Biological
molecules
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Carbohydrates
contain 3 elements; carbon(C), hydrogen (H), and oxygen (O).
Carbohydrates are found in one of three forms; monosaccharides, disaccharides
(both sugars) and polysaccharides.
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Monosaccharides
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General
formula: (CH2O)n where n is a number between 3 and 9.
They are classified according to the number of carbon atoms. The monosaccharides
you will have to know fall into these categories:
C = 3 = triose;
C = 4 = tetrose;
C = 5 = pentose;
C = 6 = hexose.
Trioses: (e.g. glyceraldehydes), intermediates in respiration and photosynthesis
Tetroses: rare
Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids
(RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP
Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration
and as building blocks for larger molecules.
All but one carbon atom have an alcohol (OH) group attached. The
remaining carbon atom has an aldehyde or ketone group attached.
Chain
form
Ring form
Due to the bond angles between the carbon atoms, it is possible for pentoses
and hexoses to form stable ring structures. The carbon atoms are
numbered 1 to 5 in pentoses and 1 to 6 in hexoses.
Depending
on the orientation of the OH group on carbon 1, the monosaccharide can
have either a or b configurations.
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Disaccharides
and glycosidic bonds
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These
are formed when two monosaccharides are condensed together. One
monosaccharide loses an H atom from carbon atom number 1 and the other
loses an OH group from carbon 4 to form the bond.
The reaction, which is called a condensation reaction, involves the loss
of water (H2O) and the formation of a 1,4-glycosidic bond.
Depending on the monosaccharides used, this can be an a-1,4-glycosidic
bond or a b-1,4-glycosidic bond.
The reverse of this reaction, the formation of two monosaccharides from
one disaccharide, is called a hydrolysis reaction and requires one water
molecule to supply the H and OH to the sugars formed.
Examples of Disaccharides:
Sucrose: glucose + fructose,
Lactose: glucose + galactose,
Maltose: glucose + glucose.
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Polysaccharides
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Polysaccharide
Function
Structure
Relationship
of structure to function
Starch
Main storage
polysaccharide in plants
Made of
2 polymers amylose and amylopectin
Amylose: a polymer of glucoses joined by a-1,4-glycosidic
bonds. Forms a helix with 6 glucose molecules per turn and about
300 per helix.
Amylopectin:
a polymer of glucoses joined by a-1,4-glycosidic
bonds but with branches of a-1,6-glycosidic
bonds. This causes the molecule to be branched rather than helical.
Insoluble
therefore good for storage.
Helix
is compact.
The branches mean that the compound can easily hydrolysed to release the
glucose monomers.
Glycogen
Main storage
polysaccharide in animals and fungi
Similar
to amylopectin but with many more branches which are also shorter
The number
and length of the branches means that it is extremely compact and very
fast hydrolysis
Cellulose
Main structural
constituent of plant cell walls
Adjacent
chains of long, unbranched polymers of glucose joined by
b-1,4-glycosidic bonds hydrogen bond with each other to form microfibrils
The microfibrils
are strong and so are structurally important in plant cell walls.
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Functions
of Carbohydrates
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1)
Substrate for respiration (glucose is essential for cardiac tissues)
2) Intermediate in respiration (e.g. glyceraldehydes)
3) Energy stores (e.g. starch, glycogen)
4) Structural (e.g. cellulose, chitin in
arthropod exoskeletons and fungal walls)
5) Transport (e.g. sucrose is transported
in the phloem of a plant)
6) Recognition of molecules outside a cell
(e.g. attached to proteins or lipids on cell surface membrane).
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Lipids
Lipids are made up of the elements carbon, hydrogen and oxygen but in different
proportions to carbohydrates. The most common type of lipid is the triglyceride.
Triglycerides
These are made up of 3 fatty acid chains attached to a glycerol molecule.
Fatty acids
are chains of carbon atoms, the terminal one having an OOH group attached making
a carboxylic group (COOH). The length of the chain is usually between 14 and
22 carbons long (most commonly 16-18).
Three of these
chains become attached to a glycerol molecule which has 3 OH groups attached
to its 3 carbons. This is called a condensation reaction because 3 water molecules
are formed from 3 OH groups from the fatty acids chains and 3 H atoms from the
glycerol. The bond between the fatty acid chain and the glycerol is called an
ester linkage.
The 3 fatty
acids may be identical or they may have different structures.
In the fatty
acid chains the carbon atoms may have single bonds between them making the lipid
saturated. These are usually solid at room temperature and are called fats.
If one or more
bonds between the carbon atoms are double bonds, the lipid is unsaturated. These
are usually liquid at room temperature and are called oils.
Functions of Lipids
1) Storage - lipids are non-polar and so are insoluble in water.
2) High-energy
store - they have a high proportion of H atoms relative to O atoms and so yield
more energy than the same mass of carbohydrate (see Respiration QuickLearn for
the role of H atoms in making ATP).
3) Production
of metabolic water - some water is produced as a final result of respiration.
4) Thermal insulation
fat conducts heat very slowly so having a layer under the skin keeps metabolic
heat in.
5) Electrical
insulation the myelin sheath around axons prevents ion leakage
6) Waterproofing
waxy cuticles are useful, for example, to prevent excess evaporation from
the surface of a leaf.
7) Hormone production
steroid hormones. Oestrogen requires lipids for its formation, as do other
substances such as plant growth hormones.
Phospholipids
A phosphate-base group replaces one fatty acid chain. It makes this part of
the molecule (the head) soluble in water whilst the fatty acid chains remain
insoluble in water.
Due to this
arrangement, phospholipids form bilayers (the main component of cell and organelle
membranes).
Proteins
Different proteins can appear very different and perform diverse functions (e.g.
the water-soluble antibodies involved in the immune system and the water-insoluble
keratin of hair, hooves and feathers). Despite this, each one is made up of amino
acid subunits.
There about
20 different amino acids that all have a similar chemical structure but behave
in very different ways because they have different side groups. Hence, stringing
them together in different combinations produces very different proteins.
Each amino acid
has an amino group (NH2) and a carboxylic acid group (COOH). The R group is
a different molecule in different amino acids which can make them neutral, acidic,
alkaline, aromatic (has a ring structure) or sulphur-containing.
When 2 amino
acids are joined together (condensation) the amino group from one and the acid
group from another form a bond, producing one molecule of water. The bond formed
is called a peptide bond.
Primary structure of proteins
Due to the bonding and the shape and chemical nature of different amino acids,
the shape of a whole chain of amino acids (a polypeptide or protein) is specific.
This will affect
the properties of the protein, just as the type of a necklace depends on the
type of beads and how they are strung together. Therefore, the primary structure
depends on the order and number of amino acids in a particular protein.
For Example:
Haemoglobin is made up of 4 polypeptide chains, 2chains and 2chains, each with
a haem group attached. There are 146 amino acids in each chain. If just one
of these is wrong, serious problems can arise (e.g. sickle cell anaemia). The
red blood cells become distorted, the amount of oxygen they can carry is reduced
and blood capillaries can be blocked, leading to acute pains called crises.
Secondary structure of proteins
This is the basic shape that the chain of amino acids takes on. The 2 most common
structures are the -helix and the -pleated sheet.
This has a regular
coiled structure like a spring, with the R groups pointing towards the outside
of the helix. Hydrogen (H) bonds are relatively weak but because there are so
many, the total binding effect is strong and stable. The helix is flexible and
elastic.
This is composed of side by side chains connected by H bonds. All the peptide
linkages are involved in inter-chain H bonding so the structure is very stable.
Tertiary structure of proteins
This is the overall 3-D structure of the protein.
The shape of
the protein is held together by H bonds and other weak interactions between
the R groups (side chains).
These interactions
may be electrostatic attractions between charged groups e.g. NH3+ and O- or
Van de Waals forces.
Fibrous proteins
are made of long molecules arranged to form fibres (e.g. in keratin). Several
helices may be wound around each other to form very strong fibres.
Globular proteins
are made of chains folded into a compact structure. Although these folds are
less regular than in a helix, they are highly specific and a particular protein
will always be folded in the same way. If the structure is disrupted, the protein
ceases to function properly and is said to be denatured.
A globular protein
based mostly on an -helix is haemoglobin.
A globular protein
based mostly on a -pleated sheet is the immunoglobulin antibody molecule.
Quaternary structure of proteins
If a protein is made up of several polypeptide chains, the way they are arranged
is called the quaternary structure. Again, each protein formed has a precise
and specific shape (e.g. haemoglobin)
Prosthetic groups
The majority of proteins are assisted in their functions by a prosthetic group.
This may a simple metal ion such as zinc in the enzyme carboxypeptidase, or
it may be a complex organic molecule, such as the haem group in haemoglobin.
Functions of Proteins
1) Virtually all enzymes are proteins.
2) Structural
e.g. collagen and elastin in connective tissue, keratin in skin, hair and
nails.
3) Contractile
proteins actin and myosin in muscles allow contraction and therefore
movement.
4) Hormones
many hormones have a protein structure (e.g. insulin, glucagon, growth hormone).
5) Transport
for example, haemoglobin facilitates the transport of oxygen around the body,
a type of albumin in the blood transports fatty acids
6) Transport
into and out of cells carrier and channel proteins in the cell membrane regulate
movement across it.
7) Defence
immunoglobulins (antibodies) protect the body against foreign invaders; fibrinogen
in the blood is vital for the clotting process
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