Summary
IB Biology: Molecular Biology Summary
Molecular Biology (Unit 2) Summary
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2.1 MOLECULES TO METABOLISMAn organic compound is a compound that
contains carbon and is found in living things.
Exceptions include carbides (e.g. CaC2),
carbonates (CO32–
), oxides of carbon (CO,
CO2) and cyanides (CN–)
Carbon forms the basis of organic life due to
its ability to form large and complex
molecules via covalent bonding. Carbon
atoms can form four covalent bonds, with bonds between carbon atoms being
particularly stable (catenation). These properties allows carbon to form a wide variety
of organic compounds that are chemically stable.
There are four principal groups of organic compounds that contribute to much of the
structure and function of a cell:
Carbohydrates
★ Most abundant organic compound found in nature, composed primarily of C,H and
O atoms in a common ratio – (CH2O)n
★ Principally function as a source of energy (and as a short-term energy storage
option)
★ Also important as a recognition molecule (e.g. glycoproteins) and as a structural
component (part of DNA / RNA)
Carbohydrates are composed of monomers called monosaccharides ('single sugar
unit')
★ Monosaccharides are the building blocks of disaccharides (two sugar units) and
polysaccharides (many sugar units)
★ Most monosaccharides form ring structures and can exist in different 3D
configurations (stereoisomer)
The structure of complex carbohydrates may vary depending on the composition of
monomeric subunits
Polysaccharides may differ according to the type of monosaccharide they possess
and the way the subunits bond together
★ Glucose monomers can be combined to form a variety of different polymers –
including glycogen, cellulose and starch
,Lipids
★ Non-polar, hydrophobic molecules which may come in a variety of forms (simple,
complex or derived)
★ Lipids serve as a major component of cell membranes (phospholipids and
cholesterol)
★ They may be utilised as a long-term energy storage molecule (fats and oils)
★ Also may function as a signalling molecule (steroids)
Lipids exist as many different classes that vary in structure and hence do not contain
a common recurring monomer
However several types of lipids (triglycerides, phospholipids, waxes) contain fatty
acid chains as part of their overall structure
★ Fatty acids are long chains of hydrocarbons that may or may not contain double
bonds (unsaturated vs saturated)
Lipids can be roughly organised into one of three main classes:
★ Simple (neutral) lipids – Esters of fatty acids and alcohol (e.g. triglycerides and
waxes)
★ Compound lipids – Esters of fatty acids, alcohol and additional groups (e.g.
phospholipids and glycolipids)
★ Derived lipids – Substances derived from simple or compound lipids (e.g. steroids
and carotenoids)
Nucleic Acids
★ Genetic material of all cells and determines the inherited features of an organism
★ DNA functions as a master code for protein assembly, while RNA plays an active
role in the manufacturing of proteins
Nucleic acids are composed of monomers called nucleotides, which join together to
form polynucleotide chains
Each nucleotide consists of 3 components – a pentose sugar, a phosphate group
and a nitrogenous base
★ The type of sugar and composition of bases differs between DNA and RNA
Nucleotides form bonds between the pentose sugar and phosphate group to form
long polynucleotide chains
★ In DNA, two complementary chains will pair up via hydrogen bonding between
nitrogenous bases to form double strands
, ★ This double stranded molecule may then twist to form a double helical
arrangement
Proteins
★ Make over 50% of the dry weight of cells; are composed of C, H, O and N atoms
(some may include S)
★ Major regulatory molecules involved in catalysis (all enzymes are proteins)
★ May also function as structural molecules or play a role in cellular signalling
(transduction pathways)
Proteins are composed of monomers called amino acids, which join together to form
polypeptide chains
★ Each amino acid consists of a central carbon connected to an amine group (NH2)
and an opposing carboxyl group (COOH)
★ A variable group (denoted ‘R’) gives different amino acids different properties (e.g.
may be polar or non-polar, etc.)
Amino acids join together by peptide bonds which form between the amine and
carboxyl groups of adjacent amino acids
The fusion of two amino acids creates a dipeptide, with further additions resulting in
the formation of a polypeptide chain
★ The subsequent folding of the chain depends on the order of amino acids in a
sequence (based on chemical properties)
Vitalism was a doctrine that dictated that
organic molecules could only be
synthesised by living systems. It was
believed that living things possessed a
certain “vital force” needed to make organic
molecules. Hence organic compounds were
thought to possess a non-physical element
lacking from inorganic molecules
Vitalism as a theory has since been
disproven with the discovery that organic
molecules can be artificially synthesised. In 1828, Frederick Woehler heated an
inorganic salt (ammonium cyanate) and produced urea. Urea is a waste product of
nitrogen metabolism and is eliminated by the kidneys in mammals. The artificial
synthesis of urea demonstrates that organic molecules are not fundamentally
different to inorganic molecules.
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