Page 35 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

13. The functions of chloroplasts are as follows:
(a) The major function of chloroplast is to carry out photosynthesis
producing organic compounds especially carbohydrates.
(b) Chloroplasts carry out their functions by using the membranes
of thylakoids and lamellae to trap lights and convert them to
chemical energy mainly in the form of ATP. They carry out
2 photoactivation and photophosphorylation through Calvin cycle.
(c) The ATP is then used to perform the fixation of carbon dioxide
to become organic compounds in the stroma.
(d) The DNA and the protein synthetic system in the chloroplasts
produces some of the specific proteins used in photosynthesis.
The chloroplasts still depend on the nucleus to obtain most
proteins within.
(e) The chloroplast can divide especially in their premature
protoplastid stage in the meristem. Mature chloroplasts do divide.
Centrioles (Centrosome)

1. Centrioles are organelles that assemble spindle fibres in animal cells
but are not found in plant cells.
2. They are found in all animal cells except the nerve cells. They are also
found in fungal and algal cells.
3. One pair of centrioles is usually located beside the nucleus.
4. The two centrioles are cylindrical in shape, arranged perpendicular
to one another as shown in Figure 2.22.

Figure 2.22 The structure of centrioles
5. They are small and can be observed as a dot under a light microscope.
Their length is about 0.3-0.4 µm with a diameter of about 0.2 µm.
6. Each is made up of 9 triplets of microtubules, which are attached
lengthwise together as shown as a cross-section in Figure 2.23.

Centriole cylinder
consisting of 9 sets of
microtubules. Each 1 set (3 microtubules)
set has 3 microtubules
Third microtubule
Second microtubule
First microtubule

Figure 2.23 9 triplets of microtubules
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