Cells are the structural and functional units of any living organism. Cells form the basic building blocks of living things. It was discovered by Robert Hooke. All living organisms are made up of cells. There are about 100 trillion or 1014 cells in the human body. The size of a single cell is about 10micrometer. Within every cell are present several cell organelles that control the activities and functions of the cell. The heart of any cell is the Cell Nucleus. Cell nucleus controls the activities of the cell organelles. All animal cells have a definite plasma membrane that acts as a boundary for the various cell organelles within the cell. Plant cells have an additional cell wall outer to the plasma membrane. These cells undergo Cell division to enable the organism to grow and reproduce. Cell division occurs by means of two processes namely Mitosis and Meiosis. The cell being the functional unit of living organism contains all the hereditary information in it which gets transmitted to the next generation during cell division. Living organisms may be unicellular or multicellular.
Unicellular organisms are those that contain just one single cell. Their body is made up of just one single cell. E.g. - Amoeba, Paramecium. The oldest forms of life are the unicellular organisms and they existed 3.8 billion years ago. Bacteria, Amoeba, protozia, viruses, diatoms are the five different types of unicellular organisms. This single cell regulates the various functions of the body. Unicellular organisms are invisible to the naked eye. Unicellular organisms vary in size.
The smallest bacteria measuring only a third of a micron (300 nanometers) across, ranging up to 20 cm (8 in) across. They can be viewed under a microscope only. Unicellular Organisms were the first forms of life that developed on earth. These organisms usually have cilia,flagella or pseudopodia that helps them in locomotion. They have a simple body anatomy with the most basic structures. They reproduce both sexually and asexually. They get their food by means of a process called phagocytosis where they engulf the food particles and store them in vacuoles present in them.
Multicellular organisms are those living organisms that are made up of more than one cell. Their body contains many cells hence the name multicellular. Eg - Human Beings. In multicellular organisms all the cells coordinate with each other in order to regulate the various functions of the body. Multicellular organisms are visible to the naked eye unlike unicellular organisms. Because the first multicellular organisms that existed lacked hard body parts, there are no fossil records for them. There are many theories explaining the evolution of multicellularity;
1.Symbiotic Theory - This states that first multicellular organisms arose from symbiosis of different species of single celled organisms, each with different functions.
2. Cellularization Theory - This theory states that a unicellular organism could have developed membrane boundaries/partitions around each of its nuclei,thus giving rise to multicellularity.
3. Colonial Theory -This theory states that the symbiosis of many organisms of the same species led to a multicellular organism.
Multicellular Organisms reproduce by means of sexual reproduction. Multicellular Organisms have a complex anatomy because of the various cell organelles that control the functions of the organism. Since there are more than one cell in their body, they are classified into germ cells that are the reproductive cells and somatic cells that help in body growth by cell division. Multicellular organisms have a very well coordinated cell division process in order to prevent abnormal cell division and growth.The different cells in a multicellular organism may be the Nerve Cells, Skin cells, Cardiac cells, Epithelial cells, Connective Tissue Cells,Blood Cells,Bone Cells, Muscle Cells etc.Each of these cells have their own specific functions which helps in the overall functioning of the body.