4.1.
1 Chemicals & Life
Chemical Elements
Most of the molecules in living organisms fall into three
categories: carbohydrates, proteins and lipids
These all contain carbon and so are described as organic molecules
Chemical Elements Table
Large Molecules are Made from Smaller
Molecules
Carbohydrates
Long chains of simple sugars
Glucose is a simple sugar ( a monosaccharide)
When 2 glucose molecules join together maltose is formed (a
disaccharide)
When lots of glucose molecules join together starch, glycogen or
cellulose can form (a polysaccharide)
Summary
Carbohydrates are organic compounds used to store
energy.
A monosaccharide, the monomer of carbohydrates, is a
simple sugar, such as fructose or glucose.
Complex carbohydrates have two main functions:
storing energy and forming structures of living things.
Glycogen, cellulose and starch are all made from glucose molecules
Fats
Most fats (lipids) in the body are made up of triglycerides
Their basic unit is 1 glycerol molecule chemically bonded to 3 fatty
acid chains
The fatty acids vary in size and structure
Lipids are divided into fats (solids at room temperature) and oils (liquids
at room temperature)
Types of Lipids
Lipids may consist of fatty acids alone, or they may contain other
molecules as well. For example, some lipids contain alcohol or
phosphate groups. They include
1. Triglycerides: the main form of stored energy in
animals.
2. Phospholipids: the major components of cell
membranes.
3. Steroids: serve as chemical messengers and have other
roles.
[Figure 7]
Triglyceride Molecule. The left part of this triglyceride molecule represents
glycerol. Each of the three long chains on the right represents a different fatty
acid.
Structure of a triglyceride
Summary
Organisms use lipids to store energy. There are two
types of fatty acids: saturated fatty acids and
unsaturated fatty acids.
Animals use saturated fatty acids to store energy.
Plants use unsaturated fatty acids to store energy.
Phospholipids are the major components of cell
membranes.
Excess dietary lipids can be harmful.
Proteins
Long chains of amino acids
There are about 20 different amino acids
They all contain the same basic structure but the ‘R’ group is
different for each one
When amino acids are joined together a protein is formed
The amino acids can be arranged in any order, resulting in
hundreds of thousands of different proteins
Even a small difference in the order of the amino acids results in
a different protein being formed
General amino acid structure
Amino acids join together to form proteins
Summary
Proteins are organic compounds made up of amino
acids, the monomers of proteins.
A protein may have up to four levels of structure. The
complex structures of different proteins give them
unique properties.
Enzymes are proteins that speed up biochemical
reactions in cells. Antibodies are proteins that target
pathogens for destruction.
4.1.2 Food Tests
Test for glucose (a reducing sugar)
Add Benedict's solution into sample solution in test tube
Heat at 60 - 70 °c in water bath for 5 minutes
Take test tube out of water bath and observe the colour
A positive test will show a colour change from blue to orange or brick
red
The Benedict's test for glucose
Test for starch using iodine
We can use iodine to test for the presence or absence of starch in a food
sample.
The iodine test for starch
Add drops of iodine solution to the food sample
A positive test will show a colour change from orange-brown to blue-
black
Testing a potato to prove the presence of starch
Test for protein
Add drops of Biuret solution to the food sample
A positive test will show a colour change from blue to violet / purple
The Biuret test for protein
Test for lipids
Food sample is mixed with 2cm of ethanol and shaken
3
The ethanol is added to an equal volume of cold water
A positive test will show a cloudy emulsion forming
The ethanol test for lipids
Test for vitamin C
Add 1cm of DCPIP solution to a test tube
3
Add a small amount of food sample (as a solution)
A positive test will show the blue colour of the dye disappearing
The DCPIP test for vitamin C
Exam Tip
When describing food tests in exam answers, make sure you give the starting
colour of the solution and the colour it changes to for a positive result.
4.1.3 DNA Structure:
Structure of a DNA Molecule:
DNA, or deoxyribonucleic acid, is the molecule that contains the
instructions for the growth and development of all organisms
It consists of two strands of DNA wound around each other in what is
called a double helix
DNA, chromosomes and the nucleus
A nucleotide
The individual units of DNA are called nucleotides
All nucleotides contain the same phosphate and deoxyribose
sugar, but differ from each other in the base attached
There are four different bases, Adenine (A), Cytosine (C),
Thymine (T) and Guanine (G)
The bases on each strand pair up with each other, holding the
two strands of DNA in the double helix
The bases always pair up in the same way:
o Adenine always pairs with Thymine (A-T)
o Cytosine always pairs with Guanine (C-G)
DNA base pairs
The phosphate and sugar section of the nucleotides form the ‘backbone’
of the DNA strand (like the sides of a ladder) and the base pairs of each
strand connect to form the rungs of the ladder
The DNA helix is made from two strands of DNA held together by hydrogen
bonds
It is this sequence of bases that holds the code for the formation of
proteins
Exam Tip
You do not need to learn the names of the bases, just their letter. Make sure
you know which bonds with which, as this is the most commonly asked
question about this topic.
Summary
Transcription is the DNA → RNA part of the central
dogma of molecular biology.
Transcription occurs in the nucleus of eukaryotic cells.
During transcription, a copy of mRNA is made that is
complementary to a strand of DNA. In eukaryotes,
mRNA may be modified before it leaves the nucleus of
eukaryotic cells.
