Learn Class 8 with Free Lessons & Tips

                                                                     Chapter – 8
                                                   Cell — Structure and Functions

• Cell: The smallest structural and functional unit of an organism, which is typically microscopic and consists of cytoplasm and a nucleus enclosed in a membrane.
  
• All organisms are made of smaller parts called organs.
• Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
• Cells were first observed in cork by Robert Hooke in 1665.
• Cells exhibit variety of shapes and sizes.
• Number of cells also varies from organism to organism.
• Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.
• Some organisms are single-celled, while others contain large number of cells.
• The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multi cellular organisms.
• The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.
   
• Cell membrane: The basic component of a cell. The cytoplasm and nucleus are enclosed within cell membrane. There is an outer thick layer in cells of plants called cell wall.
• Cytoplasm: The jelly-like substance present between the cell membrane and the nucleus. Various organelles present in the cytoplasm are:
  (i) Mitochondria
  (ii) Plastids (present only in plant cell)
  (iii) Endoplasmic reticulum (ER)
  (iv) Ribosome
  (v) Lysosomes
  (vi) Vacuole
  (vii) Golgi body
  (viii) Centrosome (present only in animal cell).
   
• Nucleus: Nucleus is separated from cytoplasm by a nuclear membrane. It is generally spherical in the centre of the cell.
• Nuclear membrane: Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.
• Nucleolus: Nucleus contains a still smaller round body known as nucleolus.
• Chromosomes: Nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of character from the parents to the off springs
• Cells without well organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.
• Plant cells differ from animal cells in having an additional layer around the cell membrane termed cell wall.
• Coloured bodies called plastids are found in the plant cells only. Green plastids containing chlorophyll are called chloroplasts.
• Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.
• Size of cells: The size may be as small as a millionths of a metre or may be as large as a few centimetres. Size of the cells has no relation with the size of the body of the animal or plant. It is related to its function.

 

read less

6. Which of the following need a histogram to show the data? (i) The number of letters for different areas in a postman’s bag. (ii) The height of competitors in an athletics meet. read less

6.State whether True or False. Correct that is false. (i) A point whose x coordinate is zero and y-coordinate is non-zero will lie on the y-axis. (ii) A point whose y coordinate is zero and x-coordinate is 5 will lie on y-axis. (iii) The coordinates of the origin are (0, 0) read less