Chapter 4: Biological Molecules
Water
Water is a key solvent in our body as it helps metabolic reactions take place in
our body.
Plasma, the liquid part of blood, contains a lot of water, so substances like
glucose can dissolve in it. These dissolved substances are transported around
the body.
Helps in dissolving enzymes and nutrients in the alimentary canal.
Helps get rid of waste products from our bodies.
Carbohydrates
Substances that include sugars, starch, and cellulose; contain carbon and
hydrogen
1 molecule of carbohydrate contains about twice as many hydrogen atoms as
carbon or oxygen
Simplest carbohydrates: sugars, monosaccharides, or glucose.
If two simple sugar molecules join together, a larger molecule called a complex
sugar or disaccharide is made.
Examples of complex sugars: sucrose and maltose.
Polysaccharides:
A very large complex sugar
The cellulose of plant cell walls is a polysaccharide and so is starch.
Animal cells have a polysaccharide called glycogen.
Used for:
Needed for energy.
The carbohydrate that is normally used in respiration is glucose.
Plants store carbohydrates as starch while animals do not.
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Testing for carbohydrates:
Benedict’s test:
We can test for the presence of sugars by adding Benedict’s solution to food
and heating it.
If the food contains reducing sugar (such as glucose or maltose), then a
brick-red color will be produced.
The mixture changes gradually from blue, through green, yellow, and orange,
and finally brick red.
If there is no reducing sugar, then Benedict’s solution remains blue.
Adding iodine solution to a sample of the food. If there is starch present, a blueblack color is obtained. If there is no starch, the iodine solution remains orangebrown.
Fats and oils
Also known as lipids that are insoluble in water
Liquid at room temperature (oil).
Fats and oil contain carbon, hydrogen, and oxygen. Each molecule contains less
oxygen than is found in a carbohydrate molecule
Fats are made of fatty acids and glycerol
Used for:
They are used as energy stores
In mammals, some cells underneath the skin, store drops of fats and oils. This
fat-containing tissue is both an energy store and a heat-insulating layer. This
layer of cells is called adipose tissue.
Plants store oil in their seeds. It provides a good store of energy for germination.
Testing the presence of alcohol using fats and oils: Emulsion
test
Fats and oils do not dissolve in water
However, they do dissolve in ethanol. We can use this to detect their presence in
food
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Step 1: the food is shaken with ethanol, to allow any fats in it to dissolve in the
alcohol
Step 2: the ethanol is poured into a clean tube containing water. If there are fats in
the ethanol, they form tiny droplets in the water, which give it a milky appearance.
The mixture of tiny droplets in water is called an emulsion
Proteins
A substance whose molecules are made of many amino acids linked together
Each different protein has a different sequence of amino acids
Amino acids are substances with molecules containing carbon, hydrogen,
oxygen, and nitrogen
There are 20 different types of amino acids
Used for:
All enzymes are proteins
Antibodies, which help to protect the body against pathogens, are proteins
Protein is present in haemoglobin
Important for forming cell membranes in all organisms
In humans, hair and fingernails are made from a protein called keratin
Testing for proteins
Biuret test: involves mixing the food in water, and then adding dilute copper sulfate
solution. Then dilute potassium hydroxide solution is gently added. A purple color
indicates that protein is present. If there is no protein, the mixture stays blue.
Test for vitamin C
DCPIP: a purple liquid that becomes colorless when mixed with vitamin C
DNA
Stands for deoxyribonucleic acid.
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DNA is the material that makes up our genes and chromosomes. We inherit DNA
from our parents
DNA is made of smaller molecules called nucleotides. Each nucleotide contains
a base.
The sequence of bases in the DNA molecule determines the sequence of the
amino acids that are used to build a protein
Nucleotide: molecules that are linked together into long chains, to make up a
DNA molecule.
Complementary base pairing: the way in which the bases of the two strands of
DNA pair up; A always pairs with T, and C with G.
Bases:
one of the components of DNA
Four bases: A, C, G, and T.
Their sequence determines the proteins that are made in a cell
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Carbohydrates
Fats
Proteins
Elements they are
made of
Carbon, Hydrogen,
and Oxygen
Carbon, Hydrogen,
and Oxygen
Carbon, Hydrogen,
Oxygen, Nitrogen, and
Sulfur
Smaller molecules
of which they are
made
Simple sugars
(monosaccharides)
Fatty acids and
glycerol
Amino acids
Easily available
energy (17 kJ/g)
Storage of energy
(39kJ/g); insulation;
making cell
membranes
Making cells,
antibodies, enzymes,
and hemoglobin; also
used for energy
Why do organisms
need them
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