STD-11 UNIT-3 CHA-8
CELL : STRUCTURE AND FUNCTIONS
CELL - INTRODUCTION & CELL THEORY
CELL - INTRODUCTION
- Cell is the basic unit of life and structural and functional unit of an organism .
- It is the smallest unit capable of independent existence and performing the essential functions of the life .
- Anton von Leeuwenhoek first saw and described a live cells .
- Robert Brown later discovered the nucleus .
- The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell
- Cell is a basic unit of life as no living organism can have life without being cellular because cell is a unit of both its structure and function .
- All life begins as a single cell .
- A number of organisms are made of single cells . They are called unicellular or acellular , e.g. . Amoeba , Chlamydomonas , Acetabularia , bacteria . yeast .
- Here a single cell is capable of independent existence and able to perform all the essential functions of life . Anything less than a complete cell can neither lead an independent existence nor perform all the functions of life.
- A multicellular organism is made of many cells .
- A higher animal or plant contains billions of cells . For example , a newly born human infant has 2 x 10 cells . The number increases to 100 trillion
- A drop of blood contains several million cells .
- The large sized organisms do not have large sized cells . Instead they possess higher number of cells .
- In multicellular organisms , cells are building blocks of the body or basic units of body structure . Of course , they become specialized for performing different func tions . Human body has some 200 types of cells , e.g. , erythrocytes , leucocyte types , epithelial cell types , muscle cells , nerve cells , fat cells , cartilage cells , bone cells , connective cells , gland cells , germinal cells , pigment cells , etc.
CELL THEORY
- Cell theory was jointly put forward by Schleiden and Schwann in 1839.
- Cell theory states that the bodies of all organisms are made up of cells and their products so that cells are units of both structure and function of living organisms .
- Rudolf Virchow ( 1855 ) first explained that cells divide and new cells are formed from pre - existing cells ( Omnis cellula - e - cellula ) .
- He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape .
- All living organisms are composed of cells and their products
- Each cell is made of a small mass of protoplasm containing a nucleus in its inside and a plasına membrane with or without a cell wall on its outside .
- All cells are basically alike in their chemistry and physiology .
- Activities of an organism are the sum total of activities and interactions of constituent cells .
- It is also known as cell doctrine or cell principle .
- Modern cell theory states that
- The bodies of all living beings are made up of cells and their products
- Cells are units of structure in the body of living organisms .
- Every cell is made up of a mass of protoplasm having a nucleus
- organelles and a covering membrane .
- Cells are units of function in living organisms, that is, the activities of an organism are the sum total of the activities of its cells.
- While a cell can survive independently, its organelles cannot do so.
- The cells belonging to diverse organisms and different regions of the same organism have a fundamental similarity in their structure, chemical composition and metabolism.
- Genetic information is stored and expressed inside cells.
- Life passes from one generation to the next in the form of a living cell.
- New cells arise from pre-existing cells through division.
- All new cells contain the same amount and degree of genetic information as contained in the parent cell.
- All the present day cells / organisms have a common ancestry because they are derived from the first cell that evolved on the planet through continuous line of cell generations.
- Basically the cells are totipotent (i.e., a single cell can give rise to the whole organism) unless and until they have become extremely specialized.
- Viruses are acellular and do not have a cellular machinery. Even then they are considered to be organisms.
- In some organisms, the body is not differentiated into cells though it may have numerous nuclei (coenocytes, eg Rhizopus).
- Protozoans and many thallophytes have a uninucleate differentiated body (eg .. Acetabularia) which cannot be divided into cells. They are acellular.
- Bacteria and cyanobacteria do not have nucleus and membrane bound organelles.
- RBCs and sieve tube cells continue to live without nucleus.
- Protoplasm is replaced by nonliving materials in the surface cells of skin and cork.
- Schleiden and Schwann did not know the mechanism of cell formation. Schwanny believed cells to develop spontaneously like a crystal. Schleiden thought new cells to deve'p from cytoblast or nucleus.
- There is a structural similarity in cells belonging to diverse groups of organisms.
- All the cells perform similar metabolic activities in Life exists only in the form of cells.
- Life passes from one generation to the next as cells. All living beings are descendants of a primitive cell that developed on earth as the first eukaryote and prior to that as the first prokaryote.
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