Cell is the lowest level of organization. It is capable of performing all the activities Df life. The cell has a very special place in the hierarchy of biological organization. All the organisms are composed of cells. The cells occur in two ‘orms. In either case, the cell is basic unit of structure and function in organisms: (a) Unicellular: Cell occur singly as a great variety of on unicellular organisms
(b) Multicellular: They occur as subunits of organs and tissues in animals, and other multicellular organisms. Discovery of cells
- Robert Hooke:
Robert Hooke was an English scientist. He observed a slice 70-452 pdf of cork (bark from an oak tree) under his microscope. That microscope magnifieo 20 fimes (30x). Thus he first time described and named cells in 1665. He believed that these tiny boxes or “cells” were present only in cork Hooke never realized the significance of his discovery.
- Anton van Leeuwenhoek
He discovered microscopic organisms under his microscope. These organisms were single celled. He used grains of sand as a lens in his microscope. He polished these grains and formed magnifying glasses. This magnifying glass was as powerful as 300x. Leeuwenhoek discovered a microbial world in a drop of pond water. He also observed the blood cell and sperm cells of animals.
- Matthias Schleiden and Theodor Schwann
They were two German biologists. Schleiden and Schwann summarized their own and others microscopic studies. They concluded that all living things consist c` cells. It formed 70-453 the basis of the cell theory. Cell theory is defined as, “the bodies of the organisms are composed of cell and cell products”. This theory was later expanded. It was included the idea that all cells come from other cells. The ability of cells to divide to form new cells is the basis for all reproduction, growth and repair of multicellular organisms, such as humans. Cells as unit of structure and function Cell is unit of structure and function. Cell is building block of the bodies of the organisms. It forms the basis of organization of the bodies of the all the organisms. Cells are organized to form tissue, . organ and organ system. In unicellular organisms only cellular organization exists. Their bodies are composed of single cells This cell performs all the activities of the organism. In multicellular organization, the cells form tissue and organs level organization . The cells in each tissue or organ perform their function individually. For example in muscle tissues, all the muscles cells contract and relax separately. The collective efforts of all the muscle tissues produce contraction in the muscle. It indicates that cell is also unit of function. Cells as a unit of evolution The formation of complex organism form simple organisms through gradual changes is called evolution. Cell is also a unit of evolution. All the organisms have same common origin. This common ‘origin is present in the form of simple cell (prokaryotes). It is believed that all other complex organism has formed from this simple cell through evolution. Adaptation and variations are produced at cell level. These adaptations and variations form the basis of natural selection. Natural selection L mechanism of evolution. Therefore, cell is also a unit of evolution. Ultra structure of cell Electron microscope was discovered 50 years ago. Electron microscope has revolutionized the study of cells. It has revealed ultra structure of cell. Cell fractionation technique is also used to study different organelles of the cells There are three main parts of the cell
- Cell membrane: All cells are enclosed by a membrane. This membrane regulates the passage of materials between the cell and its surroundings.
- Cytoplasm: It is composed of soluble part called cytosol and cell organelles like mitochondria, Golgi bodies, Encloplasmic reticulum etc. These organelles may be granular like ribosome or membranous bound like mitochondria Membranbus bound organelles are absent in prokaryotic cells.
3.. Nucleus: Ever; cell contains DNA It is a heritable material. It directs the – cell’s many activities DNA is present in nucleus in eukaryotic cells but it is “freely dispersed in the prokaryotic organelles Types of Cells There are two major kinds of cells prokaryotic cells and eukaryotic cells. They cip be distinguished an the basis of their structural organization. The cells of the n—crootganisins known as bacteria are prokaryotic All other forms of life are composed of eukaryotic cells to structures called chromosomes. These chromosomes are present within a ucleus. Surrounding the nucleus is the cytoplasm. Cytoplasm is a thick fluid. arious organelles are suspended in the cytoplasm. These organelles perform ost of the cell’s functions, Some eukaryotic cells like plants have tough walls xternal to their membranes. Animal cells lack walls. rokaryotic cellst is much simpler cell. The DNA is not separated from the rest of the cell into a ucleus. Prokaryotic cells also lack the cytoplasmic organelles typical of ukaryotic cells. Almost all prokaryotic cells (bacteria) have tough external cell ails,he eukaryotic and prokaryotic cells show differences on the basis of their omplexity. But still’ they have some key similarities. Cells are varying widely in ize, shape. and specific structural basis of life, All cells are highly ordered tructures. They carry out all the complicated processes, These processes are ecessary for maintaining of life. Thus cell is a fundamental unit of life. (Adoptedrom Biology by Neil. A Campbell)