Download Notes. - Glow Blogs

Carbohydrates and related Substances
Plants make carbohydrates by photosynthesis and release oxygen.
Plants use sunlight, carbon dioxide and water to make a family of foods called carbohydrates in a
process called photosynthesis.


The process is endothermic as the energy of the sun is needed.
Glucose is a carbohydrate made by photosynthesis
The importance of chlorophyll
Chlorophyll, a green substance in leaves, absorbs the energy from sunlight to allow
photosynthesis to take place.
sunlight + carbon dioxide + water ----------> carbohydrate + oxygen
6CO2 + 6H2O ----------> C6H12O6 + 6O2
The importance of Carbohydrates



Carbohydrates are an important food for animals.
Respiration is the process which releases energy in plants and animals
Respiration is important for all living things because it provides energy when glucose is
burned or broken down in the body.
Glucose + oxygen ----------> carbon dioxide + water + energy
C6H12O6 + 6O2 ----------> 6CO2 + 6H2O
Burning a Carbohydrate
Carbohydrates release energy when burned, and make carbon dioxide and water.
The release of energy can be seen in the custard-powder tin experiment.
Using this energy
Animals use this energy for many things such as movement, and warmth to keep our body
temperature at 37°C.
Elements in a Carbohydrate
Burning a carbohydrate makes carbon dioxide and water. This proves that a carbohydrate
contains carbon and hydrogen.
Plants and the regulation of oxygen and carbon dioxide levels in the atmosphere
Respiration by living things and combustion of fossils
fuels use oxygen and release carbon dioxide but the
level of oxygen remains at 21% and carbon dioxide at
0.03%.
Photosynthesis balances this by using up carbon
dioxide and releasing oxygen.
There is a lot of concern that giant rain-forests are being
chopped down because these forests are seen as the
oxygen factories of the Earth.
Examples of Carbohydrates
Carbohydrate Formula
Glucose
C6H12O6
Fructose
C6H12O6
Maltose
C12H22O11
Sucrose
C12H22O11
Starch
(C6H10O5)n
Glucose/fructose and maltose/sucrose are Isomers
The atoms in glucose (C6H12O6) and fructose (C6H12O6) are joined together differently and the two
forms are called isomers. (No need to learn these structures above)
The atoms in maltose (C12H22O11) and sucrose (C12H22O11) are joined together differently and the
two forms are called isomers.
Carbohydrates contain carbon, hydrogen and oxygen
The previous table shows that carbohydrates all contain carbon, hydrogen and oxygen.
In every carbohydrate, the ratio of hydrogen:oxygen is always 2:1 - the same as in water ('hydrate'
refers to water).
Properties of carbohydrates
The table below shows the properties of various carbohydrates some of which apply at General
level only.
The iodine test is performed by adding brown iodine solution to the substance being tested. A
positive test is the appearance of a blue-black colour.
The Benedict's (or Fehling's) test is performed by warming the carbohydrate solution with blue
Benedict's (or Fehling's) solution. A positive test is the change in colour from blue to red-brown.
Carbohydrate Appearance Solubility in water
Type of
solution
Iodine test
Benedict's
test
Glucose
White solid
Soluble
True *
No change
Blue to brown
Fructose
White solid
Soluble
True
No change
Blue to brown
Maltose
White solid
Soluble
True
No change
Blue to brown
Sucrose
White solid
Soluble
True
No change
No change
Starch
White solid
Does not dissolve
well
Colloid #
Turns blueblack
No change
Starch can easily be identified from the other carbohydrates as it turns iodine solution to a blue
black colour.
Distinguishing glucose and sucrose
Glucose can be distinguished from sucrose by the Benedict's test.
Distinguishing glucose, maltose and fructose from sucrose
Only sucrose does not give a positive result with Benedict's solution.
Distinguishing a true (*) and a colloidal (#) solution
Light rays travel through a true solution unseen, but can be seen and are scattered when they
pass through a colloid.
To show that starch is made in leaves by photosynthesis
1.
2.
3.
4.
5.
6.
Put plant in dark for 2 days to remove all food from the leaf.
Put plant in the light for one day to let it make food.
Remove one leaf and boil in water to kill it.
Put in hot alcohol to remove chlorophyll - the leaf will be white.
Wash in water to remove alcohol and add iodine solution to it.
The leaf will be blue-black proving that starch had been made.
Starch is a polymer of glucose molecules
Glucose molecules can join together in living things to make a big molecule called starch, a
polymer.
The process is called polymerisation.
Water is lost when glucose molecules join together
One water molecule is lost from each glucose molecule when they join together.
This is why the formula of glucose is represented as C6H12O6, while the formula of starch is
(C6H10O5)n.
The joining of glucose molecules to make starch is called condensation polymerisation
When molecules join together and water molecules are eliminated when they join, the reaction is
called condensation.
If many glucose molecules join together and a polymer is made, the reaction is called
condensation polymerisation.
Digestion of Starch




Digestion is the chemical breakdown of food in the body into smaller molecules (glucose)
which pass through the gut wall and are absorbed by the blood.
Digestion is helped by acid or an enzyme
Digestion of starch in food begins in the mouth where the starch meets saliva which
contains the enzyme amylase.
Digestion continues in the stomach where there is acid.
When starch is changed into glucose, water molecules are involved in splitting the starch polymer
and the reaction is called hydrolysis.
(C6H10O5)n + nH2O ----------> nC6H12O6
Polysaccharide molecule + water molecules ----------> monosaccharide molecules
Sucrose can also be split up (hydrolysed) into simpler sugars such as glucose using an enzyme or
acid.
C12H22O11 + H2O ----------> C6H12O6 + C6H12O6
disaccharide molecule + water ----------> two monosaccharide molecules
Monosaccharides and Disaccharides
Monosaccharides all have six carbon atoms in each molecule and the formula C 6H12O6.
Glucose and fructose are monosaccharides.
Disaccharides have twelve carbon atoms and the formula C12H22O11
Disaccharides are formed when two monosaccharides join together.
e.g. Sucrose is made when a glucose and fructose molecule join.
C6H12O6 + C6H12O6 ----------> C12H22O11 + H2O
Glucose + fructose ----------> sucrose + water
e.g. Maltose is made when two glucose molecules join.
The making of a disaccharide from two monosaccharides is an example of condensation.
Alcohol can be made from sugars such as glucose or starch by fermentation
Enzymes present in yeast help the reaction but can only produce an alcohol content of about 15%.
The yeast dies when the alcohol content reaches about 15%,
Glucose ----------> alcohol + carbon dioxide + energy
Sources of sugars for fermentation
Alcoholic drink Alcohol content
Source of Carbohydrate
Beer
4% (varies)
Hops, barley
Cider
4% (varies)
Apples
Wine
about 10%
Grapes, other fruit and vegetables
Whisky
40%
Barley
Vodka
37.5%
Potatoes
Brandy
40%
Grapes
Distillation is used to increase alcohol concentration and drinks made by this method are called
spirits.
Alcohol boils at 79°C and water boils at 100°C.
In a distillery, the water/alcohol mixture is heated to 80°C
and the alcohol collected by distillation.
Alcohol is a member of the Alkanol family
The alcohol present in alcoholic drinks is called ethanol,
C2H5OH.
Alkanols are similar in structure to the alkanes but have an OH group instead of one of the
hydrogen atom.
New terms and their meanings
PHOTOSYNTHESIS - a process in plants in which carbon dioxide and water are changed into
carbohydrates and oxygen with the help of sunlight and chlorophyll.
CARBOHYDRATE - a compound containing carbon, hydrogen and oxygen in which the ratio of
hydrogen:oxygen is the same as in water.
ENDOTHERMIC - a reaction in which energy is absorbed (or taken in).
CHLOROPHYLL - a green chemical in the leaves of plants which is able to trap the energy of the
sun and use this energy to make carbohydrates.
RESPIRATION - a process in living things where oxygen is used to break up food and produce
water, carbon dioxide and energy
ISOMERS - compounds with the same molecular formula but with a different structural formula.
TRUE SOLUTION - where the solute breaks up into small pieces which can fit into the spaces
between the water molecules. Such solutions are 'clear',
COLLOID - where a solute is too large to fit into the spaces between water molecules. Such a
solution is not clear.
MONSACCHARIDE - single sugar molecules such as glucose or fructose (C6H12O6)
DISACCHARIDE - two sugar molecules joined together as in sucrose or maltose (C12H22O11)
CONDENSATION - the joining of sugar molecules with the elimination of water molecules.
POLYMERISATION - the joining of many sugar molecules together e.g. when glucose is changed
into starch.
DIGESTION - a process in living things where large molecules are broken into smaller molecules
with the help of enzymes.
ENZYMES - biological catalysts which are used to help reactions such as photosynthesis and
digestion in living things.
HYDROLYSIS - the breakdown of molecules by splitting them with water e.g. sucrose into
molecules of glucose and fructose.
FERMENTATION - a reaction between sugars and enzymes in yeast which produces alcohol and
carbon dioxide.
ALKANOL - a compound made by fermentation e.g. ethanol.
ETHANOL - a member of the alkanol family of formula C2H5OH.