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Document with link to ANKI flashcards for OCR A Level Biology A, year 2
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Chapter 3: Biological molecules3.1: Biological elements
Elements
Different types of atoms are called elements. Elements are distinguished by the number of protons in their
atomic nuclei.
There are over a hundred known elements in the universe but only a small percentage of these are present
in the living world.
If you have ever built a model using interlocking bricks, you will know how useful the bricks that make the
most connections are at the start of a new build.
In the same way that complex models can be built from a small range of simple bricks, all living things are
made primarily from 6 key elements - C, H, O, N, P, Na, K, Ca, Fe and S
Bonding
Atoms connect with each other by forming bonds.
Atoms can bond to other atoms of the same element, or atoms of different elements, provided this follows
the 'bonding rules'
When two or more atoms bond together the complex is called a molecule.
A covalent bond occurs when two atoms share a pair of electrons.
The electrons used to form bonds are unpaired and present in the outer orbitals of the atoms.
Bonding follows some simple rules, determined by the number of unpaired electrons present in the outer
orbitals of different elements:
o Carbon atoms can form four bonds with other atoms.
o Nitrogen atoms can form three bonds with other atoms.
o Oxygen atoms can form two bonds with other atoms
o Hydrogen atoms can only form one bond with another atom.
Ions
An atom or molecule in which the total number of electrons is not equal to the total number of protons is
called an ion.
If an atom or molecule loses one or more electrons it has a net positive charge and is known as a cation. If an
atom or molecule gains electrons, it has a net negative charge and is known as an anion.
In ionic bonds, one atom in the pair donates an electron and the other receives it.
This forms positive and negative ion that are held together by the attraction of the opposite charges.
Ions in solution are called electrolytes
, Polymers
Biological molecules are often polymers.
Polymers are long-chain molecules made up by the linking of multiple individual molecules in a repeating
pattern.
In carbohydrates the monomers are sugars and in proteins the monomers are amino acids.
3.2: Water
The bonds of life
Atoms join to form molecules by making bonds with each other.
They form negative or positive ions that are held together by the attraction of the opposite charges.
Covalent bonds occur when atoms share electrons.
However, the negative electrons are not always shared equally by the atoms of different elements.
In many covalent bonds, the electrons will spend more time closer to one of the atoms than to the other.
The atom with the greater share of negative electrons will be slightly negative compared with the other
atom in the bond, which will therefore be slightly positive
Molecules in which this happens are said to be polar - they have regions of negativity and regions of
positivity.
Oxygen and hydrogen are examples of elements that do not share electrons equally in a covalent bond.
Oxygen always has a much greater share of the electrons in a 0-H bond.
Many organic molecules contain oxygen and hydrogen bonded together in what are called hydroxyl groups
and so they are slightly polar.
Water is an example of such a molecule, in fact, water contains two of these hydroxyl groups
Polar molecules, including water, interact with each other as the positive and negative regions of the
molecule attract each other and form bonds, called hydrogen bonds.
Hydrogen bonds are relatively weak interactions, which break and reform between the constantly moving
water molecules.
Although hydrogen bonds are only weak interactions, they occur in high numbers.
Hydrogen bonding gives water its unique characteristics, which are essential for life on this planet.
Characteristics of water
Water has an unusually high boiling point; Water is a small molecule, much lighter than the gases carbon
dioxide or oxygen, yet unlike oxygen and carbon dioxide, water is a liquid at room temperature.
This is due to the hydrogen bonding between water molecules.
It takes a lot of energy to increase the temperature of water and cause water to become gaseous
When water freezes it turns to ice.
Most substances are denser in their solid state than in their liquid state, but when water turns to ice it
becomes less dense.
This is because of the hydrogen bonds formed. As water is cooled below 4°C the hydrogen bonds fix the
positions of the polar molecules slightly further apart than the average distance in the liquid state.
This produces a giant, rigid but open structure, with every oxygen atom at the centre of a tetrahedral
arrangement of hydrogen atoms, resulting in a solid that is less dense than liquid water. So, ice floats.
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