CBSE Class 8 Science Cell Chapter Notes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.
Definition :- The structural & functional unit of living beings is called cell.
A mass of protoplasm bounded by a plasma membrane.
An unit of biological activity, delimited by a differentially permeable membrane and capable of self reproduction.
DISCOVERY OF CELL
1. Robert Hooke (1665) :– An English man and first curator of Royal society of London. Observed a thin transverse section of bark of a tree under self designed microscope. He noticed honey - comb like compartments. He coined the term cell. He wrote a book - Micrographia. He actually observed dead cells.
2. Antony Van Leeuwenhoek (1674) was first to observe living cells like bacteria [from tartar of teeth] erythrocytes [fish], sperms and protozoans [eg. Vorticella]
3. N. Grew (1682) :– Proposed cell concept which states that cell is unit of structure of organisms.
4. Rudolf Virchow (1858) :– Proposed that new cells formed from the pre-existing cells.
A microscope is an instrument to view small objects by magnifying them. It enables us to see the different types of living cells and the structures they contain.
TYPES OF MICROSCOPES
There are mainly three types of microscopes. They are :
• Light microscope : The light microscope uses light to produce images.
• Transmission Electron Microscope (TEM) : The electron microscope was designed by Knoll & Ruska (1932). A TEM makes use of a beam of highly energetic electrons to examine objects. The image produced is of a very fine scale.
• Scanning Electron Microscope (SEM) : Like the TEM, the SEM also uses electrons to produce images. In the case of a SEM, electrons are reflected off the surface of the specimen, because of which SEM images usually manage to capture the physical features of a cell in great detail.
How to make a Microscopic Slide?
Most of the specimens are examined under the microscope by preparing a wet mount using water. The basic steps involved in preparing a wet mount are given below :
1. Clean the glass slide and place on a flat surface of the table.
2. Place a drop of water in the centre of the slide.
3. Place a thin piece of the specimen to be viewed on the water drop on the slide.
4. A drop of stain or dye may be added with a dropper on the slide. Staining highlights biological tisues and specific regions in the cells which makes it easier for us to view the details. Iodine, methylene blue and crystal violet stains are commonly used.
5. Hold the cover slip over the object in such a manner that it touches the edge of the drop of water. Gently lower the cover slip so that it spreads out the water and no air bubble is trapped. The coverslip serves the following purposes.
(i) It protects the microscope's objective lens from contacting the specimen sample.
(ii) It helps to create an even thickness for the sample.
6. Dry the extra water that comes out of the cover slip with the help of a blotting paper.
7. Take care that the slide thus prepared is clean and dry.
8. Now place the slide under the microscope and examine the specimen.
Basic criteria for defining the cell –
(i) Presence of genetic material. (ii) Presence of limiting plasma membrane.
(iii) Presence of a metabolic machinery.
• Cell is called structural unit of living being, because body of all living organisms is made up of one or more cells.
• Cell is called functional unit of all organisms, because all the vital activities or physiological activities
[i.e. respiration, digestion, excretion, circulation, etc are performed at the level of cell].
1. Cut an onion into small pieces. Take a fleshy leaf and break it from concave to convex side.
2. Peel off the thin epithelial membrane with the help of forceps and transfer it into a watch glass containing methylene blue solution for a few seconds.
3. Transfer the stained peel into another watch glass containing clean water to wash off the extra stain.
4. With the help of a brush and needle, place this peel in the centre of the glass slide in such a way that the membrane is not folded. Immediately put a drop of glycerine on the peel.
5. Cover the peel gently with the coverslip to avoid the entry of air bubbles.
6. Gently press the coverslip with a needle so as to spread the glycerine evenly.
7. Remove excess glycerine from the edges of the coverslip using a blotting paper.
8. Examine the slide under low power of a microscope.
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