A unicellular organism is one that only has one cell for example bacteria,
protozoa and fungi.
Bacteria are very small cells, they differ from normal animal cells as they do
not contain a nucleus so the DNA is either floating free or kept in small
containers called plasmids they also may have a tail like structure usually
called a Flagellum allowing the cell to move.
Protozoa are also unicellular organisms however it has certain characteristics
that are similar to animal cells. It has feet like projections that allow it to move
and take food into the cell.
Yeast is commonly used in bread making and making alcohol as it converts
sugar into alcohol.
Yeast is similar to plant cells in that, they have cells walls, which means they
can absorb sugars.