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Important points From the lesson
CLASS 9 NCERT FUNDAMENTAL UNIT OF LIFE

Facts that Matter

The smallest functional unit of life is a cell, discovered by Robert Hooke in 1665. A cell can independently perform all necessary activities to sustain life. Hence cell is the basic unit of life.
There are two types of cells plant cell and animal cell. The different cell organelles and their functions are as follows:
1. Plasma/Cell membrane: This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell so the cell membrane is called a selectively permeable membrane....
Some substances like CO2 or O2 gases can move across the cell membrane by a process called diffusion. The movement of water molecules (liquid) through such a selectively permeable membrane is called osmosis. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
If the medium surrounding the cell has a higher water concentration than the cell, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.
The plasma membrane is flexible and is made up of organic molecules called lipids and proteins. The flexibility of cell membrane also enables the cell to engulf in food and other material from its external environment. Such process is known as endocytosis. It is Observed in Amoeba.

CELL WALL(Protective wall)
Plant cells addition to the plasma membrane have another rigid outer covering called cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. It is a complex substance and provides structural strength to plant cells. When a living plant loses water through osmosis there is shrinkage or contraction of contents of the cell away from cell wall. This phenomenon is known as plasmolysis

Nucleus:- The nucleus has adouble-layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, i.e., to the cytoplasm
The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
Chromosomes — contain information for inheritance of features from parents to next generation in form of DNA [Deoxyribo Nucleic Acid] molecules. Chromosomes are composed of DNA and protein. Functional segments of DNA are called genes. The nucleus plays a central role in cellular reproduction.

Prokaryotic Cells:In some organisms like bacteria, the nuclear material is not enclosed by nuclear membrane and membrane bound cell organelle are absent. Such nucleus is called nucleoid and such cells are known as prokaryotic cell. Such cells have single chromosome.
Eukaryotic Cells:- Cells having well defined nucleus and having membranebound cell organelle is termed as eukaryotic cell. Such cells have more than one chromosomes.

Cytoplasm:- The cytoplasm is the fluid content inside the plasma membranealso contains many specialised cell organelles.Each of these organelles performs a specific function for the cell.

Cell Organelles:- Every cell has a membrane around it to keep its contentseparate from the external environment. The differentcomponents of cell perform different function and these components are called cell Organelles

(i) Endoplasmic Reticulum (ER) (Channels, Network for transport): The ER is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags.
There are two types of ER-Rough endoplasmic reticulum [RER] and smooth endoplasmic reticulum [SER]. RER has particles called ribosomes attached to itssurface. The ribosomes Endoplasmic Reticulum are the sites of protein manufacture.
The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. Thisprocess is known as membrane biogenesis.Some other proteins and lipids function as enzymes and hormones The one function of ER is to serve as channels for the transport of materialsbetween various regions of the cytoplasm orbetween the cytoplasm and the nucleus. The ER also functions as a cytoplasmicframework providing a surface for some of
the biochemical activities of the cell



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