$(a)$ Name the organelle which provides turgidity and rigidity to the plant cell. Name any two substances which are present in it.
$(b)$ How are they useful in unicellular organisms?

(N/A) The organelle is the $Vacuole$. Two substances present in it are $Amino \ acids$ and $Sugars$ (or $Proteins$, $Organic \ acids$).
$(b)$ $(i)$ In $Amoeba$, food vacuoles contain the food items captured by the organism.
$(ii)$ In many other unicellular organisms, specialized vacuoles (like contractile vacuoles) play an important role in expelling excess water and some wastes from the cell.