All living things are made up of cells. But the number and variety of cells differ greatly from one kind of organism to another. Some organisms are made up of only one cell. Others, such as human beings and animals, are made up of trillions of cells. In this article, first, I am going to explain the history of cells and the most important progresses of cell, second, the cell theory, and third, kind of cells.

First, I want to elobarate you about the discovery of cells and the history of cells. Living organisms are made up of basic units called cells. The characteristics associated with the life depend on activites within cells. Some small organisms are made up of only one cell. Single-celled organisms are called unicellular organisms. The life activities of unicellular organisms happen in only one cell. Larger organisms are made up of many cells and are called multicellular organisms. The life activities of a multicellular organism are divided among the many cells.

Most cells are so small that they can not be seen by the human eye. It was not discovered and studied until the invention of microscope. The first microscopes were built around 1600 (17th century). The Italian scientist Galileo observed insects with a microscope, which he invented before. An English scientist, Robert Hooke improved microscope, and he observed everything that he found. After a while, with the help of previous research, he droved the microscopic picture of slice of corks. It inculuded small rooms which now we called cells. Thus, biologists accept him as the founder of a cell and compound microscope which he conducted his experiments with.

A few years after Hooke's observations, cells were seen by a Dutch Merchant, Anton van Leeuwenhock (LAY vehnhook). Hooke's compound microscope had magnified objects approximately 30 times, but he built simple microscope, which magnified objects up to 200 times, is containing single lens. Also he observed blood cells, bacteria, and single-celled creatures in water.

After years, by the nineteenth century, microscopes had been greatly improved. That was the begining of the cell theory, which is the second part of my lecture. Meanwhile, scietists were able to study structures that had not previously seen in cells. In 1833, a Scottish botanist, Robert Brown, discovered that central structure now is called the nucleus (NOO Klee uhs). A few years later the term protoplasm (PROH tuh plaz uhm) was used to refer to the living material within cells. In 1838, a German botanist, Matthew Schleiden, as a result of observations of plant tissue, proposed the hyphotesis that all plants are made up of cells. The next year, Theodor Schwann, after observations of animal tissue, extended this hypothesis by proposing that animals are made up of cells too. Schwann also proposed that the life process of organisms take place in the cells. In 1858, Rudolf Virchow provided evidence that cells reproduce to form new cells.

The results of many investigations, including the work of Schleiden, Schwann and Virehow, let to the development of the cell theory. The cell theory can be summarized by these statements: All organisms are made up of one or more cells, cells are the basic units of structure and function of organisms, new cells come from existing cells by cell reproduction.

And the final part of my lecture is about kinds of cells. Most cells contain structures, called organelles, which carry out specific functions. Today cells are classified into two groups according to whether or not they contain specialized membrane-bound organelles. A membrane is a structure that encloses a cell or part of a cell. The cells, which do not contain membrane-bound organelles, is called Prokaryotic (proh kar ee AHT ik) cells. They are small cells, averaging 1 micrometer in diameter. Bacteria are classified as prokaryotes (proh KAR ee ohts), or organisms that are made of prokaryotic cells. Prokaryotes are the oldest known forms of life. Cells that contain membrane-bound organelles are called eukaryotic (yoo KAR ee ohts). Eukaryotic cells are larger than prokarotic cells, averaging 20 micrometer in diameter. Plants, fungi and animals are eukaryots which consist of eukaryotic cells.

Both eukaryotes and prokaryotes contain nucleic acid in eukaryote is enclosed within the membrane-bound structure called the nucleus. The nucleus is the organelle that controls the cell's activities. The information for controlling a cell's activities is contained in the nucleic acid, but prokaryote does not contain a nucleus. The cell's nucleic acid is not contained within a nuclear membrane.

The jellylike material found within prokaryotic and eukaryotic cells is called cytoplasm (SI tuh plaz uhm). Notice that a number of organelles can be seen in the cytoplasm of the eukaryotic cell. In prokaryotic cells, most of the activities that take place in organelles in eukaryotic cells are associated with the cell membrane, the structure that encloses a cell.

All in all, in my lecture, I explained the history of cell, the cell theory, and kinds of cells. In my opinion, cell is the most important construction material of our body. Our bodies make up of millions of cells. Also, some of the cell process' keep their mystery, for example, we still could't know how the DNA creates and give characters to the owner.