Important Notes for CBSE Class 8 Science Chapter 8 – Cell Structure and Functions. Cell Structure and Functions Class 8 Notes and Study Material Download in PDF form.
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Cell Structure and Functions Class 8 Notes
- The basic structural unit of an organ, which is the cell. Cells are the building blocks of the body which assemble to make an organism.
Discovery of cell
- The 1st cell was observed by Robert hook in 1965 in cork using simple magnifying glass.
- The cell resembled honey comb boxes which were separated by a wall or a partition.
- Cells have different shapes and sizes. Cells are usually microscopic
- The egg in a hen represents a single cell and is big enough to be seen by the unaided eye.
Number of cells:
- Human body has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions.
- Unicellular organisms: The single-celled organisms are called unicellular organisms. The word root is uni: one; cellular: cell.
- Multicellular organisms: Organisms in which are made up of multiple cells are called multicellular organism
Shape of cells:
- Cells can be round, spherical or elongated, quite long, branched like the nerve cell, long and pointed at both ends, spindle shaped etc.
- The different shape account different functions.
- The cell membrane is responsible for the shape of the cell.
- Apart from this plant cell wall also helps it gives shape and rigidity.
Size of Cells:
- Size can vary from millionth of a meter or may be as large as a few centimeters.
- Cells are microscopic in size and are not visible to eyes and is observed by a microscope.
- Bacterial cell size is 0.1 to 0.5 micrometers
- The largest cell measuring170 mm ×130 mm, is the egg of an ostrich.
- The size of the cells has no relation with the size of the body of the animal or plant.
Cell Structure and Function:
- Living things are made of cells. These cells can function together as organs.
- Each organ in turn performs different functions.
Parts of cells
(i) Cell Membrane:
- It is also called plasma membrane and encloses the cytoplasm and the nucleus.
- It is of porous nature so as to allow the movement of substance inside and outside the cell.
(ii) Cell wall:
- Plants have an extra membrane outside the cell membrane for protection against variations in temperature, high wind speed, atmospheric moisture etc.
(iii) Gene:
- It is a unit of inheritance in living organisms.
- The transfer of a hereditary characters from parents to offspring is through genes.
(iv) Cytoplasm:
- The fluid component of the cell between the cell membrane and thenucleus.
- Organelles of cells are present in the cytoplasm E.g., Mitochondria, Golgi bodies, ribosomes, etc.
(v) Nucleus:
- It is spherical in shape and located in the center of the cell.
- It can be stained and seen easily with the help of a microscope.
- The nuclear membrane: Nucleus is separated from the cytoplasm by this membrane
- The nucleolus is located inside the nucleus.
- Chromosomes: Compacted strands of DNA which carry genes are called the chromosomes.
- Nucleus is the control center of the activities of the cell.
- Prokaryotic cells: Cells having nuclear material without nuclear membrane.
- Eukaryotic cells: Cells having well-organized nucleus with a nuclear membrane is a eukaryotic cell.
(vi) Protoplasm:
- The entire content of a living cell is known as protoplasm. Protoplasm is called the living substance of the cell.
(vii) Vacuole:
- Cells have smaller vacuoles as well as large vacuoles are common in plant cells.
- Vacuoles in animal cells are much smaller.
(viii) Plastids:
- Plastids contain green pigment called chlorophyll; it may have different colors. Green colored plastids are called chloroplasts.
- They provide green color to the leaves which is essential for photosynthesis.