What is a cell

WHAT IS A CELL ?

Cell is the fundamental structural and functional unit of all living organisms. Let us observe a unicellularorganism (e.g., Amoeba) under a microscope. We will find that a single cell is capable of independentexistence and also performing the essential functions of life. In fact, all cells have certain structures in common to carry out basic life processes, but differential distribution of  organelles gives cells distributionof organelles cells distinct characteristics.

Why Do the Organisms Have Cells ?

In the living organisms, the metabolic reactions can occur only in a delicately balanced environmentthat is different from any found in the non-living objects. Cells are the life-supporting chambers which havesuch a special environment. A living cell keeps its chemical composition steady within its boundary. Thiscondition is called homeostasis (G. homoios= same; stasis= standing). In the controlled environment of acell. all life activities can occur in a desired manner.

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Cytology/Cell Biology.

Discovery of the cell diverted the attention of the biologists from morphology (the study of form andgross structure of organisms) to cytology (the study of cell). For many years, cytology somehow remainedconfined to the structure of the cell as seen through a microscope. However, it now covers cell physiologybiosynthesis, energetics, motility, growth), cell reproduction and cell chemistry also. With these additions to itsscope, cytology has been renamed cell biology. The cell biology may be briefly defined as the study of cell

structure, chemical composition and function. A cell biologist generally describes fundamental processescommon to all cells. Thus, cell biology is, in fact, the study of life.

CELL THEORY

1. History of Cell Theory

if we go back to the scientific discoveries, the study of cell has continued for the last more than threeand a half centuries. For such a study, basic requirement was that of a microscope or instruments with goodresolving power and magnification. Other needs included preservation, sectioning, staining and mounting

techniques to study and distinguish various cellular components. Continuous improvement in the tools and techniques in all these periods has enhanced our knowledge about cell biology.

The term cell was introduced by an English scientist Robert Hooke in his book Micrographia publishedin London in 1665. With this, he launched the study of microscopic anatomy. He examined thin slices ofcork under primitive microscope he had assembled  . He saw tiny, empty compartments in the slices , He called them cellulae, now termed cells ; and thought them to act as passages for conductinng
fluids. Since cork is the dead bark of a spanish oak, Quercus, Hooke saw only the dead walls of plant cells.

These cells had lost their livíng contents and looked like small rooms. Hence, he gave each compartment an appropriate name, the cell (L. cella =chamber, cellula= dim. of cella). Hooke also observed many other plants under the microscope and found the same structure. We now know that the really important parts of a
cell are its contents, and it may or may not have a well Thus, the term cell is a misnomer. 

However, the term is still used because it is simple and a suitable substitute has not been found .After Hooke  the early microscopists started studying the fine structure of the organisms.

*Protists are better described as acellular or noncellular organisms because their body is not divided into cells.

The term unicellular assumes that a protist is comparable to one cell of a multicellular organism. 

This is not true. A protist is a complete organism carrying on all the functions of life, whereas, a cell of a multicelular organism is merelya specialized unit of the body performing a particular function.

 Statement of Cell Theory

The cell theory states that___

1.all living things are composed of minute units, the cells, which are the smallest entities that can be called "living"

2.a cell is a mass of protoplasm containing a nucleus and bounded by a cell membrane, and in many

Cases by a cell wall also;

3. all cells are basically alike in structure and metabolic activities;

4. the function of an organism as a whole is the result of the activities and interactions of the constituent

cells.

Like the theory of evolution, the cell theory is surely one of the great unifying principles of biology.

Although the cell theory is substantially correct, it does not apply to all organisms.

CELL PRINCIPLE

The cell principle is the modified or modern form of the cell theory.

. Need for Modification in Cell Theory

Schwann (1810-1882) held that cells could arise spontaneously. A German physiologist Rudolph Virchow

(1821-1902) found in 1858 that all living cells arise from the preexisting cells, omnis cellula e cellula. This

conclusion suggested a continuous line of cell generations from the very beginning of life. This 1s called

theory of cell lineage. This theory, points out to the evolutionary implication that all cells, and hence all

organisms, have a common ancestry. It also made the cell a vehicle for the transmission of hereditary traits

from one generation to the next. Haeckel, in 1866, announced that the nucleus stores and transmits hereditaryintormation.

Better observational instruments and study techniques in the twentieth century have revealed many

details about the cell structure and function. This necessitated modification in the cell theory. In the modified

form, the cell theory applies almost fully and universally to life, and has, therefore, been given the status of a

cell principle, or cell doctrine. It is also referred to as the modern cell theory

Statement of Cell Principle

The cell principle states that

1. Life exists only in cells.

2. Living objects are composed of cells and cell products, or are multinucleate mass of protoplasm, or are like a single free cell.

3. A cell is a small mass of protoplasm usually containing a nucleus or nuclear material and some other

organelles, and is bounded by a cell membrane. A cell organelle does not survive alone.

4. Cell is also a unit of function, reproduction, heredity and disease, besides being a unit of structure.

5. Cells always arise from the preexisting living cells by division. They never arise de novo. The new

cells are like the parent cell in all respects.

6. All cells existing to-day can trace their ancestry back to the original cells of ancient time. This also

holds good for the cell organelles called chromosomes.

7. All cells have fundamental similarity in physical structure (organelles)chemical composition and basic metabolic reactions.