Discuss the advantages and disadvantages of cell specialization.

Cell specialization, or rather, cell differentiation, is the process by which unspecialized structures become modified and specialized for the performance of specific functions. This process usually occurs in multicellular organisms, which consist of many groups of specialized cells making up their tissues and organs. Unicellular organisms consist of one general cell under which all activities are housed.

In animals, the development of embryos can be divided into three phases; cleavage, gastrulation and organogeny. During cleavage, the zygote undergoes repeated mitotic division to form a hollow ball of cells. In gastrulation, these cells then continue to divide and become rearranged into special layers, called germ layers, within the embryo. It is during the third phase of organogeny that the cells of the germ layers within the enlarging embryo begin to undergo further cell division as well as cell specialization to form the organs of the embryo. This results in the formation of the epithelia, connective tissues, nerves and muscle. Other specialized cells such as red blood cells are also formed.

In flowering plants, the embryo is formed in the seed, which develops from a fertilized ovule. Initially, all the cells of the embryo are meristemic – capable of further cell division. However, as the embryo begins to grow into an independent seedling, its cells become restricted to specialized regions called primary meristems. Occurring at the tips of stems and roots, these cells are responsible for growth in the length of the stem and root. Cells cut off from the primary meristems form primary plant tissues which are either simple (consisting of one cell type only) or compound (consisting of more than one type of tissue) plant tissues. Examples of such would be the parenchymal tissue and xylem tissue respectively. Additionally, a lateral meristem forms from the procambial strand, which runs the length of the stem and the root. This is responsible for the formation of the vascular tissue in the stem and for the growth of the girth of the stem later on.

Single-celled organisms such as protists (Chlamydomonas and Euglena) contain one general cell for all necessary functions and do not undergo cell specialization. They have their own mechanisms for procuring food, for example, with the use of chloroplasts, and for moving about – Chlamydomonas contain a pair of hair-like flagella whereas Euglena contains a single flagellum. They also have light-sensitive eyespots, excretory systems (contractile vacuoles) and are capable of reproduction on their own. Although these organisms are incapable of growing to a much larger size, they do hold benefits of being unicellular. As there are always two separate angles to view the same object, cell specialization also has its advantages and disadvantages.

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When a cell undergoes specialization, it changes its shape, structure and physiology to better carry out its function. This is an added benefit to the group of cells, which can then carry out its task more efficiently. For example, muscle cells are specialized to pull and cause movement. Consisting of elongated cells or fibers held together by connective tissue, they are highly specialized to the point that they are able to shorten to half or even a third of their normal ‘resting’ length. Muscle tissue makes up about 40 per cent of the mass of an adult body and is crucial to the functioning of an organism. Another example of specialized cells would be red blood cells, or erythrocytes. These small circular biconcave discs begin as cells containing a nucleus and little haemoglobin. Upon maturity, haemoglobin content increases to almost 90 per cent of the dry mass of the cell. The nucleus is then squeezed out to provide more space for the haemoglobin. These cells are specialized in carrying oxygen, combining it with haemoglobin to form oxyhaemoglobin. This is a method of transportation of oxygen from areas of high oxygen tension to lower oxygen tensions in respiring tissue all around the mammalian body. Erythrocytes are also able to carry carbon dioxide as hydrogencarbonate from regions of high to low carbon dioxide tensions in the lungs.

But it must be noted that when these cells are differentiated from the general cell, they are specialized and changed at the expense of performing another task. Though the muscle cells can contract and relax, these actions are merely in obedience to other cells, such as nerve cells, which cause them when to contract. Also, the muscle cells are not able to get their own food, showing further reliance on other cells. The mature erythrocyte, which lacks a nucleus, is unable to regulate cell activity and is destroyed after 3 to 4 months in the body. Unicellular organisms on the other hand, are able to control all activities within one independent cell and are not reliant on any other cells for survival.

The evolution of cell specialization in multicellular organisms gives living organisms a whole new potential for the exploitation of their environment, a level totally beyond the reach of prokaryotes. One of the most primitive multicellular eukaryotes, the cellular slime mould Dictyostelium, is a good illustration of the advantage of cell specialization in the division of labour to exploit the environment fully. At most times during their life cycles, Dictyostelium cells exist as solitary, independent amoebae wandering about over their substrate. Each cell represents a complete, self-sufficient organism. However, when the food supplies become scarce, an entirely new type of activity is triggered amongst the cells and they stream towards each other to form an aggregate, becoming a small part of a multicellular individual. This new stage in the life cycle is termed a pseudoplasmodium, which migrates for a period over the surface of its container. Close examination of the cells of the pseudoplasmodium shows that the cells are no longer a homogenous population. After a while, the pseudoplasmodium will stop moving about and begin to round up on the substrate and rise into the air as an elongated fruiting body. This body consists of a slender stalk supporting a round mass of cells towards its tip. Close examination indicates that the stalk cells and spore cells are not only very different under the microscope, but are also very different in function. The stalk cells support the spore mass above the substrate whilst the spore cells ensure the continuity of the population into the next generation.

However, as the evolution of cell specialization causes the cells to become more profoundly specialized in its function, it also decreases the possibility of the cells changing their functions if the unusual need arises. Changes to cells are sometimes drastic and irreversible, such as the loss of the nucleus in erythrocytes as mentioned previously or the total loss of living contents in mature xylem vessels of flowering plants. Xylem vessels in plants have thickened walls, impregnated with lignin and serve to conduct water and mineral salts through the plant and to provide mechanical support. Formed from procambial strand cells on the inner side of the strand in the developing stem, the xylem vessels effectively become continuous tubes when all the transverse walls have been broken down. Highly specialized to effectively serve its function, these cells can no longer change their specified functions. If the natural environment changes drastically to that of a desert terrain, most plants with xylem vessels would not be able to fully utilize them since there would be no water to transport in the first place. Also, the support given by the xylem vessels may be unwanted since it would be exposing more of the plant towards the sun and risk even greater water loss.

Yet another possible advantage of specialization would be the capability of the organism to increase in size. Protists, which are not able to grow larger without organ systems, become easy targets for predatory forms. However, with the increase in size of an organism comes an increase in the complexity of its form. Such organisms would then require complex nutrients in their diets such as vitamins, and thus are unable to live off simple carbon and nitrogen sources. The nervous system consists of nervous tissue made up of cells known as neurones, which are specialized for the transmission of electrical impulses, together with associated neuroglia cells. Neurones are the pathways of communication between the brain and the body. Electrical impulses pass along the neurones from organs that receive stimuli, to organs that effect change. Impulses are also generated spontaneously in the brain and are conducted to effectors via the peripheral nerves. For proper nervous system function, vitamins such as folic acid and pyridoxine (B6) must be present. Other minerals such as magnesium, niacin and potassium play a vital role to support the heart muscle and its force of contraction. Besides the need of more complex nutrients, multicellular organisms with specialized cells would also require a larger repertoire of enzymes. In Escherichia Coli (E. Coli), over 629 different types of enzymes exist for its 159 metabolic pathways and 946 different reactions. Compared to a simpler unicellular organism, the number of reactions occurring and enzymes required would be insignificant. The more complex an organism evolves into, the more complex its repertoire of enzymes.

Due to cell specialization, reproduction of multicellular organisms is no longer as simple as if they were unicellular. The choosing of an appropriate mate and the raising of young pose a more tedious situation to the multicellular organism compared to the meeting, fusing of single cells to grow through meiosis. However, due to this, more variability would be possible for even greater evolutionary adaptation. Also, in unicellular organisms, only a net gain of two cells is obtained from each sexual cycle since the original two cells are taken off the four produced cells. But in multicellular organisms, cells are specialized to form gametes such as the sperm and egg. These cells fuse and then go through meiosis to form four new individuals in each meiotic cell division. The two original parents are not directly involved in the reproduction cycle. If this process were to be plotted against the number of generations, the number of individuals in a population for a unicellular population versus a simple multicellular population, it can then be easily seen that the number of multicellular organisms produced are compared to unicellular for every cycle. This is advantageous in the detailed sense of cellular reproduction, which is much faster. However, the number of cells needed to make up a multicellular organism and a unicellular one balances the scale yet again.

Cell specialization not only helps an organism to grow larger, but also enables its cells to carry out functions efficiently and effectively. This main advantage does comes with a price – not being able to change to adapt to a sudden change in environment and the need to take in more complex nutrients such as vitamins and minerals. But no matter what disadvantages it may hold, the cons of specialization are outweighed by advantages since multicellular organisms are able to achieve more than just simple survival. Humans are able to make active choices as to what to do, what to eat or even whom to choose as their mates. Paramecium, a unicellular organism, on the other hand, cannot. Plants are able to adapt to their present environments through more complex systems of transport using specialized phloem and xylem tissue. Though these cells have to rely on other cells for other basic needs, it does not put them at any direct disadvantage. All cells are able to work together for the proper functioning of the organism, which may be seen as a better version of the single-celled organism. Thus, advantage of cell specialization greatly outweighs its diadvantage and nature coule be said to have made a rgith decision when the evolution of cell specialization in organisms came about.

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Michele Ng

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