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Summary IB Biology HL Topic 2 Molecular Biology Notes
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50 pages of Molecular Biology notes according to syllabus points. Includes diagrams, tables, explanations, examples.
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UNIT 2 : MOLECULAR BIOLOGY2.1 - Molecules to metabolism
Carbon:
- Can form four covalent bonds as carbon-carbon bonds are extremely strong and
stable
- So, carbon can make up many of the basic building blocks of life (sugars, fats,
proteins)
- All bonding in hydrocarbons is covalent
- Carbon allows a diversity of stable compounds to exist
- Carbon compounds can form covalent bonds with oxygen, hydrogen, nitrogen and
sulfur
- Carbon compounds can form small single subunits (monomers) that bond with many
repeating subunits to form large molecules (polymers) through polymerisation
- Covalent bonds are chemical bonds formed by the sharing of a pair of electrons
between atoms
- The nuclei of two different atoms are attracting the same electrons
Life is based on carbon compounds including carbohydrates, lipids, proteins and nucleic
acids.
Carbohydrates:
General formula: (CH2O)n
- Composed of carbon, hydrogen, and oxygen
- Used for short term energy or structural purposes
- Monomers are usually ring shaped molecules
- Soluble in water, so they are easily transported around the body
- Fast and easy to digest so their energy is useful if the body requires energy quickly
Examples: starch, glucose, cellulose, glycogen
Lipids:
- Used for long-term energy storage (fat in humans, oil in plants) which can release
fuel for cellular respiration
- Used as heat insulation (fat under the skin reduces heat loss)
- Allow buoyancy (ability to float) as they are less dense than water, so animals can
float on water
- Insoluble in water, so they don’t affect osmosis which prevents problems within cells
in the body
- Soluble in nonpolar solvents
- Contain x2 the amount of energy of carbohydrates and proteins (9 kcal/gram)
- Animal fats are saturated and solid at room temperature
- Plant fats are unsaturated and liquid at room temperature
Examples: triglycerides, steroids, waxes, phospholipids
,Proteins:
- Contain carbon, hydrogen, oxygen, nitrogen (and sometimes
sulfur)
- Composed of amino acids arranged into one or more linear
chains
- R is the protein
Nucleic acids:
- Composed of smaller units (nucleotides) which are linked together to form a larger
molecule (nucleic acid)
- Each nucleotide contains a base (A/T/U/G/C), a sugar (DNA/RNA), and a phosphate
group
- Made from carbon, hydrogen, oxygen, nitrogen and phosphorus
- If the sugar is ribose, the nucleic acid formed is RNA
- If the sugar is deoxyribose, the nucleic acid formed is DNA
Amino acids:
- Composed of an amine (NH2) group, a
carboxyl (COOH) group, a central carbon
atom, a simple H group, and an R group
- Each amino acid differs because the R groups
are different
Glucose
- Is a reducing sugar that contains C6H12O6
- Mostly found in a ringed structure and is the main product
formed by photosynthesis
- Is an energy molecule used in aerobic respiration
- A monomer of starch, glycogen and cellulose
Ribose:
- Pentose (5 carbon) sugar of RNA and RuBP (Calvin cycle)
- C5H10O5
- Differs from deoxyribose (sugar in DNA) because it has an extra -OH
hydroxyl group on the 2nd carbon of the ring
Fatty acids:
- The main component of triglycerides and phospholipids
- Are non-polar, so they are hydrophobic
- Chains consist of covalently bonded carbon with hydrogen
Unsaturated fatty acids:
- All double bonds
,Metabolism:
Metabolism - all enzyme-catalysed reactions within the cells of living organisms
- Catalyzed (caused) by enzymes and allow organisms to grow and reproduce,
maintain their structures, and respond to their environments
- Many of these reactions occur in the cytoplasm
- There are some extracellular metabolic reactions that are between different cells
Metabolism is divided into two components: anabolism and catabolism
Anabolism: building large molecules from smaller ones
- Anabolic reactions require energy as large molecules are being built from small ones
Eg. protein synthesis, DNA synthesis and replication, photosynthesis, building complex
carbohydrates (cellulose, starch, glycogen)
Catabolism: breaking down large molecules into their component parts
- Catabolic reactions release energy (sometimes as ATP)
Eg. digestion of food, cellular respiration, breakdown of carbon compounds by decomposers
Urea:
Urea is produced by living organisms but can also be artificially synthesized.
- Is a component of urine which is produced when there is an excess of amino acids in
the body
- A series of enzyme-catalyzed reactions produce urea in the liver where it is
transported by blood to the kidney, where it is filtered out and excreted in the urine
- Urea can be produced artificially through different chemical reactions, but the product
is the same
- Urea is mainly used as a nitrogen source in fertilizers
Monosaccharides:
- Glucose
- Fructose
- Galactose
- Ribose
Disaccharides:
- Sucrose
- Maltose
- Lactose
Polysaccharides:
- Cellulose
- Glycogen
- Starch
, 2.2 - Water
Water molecules are polar and hydrogen bonds form between them.
- A water molecule consists of an oxygen atom covalently bonded to two hydrogen
atoms
- Since O is more electronegative than H, an unequal sharing of electrons occurs
- This creates a polar covalent bond, with H having a partial positive charge and O
having a partial negative charge
- Water is also bent so the positive charge exists more or less on one side and the
negative charge from the O exists on the opposite side
- The partial positive charge is attracted to the partial negative charge, which creates
an intermolecular attraction between the water molecules called a “hydrogen bond”
- H-bonds are the strongest of intermolecular bonding but is considered a weak bond
Thermal properties of water:
- High specific heat capacity due to the extensive hydrogen bonding between the
water molecules
- Water can absorb a lot of heat and give off a lot of heat without drastically changing
the temperature of the water
- Has a high latent heat of vaporization (takes a lot of heat to evaporate water from a
liquid to a vapour)
- While sweating, water droplets absorb heat from our skin causing the water to
evaporate and our bodies to cool down
Cohesive properties:
- Water is a polar molecule with a negative oxygen end and a positive hydrogen end
- The hydrogen bonds that exist between water molecules create a high level of
attraction linking water molecules together. This attraction between two of the same
molecules is called cohesion
- These cohesive forces allow water to move up vascular tissue in plants against
gravity. It also creates surface tension on the water that allows some organisms to
walk on water.
Adhesive properties:
- Water binds strongly to itself and also forms H-bonds with other polar molecules
(adhesion)
- Important in transpiration as water adheres to the cellulose in the walls of the xylem
vessels
- As water is evaporated from the stomata, the adhesion can help the water move up
through the xylem
Solvent properties:
- Water is known as the “universal solvent” because as it is able to dissolve many
substances due to its polarity
- Water is able to dissolve other polar molecules (carbohydrates, proteins, DNA,
charged ions)
- This is essential because it allows water to act as a transport medium of important
molecules in biological organisms
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