Biochemistry
Biological Molecules
Carbohydrates
Monosaccharides
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Elements: carbon (C), hydrogen (H) and oxygen (O).
The ratio of H to O is always 2 : 1.
Monosaccharides are called simple sugars because they are the simple subunits joined together to make larger carbohydrates - and they taste sweet.
The general formula of a monosaccharide is
a pentose, if n = 6 the sugar is called a hexose.
. If n = 5 then the sugar is called
Examples
Pentoses : ribose and deoxyribose found in DNA and RNA.
Hexoses : glucose, galactose and fructose.
Glucose is our major fuel. It is the sugar which is transported in our bloodstream to
our cells where it is broken down during respiration to release energy.
Dissaccharides
A disaccharide is made of two monosaccharides joined together by a glycosidic
bond. The molecular formula for disaccharides is C12H22O11.
The monosaccharide components of each of the disaccharides are:
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sucrose = glucose + fructose
lactose = glucose + galactose
maltose = glucose + glucose
If sucrose is given the value of one 'spoonful' for sweetness, then
this is how other natural sugars, and some commercial sweeteners,
compare:
.
One spoon of substance
Equivalent to spoons of
sucrose
Fructose
1.7
Glucose
0.7
Maltose/galactose
0.3
Lactose
0.2
Aspartame
180
(artificial sweetner in fizzy drinks
and tabletop sweeteners)
Saccharin
400
(artificial sweetner in fizzy drinks
and tabletop sweeteners)
Polysaccharides
Polysaccharides are large carbohydrate molecules made up of many
monosaccharides held together by glycosidic bonds. Starch, made up of glucose, is
shown below.
Starch
Cellulose
Two molecules make
up starch:
Long unbranching
chains which lie
parallel to each other
and are connected by
hydrogen bonds
forming cross links
between the chains.
Glycogen
Monomer
Molecular structure
Highly
branched,
tree-like
Amylopectin
(branched)
Amylose
(coiled not branched)
Found in
Function
The chains are layered
into bundles which are
laid down at right
angles to each other in
the cell wall
Plants only: all plant
cell walls
Animals
Plants only, found as
only, found starch granules in
as tiny
photosynthetic cells in
granules in particular
muscle and
liver cells
Storage form Energy reserve - found One of the main
of glucose in in storage organs, e.g. components of the cell
muscles and potato
wall
liver hydrolysis of
the glycogen
releases
glucose
when blood
glucose
levels drop
Roles of Carbohydrates
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Monosaccharides, glucose in particular, are an important source of energy
for cells.
Ribose and deoxyribose sugars make up part of RNA and DNA.
The soluble monosaccharides are essential for the transport of food
around animals (glucose).
Mammalian milk contains 5% lactose, so it's a major source of
carbohydrates for babies.
Glycogen and starch are important energy reserves for animals. Both are
insoluble in water which means they remain inside cells.
Glycogen and starch are compact structures. Branched molecules can
take up less space in a cell but are made of more glucose monomers. This
means more energy is stored in less space!
The cellulose in cell walls has a high tensile strength - this means it is
tough. But it is permeable to water and most substances can pass straight
through it.
Proteins
Structure of proteins
The basic structure of a protein is a polypeptide chain. Many amino acids (from as
few as forty, to a few hundred) join together by means of peptide bonds in a specific
sequence (order) to form a polypeptide chain.
There are 20 different amino acids found in proteins, yet over tens of thousands of
different possible proteins. This variety is possible because there are over
8 000 ways that 20 amino acids can be arranged in a tripeptide, i.e. just 3 amino
acids.
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Enzymes speed up chemical reactions, e.g. salivary amylase
hydrolyses starch to maltose.
Proteins are essential constituents of many parts of the body,
e.g. hair, nails, skin, bones, tendons, cartilage and ligaments.
Some globular proteins are antibodies which bind to foreign
microbes.
Other globular proteins are found in plasma membranes - they
aid the transport of large molecules into and out of cells.
Haemoglobin and myoglobin are transport proteins - carrying
oxygen in the blood and muscles respectively.
Some hormones are proteins, e.g. insulin, which controls
blood sugar levels.
Proteins are amphoteric: they can act either as an acid or a
base. So they make good buffers - resisting changes in pH,
e.g. in tissue fluids, proteins keep pH levels steady (an
important aspect of homeostasis).
Lipids
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a mixed group of organic compounds
are grouped together because they have a high proportion of CH2
are, as a result, not soluble in water but are soluble in non-polar (organic)
solvents, e.g. ether or chloroform
divided into several groups:
o fats, oils and waxes
o steroids
o phospholipids
Fats, oils and waxes
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Fats and oils are composed of C, H and O, as in carbohydrates. But the ratio
of H : O is higher in fats (i.e. more H atoms per O atom in fats than in
carbohydrates). The main difference between fats, oils and waxes is their
melting points.
Monomers are alcohols (glycerol) and fatty acids.
A condensation reaction produces a triglyceride and water molecule when
glycerol and three fatty acids react together.
An unsaturated fatty acid contains one or more carbon double bonds.
There is potential for more hydrogen atoms to become part of the tail
structure if the carbon double bond were to be broken.
A saturated fatty acid is 'filled' with hydrogen atoms - every available
carbon bond of the central chain is taken up by hydrogen atoms.
Fats (triglycerides) in the diet are classified either as saturated or
unsaturated.
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A saturated fat is formed when saturated fatty acids react with
glycerol.
An unsaturated fat is made when unsaturated fatty acids react
with glycerol.
Along with proteins, phospholipids are essential components of all
cell membranes. A phospholipid is composed of two fatty acids, a
glycerol and a phosphate group,
The cell membrane is composed of a bilayer of phospholipids. The
hydrophobic tails face each other (pointing inwards) and the
hydrophilic heads point outward and are in contact with the aqueous
external and internal cellular environments