helicase
2. Single strand acts as a template upon which
free nucleotides bond to; caused by the enzyme
Polymerase
3. New strands of DNA form double helix

17. Differences Between DNA and RNA
DNA RNA
Two strands of nucleotides to form a
double helix
One strand only
Deoxyribose sugar Ribose sugar
Use bases Adenine, Guanine,
Cytosine, and Thymine
Uses bases Adenine, Guanine,
Cytosine, and Uracil (instead of
Thymine)

18. Genes and Polypeptides
• Polypeptides are made up of amino acids
• Information for making polypeptides are
stored in a coded form in the genes of the
amino acids
• The sequence of bases in a gene codes
for the sequence of amino acids in a
polypeptide

19. Transcription
• Transcription: Copying of a base sequence of DNA
by making RNA
• Uses complementary base pairing; replacing
Thymine with Uracil when bonded to Adenine
• Stages:
1. DNA uncoils and separates
2. Free RNA nucleotides assemble using one strand as
a template
3. RNA nucleotides link together
4. The RNA strand then separates from DNA; it is now
messenger RNA (mRNA)
5. DNA reforms a double helix

20. Translation
• Translation: Translates the RNA genetic code
(which are in groups of codons) into an amino acid
chain
• Stages:
1. mRNA binds to the small subunit a of ribosome;
mRNA contains codons
2. Free transfer RNA (tRNA) molecules have
anticodons that are complementary to certain mRNA
codons; tRNA also carries amino acids
3. tRNA bonds to the ribosome if it is complementary to
the mRNA codon; these codons and anticodons
form hydrogen bonds
4. The amino acids carried by the bonding tRNA
molecules bond together into peptides

21. One Gene-One Polypeptide Hypothesis
• There is almost always a single gene to
code for a polypeptide that does not code
for any other polypeptide

22. Introducing Enzymes
• Enzymes: globular proteins that catalyze
chemical reactions
• By making only certain enzymes, cells can
control what chemical reactions take place
• Denaturation: changing the structure of an
enzyme so it can no longer carry out its
function
• Substrates: the reactants in enzyme
reactions

23. Enzyme-Substrate Specificity
• Most enzymes are specific and only catalyze
certain reactions with certain substrates
• Substrates bond to active site of an enzyme
• Active site: region on the surface of an
enzyme to which substrates bind to catalyze
a chemical reaction with the substrate
– Only certain substrates can fit
the shape (like a key fitting into
a lock)

24. Factors Affecting Enzyme Activity
• Temperature
– As temp increases, so does enzyme activity
– At a certain temp enzyme activity will drop
• pH
– Optimum pH is 7
– pH levels closest to 7 have highest activity
• Substrate concentration
– The greater the amount of substrates available to
bind to, the greater the activity
– Enzyme activity eventually plateaus

25. Lactase and Lactose-Free Milk
• Lactose: natural sugar in milk
• Lactase: converts lactose into glucose and
galactose
• Biotechnology can extract lactase to
prevent possible negative effects caused
by lactose

26. Energy and Cells
• All living cells need continuous energy
• Cell respiration: controlled release of ATP
energy from organic compounds in cells
• Can be aerobic (with oxygen) or
anaerobic (without oxygen)

27. Use of Glucose in Respiration
• Cell respiration often uses glucose
• Glucose is broken down into pyruvate (a
simpler ogranic compound)
• This produces a small amount of ATP
energy that is released by glucose

28. Anaerobic Cell Respiration
• If oxygen is unavailable, pyruvate is
converted into a waste product
– Waste product is either lactate (in humans); or
ethanol or carbon dioxide (in yeast)
• No ATP is produced

29. Aerobic Cell Respiration
• If oxygen is available, the pyruvate is
absorbed into the mitochondria and is
broken down into Carbon Dioxide and
Water
• ATP is produced

30. Introducing Photosynthesis
• Photosynthesis: process used by plants to
produce organic substances from light
energy and inorganic substances
• Light energy is converted to chemical
energy
• Chlorophyll is the main pigment that
absorbs light
• Some of the absorbed energy makes ATP
and other split into water molecules

31. Measuring Rates of Photosynthesis
• Can measure the rate of photosynthesis
with:
– Production of Oxygen
– Uptake of Carbon Dioxide
– Increase in Biomass

32. Factors Affecting Photosynthesis
• Light intensity
– As light intensity increases, the rate of photosynthesis
increases
– Eventually plateaus
• Carbon Dioxide
– As CO2 concentration increases, the rate of
photosynthesis increases
– Eventually plateaus
• Temperature
– As temperature increases, the rate of photosynthesis
increases steeply
– Reaches an optimum temperature then the rate drops