Page 31 - Biology_F5

Biology for Advanced Level Secondary Schools

(b) 80s ribosomes: These have the “translate” the message contained in
sedimentation coefficient of 80s the mRNA codes.
and the molecular weight of 40x10 6 (c) They have enzymes that catalyse the
Daltons. The 80s ribosomes occur in synthesis of peptide bonds.
eukaryotic cells.

Lysosomes
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The ribosomes of mitochondria and Lysosomes are membrane-enclosed
chloroplasts are always smaller than organelles that contain an array of enzymes
cytoplasmic ribosomes and are comparable capable of breaking down all types of
to prokaryotic ribosomes in both size and biological polymers such as proteins,
sensitivity to antibiotics. nucleic acids, carbohydrates, and lipids.
The word lysosomes come from the two
Large subunit words lyso meaning ‘digestive’ and soma
meaning ‘body.’ They are also referred to
as suicide bags since they undergo self-
destruction, as they can digest the entire
cell (autolysis). They mostly occur in
animal cells and few or rare in plant cells.
Lysosomes are not present in bacteria
and mature erythrocytes, while a few of
them occur in muscle cells. Leucocytes,

Small subunit especially granulocytes, are particularly
rich sources of lysosomes. In addition,
Figure 1.13: Structure of a ribosome they are numerous in the epithelial cells
of lungs and uterus.
Functions of ribosomes
(a) Ribosomes are sites for protein Structure of lysosomes
synthesis; they provide surface Lysosomes are round, with vacuolar
area or space for polypeptide chain structure, which remains filled with dense
construction and enzymes for the material and bounded by a unit membrane.
synthesis of peptide bonds between They consist of digestive enzymes,
amino acid monomers. Thus, they which are synthesised in the ribosomes
are known as protein factories. The of the rough endoplasmic reticulum. The
ribosomes are adapted for protein enzymes, which always have acidic pH,
synthesis, as they have ribosomal RNA are transported to the Golgi apparatus
(rRNA) which provides attachment for modification. The Golgi apparatus in
points for both messenger RNA the cytoplasm concentrates and packs the
(mRNA) and transfer RNA (tRNA). modified enzymes in the Golgi vesicles,
(b) Ribosomes have receptor sites on the which pinch off as lysosomes. Their shape
small sub-unit where mRNA binds and density vary greatly from cell to cell
itself and they are able to “read” and and from time to time.

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