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# Cell Structure and Organization ## Can you recall? 1. Who observed cells under the microscope for the first time? 2. Who made the first microscope? 3. How do onion peel cells and our body cells differ? 4. Why bacterial nucleus is said to be primitive? ## 5.1 Cell: - Cell is called a structura...

# Cell Structure and Organization ## Can you recall? 1. Who observed cells under the microscope for the first time? 2. Who made the first microscope? 3. How do onion peel cells and our body cells differ? 4. Why bacterial nucleus is said to be primitive? ## 5.1 Cell: - Cell is called a structural and functional unit of life of all living organisms capable of independent existence and can perform all functions of life. - To see cells clearly we need a microscope. Larger cells can be seen through a simple microscope but to see smaller cells, we require a compound microscope. - A simple microscope can magnify an image 50 to 100 times, but a compound microscope can do so 1000 times or more. - In the microscope we use in the laboratory, a beam of light is used to make things visible, hence it is a light microscope. To see the interior of a cell we need an electron microscope. - It can magnify an image 500000 times. - There is no typical shape of a cell. Cells may be spherical, rectangular, flattened, polygonal, oval, triangular, conical, columnar etc. - Cell size varies greatly in various plants and animals. Some of them are not visible to the naked eye. Some are barely visible while some are macroscopic. The smallest cell size can be seen in mycoplasma (0.3 µm in length), bacterial cell size is 3 to 5 µm, while the largest size of cell is seen in Ostrich egg (nearly 15 cm). The longest cells are nerve cells. - You already know that cell theory was proposed by Schwann and Schleiden. - However, in this theory, there was no explanation about the formation of new cells. It was Rudolf Virchow (1855) who explained for the first time that new cells are formed by cell division from preexisting cells (Omnis cellula-e-cellulla). - In later years, advanced research in cytology led to modifications in cell theory, which is now known as Modern Cell Theory. ## Fig. 5.1 Microscope - Leeuwenhoek Microscope (late 1600s) ## Fig. 5.2 Cell size - Atom - C60 Protein - Flu Virus - Mitochondria - Lipids - Bacteria - Red blood cell - Animal cell - Pollen - Human egg - Plant cell - Frog egg - Relative Size on a logarithmic scale: | | | |---|---| | 0.1 nm | 1 nm | | 10 nm | 100 nm | | 1 µm | 10 µm | | 100 µm | 1 mm | ## Find out: 1. How does a combination of lenses help in higher magnification? 2. When do we use plane and concave mirror and diaphragm? 3. What is the difference between magnification and resolution? ## 5.2 ## Totipotency: - It is the capacity of the living nucleated cell to differentiate and divide to form or to produce any other type of cell or a complete new organism. - A cell is totipotent because it contains entire genetic information of the cell's nucleus. - Embryonic animal cells are totipotent and termed as stem cells which have great medical applications for many diseases. ## Know the Scientific History - A German scientist [**Schleiden (1838)**] examined the plant parts and concluded that various parts of plants are composed of different kinds of cells. - At the same time, a British zoologist [**Theodor Schwann (1839)**] proposed that all living organisms have a thin membrane, the plasma membrane. He proposed that plants are composed of cells and their tissues are formed by the division of pre-existing cells, this is a basic concept of cell theory. ## Postulates of Cell Theory: - All living organisms are composed of cells. - Living things may be composed of a single cell or many cells. - A cell is the structural and functional unit of life - All cells arise from pre-existing cells. - Total activities of an organism are the sum of activities of all cells. - Each cell is capable of independent existence. - Cells are the basic unit of structure and function in living organisms.