Class 9 Science Chapter 5 The Fundamental Unit of Life NCERT Notes

Chapter 5 The Fundamental Unit of Life Class 9 Science CBSE NCERT Notes

Who Discovered Cell?

Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Cork is a substance which comes from the bark of a tree. This was in the year 1665 when Hooke made this chance observation through a self-designed microscope. Robert Hooke called these boxes cells.

The first man to witness a live cell under a microscope was Anton van Leeuwenhoek, who in 1674 described the algae Spirogyra.

In 1831 a Scottish botanist discovered the nucleus of the cell and gave the earliest descriptions of the cell nucleus.

Schleiden and Schwann presented the cell theory which states that all plants and animals are composed of cells and cell is the basic unit of life.

In 1839 Purkinje coined the term ‘Protoplasm” for the fluid substance of the cell.

What is a Cell Made Up of?

Cells are the basic unit bounded by the membrane that consists of the fundamental molecules of life of which all living organisms are made up of.

Each cell contains a cytoplasm which is enclosed by a membrane and contains several biomolecules like proteins, nucleic acids, etc.

Types Of Cell

Prokaryotic Cell

A prokaryotic cell possesses a poorly defined nucleus due to the absence of nuclear membrane. The nuclear region is called Nucleoid. These cells also lack most of the membrane bound cytoplasmic organelles so that the various functions are performed by poorly organized parts of the cytoplasm.

Eukaryotic Cell

The Eukaryotic cell possesses well defined cellular organelles to perform different functions. They have true nucleus with nuclear membrane. They are on an average 10 times larger than a prokaryotic cell and have complex DNA which enable them to do complex jobs.

Difference Between Prokaryotic Cell and Eukaryotic Cell

Prokaryotic Cells	Eukaryotic Cells
Very minute in size. (1 to 10 m)	Fairly large in size. (5-100μm)
Nuclear region (nucleoid) not surrounded by a nuclear membrane.	Nuclear material surrounded by a nuclear membrane.
Always Unicellular	May be unicellular or multicellular.
Single Chromosome present.	More than one chromosome present.
Nucleolus absent.	Nucleolus present.
Cell division by fission or budding.	Cell division by mitosis or meiosis.
Membrane bound cell organelles are absent.	Membrane bound cell organelles are present.
Example include Bacteria	Example includes All Plants, Animals, Amoeba etc.

Components of Cell

Cell Membrane

Plasma membrane is selectively permeable in nature, means it allows or permits the entry and exit of some materials in and out of the cell.

It is the limiting boundary of each cell which separates cytoplasm from its surroundings. It is found in both plant as well as animal cells.

It is the outermost covering of a cell in case of animals and lies below the cell wall in case of plants.

Cell Wall

Cell wall is only present in the plant cells.

In plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall.

It is mainly composed of cellulose which provides strength to the plants.

Functions of Plasma Membrane

It regulates the movement of molecules inside and outside the cell.
It helps in maintaining the distinct composition of the cell.

The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis.

Transportation of molecules across the Plasma membrane

Diffusion: Movement of solutes or ions from a region of higher concentration to a region of lower concentration is called as diffusion. It does not require energy therefore, it is called as passive transport.

Osmosis: The movement of solvent or water from its higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as osmosis.

Types of Solutions on the Basis of Concentration and its effect on cell

Isotonic Solution: When concentration of a solution outside the cell is equal to the concentration of cytoplasm of the cell, it is called as isotonic solution.

Hypertonic Solution: When concentration of a solution outside the cell is more than inside of the cell. Due to this, cell loses water and becomes plasmolysed.

Plasmolysis: Shrinking of the protoplasm away from the cell wall due to Excessive loss of water (Exosmosis)

Nucleus

Nucleus is the most important cell organelle which directs and controls all its cellular activities. It is called as ‘Headquarter of the cell’.

It was discovered by Robert Brown in 1831.

In Eukaryotes, a well-defined nucleus is present while in Prokaryotes, a well- defined nucleus is absent.

Prokaryotes contain a primitive nucleus called Nucleoid.

It has double layered covering called as nuclear membrane.

Nuclear membrane has pores which regulate the movement of materials in and out of the Nucleus.

Besides nuclear membrane, nucleus also contains nucleolus and chromatin material. Chromatin is made up of DNA and Protein, that ultimately condense and forms chromosome.

Chromosomes or chromatin material consists of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.

Functions of Nucleus

It controls all the metabolic activities of the cell and regulates the cell cycle.
It helps in transmission of hereditary characters from parents to their offsprings.

Cytoplasm

Cytoplasm is the site of both biosynthetic and catabolic pathways (Metabolic activities) It can be divided into two parts:

Cytosol: Aqueous soluble part contain various fibrous proteins forming cytoskeleton. It contain about 90% water, 7% Protein 2% carbohydrates & 1% etc.
Cell organelles: Living part of the cells having definite shape, structure and function bounded by plasma membrane. There are single membrane bound, double membrane bound and non membrane bound Cell organelles.

In prokaryotes, beside the absence of a defined nuclear region, the membrane-bound cell organelles are also absent. On the other hand, the eukaryotic cells have nuclear membrane as well as membrane-enclosed organelles.

Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.

Cell organelles

Every cell has a membrane around it to keep its own contents separate from the external environment.

Some important examples of cell organelles which we will discuss now are: endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids.

Endoplasmic Reticulum

It 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.
Smooth Endoplasmic Reticulum.

Smooth ER	Rough ER
Made of tubules mainly.	Made of Cisternae and vesicles.
Helps in steroid, lipids and Polysaccharide synthesis.	Helps in protein synthesis.
Ribosomes are absent.	Contains ribosome on its surface.

Function of ER

It is the only organelle which serves as a channel for the transport of materials between various regions of cytoplasm and between cytoplasm and nucleus.

It also functions as a cytoplasmic framework to provide surface for some of the biochemical activities.

It forms endoskeleton of cell.

It helps in synthesis of fats, protein, steroids, cholesterol etc.

SER plays a crucial role in detoxification of drugs and poisonous by products.

Golgi apparatus

Golgi apparatus consists of a system of membrane bounded fluid filled vesicles arranged parallel to each other in stacks called Cisternae along with some large and spherical vesicles.

It was discovered by Camillo Golgi. It is absent in prokaryotes, mammalian RBC’s and sieve cells.

Functions of Golgi apparatus

Its function include the storage, modification, packaging and secretion of products in vesicles.
It involved in the formation of lysosomes.
It is secretary in nature.
It also involved in the synthesis of cell wall and plasma membrane.
It helps in melanin synthesis.

Mitochondria

It is a rod shaped structure found in cytoplasm of all eukaryotic cells except mammalian RBC’s. These are also absent in prokaryotes.

It is also called as ‘Power House of the Cell’ or the ‘Storage Battery’.

It is double membranous structure where outer membrane has specific proteins while inner membrane is folded inside to form chambers called Cristae.

Mitochondria has its own DNA and Ribosomes

Functions of Mitochondria

Its main function is to produce, store and release the energy in the form of ATP (Adenosine Triphosphate) The energy currency of the cell.
It is the site for cellular respiration (Kreb cycle) in which ATP are produced.

Ribosomes

Ribosomes are the sites of protein synthesis. All structural and functional proteins (enzymes) coded by the nuclear DNA are synthesized upon cytoplasmic ribosomes. The DNA codes are transcripted into messenger RNA (mRNA) molecules, Which comes out of the Nucleolus and translated (Protein synthesis) by ribosomes attached to RER in the form of proteins.

Functions of Ribosomes

Ribosomes are the main site of protein synthesis. Synthesized proteins is transported by endoplasmic reticulum.

Plastids

Plastids are present only in plant cells. There are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids)

Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored. The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma.

Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

Vacuoles

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.

In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins.

Each cell thus acquires its structure and ability to function because of the organisation of its membrane and organelles in specific ways. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins. Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.

Cell Division

The process by which new cells are made is called cell division. There are two main types of cell division:

Mitosis
Meiosis

The process by which most of the cell divide for growth is called mitosis, in this process each cell called mother cell divides to form two identical daughter cell and have the same number of chromosomes as the mother cell. It helps in the growth and repair of the tissues in organisms.

Specific cells of reproductive organs or tissues in animals and plants divide to form gametes which after fertilisation give rise to offspring. They divide by a different process called meiosis. In meiosis a cell divides into four new cell instead of just two, the new cells also have half the number of chromosomes than that of the mother cells.

Difference between Plant Cell and Animal Cell

Plant Cell	Animal Cell
Contain chloroplasts for photosynthesis.	No chloroplasts
Have a cell wall to maintain structure and rigidity.	No cell wall
Usually do not contain lysosomes and Peroxisomes.	May Contain cilia and/or flagella
Cells are square and rigid or geometric shaped.	Cells are fluidic and flexible, many shapes.
Limited movement.	Cells can move around.
Have one large central vacuole.	Has small or no vacuoles. Have lysosome.