Cell is basic unit of life

Science World notes cover all important topics and diagrams. Here we cover cell theory | organelles | Plant and Animal cell | Cell is basic unit of life chapter 5 class 9 science


  1. Introduction of cell
  2. Concept of hypertonic and hypotonic
  3. Shapes of cells
  4. Cell theory and exception
  5. Plasma membrane and cytoplasm
  6. Organelles
  7. Eukaryotes and prokaryotes
  8. Plant cell and animal cell

Some important terms

  • Numerous living & non-living structure that present in the cytoplasm of cell collectively called Cytoplasmic inclusion.
  • During protein synthesis many ribosomes are aggregated due to common messenger RNA and form Polysomes.
  • Movement of substance from a region of high concentration to region of low concentration is called Diffusion.
  • Osmosis is a special type of diffusion of water that occurs through a semi –permeable membrane. It is of two types:
  • When water diffuse into the cell due to its higher concentration outside, it is called Endosmosis.
  • When water diffuses out of the cell because of its higher concentration inside the cell, it is called Exosmosis.
  • The branch of biology that deals with the various aspects of structure, chemistry, development, genetics & functioning of cells is called cell -biology.
  • That membrane which allows certain substances through it called selectively permeable membrane.
  • Instrument used to measure the osmotic pressure is called osmometer.
  • Shrinkage of protoplast of a cell from its cell wall under the influence of a hypertonic solution is called Plasmolysis.
  • DNA + HISTON PROTEIN = CHROMOSOME In human cell 46 chromosomes is present.
  • DNA is circular in prokaryotic cell while in eukaryotic cell it is linear.
  • Mycoplasma is also called PPLO
  • First microscope was built by Zacharias Janssen.
  • Pigment Carotene responsible for changing of fruit colour during ripening (from green to yellow or reddish).
  • Yellow pigment called Xanthophylls.
  • Chloroplast is found in Mesophyll cells. It is the site where photosynthesis occurs.

Cell theory

The theory was jointly put forward by Mathias Jacob Schleiden & Theodor Schwann in their paper `Microscopic investigations’ that based on the similarity of structure and growth in animals and plants. This theory state that the bodies of all living organisms are made up of cells and their products so that cells are units of both structure and function of living organisms. Schleiden and Schwann compared their findings and formulated the cell theory in their joint paper in 1839. The theory proposed that cells are the units of both structure and function of organisms. Cell theory is also called cell doctrine.

Fundamental feature of cell theory

  • All living organism are composed of cells and their products.
  • Each cell is made of a small mass of protoplasm containing a nucleus in it.
  • All cells are basically same in chemical & physiological sense.

In coming year the cell theory was to be extended by Karl Nageli. In 1846 he showed that cell arises from the division of pre-existing cells. In 1855 Rudolf Virchow gives a statement that was “omnis cellula e cellula” it means every cell from a cell thus he gave the significance of cell division in reproduction of organism. In 1858 Virchow published his classical text book Cellular Pathology. Later in 1865 Louis Pasteur gave experimental evidence to support Virchow’s extension of cell theory. The modern version of cell theory state that:

  • Growth of an organism involves growth & multiplication of its cell.
  • Life exists in the cells because all activity of life is performed by cell.
  • A cell can survive independently but its organelle cannot survive independently.
  • New cells are arising from pre-existing cells.
  • Genetic information stored & express in cell.
  • Activities of an organism are the sum total of activities and interactions of its constituent cell.


  • Virus are acellular and do not have cell machinery. Even then they considered as organism.
  • Bacteria and cynobacteria do not have nucleus and membrane bound organelle.
  • RBCs and Sieve tube cell continue to live without nucleus.

Shapes of cell

The cells vary in their shapes. They may be disc like, polygonal, columnar, cuboids, amoeboid, thread like or irregular. The shape of cell is related to its position and function. Some of shapes are:

      Bacteria                                    Amoeba                              Mesophyll cell

        Asteocyte                              White blood cell                        Sperm

Cell size

  • There is a wide variation in size & shape of cell. The smallest cell is of Mycoplasma (pleuro pneumonia like organism). They have size of 0.1—0.5µm. longest cell of human body are the nerve cells which may reach a length of 90cm.
  • Eggs are large cells because they store food for development of the embryo. Egg of ostrich is large egg cell.

Structural organization of cell

The protoplasm of cell generally divided into three parts:

Plasma membrane

it is selectively permeable covering of the cytoplasm that forms the innermost component of cell envelope. It is made up of phospholipids bilayer with protein of various types. Plasma membrane holds the liquid of cell & separates it’s from surroundings, serve as selective permeable membrane. It allows useful substance inwards and harmful substances outwards. It perform following functions:

  • It provides & maintains shapes of cells.
  • It provides mechanical support & protection.
  • It allows only useful substances to enter into the cells.
  • It provides specificity to cells.

Cell wall: in case of plant cells outer to plasma membrane is present a non-living and rigid cell wall made up of cellulose. It allows substances freely to enter or to leave it. It performs following function:

  • It maintain the shape of cell
  • It provide rigidity as well as elasticity to cell


the bulk of the cell is made of a dense, viscous, colloidal mass called cytoplasm. It presents b/w plasma membrane and nuclear membrane. It is the site where all metabolic activity takes place. Cytoplasm of unicellular organisms is differentiated into outer ectoplasm and inner endoplasm. There are various cell organelles.

Endoplasmic reticulum

A inter connected network of membrane – lined channels called endoplasmic reticulum. The name endoplasmic reticulum was coined by Porter in 1953 in liver cell.

Handmade Diagram By Science World

It is visible only under electron microscope.

It is connected with the plasma membrane, Golgi body & nuclear membrane. It helps in the formation of other organelles. It is of two types:

  1. RER–it is rough in appearance because ribosome present on its surface. Functions are following: endoplasmic reticulum provides ultra structure skeletal framework to cell and give mechanical support, it is the site for protein synthesis& it forms nuclear envelope after each nuclear division.
  2. SER–it is found mostly in the outer part of cytoplasm. It is smooth because absence of ribosome on its surface. Functions are following: synthesis of lipid, sterol metabolism, it concerned with glycogenolysis & detoxification.

Golgi body

Golgi body is also called Golgi apparatus. It was coined by Camillo Golgi& S.R. Golgi. The Golgi body occurs in all cells except the prokaryotic cells of certain fungi, bryophytes & pteridophytes and also RBCs of animal cells. Their number varies from cell to cell. They are more than 25000in algal rhizoid. In animal cells there are usually occurs a single Golgi apparatus, its no. may varies from cell to cell. It is divided basically in three parts: cisternae, tubules & vesicles. It concerned with the following function: secretions, membrane flow, formation of lysosomes and formation of vitelline membrane of primary oocytes. In plants it forms cell plate.


These are popularly known as Power house of cell. Mitochondria are granular cytoplasmic organelles of all aerobic cells of higher animals & plants and also of certain m/o including algae, protozoa and fungi.

Mitochondria By Tushar Sir
  • They also contain a specific DNA for cytoplasmic inheritance.
  • Mitochondria were first discovered by the Kolliker in 1850 in striated muscles. The number of mitochondria in a cell depends on the type and functional state of cell. Liver cell of rat contain 500 to 1600 mitochondria. Mitochondria area double membranous structure that has various enzymes because it is the site for various metabolic reactions.

Function: it is the site where ATP is formed due that it is called power house of cell


These are the granular structure that is seen under electron microscope. They occur either freely in the matrix of mitochondria, chloroplast & cytoplasm or remain attached with the membrane of the endoplasmic reticulum &nucleus. When they attached to the endoplasmic reticulum then they called RER. In 1952, G.E. Palade describes the ribosome. On the basis of sedimentation coefficient it is of two types: 70S & 80S. Ribosome helps in the protein synthesis so called protein factory.


Vacuoles are the fluid filled cavity in cytoplasm. These are covered by membrane called Tonoplast. Its fluid is called cell sap. Vacuoles can occupy up to 90 per cent of the volume of the cell. It has both organic and inorganic substances. In plants, the Tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole; hence their concentration is significantly higher in the vacuole than in the cytoplasm.

In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles. They includes amino acid, protein etc. its function is to keep the turgidity of cell, Tonoplast act as semi permeable membrane. Vacuoles in amoeba help in excretion.


The term plastid was used by the A.F.W. Schimper in 1885. He classified the plastid into three types that are:

Leucoplast—these are colourless plastids that are found in the embryonic & germ cells. They are also found in that region which not receive light. They store food materials as carbohydrate (Amyloplast) protein (Proteinoplast) &fat (Eleioplasts).

Chromoplast-these are coloured plastid containing carotenoids & other pigments. They impart colour to certain portion of plants such as petals, fruits etc.

Chloroplast—the chloroplast is the most abundant pigment molecules that provide green colour. It is the site where process of photosynthesis occur that results formation of food for plant body.

Structure of Chloroplast By Science World


These are very fine microscopic fibre that made up of protein. Microtubules provide strength, it brings about movement inside the cell, and they help in formation of spindle fibres during cell division.

  • Centrosome

It is found in the animal cells and present near the nucleus. It concerned with the cell division and formation of basal bodies of cilia and flagella.

  • Micro bodies

Many membrane bound minute vesicles called micro bodies that contain various enzymes, are present in both plant and animal cells.


Nucleus is a cell organelle that was first described by Robert Brown in 1831.The nucleus is bounded by double membrane, porous, selectively permeable nuclear envelope.

Nucleus By Tushar Sir
  • It is filled with a jelly like substance called nucleoplasm.
  • A network of thread like structure in it is called chromatin that occurs around the nucleolus.
  • Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm), called the peri-nuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.
  • The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it. Nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
  • Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed. The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.

Functions: nucleus controls the functioning of cell, it helps in the transmission of characters from parents to the offspring and it also regulates the cell cycle.


During different stages of cell division, cells show different structured chromosomes in place of the nucleus. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.


Based on the position of the centromere, the chromosomes can be classified into four types. The metacentric chromosome has middle centromere forming two equal arms of the chromosome. The sub-metacentric chromosome has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm. In case of acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal Centromere.

Types of cell


The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells. They may vary greatly in shape and size.

  • The four basic shapes of bacteria that are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
  • Organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions.
  • All prokaryotes have a cell wall surrounding the cell membrane. The fluid matrix filling the cell is the cytoplasm.
  • There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane.
  • In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria that is resistance to antibiotics. Plasmid DNA is used to monitor bacterial transformation with foreign DNA.
  • Nuclear membrane is found in eukaryotes. No organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosome. Prokaryotes have something unique in the form of inclusions. A specialised differentiated form of cell membrane called mesosome is the characteristic of prokaryotes. They are essentially infolding of cell membrane.


The eukaryotes include all the protists, plants, animals and fungi. In Eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles.

  • Eukaryotic cells possess an organised nucleus with a nuclear envelope. In addition, Eukaryotic cells have a variety of complex locomotory and cytoskeletal structures.
  • Their genetic material is organised into chromosomes. All eukaryotic cells are not identical.
  • Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in almost all plant cells.

Difference b/w Plant and Animal cell

Plant cellAnimal cell
Plant cell has a rigid wall on the outside.A cell wall is absent.
It is usually large in size.Animal cell is comparatively small in size.
It cannot change its shape.It can change its shape.
Plastids are found in plant cell.Plastids are absent.
A mature cell has large central vacuoles.Animal cell may have many small vacuoles.
Nucleus is elliptical.Nucleus is rounded.
Mitochondria are less in number..Mitochondria are numerous.
Centrioles are absentCentrioles found in animal.
Plant cell does not burst when placed in hypotonic solution.It usually burst.
Reserved food is usually starch and fat.Reserved food is usually glycogen & fat.
Cell cannot take part in phagocytosis.Lysosomes of cell take part in phagocytosis.
Glyoxysome may presentThey are absent


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