Question

Which of the following cellular parts is correctly described?
A. Centrioles – sites for active RNA synthesis.
B. Ribosomes – those on chloroplasts are larger $(80S)$ while those in the cytoplasm are smaller $(70S)$.
C. Lysosomes – optically active at a $pH$ of about $8.5$.
D. Thylakoids – flattened membranous sacs forming the grana of chloroplasts.

Answer 1

The cellular parts are all those units that comprise the functional parts of the cell. In addition, the meaning of each term should be understood based on its literal meaning and position.

Complete step by step answer: The centrioles are the two cylindrical organelles that are situated just near the nucleus. They are very minute in size and are made up of microtubules. During the process of cell division, they are the ones that produce spindle fibers. These are present in almost all eukaryotic cells. The organization of mitotic spindle by centrioles results in the completion of the process of cytokinesis. They have no role in RNA synthesis. Thus, this description is wrong.
-Ribosomes on the other hand are the units that are involved in protein production. The ribosomes present over the surface of the chloroplast are two-thirds the size of ribosomes present in the cytoplasm of the cell. They have an active role in the transcription process both in chloroplast and cytoplasm. The chloroplast DNA is transcribed by ribosomes present over it. The cytoplasmic ribosome transcribes mRNA from the nucleus of the cell. Therefore the description that ribosomes in chloroplasts are larger $(80S)$ while those in the cytoplasm are smaller $(70S)$ is incorrect.
-The lysosomes are spherical sac-like structures present in the cell. They consist of hydrolytic enzymes that break down various biomolecules. These enzymes stay inactive until the $pH$ of 4 to 5 is reached. Hence, the description of lysosomes that are optically active at $pH$of $8.5$ is wrong.
The thylakoid is a compartment situated inside the chloroplasts and Cyanobacteria. They are flattened compartments that are bound to the membrane that holds them. They act as sites where light-dependent reactions of cells take place like photosynthesis. Thus, they are described correctly.
Therefore, the right answer is option D.

Note: The actual site for the initiation of light-based reactions is the pigmented membrane of thylakoids. These are arranged in stacks called grana in a chloroplast.