Looking through the trend of classification of living organisms, one easily notices that as we move from one phylum to the next, the organisms become larger and more complex. Cells on their own are not specialised for the performance of any function of life except for repeated cell division.
Levels of organisation of life
It is from the embryonic cell that tissues, organs and later systems may be derived to form the basis for the levels of organisation of life.
Usually, the cell is bounded by a membrane, and contains a nucleus, and cytoplasm or protoplasm. Some cells are capable of independent existence, carrying out (but without specialisation) all the characteristic (life) processes of living things. Indeed, cells usually have an inherent ability to grow and reproduce, to metabolise, to receive and respond to stimulus and to show movement, but not specialised for any of those functions. Cells may have pseudopodia, cilia, flagella, etc., and other inclusions and organelles.
Among some of the best known organisms of cellular organisation are microscopic and unicellular forms such as amoeba, paramecium, euglena, chlamydomonas, etc. These exist either as one-celled or colonial forms, having a single unit of protoplasm including cytoplasm, one or more nuclei and a variety of organelles.
In multicellular organisms, there are more than one group of cells. Each group of cells are similar in structure and function. A tissue is a collection (group) of cells which are similar in structure, and perform similar functions. Tissues usually have the same origin and occupy the same position in the body of the organism. While some tissues cannot exist on their own, some can live on their own, e.g. Hydra.
Sometimes, the cells of a tissue are held together by a material called matrix, usually secreted by cells. The jelly fish, sea anemones and coral are at the tissue level of organisation. Common examples of tissues in animal and plant bodies include the following:
a. Epithelia which are made of one or several layers of cells. The cells are of different types and are either squamous, cuboidal or columnar. They are seen covering or lining the inner surfaces of the skin, body cavities, blood vessels, trachea, digestive tracts. Their function is either protective or secretive as in the goblet cells in the digestive tract.
b. Connective tissues which bind and support other body structures. Others include muscle, nervous, skeletal and blood tissues, each performing a different and specialised function. In plants, conducting tissues include xylem and phloem while supporting tissues include parenchyma, collenchyma and sclerenchyma.
As organisms grow in sizes and complexities, cells or units of cells become unable to service the needs of such organisms.
An organ is a collection of different tissues, that perform a common function or functions. Some organs carry out a single function, e.g., the function of the heart is to pump blood. Others carry out more than one function, for example, the kidney carries out excretion and osmoregulation and maintenance of the internal environment.
The onion bulb (Allium cepti) presents a good example of an organ. At a glance, we observe the roots for anchorage and absorption of water and other nutrients, the stem to link the roots with leaves and sexual reproduction, while the leaves serve the function of food production and storage.
The most advanced and complex organisms cannot be adequately serviced by tissues and organs alone, but are organised into systems of organs. A system is made up of different organs that perform a particular function. The system level of organisation is common in higher invertebrates and the vertebrates.
The following systems are common in most advanced forms: muscular integument, digestive, circulatory, skeletal, respiratory, excretory, nervous, hormonal and reproductive systems.
Complexity or organisation in higher organisms
We have observed from the above treatment that organisms stand to benefit a lot as they advance from their simple, microscopic and unicellular forms to higher multicellular and complex forms.
(i) Complexity leads to specialisation of the tissues, organs or systems.
(ii) It enhances increase in the size of the organisms, and wider adaptation to and survival in various types of environments.
(iii) Specialisation in various functions in an organism leads to division of labour.
(iv) This in turn brings about efficiency of the tissues, organs or systems.
(v) One body function does not adversely affect other body functions, as various systems operate side by side without adversely affecting the other.
(vi) Reproduction in complex organisms does not lead to the break down of the parent’s body, since that is a specialised system. But in simple unicellular organisms, parents disintegrate after reproduction or conjugation.
The main disadvantage of complexity of organisms is that cells become so specialised, that each cell, tissue or organ may not survive in isolation from others. This is as opposed to lower animals, in which if an individual is cut into parts, each part will develop into another complete individual.
Advantages of simple (cellular) organisation over complexity
(i) Usually, there is individuality of life in simpler forms, for example, amoeba over lizard cells. Each cell of amoeba can exist as an individual or integral whole and perform life processes, but individual cells of the lizard cannot survive as a unit.
(ii) Diffusion alone can meet all the physiological needs of amoeba. These advantages are because:
(b) The distance which materials travel within the cell is small or short as compared with that in the complex forms.
(c) Also, the quantities of materials moving from place to place are smaller and simpler than the complex forms.
(d) Moreover, the simple forms are in more direct contact with the environment than the complex forms.