Chapter 8 cells-structure and functions class 8 science
- 1. Cells - Structures and Functions
- 2. -The basic structure unit of an Organ is known as Cells. -Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism.
- 3. -DISCOVERY OF CELLS -In 1665 Robert Hooke observed slices of cork under a simple magnifying device. -Cork is a part of the bark of a tree. - He took thin slices of cork and observed them under a microscope. He noticed partitioned boxes or compartments in the cork slice. -Cells of living organisms could be observed only after the discovery of improved microscopes.
- 4. -THE CELLS -In every Living organisms, Cells are basic structural units. -All organisms are different from one another but all are made up of cells. -The hen’s eggs represents a single cell and is big enough to be seen by the naked eyes.
- 5. -ORGANISMS SHOW VARIETY OF CELL NUMBER, SHAPE AND SIZE -To study the detailed structure, stains(dyes) are used to colour parts of cell. -There are millions of living organisms. -They are of different shapes and sizes. -Their organs also vary in shape, size and number of cells.
- 6. -NUMBER OF CELLS -Human body has trillions of cells which vary in shapes and sizes. -Different groups of cells perform a variety of functions. -Organisms made of more than one cell are called multicellular. (multi:many, cellular:cell) -The single-celled organisms are called unicellular. (uni:single, cellular:cell)
- 7. -Both multicellular and unicellular performs same necessary functions. -A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces.
- 8. -Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. -Tissues, in turn, form organs.
- 9. -SHAPE OF CELLS -Amoeba has no definite shape, unlike other organisms. -It keeps on changing its shape. -The projections of varying lengths protruding out of its body is known as pseudopodia. -These projections appear and disappear as amoeba moves or feeds.
- 10. -A white blood cell (WBC) in human blood is another example of a single cell which can change its shape. -WBC is a cell, amoeba is a full fledged organism capable of independent existence. -The different shapes are related to their specific functions. -Cells are round, spherical or elongated.
- 11. -Components of the cell are enclosed in a membrane -This membrane provides shape to the cells of plants and animals. -Cell wall is an additional covering over the cell membrane in plant cells. -It gives shape and rigidity to these cells.
- 12. -SIZE OF CELLS -The size of cells in living organisms may be as small as a millionth of a metre (micrometre or micron) or may be as large as a few centimetres. -Most of the cells are microscopic in size and are not visible to the unaided(naked) eye, They need to be enlarged or magnified by a microscope. -The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich.
- 13. -Cell Structure and Function -Each organ in the system performs different functions such as digestion, assimilation and absorption. -Similarly different organs of a plant perform specific/specialised functions. For example, roots help in the absorption of water and minerals. -Each organ is further made up of smaller parts called tissues. - A tissue is a group of similar cells performing a specific function.
- 14. -PARTS OF CELLS -Cell Membrane -The basic components of a cell are cell membrane, cytoplasm and nucleus. -The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane.
- 15. -The plasma membrane is porous and allows the movement of substances or materials both inward and outward. -The boundary of the onion cell is the cell membrane covered by another thick covering called the cell wall. - The central dense round body in the centre is called the nucleus.
- 17. -The jelly-like substance between the nucleus and the cell membrane is called cytoplasm. -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 plants for protection. -Plant cells need protection against variations in temperature, high wind speed, atmospheric moisture etc.
- 18. -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.
- 19. -Nucleus -It is an important component of the living cell. - It is generally spherical and located in the centre of the cell. -It can be stained and seen easily with the help of a microscope. -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.
- 20. -Nucleus contains thread-like structures called chromosomes. -These carry genes and help in inheritance or transfer of characters from the parents to the offspring. -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.
- 21. -The blank looking structure in an onion is known as vacuole. -It could be single and big as in an onion cell. -Several small coloured bodies in the cytoplasm of the cells of Tradescantia leaf. They are scattered in the cytoplasm of the leaf cells. These are called plastids.
- 22. -They are of different colours. Some of them contain green pigment called chlorophyll. - Green coloured plastids are called chloroplasts. -They provide green colour to the leaves.
- 23. -Comparison of Plant and Animal Cells -The similarities and distinguishing features of plant and animal cells. (Diagram on next page)