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Organelles

Cell is the basic, structural and functional unit of life. Cells are often called the building blocks of life. There are two types of cell - prokaryotes and eukaryptes. Prokaryotes are single-celled organisms, they lack membrane bound cellular organelles like nucleus, mitochondria. Eukaryotic cells have compartmentalization in their cells. They have membrane bound compartments, which bring about specific metabolic processes in the cell.

There are many different types of organelles in a cell. Organelles like the nucleus and golgi apparatus are usually single while others like mitochondria, chloroplasts, peroxisomes and lysosomes are in large numbers about hundreds to thousands. The cellular organelles are suspended and surrounded in a gelatinous fluid cytosol that fills the cell.

 

Organelle Definition

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Organelles are specialized structures of the cell which perform specific function and they are usually enclosed separately within its own lipid bilayer membrane. Organelles are cell parts that are adapted and specialized to perform various vital activities of the cell like the organs of the human body. Both eukaryotic and prokayotic cells have cellular organelles, but the organelles of the prokaryotic cells are simpler and not membrane bound. 

Cell Organelles

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Organelles are also known as cell compartments. Eukarytoic cells have membrane bound compartments in which specific metabolic activities takes place. The organelles of the prokaryotes are not membrnae bound and are simpler structures.

Cell Orgenelles 

Eukaryotic Organelles - The cells of eukarytotic organelles are structurally complex and have organized interior compartments that are enclosed by lipid membranes. 

The list of organelles are as follows: 
Endoplasmic reticulum (ER) 
They are found in all eukarytoic cells and are structurally continuous with the nucleus of the cell. The ER is a complex network of tubes. The lumen is filled with fluid. There are two types of endoplasmic reticulum - smooth ER and rough ER. 
  • Smooth Endoplasmic reticulum - They are tubes with a smooth surface as they lack ribosomes. The smooth ER helps in calcium sequestration and release and secretion of lipids.  
  • Rough Endoplasmic reticulum - They are tubes with rough surface as the ribosomes are attached to its surface. 
The main function of the endoplasmic reticulum translation and folding of new proteins which takes place in the rough ER and the expression of lipids that happens in the smooth ER. 

Endoplasmic Reticulum

Golgi Apparatus 
  • The Golgi bodies are elongated, flattened structures called cisternae and they are stacked parallelly to one another. 
  • They are bound by a single membrane and are found close to the nucleus. 
  • The vesicle formed from the ER fuses with the membrane of the Golgi apparatus. 
  • The cavity of the Golgi body is has vessel proteins that are modified for export. 
The main function of the Golgi apparatus is sorting, packaging, processing and modification of proteins. It also forms lysosomes and peroxisomes. 

Pictorial Illustration of Golgi Complex and Lysosome

Lysosomes
  • Lysosomes  are single membrane bound structures. 
  • They are tiny sac like structures and are present all over the cytoplasm. The main function is digestion. They contain digestive enzymes. Lysosomes contain digestive enzymes that are acid hydrolases.
  • They are responsible for the degrading of proteins and worn out membranes in the cell and also help degradation of materials that are ingested by the cell. 
  • Lysosomes that are present in the white blood cells are capable of digesting invading microorganisms like the bacteria and viruses. 
  • During the period of starvation the lysosomes digest proteins, fats and glycogen in the cytoplasm.
  • They are capable of digesting the entire damaged cell containing them, hence, the lysosomes are known as "suicide bags" of the cell.
Peroxisomes
  • Peroxisomes are found in liver and kidney cells. 
  • Peroxisomes have enzymes that are responsible to get rid of the toxic peroxides from the cell. 
Peroxisomes

Ribosomes
  • Ribosomes are the site for protein synthesis of the cell.
  • It is composed of two subunits, a small subunit and a large subunit. 
  • The ribosomes subunits acts as an assembly line where the RNA from the nucleus is used to synthesize proteins from amino acids. 
  • Ribosomes are found freely floating or bound to a membrane or attached to mRNA molecules in a polysome. 
Ribosomes

Centrosome
  • Centrosomes are the cytoskeleton organizers. 
  • The centrosomes produces the microtubules of a cell, they are the key components of the cytoskeleton. 
  • Centrosomes are composed of two centrioles, they separate during cell division and they help in the formation of mitotic spindle. 
Mitochondria
  • Mitochondria are of various shapes and sizes and are numerous in the cytoplasm of all eukaryotic cells.
  •  Mitochondria are double membrane bound. The inner membrane is folded into numerous cristae.
  • Mitochondria are the power generators of the cell. 
  • They are capable of self-replication as they possess their own DNA.  
  • The main function of mitochondria is to produce energy through metabolism. 
  • In the mitochondria sugar is finally burnt during cellular respiration. 
  • The energy released in this process is stored as high-energy chemicals called adenosine triphosphate (ATP).
  • The energy is used by the body cells for synthesis of new chemical compounds.
Mitochondria

Plastids 
Plastids are cellular organelles found only in the plant cell. Plastids are of three types - chloroplasts, chromoplasts and leucoplasts.
  • Chloroplasts are elongated disc shaped organelles which contains chlorophyll.  Chlorophyll is present in green plants which helps them make food by the process of photosynthesis, which uses energy from the sunlight is converted into chemical energy.  
  • Chromoplasts are plastids which are found in fruits and are yellow, orange and red in color. 
  • Lecuoplasts are colorless plastids. They found in roots, seeds and underground stems. 
Types of Plastids

The function of chloroplast of trap solar energy for the process of photosynthesis. Chromoplasts gives color to flowers and fruits which helps in pollination by attracting pollinating agents like insects and birds. Leucoplasts acts as storage for food in the form of carbohydrates, fats and proteins. 

Nucleus
  • Nucleus is the large st organelles in a cell. 
  • Nucleus has different parts like the nuclear envelope, nucleolus and the chromosomes. 
  • The nuclear envelope is a double membrane structure, it encloses the nucleus and segregates the chromatin from the cytoplasm. 
  • The nucleus contains chromatin. It contains the genetic material of the cell which are organized as multiple long linear DNA molecules that form chromosomes. 
  • Nucleolus produce ribosomes that function in the expression of gene code into proteins. 
Nucleus

Prokaryotic Organelles - Prokaryotic cells are not complex structurally as the eukaryotes. The prokaryotes do not have compartmentalized organelles. The DNA of the prokarytoic cells are free floating in the cytosol and is surrounded by a cell membrane. 

Prokaryotic Cell
  • Nucleoid - The chromosomal DNA of the nuceloid is wrapped in binding poteins in the nucleoid. Replication of DNA and transcription of RNA occurs at the same time in the nucleoid. 
  • Pilli also known as fimbriar are thin structure that protrude from the surface. They are made up of a single protein called pilin. The function of pili is transfer of DNA, binding to surface and motility. 
  • Flagellum is a protein filament and its main function is movement in external medium. 
  • Plasmid are circular DNA but not the genomic DNA. The main function is DNA exchange.  
  • Ribosome (70S) is the RNA proteins. Its main function is translation of RNA into proteins. 
  • Mesosomes are small irregular shaped organelles contains ribosomes. It functions like the golgi bodies, centrioles etc.

Difference Between Animal Cell and Plant cell

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The main differences between a typical animal cell and a typical plant cell are as follows: 



                           Animal cell                                           Plant Cell  
 Animal cells are usually smaller is size.   Plant cells are usually larger in size. 
 Cell wall is completely absent.   Presence of cell wall is a characteristic feature of plant cell.
 Cellulose in any form is not present.    Cell wall is made up of cellulose. 
 Cytoplasm of animal cells is dense, more granular and
 it occupies most of the space in the cell. 
 In a plant cell, cytoplasm is pushed to the periphery of the cell.
The cytoplasm forms a thin lining against the cell wall. 
 Vacuoles are absent usually. If present, they are small organelles,
 they are temporary and they serve as organelles for excretion or secretion. 
 Vacuoles are prominent and large organelles in the plant cell.
One or more vacuoles may be present.
The central space in the cell may be occupied by a large single vacuole.
 Plastids are absent.  Plastids are present. They may of three types chromoplasts, chloroplasts and leucoplasts. 
 Centrosome is present.   Centrosome is absent in plant cells. Instead of centrosome,
there are two small clear areas called polar caps are present.
 Golgi complex is prominent and highly complex,
 it is present near the nucleus of the cell. 
 Golgi apparatus is present in form subunits called dictyosomes. 


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