Robert Hook was the first to observe cells in a living being. All living beings are made of tiny chambers called cells. A cell is the smallest functional and structural unit of a living organism. By functional unit, we mean that a cell works as a whole and no part of a cell can function independently of other parts. A group of cells that performs a special function for the body is known as a tissue. Many tissues make up an organ and many organs together make up an organ-system. For example, the human digestive system has many different organs, the tissues in which are highly specialized to perform digestive functions and not functions of any other type. Many systems together make constitute a living organism.
This organization of cell- tissue- system-body is not uniform throughout the living world. There are many animals which consist of just one cell (e.g. protozoa) and there are creature that have many cells, but those cells are not organized into specialized tissues. However, the typical cell- tissue- system-body outline above is true of all higher animals.
Many animals living together in a particular habitat make a population. All the different animals or plant species living together in mutual inter-link with one another in a habitat make a biological community. And a community of living organisms living together with its physical environment make it an ecosystem. The study of the interactions between the living creatures themselves and their physical environment and vice-versa is known as ecology.
Mitochondria and centriole are some major parts of a cell. In fact, the animal and plant cells do not differ much in structure and function, except that all plant cells are additionally covered by a cell wall and that they lack centriole. The nucleus controls all the cell functions while the actual production of energy takes place on the mitochondria. This energy is released in the form of ATP molecules (Adenosine Triphosphate) and that’s why mitochondria is often known as the powerhouse of the cells. Inside the nucleus, a thread-like network of chromatin can be seen. During cell division, this network is clearly differentiated into chromosomes, bodies that contains genes responsible for transmission of traits from one generation to the next.
Every species has a fixed number of chromosomes in all body cells. For example, in case of humans, the number of chromosomes is 46, which is organized into 23 pairs. The chromosomes and the genes always occur in pairs.
A cell keeps on growing until a certain stage, beyond which it has to divide. The cell can divide in two ways: mitosisi and meiosis.