CELL(Biology) - Definition, Types, Structure and Functions

When we read any topic in biology, we frequently meet with the term "cell". In this post, I discuss about Cell.

What is Cell:

Well, I think we are all aware of how a brick wall is made of. Simply put, a brick
 wall is made of, joining bricks.            

Brick wall.

Body made up of cell.

Similarly, living organisms are made up of cells. Body of living organism is build by cells.
There are Unicellular Organism( which is consisted of only one cell.) and Multicellular Organism (This type of Organism made up of many cell.)

Amoeba; It is a Unicellular organism.

   We, human being are multicellular organism. Think about our skin. Our skin is made up of many many cell. In fact, all organs as well as whole body are made up of cell. Although they (cells)  all are not same in size, shape,m functions.
       All living organisms are made up of cells. In fact, all work performed by a living organism, are accomplished by cell, directly or indirectly.
So what is the definition of cell:
The cell is the basic structural, functional and biological unit of a living organism.
"Cell" is derived from a Latin word "Cella" meaning "small room". 

Who discovered Cell:

 In 1965 Robert Hooke first discovered that the structure of cork( it is a substance which comes from the bark of the tree) was like honeycomb( i.e. made of small boxes). He made this chance observation through a self-designed microscope. 

Honeycomb

Magnified view of the cell of the Cork.

Robert Hooke named these boxes as "cell". The word cell come from Latin word "Cella"( meaning "small room"). So it can be said that Robert Hooke First discovered cell.
  In fact, due to lower magnification power of microscope, he unable to observe the living matter and real function of living cells.

Modern microscope

Digital microscope

 Anton Van Leeuwenhoek  was another scientist who first observed and describe the living cell.

 

Cell Theory:

In biology, Cell Theory is a basic scientific theory about cell. From this theory, we can know about some fundamental facts about cell. 

Two scientists,  Matthias Schleiden and Theodor Schwann proposed the cell theory. But it did not explain how new cells are formed. Rudolf Virchew explained in 1855 that new cells are formed from pre-existing cells. He modified the Cell theory,.

The three tenets of the Cell Theory are:

• All living organism are composed of one or more cell.
• The cell is the basic structural and functional unit.
• All cells arise from pre-existing cells ( The process of producing new cell from pre-existing is known as cell division).

An overview of Cell:

Basic diagram of Cell

• All cells are covered by a living membrane which is called Cell membrane.

• Plant cell have an extra non-living rigid cover, outside of the cell membrane. This non-living outermost cover is known as Cell wall.

                * Animal cell has no cell wall.

                * Cell wall gives the structure and protection to plant cell.

• Cell membrane encloses a matter known as Protoplasm.

•  Protoplasm has two part -   Cytoplasm  and Nucleus.

•  Cytoplasm is jelly-like matter stayed inside the cell membrane but outer to the nucleus ( nuclear membrane).

•  Nucleus  is sphere like structure embedded in the cytoplasm.                          
• Cytoplasm contains many  Cell Organelles( living things which help to carry out various works in cell).  

•  Nucleus bounded by a membrane known as Nuclear membrane.

•  There are Nucleoplasm ( which contains Chromatin material and  Nucleolus) inside the nuclear membrane. 

Types of Cell:

There are many types of cell (i.e. Plant cell, animal cell, germ cell, etc.). In general, Cells are of two types.

                            1) Prokaryotic Cell

                            2) Eukaryotic Cell
                                                                  

1) Prokaryotic Cell:

This type of cell does not have nuclear membrane and other membrane bound organelles.  Prokaryotic Cell are not fully developed. All living organism build by this type of cell are unicellular. Prokaryotic cells are found in bacteria, cyanobacteria etc.

2) Eukaryotic Cell:

This type of cell has nuclear membrane and other membrane-bound cell organelles. Eukaryotic Cell are fully developed cell. They can form multi-cellular organism. This types of cells are found in animals and plants.

Structure of Cell:

We have known , so far, the basic structure of a cell. Here, I discuss the structure of cell in details.

  An ideal cell( there are many types of cell. Hence, there also exist many differences. Idea cell refers a cell which have all features.) have

1. Cell wall 
2. Cell Membrane
3. Protoplasm 

                           i) Cytoplasm

                           ii) Nucleus

1) Cell Wall:

• Cell wall is the outer most covering of the cell.

• Cell wall of prokaryotic cell is made up of Peptidoglycans and the cell wall of Eukaryotic cell is mainly made up of Cellulose.

•  In the division of Eukaryotic cell, only Plant Cell have Cell Wall (i.e. animal cell has no cell wall).

   

• Based on the response of the cell wall Staining Technique developed by scientist Gram,  bacterial cell (Prokaryotic cell) are of two types

                                   i) Gram Positive bacteria

                             ii) Gram negative bacteria

 Difference between Gram Positive bacteria and  Gram negative bacteria

 

Gram Positive bacteria	Gram Negative bacteria
These bacteria retain a purple/ blue color after washing with alcohol.	These bacteria do not retain any color after washing with alcohol.
Cell wall is thick about 20 - 80 nm in thickness.	Cell wall is thin about 8 - 12 nm in thickness.
70 - 80% component of the cell wall is peptidoglycan.	20 - 30% component of cell wall is peptodoglycan.

      

  Function of Cell wall:

•  It provides the mechanical support. 

• It act as a barrier to unwanted molecule.

2)Cell Membrane:

Cell membrane is a biological membrane which encloses the protoplasm. It is also called Plasma membrane.
Cell membrane is mainly consist of Lipid and Protein.

 Percentage of presence of lipid in cell membrane vary type to type. Roughly, lipid may contain 20 - 80 % in cell membrane. There is two layer of lipid in cell membrane.

Lipid bilayer

They are arranged such that polar heads ( hydrophilic head)  of lipid ( of two layer ) are placed in outer-side (i.e. inner and outer part of cell membrane) and Non-polar tail ( hydrophobic tail) of lipid are placed in inner side ( i.e. joining side of two layer).

Cell membrane also contains Protein. Based on the ease of extraction of Protein from the membrane,  Proteins are classified as Peripheral Protein and Integral Protein.

Cell membrane

 

                                                                  

Peripheral Protein:

This type of proteins lie on the surface of membrane and it can be easily extracted.

Integral Protein:

Integral protein partially or totally embedded into the membrane. This type of Protein cannot be easily extracted.

Function of Cell membrane:

• Cell membrane encloses the protoplasm.

• Cell membrane permits the entry or exit of some material in or out of the cell. This is why, it is called selectively permeable membrane.

There are two method of transport i.e. Active Transport and Passive Transport.

Active Transport	Passive Transport
This type of transport occurs using the metabolic energy.	Fir this type of transport no energy is required.
The transportation can be taken place against concentration gradient (i.e. material can be transported from lower to higher concentration).	It occur only following the concentration gradient (i.e. material can only be transported from higher concentration to lower concentration).
It occurs rapidly.	It occurs slowly.

 

3)Protoplasm:

The matter bounded by cell membrane is called Protoplasm.

                                               

It is divided into two part.

i) Cytoplasm
ii) Nucleus






i) Cytoplasm:

Cytoplasm is the part of protoplasm except nucleus. Usually it is a jelly-like substance. But, it consist of Cytosol, Organelles and Cytoplasmic Inclusion.


Cytosol:

Basically it is a fluid like substance composed of water, salts and organic molecule. It is the medium where organelles and cytoplasmic Inclusions are stayed.

Organelles:

Organelles means "little organs". Organelles are the membrane bound living things which perform specific function inside the cell. They are suspended in the cytosol. The name of some organelles are Mitochondria, Golgi apparatus(bodies), Lysosome, Plastid, Ribosome( it is and organelles which is not membrane bound.).


Cytoplasmic Inclusion:

 Cytoplasmic Inclusions are non-living tiny particle, suspended in cytosol. Many types of inclusions are present in cytosol, e.g. glycogen( a form of glucose) in muscle cells, lipid droplet in fat cells, various pigment particle in certain cell( skin, hair, blood).


Function of Cytoplasm:

• Cytoplasm provides site for many important biochemical reaction that are necessary for maintaining life.

•  Cell organelles of cytoplasm accomplish various work. For example-

             Ribosome synthesis protein.

             Mitochondria  produce energy(ATP). 

• Cytoskeleton ( it is a complex network made of various filament and tubules. It extended throughout the cytosol.)Provides the shape of the cell.

• Cytoplasmic Inclusions help to store food( fat, sugar) in the cell.

ii) Nucleus:

Nucleus is a membrane bound structure which contains the genetic material( which carry the hereditary characteristics i.e. Gene ). It controls the growth, development and reproduction of cell. This is why, nucleus is called "the brain" of a cell.

Prokaryotic cell do not have nucleus.The area of the cytoplasm ( of prokaryotic cell) where genetic material are stayed, is called Nucleoid. It is not surrounded by any membrane. 

 There are some Eukaryotic cell which have no nucleus. This type of cells are called anucleated cell. Anucleated cells are not divided(i.e. they cannot produce daughter cells). The well known example is red-blood cell of mammals.
Nucleus has four parts -

• Nuclear membrane                             
• Nucleoplasm 
• Chromatin reticulum
• Nucleolus 
 

 

Nucleus

 

 

 

Nuclear Membrane:

Nuclear membrane are encloses the internal matter of the nucleus. It is double membrane boundary. Two membrane are parallel with a space between them .
                                                           

A little part of Nuclear membrane

It is not entirely continuous. There are some minute pores which are formed by the fusion of its two membrane. These pores help in exchanging material between cytoplasm and nucleoplasm.















Nucleoplasm:

It is semi-aqueous substance which contains chromatin materials and nucleolus.



Chromatin Reticulum:

Chromatin reticulum is a network of nucleoprotein fibres (chromatin fibres). Chromatin is the combination of DNA ( Deoxyribonucleic Acid), some proteins and RNA ( Ribonucleic Acid). DNA store the genetic information.

DNA

 When cell is "resting" (i.e. not dividing), the genetic material spreads over the nucleoplasm as chromatin reticulum(or network). But, when it start division, chromatin condensed into rod-like structures known as Chromosome. Every species has certain number of chromosome. A cell of human has 46 chromosome.

Nucleolus:


Nucleolus ( in plural, Nucleoli ) is dense, membrane-less spherical structure placed inside the nucleus. It helps to synthesize the ribosomal RNA.


Function of Nucleus:

• It stores the hereditary material.

•  Using the messenger RNA (mRNA) nucleus regulates the synthesis of protein in cytoplasm.

•  It controls the overall development of cell.