Exceptions to the Cell Theory
Viruses, however, are not the only exception to the Cell Theory as formulated earlier. This has led scientists to take a much broader view of the problem. some organisms like a bacterium, blue-green alga, mycoplasma, amoeba, Chlamydomonas and Mucor are composed basically of a single cell.-In such cases we face another problem. You have so far learnt that the cell is the basic structural unit of an organism, something like bricks in a building.
But in the examples cited above just a single cell constitutes the whole animal, which amazingly performs all the body functions like respiration, digestion, excretion, etc. The problem is further complicated by the occurrence of many multinucleate protozoans, multinucleate animal tissues etc.For example a species of amoeba, Chaos as well as also many other protists like the ciliate Opalina have many nuclei in their body. The common. pond plant Nitella has giant cells with many nuclei in the cytoplasm. There are also tissues and cells in our body which have many nuclei.
Such cells are called syncytial. For example, the striated muscle fibres of our body are syncytial in nature. The problems listed above arose due to the fact that Cell Theory was proposed on the basis of observations on multicellular organism. This created a wrong notion that single celled organisms were totally comparable to a cell of multicellular organisms. It is however, now established beyond any doubt that as it stands today, the present unified Cell Theory given below, is universally applicable with equal force from bacteria to protozoa to humans except for viruses.