NCERT Class 8th Chapter 8 cell structure and function

  The cell was first discovered by Robert Hooke in 1665 in thin slides of cork.  He saw a beehive-like cell based on a thin layer of cork, which Robert Hooke named the cell.  In 1674, Leeuwenhoek discovered living cells in the pond.
 

Cells can be of different types in different types of organisms or in different organs of the same organism.  Generally the cell is round and elongated but some cells are very long and sharp at the edges, that is, the shape or size of any cell depends on its function.. Most cells are microscopic. They cannot be seen with the naked eyes.
 

size

 • The smallest cell is that of a bacterium, its size is 0.1-0.5 micrometers.
 • The largest cell is the ostrich egg.  Its size is 170x130 mm It happens. 
• Viruses do not follow the cell principle.
 • The discovery of the electron microscope in the 1940s led to the discovery of the complex structure of the cell.
 

    Cell: Structural and functional unit

 

Cells can be of different types in different types of organisms or in different organs of the same organism.  Generally the cell is round and elongated but some cells are very long and sharp at the edges, that is, the shape or size of any cell depends on its function.
 

• All organisms are made up of cells.  In some organisms, only one cell does the work of the whole organism.

 Single Single-cellsm:-Bacteria whose bodies are made up of a single cell are called unicellular organisms.  E.g. amoeba, eugenics, bacteria, paramecium etc.

 • multicellular organism:- And such organisms whose body is made up of many cells, are called multicellular organisms.  Like humans, plants, animals etc. Multicellular organisms are made up of different types of cells.  Enzymes that differ in size and function are found in multicellular organisms.
 

The word ‘cell’ is derived from the Latin word ‘shellula’ which means ‘a small room’.
The cell's basic component of the body of all living things (animals and plants)
Based on the study under the microscope, the cell is divided into three part....

1. Plasma Membrane or Cell Membrane
2. Nucleus
3. Cytoplasm

 A rigid cell wall is found outside the plasma membrane in plants.

1.Plasma Membrane / Cell

Membrane) Each cell is surrounded by a layer that separates it from the environment, called the plasma or cell membrane.  This membrane is made up of phospholipid proteins.
 • Scientists named Singer and Nicholson gave the accepted theory of plasma membrane.

 

Cell membrane.

 This principle is called the fluid mosaic model.

 • The cell membrane surrounds the cytoplasm.  Hence it gives a definite shape to the cell.  And provides mechanical support to the cell.

 • This membrane is semi-permeable, so it allows some substances not all, so this membrane is called selectively permeable membrane or selectively permeable membrane

 • The entry of substances into the cell membrane through the external medium is called cytosis.  If there is ingestion of solids then it is called phagocytosis and if the ingestion of liquids is called cytoplasm.

This process is called endocytosis, due to the ability of chelate, unicellular organisms to get food from the environment.

Nucleus

The nucleus in the cell was discovered by a scientist named Robert Brown in 1831 AD.  There is a nucleus in the middle of the cell, which functions to control the biological or metabolic activities that take place in the cell, hence it is also called the manager of the cell, the controller or the brain of the cell.

Generally, the  nucleus is found in all living cells but the nucleus is not found in RBC, bacteria, virus , plasma,blue-green algae.

 Parts of the nucleus

 

1. Nuclear membrane The nucleus is separated from the cytoplasm by the nuclear membrane. A double membrane made of fat and protein is found around the nucleus, which is called the nuclear membrane. Small pores are found in the nuclear membrane, which is called nuclear pores.

 Through these pores, the cytoplasmic fluid moves in or out of the nucleus.

 Note- Organisms which do not have nuclear membrane are called prokaryotic organisms and those organisms which have nuclear membrane are called eukaryotic.

Nucleoplasm

 A thick semi-liquid transparent liquid is filled inside the nucleus, which is called Nucleoplasm.  It is also called karyolymph or nucleoplasm.  Enzymes like DNA polymerase, RNA polymerase, ribonucleoprotein, alkaline phosphatase, etc. are found in this material.

Nucleolus

The nucleus was discovered by the Saltana scientist.  Inside the nucleus, there is a small spherical structure, called the nucleus, found in eukaryotic organisms.

Chromosomes

Chromosomes are found in the nucleus.  Chromosomes are believed to be responsible for inheritance.

• Chromosomes appear before and after cell division, as long threads called chromatin. During cell division, the threads of the chromatin reticulum separate into small rod-like structures called chromosomes.

 •  The nucleus has a fine thread-like structure called the chromatin (network) reticulum. Chromosomes are made up of DNA and histone proteins.

DNA

  • The DNA molecule contains all the necessary information for the formation and organization of the cell . 

 

 

DNA carries genetic traits from one generation to the next go ,  The functional segment of DNA is called a gene.  Therefore DNA is called the genetic material and gene is called the genetic unit.

 • The nucleus protects the cell and participates in cell division.,  It produces cellular RNA (RNA) necessary for protein synthesis.

 • Synthesis of RNA takes place in the nucleus.