The Fundamental Unit of Life Notes Class 9

The Fundamental Unit of Life Class 9 Notes

Discovery of cell

Robert Hooke (1665), while examining a thin slice of cork under the primitive microscope, observed that cork consists of small box-like structures resembling a honeycomb. He called these boxes cells. The substance called cork Congress from the bark of a tree. The use of the word ‘cell’ to describe these units is used to this day in biology. Cell is a Latin word from ‘a little room’.

Cellular composition in different organisms

On the basis of the number of cells present in different organisms, they are classified into two types:

(i) Unicellular organisms (having single cell).

(ii) Multicellular organisms (having many cells).

Differences between Unicellular and Multicellular organisms

Unicellular organism Multicellular organism
A single cell constitutes the whole organism. Multiple cells are grouped together in a single body which assume different functions in the body to form various body parts.
There is no division of labour in prokaryotic Unicellular organism, but it may be seen within the cell in eukaryotic organism. All cells are specialised to perform different functions of the multicellular body so that there is a division of labour within a single cell as well as group of cells.
e.g. Amoeba, chlamydomonas, Paramecium, bacteria etc. e.g. fungi, plants, animals including humans etc.

Shape of cells

Some cells have fixed shapes (e.g. most plant and animal cells), while cells like WBCs and Amoeba keep changing their shapes. Fixed shapes cells may be of various types like elliptical (e.g. Fat cell), spherical (e.g. Ovum), spindle-shaped (e.g. smooth muscle cell).

Size of cells

The size of cells varies significantly from the smallest cell of Mycoplasma (0.1-0.5um) to very large egg cells of the ostrich (18cm).

The longest cells of the human body are the nerve cells, which may reach up to a length of 90cm.

Plasma membrane or cell membrane

This is the outermost living, thin and delicate covering of cells that separates the contents of the cell from its external environment.

The presence of lipids and proteins (as phospholipids) provides flexibility to the plasma membrane and enables cells to engulf food and other materials from the external environment.

Functions of Plasma membrane

(i) It allows our permits the entry and exit of some selective materials in and out of the cell. The cell membrane, therefore, acts as a semipermeable, selectivity permeable, partially permeable or differentially permeable membrane.

(ii) It helps to maintain the shape of the cell.

(iii) It acts as a mechanical barrier and protects the internal contents of the cell from leaking out.

Transport across the membrane by osmosis

The movement of water molecules through a selectively permeable membrane along the concentration gradient is called osmosis.

The absorption of water by plant roots is also an example of osmosis. The process of osmosis can be seen in a cell placed in solution of different concentrations (such as hyponotic, isotonic and hypertonic).

Hypotonic solution

When the medium or solution surrounding the cell has high water concentration compared to inside of the cell (or, the outside solution is very diluted). As a result, the cell will gain water and swell up via ebdosmosis. This happens because the water molecules are free to pass through the cell membrane in both directions. More water however enters the cell than that leaving it.

Isotonic solution

When the medium surrounding a cell has the same concentration of water as that present inside the cell.

Water crosses the cell membrane in both directions, but the amount moving in is the same as the amount moving out, so there is no overall movement of water. As a result, no overall change is observed and the cell will stay in the same size.

Hypotonic solution

When the medium surrounding a cell has a lower concentration of water than the cell (i.e. Outside solution is very concentrated). Water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. As a result, the cell will shrink (exosmosis).

Cell wall

It is a tough, non-living covering outside the plasma membrane. It is found only in plant cells and is freely permeable. It is mainly made up of cellulose, a complex substance that provides structural strength to plants.

Functions of cell wall

(i) Cell wall permits the cell of plants, fungi and bacteria to withstand hypotonic without bursting. In hypotonic media, the cells tend to take up water by osmosis. The cell swells up, building up pressure against the cell wall. The wall exerts equal pressure against the swollen cell. Due to the presence of the wall, plant cells can tolerate much greater changes in the surrounding medium than animal cells.

(ii) It has narrow pores, called pits, through which fine stand of cytoplasm (or cytoplasmic bridges) called plasmodesmata are able to cross the cell walls. Plant cells interact with each other through these cytoplasmic channels.


It is popularly called as the brain of cell. It is composed of a double-layered covering called nuclear membrane. It has numerous pores called nuclear pores for the transfer of materials from inside the nucleus to cytoplasm. The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. It encloses a liquid ground substance called nucleoplasm, which contains nucleolus and chromatin material.

Nucleolus is a more or less round structure found inside the nucleus and does not have covering membranes.

Chromatin is an entangled network of long, thread-like structures, which condenses to form chromosomes during cell division.

Chromosomes contain information for the inheritance of features from parents to next generation in the form of DNA molecules (Deoxyribonucleic Acid). Chromosomes are composed of two components, i.e. DNA and protein. The DNA molecule contain information necessary for constructing and organising cells. The functional segments of DNA at called genes. Nucleus also contains RNA that directs protein synthesis.

Functions of Nucleus

(i) Nucleus plays an important role in cellular reproduction, in which a cell divides to form two new cells.

(ii) It also determines the cell development and maturity by directing the chemical activities of the cell.

(iii) It helps in the transmission of hereditary traits from parents to offsprings.

(iv) It controls all metabolic activities of cell and if it is removed, the protoplasm dries up.

Differences between Prokaryotic and Eukaryotic cells

Feature Prokaryotic cell Eukaryotic cell
Size Generally small (1-10um). Generally large (5-100um).
Nuclear region Poorly developed, no nuclear membrane and called as nucleoid. Well-defined, surrounded by nuclear membrane.
Chromosome Single More than one
Nucleolus Absent Present
Membrane-bound cell organelles Absent Present, e.g. mitochondria, plastids, endoplasmic reticulum etc.
Centriole Absent Present in animal cells.
Organisms Found in bacteria, blue-green algae etc. Found in fungi, plants and animals.
Cell division Takes place by binary fission or budding. Takes place by mitotic or meiotic cell division.


The large region of each cell enclosed by cell membrane is called cytoplasm. It is the fluid content present inside the plans membrane. It also contains many specialised cell organelles, each of which performs a specific function for the cell.

Functions of cytoplasm

(i) It helps in the exchange of material between cell organelles.

(ii) It acts as a storehouse of vital molecules such as amino acid, glucose, vitamin, iron etc.

(iii) It acts as the site for certain metabolic pathways such as glycolysis etc.

Cell organelles

Cell use membrane-bound structures performing specialised functions within themselves called as cell organelles.

Endoplasmic Reticulum (ER)

It is a large network of membrane-bound tubes and sheets extending from outer nuclear membrane into the cytoplasm. Depending upon nature of its membrane, ER is of two types:

(i) Rough Endoplasmic Reticulum (RER): It contains ribosomal particles, due to which is surface is tough. The ribosomes are the site of protein synthesis. RER is mainly formed of cisternae. It’s surface is smooth due to the absence of ribosomes.

(ii) Smooth Endoplasmic Reticulum (SER): It helps in manufacture of fat molecules or lipids. It is formed of vesicles and tubules. It’s surface is smooth due to the absence of ribosomes, ER appears in varying forms in different cells, but it always form a network system of vesicles and tubules.

Functions of Endoplasmic Reticulum

(i) Ribosomes present in all active cells act as sites for protein synthesis. Proteins manufactured here are transported throughout the cell by endoplasmic reticulum.

(ii) Fat and lipid molecules manufactured by SER helps in building cell membrane. This process is called membrane biogenesis.

(iii) Some other proteins and lipids synthesised by ER function as enzymes and hormones.

Golgi apparatus

It consists of a system of membrane-bound, fluid-filled vesicles, large spherical vacuoles and smooth, flattened cisternae, which are stacked parallel to each other. Each of these stacks is called a cistern. The Golgi apparatus arises from the membrane of smooth ER and therefore, constitutes another portion of a complex cellular membrane system. The material that is synthesised near Endoplasmic Reticulum (ER) is packaged and dispatched to various parts, i.e. inside and outside the cell through Golgi apparatus.


These are a kind of waste disposal system of the cell. Lysosomes are membrane-bound sacs that are filled with digestive enzymes. These enzymes are made by rough endoplasmic reticulum. Lysosomes are also called the suicidal bags of a cell because during the disturbance in cellular metabolism or when the cell gets damaged, lysosomes may burst and the enzymes can digest their own cell.

Functions of Lysosomes

(i) They help to keep the cell clean by digestive any foreign material that enters the cell as well as worn out cellular organelles. Hence, called scavengers and cellular housekeepers.

(ii) They remove up bacteria or any foreign material entering the cell by breaking it into small pieces through its powerful digestive enzymes, which can breakdown all organic materials.

(iii) During starvation, the lysosomes suggest stored food contents by autophagy and supply energy to the cell.

Mitochondria: it is know as the power house of the cell.

Functions of Mitochondria

(i) It generates energy for various activities of cell and is known as the powerhouse of the cell. Mitochondria are sites of cellular respiration and release the energy required by the cell in the form of ATP (Adenosine Triphosphate) molecules. This ATP is known as energy currency of the cell.

(ii) Whenever, the cell requires energy, ATP molecule breaks down generating energy to be used for metabolic activities of body.

(iii) Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes hence, are able to make some of their own proteins.

(iv) They provide intermediates for the synthesis of various chemicals like fatty acid, steroids, amino acids etc.


These are found only in plant cells. The internal organisation of plastids contains numerous membrane layers embedded in a material called the stroma. Plastids are similar to mitochondria in external structure.

Types of plastids

(i) Chloroplasts: These are the plastids containing chlorophyll (a green pigment), which gives green colour to the plant. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. It is also a semiautonomous organelle. Chloroplasts are also known as the kitchen of cells.

Function: These are important for photosynthesis in plants.

(ii) Leucoplasts: These are the white or colourless plastids. They can change into other types of plastids.

Function: Leucoplasts store materials such as starch (amyloplasts), oils (elaioplasts) and protein granules (aleuroplasts).

(iii) Chloroplasts: These are coloured plastids (except green).

Function: Chloroplasts impart colour to flowers and fruits. They are rich in carotenoid pigments and lipids.


These are the storage sacs for solid or liquid contents. In animal cells, vacuoles are small-sized, but in plants, the vacuoles are large-sized and some may occupy 50-90% of the total cell volume.

Functions of vacuoles:

(i) Vacuoles are full of cell sap and provide turgidity and rigidity to the cell in plants.

(ii) Many substances like amino acids, sugars, organic acids and proteins are stored in vacuoles.

(iii) In Amoeba, consumed food items are stored in food vacuoles.

Try To Solve These Questions

Q1. List Three Differences between Unicellular and Multicellular organisms.

Q2. Functions of cell wall?

Q3. List Three Differences between Prokaryotic and Eukaryotic cells.

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