Cell wall is absent in:

The mycoplasma is an organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many mycoplasmas are pathogenic i.e. cause various diseases in plants (Pleuropneumonia), animals (witches broom) and human beings. These are pleomorphic (can change shape due to the absence of cell wall) and were called PPLO (Pleuropneumonia like organisms) or jokers of the plant kingdom. The bacterial structure is very simple but they are very complex in behaviour compared to many other organisms, bacteria as a group show the most extensive metabolic activity.