Organic compounds
Organic molecules are often large molecules made of smaller sub-units that are bonded together in various ways.
Compounds formed in this way are called Polymers. The sub-units are called monomers. We can classify molecules
on the basis of the kind of sub-units they contain.
Refer to page 56.
Carbohydrates
The basic unit of carbohydrates is a sugar molecule also called a monosaccharide. Carbohydrates containing one or
two sugar units are sometimes called simple; those containing many sugar units are called complex carbohydrates
or polysaccharides. Refer to page 56
Carbohydrates are important in plants as structural material. Cellulose is the most abundant organic compound on
Earth. Carbohydrates are also stored and used as a source of energy for plant and animal cells. Refer to page 57
Proteins
Proteins are large molecules built of sub-units called amino acids. There are 20 naturally occurring amino acids.
Amino acids join together when they peptide bond forms between them. When a number of amino acids are joined
this way, a polypeptide is formed.
Each type of Protein has its own
particular sequence of amino
acid. Polypeptide chains become
folded in different way
depending on their function.
Refer to page 58
Lipids
Proteins may be structural, for example in cell membranes. Refer to page 58
The enzymes that take part in all the metabolic processes of every living cell are also proteins.
Lipid is the general term for fats, oils and waxes. They have a little affinity for water.
A fat molecule is made of two kinds of molecules, fatty acids and glycerol.
On a weight basis, fat stores twice as much energy as the same weight of polysaccharide so fats are important
energy stores in cells.
Nucleic acids
There are two kinds of Nucleic acids; one is Deoxyribonucleic acid (DNA) and the second kind is Ribonucleic acid.
Deoxyribonucleic acid
The genetic material deoxyribonucleic acid is a polymer of nucleotides. Each nucleotide unit has a sugar part, a
phosphate part and a Nitrogen containing base. The sugar and phosphate parts are the same in each nucleotide.
There are four different kinds of nucleotides because four different kinds of N-containing bases are involved. The
four different N-containing bases are adenine, thymine, cytosine and guanine and the four different nucleotides are
denoted by the letters
A, T, C and G because of
the kind of base each
contains. Refer to
page 59
Ribonucleic acid
•
Ribonucleic acid is also a polymer of nucleotides. It differs from DNA, in that it is an unpaired chain of
nucleotide bases and it exist in three different forms. The three different forms of RNA are:
•
Messenger RNA (mRNA), formed against DNA as a template. mRNA carries the genetic message to the
ribosomes where the message is translated into a particular protein.
•
Ribosomal RNA (rRNA) which, together with particular proteins, makes the ribosomes found in cytosol.
•
Transfer RNA (tRNA), molecules that carry amino acids to ribosomes where they are used to construct
proteins. The strand of nucleotides in each of the RNAs is folded in a different way.
Key ideas
The major compounds that make up living cells are different kinds of carbohydrates, lipids, proteins and nucleic
acids. Each major compound has a specific role. Some exist only in either plant or animal cells; others play an
important role in both.