In the given figure,what do $a$ and $b$ represent respectively?

Which of the following is not a component of the endomembrane system?

Choose the correct statements regarding cell organelles and their inclusions.
$A$. The endomembrane system includes Golgi complex, endoplasmic reticulum and mitochondria.
$B$. Rough endoplasmic reticulum bears ribosomes on its surface.
$C$. Both mitochondria and plastids have circular $DNA$.
$D$. $A$ network of microtubules, microfilaments and intermediate filaments present in the cytoplasm is called cytoskeleton.
$E$. Mitochondrion is a single membrane-bound structure.
Choose the correct answer from the options given below:

Assertion $(X)$: The cell is a wonderful marvel of the world.
Reason $(Y)$: It converts nutrients into free energy and contains its own information in the form of genetic material.

Which of the following organelles is not considered a component of the endomembrane system?

Differentiate between a prokaryotic cell and a eukaryotic cell.