1.1, Introduction to biological molecules

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Bonding and the formation of molecules

Covalent bonding: atoms share a pair of electrons in outer shell, making a more stable compound (molecule). 


Ionic bonding: ions with opposite charges attract one another. This electrostatic attraction is known an ionic bond. 


Hydrogen bonding: electrons within molecules are not distributed evenly, spending more time in one position, making it more negatively charged than the rest of the molecule. A molecule with an uneven distribution of charge is said to be polarised. The negative area of one polarised region and the positive area of another attract each other, forming a weak electrostatic bond between them. Although each bond is individually weak, they can collectively form important forces that alter the physical properties of molecules. 

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Polymerisation and the formation of macromolecules

Monomers can be joined to form  polymers. This is polymerisation.

The monomers of a polymer are usually based on carbon.  

Many polymers are industrially produced. Others are made naturally by living organisms.  

The general sub-unit for polysaccharides are monosaccharides or single sugars such as glucose.

Polynucleotides are formed by mononucleotide sub-units.

Polypeptides are formed by linking peptides that have animo acids as their basic sub-unit. 

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Condensation and hydrolysis reactions

In the process of polymerisation in organisms, each time a sub-unit is attached, a molecule of water is formed.

Reactions that produce water in this water are called condensation reactions


Polymers can be broken down through the addition of water.

Water molecules are used when breaking the bonds that link the sub-units of a polymer, thus splitting the molecule. This type of reaction is called hydrolysis

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All chemical processes that take place in living organisms are collectively called metabolism.

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The Mole and molar solution

The mole measures the amount of a substance.

Abbreviated to mol 


One mole contains the same no. of particles as there are in 12g of carbon-12 atoms (contains 6.022x1023 carbon atoms). This number is called the Avogadro constant/number. 


molar solution (M) is a solution that contains one mole of solute in each litre of solution.

A mole is the molecular mass expressed in grams. 


For example, sodium chloride has a molecular mass of 58.5 and therefore a 1 M solution of sodium chloride contains 58.5g of sodium chloride in a 1 litre solution.

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Atoms are the smallest units of a chemical element that exist independently. An atom consists of a nucleus and electrons, which orbit the nucleus. 


Protons: same mass as neutrons, but with a positive charge. 


Neutrons: same mass as protons, but without an electrical charge. 


Electrons: orbit in shells around the nucleus. Small mass. Negatively charged. Their no. determines the chemical properties of an atom. 


In an atom, the number of protons and electrons is the same, therefore giving the atom no overall charge. 


Atomic number:  the number of protons in an atom 


Mass number:  the total number of protons and neutrons in an atom.

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While the proton number in an element always stays the same, the number of neutrons can vary. The different types of the atom produced are isotopes. 


Isotopes of an element have the same chemical properties but differ in mass. 


Isotopes, especially radioactive ones, are very useful in biology for tracing the route of certain elements in biological processes and for dating fossils.

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The formation of ions

If an atom loses/gains an electron, it becomes an ion. 


The loss of electrons lead to the formation of a positive ion. 

e.g. H+ 


The receiving of an electron leads to the formation of a negative ion.  

e.g. Cl-

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