Organisation of Life
Plants and animals vary in terms of complexity. True, the cell is the functional unit of Life. But while some organisms are unicellular (single celled), some others are multicellular consisting of many cells. Other still have unicellular functions but occur and thrive is a collection in a colony.
At higher level cells are organised into tissues, tissues into organs and
into organ system. But whatever the level of organisation, all the forms perform the basic functions of life.
The Cell Level of Organisation
Cell is the smallest unit of life. At the lowest level of organisation are unicellular organisms such as chlamydomonas, amoeba euglena and paramecium.
These organisms do not have organs for performances of life functions but rather the cell as a functional unit is able to perform these functions by making use of sub- cellular structures called organelles, which are membrane bound.
Typically, cell is made up of a centrally located nucleus surrounded by a membrane in the cytoplasm which is bound or separated from the environment by cell membrane.
At the cellular level all functions are performed by organelles like mitochondria, golgi bodies, lysosomes, endoplasmic reticulum, ribosomes, plastids, cilia, flagella, micro fibule, micro filaments etc.
Examples of Organisms at Cellular Level of Organisation are Euglena, Paramecium, Amoeba
Tissue Level of Organisation
Group of cells that are similar in structure and perform specific function are called tissue. The tissue level of organisation is a higher level than the cellular level. The cells in a tissue maybe held together by a matrix secreted by the cells. Simple multicellular organisms such as hydra and other members of the coelenterates like obelia, jelly fishes and sea anemones are of the tissue level of organisation.
Organ level of Organisation
Organ is the next higher level to tissue level of organisation. A group of tissue working together as a unit is called organ. Examples of organs include kidney, lung, heart, fruit and tuber.
The need for organ level of organisation arose from increasing size and complexity of organism at higher Levels.
Organ of a plant, such as leaf or a stem, are made up of various tissues. A muscle is an organ of an animal containing long cells.
A typical example of organ level is the onion bulb (Allium capa). A vertical section through an onion bulb is shown below.
The bulb is made up of various tissues each performing specialised functions.
The tissues include those of roots which function for anchorage and absorption of nutrients from soil. The stem is compressed and serves as substratum for the leaves. The swollen succulent leaves serve the purpose of food storage, while the green leaves are for photosynthesis. The auxiliary bud is for asexual reproduction. The flower is also involved in sexual reproduction. The outer scaly leaves protect the inner fleshy parts.
System Level of Organisation
A group of organs whose functions are co-ordinated to produce effective action in the organism.
At the highest level of cellular organisation, organs are grouped into systems which work as a unit to perform a specific function.
For example, the heart and blood vessels constitute the circulatory system.
Other examples include the root systems in plants, the ferns, the snail, the nervous system, the respiration digestive system, excretory system etc.
The circulatory system in animal is composed of the heart, blood vessels and blood (See diagram below). The excretory system of man for example is composed of the kidney, the urethra, the urinary bladder and the urinary tract.
Circulatory system of man
Complex multicellular organisms such as mammals and flowering plants are composed of several organ systems. Complexity allows increased ability in terms of structure, function and behaviour. Smaller organisms are limited in their abilities, although they can perform all the functions performed in higher organisms. They are confined to simple modes of existence and functions.
However, the complex forms are disadvantaged in that their ability to regenerate or re-grow damaged part is limited. For example, when an organism like earthworm is cut, it can easily regenerate itself. Also a plant stem, for instance can produce whole part by vegetative propagation. But this is not so for mammals.
Complexity of organization in higher organisms
Although unicellular organisms may show a surprising degree of structural differentiation and function, there is a limit to the possibilities.
All higher organisms are multi-cellular, and have overcome many of the limitations imposed by size and shape in single cells
Advantages of Complexity of Organisation in higher organisms
1. Higher organisms have their protoplasm sub-divided into cells. This many-celled structure allows for increase in size of the organism
2. Cellular construction divides a large mass into a great number of cells, making it possible to have spaces between them and thereby increasing the surface through which substances can diffuse in and out of the protoplasm
3. There is division of labour among many cells making for greater efficiency. It also leads to structural specialization.
4. There is greater efficiency in the performance of complicated physiological functions such as movement.
Disadvantages of Complexity of Organisation in higher organisms
1. The functions of the different cells must all be organised and co-ordinated to ensure the efficient working of the whole complex organism.
2. The body must have adequate support, which leads to requirement of skeletal.
3. Bigger sizes of higher organisms lead to requirement of more resources than unicellular organisms.
4. Complexity leads to longer generation time.
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