Page 33 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

5. They are very small, about 22 nm in diameter for the eukaryotic 80S
types and 18 nm diameter for the prokaryotic 70S types which are
found in prokaryotic cells as well as mitochondria and chloroplasts.
6. The subunits can be attached to form bigger functional units in the
presence of magnesium ion.

For eukaryote: 60S + 40S  80S

2 For prokaryote: 50S + 30S  70S
7. They are made of RNA and protein synthesised in the nucleolus. As
for the 80S types, the 60S subunits contain about 3 types of RNA and
equal number of types of proteins. The 40S smaller subunits contain
1 type of RNA and protein.
8. As for the 70S types, the larger 50S subunits contain 3 types each of
different RNA and proteins. The smaller 30S subunits contain only a
single type each of RNA and protein.
9. The function of ribosomes is to provide the site for the formation of
peptide bonds in which amino acids are joined to form polypeptide
or protein. The subunits can form a complex with mRNA. Two sites
are found on the surface where two tRNAs will each bring an amino
acid to the corresponding site, matching the codons of mRNA to that
of the anti-codon of the tRNA. Therefore, ribosomes can ‘read’ the
codons on the mRNA and join specific sequence of amino acids to
form specific protein.

Chloroplasts
1. Chloroplasts are plastids, organelles that contain chlorophyll and
carry out photosynthesis.

2. Chloroplasts are found in the part of the plant that is green in
colour, especially in the mesophyll cells of leaves, parenchyma of
young stems, fruits, sepals and even aerial green roots.
3. Their locations in cells are not fixed, they can move and orientate
themselves with their larger surface towards the sunlight. This is to
enable them to obtain the maximum amount of sunlight.
4. There are about 100 of them in a palisade mesophyll cell of flowering
plants. The number can be increased when light intensity increases,
or decrease if the light intensity decreases.
5. They have biconvex circular shapes like that of lenses.
6. They are reasonably big to be observed under a light microscope.
They are about 3-10 µm in diameter and 2-3 µm thick.
7. The envelope consists of two layers of lipoprotein membranes that
are smooth, with no foldings or granules.
8. There is an internal membrane system inside the chloroplast called
the thylakoid system within a liquid called stroma. The internal
structure is shown in Figure 2.21.

78

02[STPM Bio T1].indd 78 3/29/18 5:08 PM