After incorporating two major variations (internal membrane system and cell wall) to the basic cell design, all further variations are what I call “icing on the cake.”
So what can we consider as “icing on the cake” variations? Well, in animal cells, further variations include changes in size, shape, structure, etc. Some cells stretched out, flattened, became spherical, developed processes, merged, branched, and exhibited other changes. With every change emerge a new cell. Thus we have so many different kinds of animal cells like: muscle cell, nerve cell, lung cell, red blood cell, liver cell, bone cell, egg cell, sperm cell, etc. In the human body for example, there are more than 200 different cell types.
What about plant cells, what kind of “icing on the cake” variations occurred? Well, because of their cell wall, plants cells cannot change shape or stretch out or flatten. So how do plant cells vary? Well, their variations are mostly in their cell walls. Some have very thick or very thin cell walls, while others have added new substances, or even spaces to their cell walls. Thus, we recognize plant cells as: collenchyma, parenchyma, or sclerenchyma. Their main differences are simply in their cell wall. Of course plant cells also differ from animal cells by having chloroplasts and a large vacuole in their cytoplasm.