Module 1 - Biological Molecules 2.1.1 - Biological Molecules

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Module 1: Biological Molecules
Proteins, carbohydrates, lipids and nucleic acids are key biological
macromolecules with important roles in living organisms.
2.1.1 Biological Molecules
A.Water
Structure
Water is a very small molecule consisting of two hydrogen atoms covalently
bonded to an oxygen atom. The covalent bond is very strong, making it very
difficult to split the hydrogen and oxygen atoms up. The electrons shared in
these bonds aren't shared evenly they're pulled closer to the oxygen atom
due to its larger positive charge in its nucleus. This leaves the oxygen with a
very slight negative charge, and the hydrogen atoms a very slight positive
charge. Water molecules are therefore dipolar as they have two charges. This
means that water molecules are attracted to each other, with the force of
attraction being known as a hydrogen bond. This is a weak electrical attraction
between a hydrogen atom in one molecule and an oxygen atom in another.
These bonds are about 10% of the strength of covalent/ionic bonds, meaning
that in liquid water, as molecules are continuously moving around they are
continuously making and breaking hydrogen bonds.
Properties
The polar nature of water molecules is responsible for their properties:
Property Reason Advantage
High A lot of energy is required to Water is therefore very good
Specific break the hydrogen bonds at maintaining constant body
Heat between water. As so much temperatures in living
Capacity heat energy is used for this, organisms. This is essential
there's less energy available to as a certain internal
raise the temperature. temperature must be
Therefore, water requires a lot maintained to optimise
of heating before its enzyme activity. The water
temperature rises much. therefore acts as a
temperature buffer.
It also means that the
external environmental
temperature for organisms
that live in water is relatively
stable.
High Latent Due to the strong hydrogen When organisms sweat, the
Heat of bonds between water water absorbs energy from
Evaporatio molecules, water is liquid at their skin to gain enough
n room temperature and must be energy to evaporate, cooling
heated to 100 before the them down in the process.
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Density The hydrogen bonds between Due to their high water
water molecules pull them content, most organisms
closely together, making water have a density close to that
relatively dense ­ about 1gcm3 .
of water, making it easy for
them to swim.
Ice is less dense than water As ice is less dense than
because when water freezes, water, bodies of water freeze
each water molecule forms 4 form the top down.…read more

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Water can form ions: These ions can be used in
H2O OH
+ H+ reactions such as
photosynthesis.
Cohesion Water forms hydrogen bonds Cohesion and adhesion allow
& with other polar substances, water to enter and move
Adhesion sticking to them. This is known along very narrow spaces in
as adhesion. a process called capillarity.
Water molecules also form For example, continuous
hydrogen bonds between columns of water can be
themselves due to their dipolar pulled all the way up to the
nature.…read more

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C.Peptide Bonds
Making Peptide Bonds
Amino acids can join together to form
long chain polypeptides and proteins.
This occurs through protein synthesis
on the ribosomes of cells. The amino
acids are joined together using a
condensation reaction (where water is
also formed). This joins the amino
group form one amino acid and the carboxyl group from the other to form a
peptide bond. Peptide bonds are very strong as they involve covalent bonds.…read more

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The shape of an alpha helix or beta fold is maintained by hydrogen bonds
forming between the amino acids in the backbone of the chain, not between
the R groups. Although hydrogen bonds are weak individually, when many of
them are present it creates a strong structure.
Alpha Helix
The chain twists into a regular helix, with
hydrogen bonds between adjacent NH and C=O
groups holding it in position.…read more

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Examples of proteins with a tertiary
structure are antibodies, channel proteins and enzymes.
The polypeptide chains in haemoglobin also
have a tertiary structure. They each wind
themselves around a haem group (a little group
of atoms with an iron ion in the centre), curling
up into a ball, forming a globular protein.
G. Quanternary Structure
Quaternary Structure: Occurs when 2+
polypeptide chains are bonded together to form a larger molecule.
This is the final and most complex level of protein structure.…read more

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Haemoglobin
In a haemoglobin molecule, the quaternary structure consists of 4 polypeptide
subunits, including 2 alpha globin chains and 2 beta globin chains. These fit
together, forming the complete structure of this globular protein.
Haemoglobin contains 4 haem groups, which aren't made up of amino acids
but form an essential part of the molecule. These are known as prosthetic
groups, making haemoglobin a conjugated protein.…read more

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Must have a very precise shape as this determines their function. They
therefore have a very precise primary structure, always being made up of
exactly the same sequence of nonrepeating amino acids, in a chain that is
always of the same length.
E.g. Haemoglobin, enzymes, antibodies and some hormones such as
insulin and glucagon.
Fibrous Proteins
Have a more variable primary structure with a limited range of different
repeated amino acids and can be joined together to form chains of varying
lengths.
Insoluble in water.…read more

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Monosaccharides
These are simple reducing sugars made up of one unit. They have the general
formula of (CH
2O) n, where n =39, and include glucose, fructose and
galactose.
Glucose is a sugar which contains 6 carbon atoms, so is a hexose. It can exist
in two forms (isomers), known as and glucose. Glucose is the most
abundant hexose sugar and has a general formula of C
6H
12O6.…read more

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Lactose = galactose + glucose
Sucrose = fructose + glucose. This is a small and soluble molecule used for
transport in the phloem in plants, and to make fruits sweet.
Polysaccharides
Polysaccharides are macromolecules which are insoluble in water and don't
taste sweet. They are noncrystalline structures and are all nonreducing.
They are formed by linking many glucose molecules together in different ways,
making them the
most complex
carbohydrates.…read more

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