NCERT Class 8:Cell-structure and function

NCERT
Class 8
Cell-structure and function

1. CELL-STRUCTURE AND FUNCTION

2. Catalogue Discovery of the cell The cell Shape of cells Size of cells Cell functions Parts of cells Eukaryotic and Prokaryotic cells Comparison of animal and plant cells

3. Discovery of the cell 1. Robert Hooke (1665) - observed dead cork (wood bark)and discovered cells 2. Anton Van Leeuwenhoek (1700) invented the compound microscope, observed living cells (saliva & blood) ,and discovered algae 3. Matthias Schleiden - discovered that all plants and every part of them are made up of cells, discovered the cell nucleus 4. Theodor Schwann (1807-1893)- discovered that all animals and every part of them are made up of cells, independently announced that yeast is a living organism. 5. Rudolf Virchow (1858) -best known for his theory Omnis cellula e cellula every cell originates from another existing cell like it. "), first to recognize leukemia cells

4. The Cell Cells are the basic structural units of living organisms. Some organisms are made up of a single cell and some are made up of many cells.Human body has trillions of cells which vary in shapes and sizes.A single-celled organism performs all the necessary functions that multicellular organisms perform. 1.Organisms made up of many cells are called multicellular organisms. Eg: a tree, an elephant. 2.Organisms made up of only one cell are called unicellular organisms. Eg: amoeba, paramecium.

5. Shape Of Cells Cells are of different shapes. Some cells keep changing their shape. Eg : amoeba, white blood cells in human beings. Some cells are spherical, round, elongated, spindle shaped, or branched.

6. Size Of Cells Cells in living organisms are of different sizes. They may be as small as millionth of a meter (micrometre or micron) or large as a few centimetres. The smallest cell is the cell of bacteria (0.1 to 0.5 micrometre). The largest cell is the egg of an ostrich (17cm x 13cm).

7. Cell Functions Each living organism is made up of organs.Each organ performs different functions.Each organ is further made up of smaller parts called tissues.A tissue is a group of smaller cells performing a specific function.

8. Eukaryotic and Prokaryotic cells Cells having well organised nucleus with a nuclear membrane are called eukaryotic cells.All organisms other than bacteria and blue green algae are called eukaryotes. Cells having nuclear material without nuclear membrane are called prokaryotic cells.The organism with these type of cells are called prokaryotes.Example-bacteria,blue green algae.

9. Parts of cells 1.Cell membrane- The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. The membrane separates cells from one another and also the cell from the surrounding medium. The plasma membrane is porous and allows the movement of substances or materials both inward and outward. 2.Cell wall- In addition to the cell membrane, there is an outer thick layer in cells of plants called cell wall. This additional layer surrounding the cell membrane is required by the plants for protection. Plant cells need protection against variations in temperature, high wind speed, atmospheric moisture, etc. They are exposed to these variations because they cannot move.

10. 3.Cytoplasm-It is the jelly-like substance present between the cell membrane and the nucleus. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, golgi bodies, ribosomes, etc. 4.Nucleus-It is an important component of the living cell. It is generally spherical and located in the centre of the cell. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus. In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.

11. Nucleus, in addition to its role in inheritance, acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell. 5. Vacuoles-Vacuoles are blank looking structures in the cytoplasm. Vacuoles are large in size in plant cells and small in size in animal cells. 6.Plant cells have several small coloured bodies in the cytoplasm called plastids. The green coloured plastids are called chloroplasts. They contain chlorophyll which helps in photosynthesis.

12. Comparison of Plant cell and Animal cell

13. Thank You