The Fundamental Unit of Life : All you need to know

Fundamental Unit of Life
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Fun­da­men­tal Unit of Life

Cell

It is the struc­tur­al and func­tion­al unit of life.

→ Cell is termed as the struc­tur­al unit of life as it pro­vides struc­ture to our body.

→ Cell is con­sid­ered as the func­tion­al unit of life as all the func­tions of the body take place at cell level.

Discovery of cell:

→ Dis­cov­ered by Robert Hooke in 1665.

→ Robert Brown in 1831 dis­cov­ered the nucle­us in the cell.

Cell Theory:

Cell the­o­ry states that:

→ All liv­ing organ­isms are com­posed of cells.

→ Cell is the fun­da­men­tal unit of life.

→ All new cells come from pre-exist­ing cells.

Types of Organisms on the Basis of Number of Cells

There are two kinds of organ­isms on the basis of cells:

(i) Unicellular Organisms: 

The organ­isms that are made up of sin­gle cell and may con­sti­tute a whole organ­ism, are named as uni­cel­lu­lar organisms.

For exam­ple: Amoe­ba, Para­me­ci­um, bac­te­ria, etc.

(ii) Multicellular Organisms: 

The organ­isms which are com­posed of a col­lec­tion of cells that assume func­tion in a coor­di­nat­ed man­ner, with dif­fer­ent cells spe­cial­ized to per­form par­tic­u­lar tasks in the body, are named as mul­ti­cel­lu­lar organisms.

For exam­ple: Plants, human beings, ani­mals, etc.

Shape and Size of Cells

→ Cells vary in shape and size. They may be oval, spher­i­cal, rec­tan­gu­lar, spin­dle shaped, or total­ly irreg­u­lar like the nerve cell.

→ The size of cell also varies in dif­fer­ent organ­isms. Most of the cells are micro­scop­ic in size like red blood cells (RBC) while some cells are fair­ly large like nerve cells.

Types of Cells

The cells can be cat­e­go­rized in two types:
1. Prokary­ot­ic Cell  2. Eukary­ot­ic Cell

1. Prokaryotic cell

Prokary­ot­ic cells are cells in which true nucle­us is absent. They are prim­i­tive and incom­plete cells. Prokary­otes are always uni­cel­lu­lar organ­isms. For exam­ple, archae­bac­te­ria, bac­te­ria, blue green algae are all prokaryotes.

2. Eukaryotic Cell

Eukary­ot­ic cells are the cells in which true nucle­us is present. They are advanced and com­plete cells. Eukary­otes include all liv­ing organ­isms (both uni­cel­lu­lar and mul­ti­cel­lu­lar organ­isms) except bac­te­ria and blue green algae.

Difference Between Prokaryotic and Eukaryotic Cells:

S. No.Prokary­ot­ic cellEukary­ot­ic cell
1.Size of cell is gen­er­al­ly small (1–10 mm).Size of cell is gen­er­al­ly large (5–100 mm).
2.Nucle­us is absent.Nucle­us is present.
3.It con­tains sin­gle chromosome.It con­tains more than one chromosome.
4.Nucle­o­lus is absent.Nucle­o­lus is present.
5.Mem­rane bound cell organelles are absent.Mem­rane bound cell organelles such as mito­chon­dria, plas­tids, endo­plas­mic retic­u­lum, gol­gi appa­ra­tus, lyso­somes, etc., are present.
6.Cell divi­sion takes place by fis­sion or budding.Cell divi­sion takes place by mitot­ic or mei­ot­ic cell division.

Structure of Cell

Cell is gen­er­al­ly com­posed of three basic components:

  1. Cell wall  and cell membrane
  2. Nucle­us
  3. Cyto­plasm

(i) Cell membrane or Plasma membrane:

Plas­ma mem­brane is the cov­er­ing of the cell that sep­a­rates the con­tents of the cell from its exter­nal environment.
It is a liv­ing part of the cell and is present in cells of plants, ani­mals and microorganisms.
It is very thin, del­i­cate, elas­tic and selec­tive­ly per­me­able membrane.
It is com­posed of lipid and protein.

Func­tion:
As it is selec­tive­ly per­me­able mem­brane, it allows the flow of lim­it­ed sub­stances in and out of the cell.

(ii) Cell wall:

cell wall is non-liv­ing, thick and freely per­me­able cov­er­ing made up of cellulose.
It is present in eukary­ot­ic plant cells and in prokary­ot­ic cells.

Func­tions:

→ It deter­mines the shape and rigid­i­ty to the plant cell.

→ It pro­tects the plas­ma membrane.

→ It pre­vents des­ic­ca­tion or dry­ness in cell.

→ It helps in the trans­port of varous sub­stances in and out of the cell.

(iii) Nucleus:

Nucle­us is dense and spher­i­cal organelle.
Nucle­us is bound­ed by two mem­branes, both form­ing nuclear enve­lope. Nuclear enve­lope con­tains many pores known as nuclear pores.
The flu­id which present inside the nucle­us is called nucleoplasm.
Nucle­us con­tains chro­mo­somes and chro­mo­somes con­tain genes which are the cen­tres of genet­ic information.

Func­tions:

→ Nucle­us con­trols all the meta­bol­ic activ­i­ties of the cell.

→ It reg­u­lates the cell cycle.

→ Nucle­us is the store­house of genes.It is con­cerned with the trans­mis­sion of hered­i­tary traits from the par­ent to offspring.

(iv) Cytoplasm:

It is a jel­ly-like, vis­cous, colour­less semi-flu­id sub­stance that occurs between the plas­ma mem­brane and the nuclear membrane.
The aque­ous ground sub­stance of cyto­plasm is called cytosol that con­tains a vari­ety of cell organelles and oth­er insol­u­ble waste prod­ucts and stor­age prod­ucts, like starch, glyco­gen, lipid, etc.

Func­tions:
→ Pro­to­plasm acts as a store of vital chem­i­cals like amino acids, pro­teins, sug­ars, vit­a­mins, etc.
→ It is the site of cer­tain meta­bol­ic reac­tions, like gly­col­y­sis, syn­the­sis of fat­ty acids, nucleotides, etc.

Cell organelles:

Inside the cell there are dif­fer­ent parts per­form­ing dif­fer­ent activ­i­ties to keep the cell alive anf func­tion­able. These part are called Cell organelles. They are explained below:

1. Golgi Apparatus:

Gol­gi appa­ra­tus con­sists of a set of mem­brane bound, flu­id filled vesi­cles, vac­uoles and flat­tened cis­ter­nae (closed sacks).
Cis­ter­nae are usu­al­ly arranged par­al­lel to each other.

Func­tions:

→ Its main func­tion is to store, mod­i­fy, pack­age and dis­patch the substances.

→ It is also involved in the syn­the­sis of cell wall, plas­ma mem­brane and lysosomes.

2. Endoplasmic Reticulum:

It is a mem­bra­nous net­work of tube like struc­tures extend­ing from nuclear mem­brane to plas­ma membrane.
It is absent in prokary­ot­ic cells and matured RBCs of mammals.
There are two types of endo­plas­mic retic­u­lum:
(i) Rough Endo­plas­mic Retic­u­lum (RER): Here ribo­somes are present on the sur­face for the syn­the­sis of pro­teins.
(ii) Smooth Endo­plas­mic Retic­u­lum (SER): Here ribo­somes are absent and is meant for secret­ing lipids.

Func­tions:

→ It gives inter­nal sup­port to cell.

→ It helps in trans­port of var­i­ous sub­stances from nuclear mem­brane to plas­ma mem­brane or vice versa.

→ RER helps in syn­the­sis and trans­porta­tion of proteins.

→ SER helps in syn­the­sis and trans­porta­tion of lipids.

3. Ribosomes:

These are extreme­ly small, dense and spher­i­cal bod­ies which occur freely in the matrix (cytosol) or remain attached to the endo­plas­mic reticulum.
These are made up of ribonu­cle­ic acid (RNA) and proteins.

Func­tion:

They play a major role in the syn­the­sis of proteins.

4. Mitochondria:

They are small rod-shaped organelles.
It is a dou­ble mem­brane struc­ture with out­er mem­brane being smooth and porous where­as inner mem­brane being thrown into a num­ber of folds called cristae.
They con­tain their own DNA and ribosomes.
They are absent in bac­te­ria and red blood cells of mammals.

Func­tions:

→ They are the sites of cel­lu­lar res­pi­ra­tion, hence pro­vide ener­gy for the vital activ­i­ties of liv­ing cells.

→ They store ener­gy releas­es dur­ing reac­tions, in the form of ATP (Ener­gy cur­ren­cy of the cell). There­fore, they are also called ‘pow­er house’ of the cell.

5. Centrosome and Centrioles:

Cen­tro­some is found only in eukary­ot­ic ani­mal cells. It is not bound­ed by any mem­brane but con­sists of centrioles.
Cen­troles are hol­low cylin­dri­cal struc­tures arranged at right angle to each oth­er and made up of microtubules.

Func­tion:
Cen­tri­oles help in cell divi­sion and also help in the for­ma­tion of cil­ia and flagella.

6. Plastids:

Plas­tids are present in most of the plant cells and absent in ani­mal cells.
They are usu­al­ly spher­i­cal or dis­coidal in shaped and dou­ble mem­brane bound organelles.
They also have their own DNA and ribosomes.
Plas­tids are of three types:
(a) Chloro­plas­ts: These are the green coloured plas­tids con­tain­ing chloro­phyll. Chloro­plas­ts aid in the man­u­fac­ture food by the process of pho­to­syn­the­sis.
(b) Chro­mo­plas­ts: 
These are the colour­ful plas­tids (except green colour).
© Leu­coplas­ts: 
These are the colour­less plastids.

Func­tion:

→ Chloro­plas­ts trap solar eber­gy and utilise it to man­u­fac­ture food for the plant.

→ Chro­mo­plas­ts impart var­i­ous colours to flow­ers to attract insects for pollination.

→ Lecuo­plas­ts help in the stor­age of food in the form of starch, pro­teins and fats.

7. Lysosomes:

Lyso­somes are small, spher­i­cal, sac like struc­tures which con­tain sev­er­al diges­tive enzymes enclosed in a membrane.
They are found in eukary­ot­ic cells most­ly in animals.

Func­tions:

→ Lyso­somes help in diges­tion of for­eign sub­stances and worn-out cell organelles.

→ They pro­vide pro­tec­tion against bac­te­ria and virus.

→ They help to keep the cell clean.

→ Dur­ing the dis­tur­bance in cel­lu­lar metab­o­lism, for exam­ple when the cell gets dam­aged, lyso­somes may burst and the enzymes digest their own cell. There­fore, lyso­somes are also known as sui­cide bags of a cell

8. Vacuoles:

Vac­uoles are liquid/solid filled and mem­brane bound organelles.
In plant cells, vac­uoles are large and per­ma­nent. In ani­mal cells, vac­uoles are small In size and temporary.
In mature plant cell, It occu­pies 90% space of cell volume.
Due to its size, oth­er organelles, includ­ing nucle­us shift towards plas­ma membrane.

Func­tion:

→ They help to main­tain the osmot­ic pres­sure in a cell.

→ They pro­vide turgid­i­ty and rigid­i­ty to the plant cell.

9. Peroxiomes:

They are small and spher­i­cal organelles con­tain­ing pow­er­ful oxida­tive enzymes.
They are bound­ed by a sin­gle membrane.
They are found in kid­ney and liv­er cells.

Func­tion:

→ They are spe­cial­ized to car­ry out some oxida­tive reac­tions, such as detox­i­fi­ca­tion or removal of tox­ic sub­stances form cell.

Difference Between Animal Cell and Plant Cell:

S. No.Ani­mal cellPlant cell
1.Ani­mal cells are gen­er­al­ly small in size.Plant cells are larg­er than ani­mal cells.
2.Cell wall is absent.Plas­ma mem­brane of plant cell is sur­round­ed by a rigid cell wall of cellulose.
3Plas­tids are absent except in case of pro­to­zoan Euglena.Plas­tids are present.
4.Here vac­uoles are many, small and temporary.They have a per­ma­nent and large cen­tral sap vacuole.
5.They have cen­tro­some and centrioles.They lack cen­tro­some and centrioles.

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