CARBOHYDRATES - SUGARS - natural
STARCH and DNA
Brown's GCSE, IGCSE, O Level and KS4 science-CHEMISTRY Revision Notes on
Oil, useful products, environmental problems, introduction to
12. Natural Molecules - carbohydrates - sugars - natural polymers like starch
Separate page for amino acids and proteins
GCSE level chemistry revision
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Naturally Occurring Molecules from plants
Molecules like glucose ==>
Polymers = Macromolecules like starch
Carbohydrates (this page),
Oils & Fats are the
main nutrient constituents of food.
occurring molecules are based on the elements carbon, oxygen and
hydrogen, together with smaller proportions of nitrogen, phosphorus,
sulfur (sulphur) and sometimes metal ions like iron (in the haemoglobin
molecule) and magnesium (in the chlorophyll molecule).
important sources of energy for many living organisms and their
formation in plants via photosynthesis is the basis of many food chains.
Initially this is
formed as glucose, but this is converted to glycogen and
starch for 'storage'.
carbon dioxide +
water + sunlight energy ==> glucose + oxygen
+ 6H2O ==> C6H12O6
are a whole series naturally occurring molecules based on the elements
carbon, hydrogen and oxygen.
- They are an important source of
chemical energy in our diet.
- e.g. the respiration reaction
- Carbohydrates like glucose and fructose are
used as sweeteners in food as well as sweets themselves.
- Historically the name 'carbohydrate' comes
from the fact that all their formulae seemed to be based on Cx(H2O)y
(see key above) BUT this is not the way to think of their formula.
- They range from relatively small molecules
called monosaccharide (means one basic unit), or disaccharide
(two basic units combined) to very large natural polymers or
macromolecules called polysaccharides (many units combined). A
summary of them is shown in the key diagram above along with some familiar
names from biology.
is one of the simpler sugar molecules (a monosaccharide). The structural formula is
shown on the left and you should be able to see that there are 4 bonds
to each carbon, 2 to each oxygen and just 1 bond to each hydrogen atom.
The right-hand 'shorthand' skeletal formula version uses short
straight lines to represent bonds. Most H's and their bonds are not
shown, and at AS-A2 level it is assumed you can interpret these
structures 'back to' a full structure!, but they are handy for
describing large 'biochemical' molecules (see polysaccharide below)
is a disaccharide
made from combining two monosaccharide molecules, glucose and
fructose by the elimination of a water.
<==> C12H22O11 + H2O
- On hydrolysis sucrose
reforms the glucose and fructose.
- The formation of complex carbohydrates:
- Their formation can be described
in terms of a
large number of sugar units joined together by condensation polymerisation
e.g. starch and cellulose are natural polymers
- e.g. the 'box' diagram below shows 4 units of a natural carbohydrate polymer
- Note: Condensation polymerisation means
the joining together of many small 'monomer' molecules by eliminating an
even smaller molecule between them to form the linkage.
n HO-XX-OH ===> [-XX-O-]n- + nH2O
- n C6H12O6 ===>
n is a very large number to form the natural polymer)
many H2O etc.
This diagram of starch
or cellulose is in 'skeletal formula'
style and both are polymers of glucose - can you see the connection
between each 'unit' and the structure of
A simplified representation
- many polymeric carbohydrates have complex cross-linked structures.
The resulting natural polymer is
called a polysaccharide.
GCSE level STUDENTS DO NOT
NEED TO KNOW THESE COMPLEX STRUCTURES
Acid hydrolysis of complex
carbohydrates (e.g.. starch) gives simple sugars.
This can be brought about by
starch with hydrochloric acid solution to form glucose.
+ nH2O ==> n C6H12O6
n is a very large number)
In your body starch and other complex
carbohydrate molecules are broken down by
enzymes like amylase.
The hydrolysis products
from polysaccharides can be
paper chromatography as in
the case of amino acids.
We can digest long molecules
like starch, though they have to be broken down by enzyme action before the
smaller molecules like glucose can be used in respiration.
Carbohydrates are an
important source of energy in our diet e.g. in breakfast cereals,
typical nutrition information illustrated below ...
cereal based on oats which contains 60.8% carbohydrate and 3.2% sugar.
A breakfast cereal based on wheat which contains 53.6%
carbohydrate and 12.1% sugar. Cereal is less healthy than cereal 1 as
regards sugar, but it does have a lot of essential vitamins and minerals
added (for children?), see picture below, and the sweet taste is more
However, we cannot digest
cellulose because we don't have the enzymes to effect this process, unlike
What happens when potatoes are
Potatoes are a good source
of carbohydrates, hence a good source of energy for the body.
Raw potato is not easily
digested but cooking partially breaks the structure of potato down so
that we can digest it AND cooking also improves the texture and taste of
the potato to make it more palatable to eat.
In the cooking process the
plant cell walls of cellulose are softened and this allows hot water to
penetrate the cells.
Further cooking causes the
starch granules to swell and gelatinize as they absorb hot water.
This eventually causes the
cells to separate producing the soft texture of well-cooked digestible
Mash with milk, butter and
salt to suit taste! Cheap and yummy!
DNA - deoxyribonucleic acid
(deoxyribonucleic acid) is a large molecule essential for life and cell
replication and is another example of a natural polymer.
It is the
natural polymer found
in all living organisms and viruses too.
DNA encodes genetic instructions for the development and
functioning of living organisms and viruses e.g. every protein molecule is
synthesised by other molecules reading the genetic code and combining the right
amino acids in the right order.
Most DNA molecules consist of two polymer chains, made
from four different monomers called nucleotides, connected together in the
form of a double helix.
- Nucleotides form the building blocks of DNA or RNA
and an individual nucleotide consists of three molecular bits combined
together - a phosphate group, a base (adenine, cytosine, guanine, and
thymine), and a pentose sugar.
- In RNA the thymine base is replaced by uracil.
So a DNA molecule consists of two 'molecular' strands coiled together to form a double helix,
but how is this helix held together?
- The strands are linked by a series of
complementary base pairs joined together by weak hydrogen bonds (base-pairing H
bonds shown here as
- There are four bases in DNA holding the
structure together and the same bases are always paired together.
- (i) adenine (A)
with thymine (T) i.e. AT,
and (ii) cytosine (C) with guanine (G) i.e. CG whererepresents
the weak (but crucial) intermolecular attractive force between pairs of
bases, called the hydrogen bond.
- It is these
hydrogen bonds that holds the two DNA strands together as a double helix.
- The double helix structure is shown in the diagram on
the right, illustrating how the DNA is held together by the
Advanced level chemistry notes on amine
bases and the hydrogen bonding in DNA and RNA
GCSE biology notes on DNA and RNA structure and Protein Synthesis and an
experiment to extract DNA
Multiple Choice Quizzes and Worksheets
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry
3 linked easy Oil Products gap-fill quiz worksheets
ALSO gap-fill ('word-fill') exercises
originally written for ...
... Ex AQA GCSE Science
Useful products from
crude oil AND
Ex OCR 21st C GCSE Science
Worksheet gap-fill C1.1c Air
pollutants etc ...
... Edexcel GCSE Science
Crude Oil and its Fractional distillation
... each set are interlinked,
so clicking on one of the above leads to a sequence of several quizzes
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sugars like glucose, sucrose and cellulose what are monosaccharides,
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revising KS4 science structure of DNA