NCERT Class 8 Science Chapter 8 - Cell - Structure and Functions Notes

Chapter 8 - Cell - Structure and Functions Notes

1. In 1665, Robert Hooke observed slices of cork from the bark of a tree under a simple magnifying glass. He noticed small boxes or compartments the cork slice. The boxes were separated from each other by a wall or partition. He named each box as a cell. The cells which Hooke observed were actually dead cells of plants.

2. Cells are the basic structural units of living organisms. Organisms made up of many cells are called multicellular organisms. Example: a tree, an elephant. Organisms made up of only one cell are called unicellular organisms. Example: amoeba, paramecium.

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

4. 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 centimetre. 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).

5. The main parts of a cell are cell membrane (plasma membrane), cytoplasm and nucleus.
The cell membrane gives shape to the cell, protects the inner parts of the cell and allows movement of materials in and out of the cell. In plant cells the cell membrane is covered by another thick covering called cell wall.

The cytoplasm is a jelly like substance between the cell membrane and nucleus. It contains other components (organelles) like mitochondria, golgi bodies, ribosomes etc.

The nucleus is a spherical body generally found in the centre of the cell. It has a membrane called nuclear membrane. It has a smaller spherical body called nucleolus and thread like structures called chromosomes. The chromosomes carry genes which transfer characters from the parents to the off springs.

Vacuoles are blank looking structures in the cytoplasm. Vacuoles are large in size in plant cells and small in size in animal cells. Plant cells have several small colored bodies in the cytoplasm called plastids. The green colored plastids are called chloroplasts. They contain chlorophyll which helps in photosynthesis.

6. Cell membrane is made up of lipoproteins that is made up of fats and proteins.

7. Cell wall is made up of cellulose that is a type of carbohydrate.

8. There are three types of plastids - Chloroplast, Chromoplast and Leucoplast.

9. Protoplasm is the living content in an organism.

10. Plant cell and Animal cell diagram

11. Difference between plant and animal cell
Animal cell
• Cell wall is absent
• Plastids are absent
• Small or absence of vacuoles

Plant cell
• Cell wall is present
• Plastids are present
• Presence of larger vacuoles

12. Comparison of Plant and Animal cell

Parts

Plant cell

Animal cell

1. Cell membrane

Present

Present

2. Cell wall

Present

Absent

3. Nucleus

Present

Present

4. Nuclear membrane

Present

Present

5. Cytoplasm

Present

Present

6. Plastids

Present

Absent

7. Vacuoles

Present

Absent (if present smaller in size)

13. Plant cell - Example: Onion Peel cells → stain used is Safranin
Animal cell - Example: Human Cheek cells → stain used Methylene blue

14. Plant cell needs cell wall whereas animal cell do not because the plants need rigid structure so that they can grow up and out. All cells have cell membranes, and the membranes are flexible. So animal cells can have various shapes, but plant cells only have the shapes of their cell walls.

15. Different types of plastids
a) Chloro means green
b) Chromo means different colors other than green
c) Leuco means transparent or white

16. Mitochondria is known as the powerhouse of the cell.

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