Cell – The fundamental unit of life – Class 9 Science Notes
9th Class Science -Cell : The fundamental unit of life:
Cell: Cell is a structural and fundamental unit of life.
- A cell is capable of independent existence and can carry out all the functions which are necessary for a living being, Such as nutrition, respiration, excretion, transportation, and
- A cell is a fundamental unit of life since unicellular organisms are capable of independent existence that shows a cell’s ability to exist independently.
- All living beings are composed of the basic unit of life, i.e. cell. Cell theory: proposed by Schleiden, Schwann, and Virchow. Which states that
- All living organisms are composed of one or more cells.
- We know that the cell is the basic unit of structure, function, and organization in all organisms.
- All cells are arising from preexisting, living cells.
For notes on Structure of Cell, click on Class 9 Science : Structure of cell
Shape and Size of Cells:-
- Cells come in all shapes and sizes.
- While most of the cells are spherical in shape, cells of various other shapes are also found.
- Most of the cells are microscopic in size, i.e. it is impossible to see them with naked eyes which can be seen through microscopic, since these microscopic organisms.
- Some cells are fairly large, e.g. a neuron in the human body can be as long as 1 meter.
- The largest known cell of a living animal is the egg of an ostrich and an average egg is 15 cm long and 13 cm wide.
- There are many cell organelles in a typical cell. Some of the main structures of a cell are as follows:
- Cell wall: The plant cell wall is made of cellulose. The cell wall is hard but permeable to most of the substances. The cell wall is present in plant cells and also in cells of bacteria and fungi.
- Plasma membrane: Plasma membrane is a semi-permeable membrane. It is composed of a bilayer of lipid and protein.
- Functions of Plasma Membrane: Plasma membrane provides a container to the cytoplasm. It facilitates passage of various substances in and out of the cell.
- Nucleus: Nucleus is covered by a double membrane; called nuclear membrane. The fluid which is inside the nucleus is called as nucleoplasm. The nucleus contains chromosomes which are very much essential for the functioning of a cell.
- Mitochondria: Mitochondrion is a capsule-like structure. It is a double membrane structure. Its inner membrane is projected into numerous finger-like structures; called cristae. Mitochondria are the sites of cellular respiration. Mitochondria produce their own protein because it has their own DNA and ribosomes.
- Functions of Mitochondria: Cellular respiration; due to this, mitochondria are also known as the ‘powerhouse of the cell’.
Watch animated videos and learn how cell looks and its structure, click 9th Class Science for demos.
DISCOVERY OF CELL AND CELL THEORY
The cell was discovered by Robert Hooke in 1665 when he saw honeycomb-like structures that organisms and plants were made up of, he named these compartments as cells.
However what Hooke actually saw was the dead cell walls of plant cells (cork) as it appeared under the microscope. Later in 1839, Robert Brown discovered the nucleus of the cell.
Learn the difference between Animal and Plant Cell, by clicking
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