Current Affairs 8th Class

Category : 8th Class

Cell - Structure and Functions

·                     Cell is the basic structural unit of life.

·                     Cells were first observed by Robert Hooke in 1665. He observed slices of cork (a part of the bark of a tree) under a simple magnifying device. He noticed partitioned boxes or compartments in the cork slice.

·                     Cells are found in wide variety of shapes and sizes. Some cells are big enough to be seen with the unaided eye like egg of a hen while some are too small to be seen by the naked eyes like an amoeba.

·                     Number of cells also varies from organism to organism. The organisms made of a single cell are called unicellular organisms while the organisms made of more than one cell are called multicellular organisms.

·                     In unicellular organisms, the single cell performs all the basic functions performed by a variety of cells in multicellular organisms.

·                     In multicellular organisms, a group of specialised cells forms a tissue which in turn forms an organ to perform various functions.

·                     The three main parts of a cell are

·                     The cytoplasm and nucleus are enclosed within the cell membrane which is also called the plasma membrane.

·                     Nucleus is separated from cytoplasm by a nuclear membrane.

·                     The entire content of a living cell is known as protoplasm.

·                     Nucleus contains thread-like structures called chromosomes. They carry genes and help in inheritance or transfers of characters from one generation to another. The chromosomes can be seen only when the cell divides.

·                      The cells without well organised nucleus are called prokaryotic cells. These cells do not have nuclear membrane. Some bacteria and blue green algae are prokaryotes.

·                     The cells having well organised nucleus with a nuclear membrane are called as eukaryotic cells. All organisms other than bacteria and blue-green algae are called eukaryotes.

·                     Plant cells have an additional layer around the cell membrane known as cell wall which is not present in animal cells.

·                     Plant cells also contain plastids. Plants cells have a big central vacuole unlike a number of small vacuoles I animal cells.