What is lacking in a virus which makes it dependent on a living cell to multiply?

$(a)$ 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?

Give differences between diffusion and osmosis.

Name the cell organelle responsible for intracellular transport.

Why is endocytosis found in animals only?

Do you agree that '$A$ cell is a building unit of an organism'? If yes,explain why.