BIOLOGICAL MOLECULES.
They are molecules made by living organisms.
All biological molecules have the element carbon.
Biological molecules can be large, often made of
repeating units called monomers.
The subunits are held by chemical bonds.
Examples of biological molecules.
Carbohydrates.
Fats.
Proteins.
Nucleic acids.
Carbohydrat Fats. Protein
es s.
Elemen Carbon. Carbon. Carbon.
ts Hydrogen. Hydrogen. Hydrogen.
presen Oxygen. Oxygen. Oxygen.
t Nitrogen
and
sometimes
Sulfur.
Basic Glucose Fatty Amino
Units acids and acids.
glycerol
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CARBOHYRATES.
Simple sugars.
Polysacharides.
1.SIMPLE SUGARS.
Monosacharides. Simplest sugars that are single –
ringed.
A commonly occurring simple sugar is glucose
which has the chemical formula C6H12O6.
Glucose often exists as a ring.
Sugar molecule formed when two simple sugars
are joined together is called a disaccharide.
All sugars are sweet to test; they are soluble in
water and are reducing sugars( except sucrose).
COMPLEX MOLECULES.
2.Disaccharides.- complex sugar molecules
made up of two simple sugars held together by a
chemical bond.
Examples of Disaccharides are maltose, sucrose
and lactose.
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3.POLYSACHARIDES.-(Complex molecules)
These are complex molecules made up of
many glucose molecules held together by
chemical bonds..
Examples: Starch , Glycogen and cellulose
All polysaccharides are insoluble in water .
Examples of Polysaccharides.
1. Glycogen –A storage carbohydrate in animal
cells.
It is stored in the liver and muscle cells.
It is compact –more molecules are stored in a
smaller space
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Less reactive- it will not affect cell metabolism
Insolubility in water makes it a suitable storage
carbohydrate as it does not affect the water
potential of the storage cell.
It is branched to increase the rate of breaking it
down to release glucose.
Is an energy store.
2. Starch- is storage carbohydrate in plants.
Its made up of many glucose molecules joined
held together by chemical bonds.
Its insolubility makes it a suitable storage
molecule as it does not affect the water potential
of the storage cells..
Its also energy store.
3. Cellulose.
It is structural carbohydrate- it has high tensile
strength making it a suitable structural
carbohydrate.
It’s used for the synthesis of plant cell walls.
Polysaccharides form parallel chains
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FUNTIONS OF CARBOHYDRATES.
Main source of energy.
Structural -Cellulose is used for synthesis of plant
cell walls.
TEST FOR REDUCING SUGARS ( SIMPLE
SUGARS)-BENEDICT’S TEST.
Test for reducing sugars is called Benedict’s test
because the chemical reagent used is Benedict’s
solution.
Procedure of Benedict’s test.
Add 2cm3 of food sample solution to a test tube.
Add an equal volume of Benedict’s solution.
Shake to mix the contents.
Boil for about 2-5 minutes.
Observation.
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Starch (Iodine test)
Add 2 cm³ of test solution + 2 drops of iodine/KI
solution.
A blue-black colour indicates the presence of
starch .
Starch is only slightly soluble in water, but the test
works well in a suspension or as a solid.
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FATS.
Fats and oils are made up of the elements
carbon, hydrogen and oxygen.
Each fat molecule is made up of one
molecule of glycerol and three fatty
acids.
Fat molecule glycerol + 3 fatty acids.
Simplified diagram of a fat molecule.
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FUNCTIONS OF FATS.
Energy stores.
Heat and electrical insulators.
Protect delicate organs eg kidneys.
Components of cell membranes.
Store fat soluble vitamins e.g Vitamin A and B.
TEST FOR FATS (Emulsion test).
Add 2cm3 of absolute ethanol.
Add 2cm3 of fat/oil into a test tube
Dissolve the lipid by shaking vigorously
Add 2cm3of cold water.
Observation.
A cloudy white suspension forms if fat is present.
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PROTEINS.
Proteins are made up of elements Carbon, Hydrogen,
Oxygen, Nitrogen and sometimes Sulfur.
They are made up of amino acids held by chemical
bonds called peptides hence are called polypeptides.
They are about 20 different kinds of amino acids that
are used to synthesise proteins.
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The order of amino acids on the protein molecule is
determined by sequence of bases on the DNA.
The order of the amino acids on the protein
determines the shape of the protein.
The shape of the protein is related to its function.
SAMPLE QUESTION
J12 P31 Q2(c) Proteins and DNA are
important nitrogen-containing compounds
in cells.
Describe the roles of DNA in cells.
[2]
MARKING POINTS.
Carries the genetic information;
control functions of the cell ;
codes for proteins ;
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Relating the shape of the protein to its function.
Enzymes.
Enzymes are biological catalysts which speed
the rate of chemical reactions.
The shape of the enzyme creates an active site,
which is complementary to the shapes of its
substrate ( molecule acted upon by the
enzyme).
The chain of amino acids take up particular
shape as a result of cross –linkages.
This makes enzymes very specific in their
action.
NB: When protein are heated , the cross-linkages break ,
the protein molecule loses its shape.
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Antibody.
Antibodies are proteins produced by
Lymphocytes.
Each antibody has a binding site , which can
lock onto the pathogen such as bacteria.
This destroys the pathogen directly.
Each pathogen has proteins called antigens on
its cell membrane with shapes complementary
to the antigen.
Structure of the antibody.
Functions of proteins.
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For growth of new tissues and replacement of cells.
To build up structures e.g keratin for formation of
hair and nails.
Formation of enzymes,
hormones
antibodies.
SAMPLE QUESTION.
N11 P31Q2(c) Proteins and DNA are
important nitrogen-containing compounds
in cells.
Describe the roles of proteins in cells.
[3]
MARKING POINTS.
enzymes ;
catalyse, reactions ;
e.g. respiration / photosynthesis
(part of cell) membranes ;
carrier proteins- allow active transport
allowing movement in and out of cell ;
haemoglobin ;
transport of, oxygen / carbon dioxide /
gases ;
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TEST FOR PROTEINS (BIURET TEST).
Procedure.
Add 2cm3 of protein solution to a test tube.
Add an equal volume of 5% potassium
hydroxide solution and mix.
Add two drops of 1% copper sulfate solution and
mix.
Observations.
If protein is present solution turns from blue to
purple/ violet/ lilac.
STRUCTURE OF DNA.
DNA- Deoxyribonucleic acid.
It is a molecule made up of long chains of
nucleotides, formed into two strands which are
twisted into a double helix.
The strands are held by crosslinks/hydrogen
bonds between bases pairs.
Adenine(A) pairs with thymine(T) and Guanine
with Cytosine(C).
COMPONENTS OF A NUCLEOTIDE.
1. 5- Carbon sugar.
2. Phosphate.
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3. Nitrogenous base- either Adenine(A),
Guanine(G), Cytosine(C) or Thymine (T).
The sugar and phosphate molecules join up and form
the backbone of the DNA strand.
Two strands of DNA are held by hydrogen bonds
formed between nitrogenous bases.
The base always pair up in the same way:
Adenine (A)pairs with thymine(T)
Guanine (G) pairs with Cytosine (C).
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Base Pairing between the DNA strands.
Copy and write the missing base pairs [2]
.
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The DNA strands twist to from a shape similar to a
spiral called a double helix.
SAMPLE QUESTION.
A sample of DNA was tested to find out
which bases were present. It was found
that 30% of the bases in the DNA were T.
Calculate the percentage of A and C in
the DNA molecule. [4]
Explain why two organisms that have
different sequences of bases in their DNA
may look different
[2]
DNA double Helix.
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SAMPLE QUESTION.
N16 P43 Q5 (a) (i) Describe the structure
of a DNA molecule. [3]
MARKING POINTS.
It is a double helix;
Two strands contain, bases / A and T
and C and G;
A joins with T and C joins with G;
strands are held by hydrogen bonds;
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J16 P41 6. A DNA molecule has two
strands as shown in Fig. 6.
Fig. 6.1
(a) (i) Fill in the boxes on Fig. 6.1 to
show the letter of the bases on strand
2 that will pair with the corresponding
bases on strand 1.
[2]
(ii) State the name for the structure of
a DNA molecule as shown in Fig. 6.1.
(iii) N12 P333 Q1(c) State precisely where
DNA is found in a cell.
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[1]
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