They r rod shaped structure that r enclosed within two membranes -outer membrane and inner membrane . the membranes r made up of  phospholipids and proteins . the space in between two membranes is called inter membrane space which has same composition as the cytoplasm of cell . however the protein content in this space differs from that in cytoplasm . 
1. outer membrane - the outer membrane is smooth unlike inner membrane and has almost same amounts of phospholipids as proteins . it has large no of special proteins called porins that allow molecules of 5000 daltons or less in weight  to pass through it .completely permeable to ATP and ADP molecules . 
2. inner membranes -  inner membrane is more complex in structure than outer membrane as it contains the complexes of of the electron transport chain and ATP syntetase complex . it is only permeable to o2 ,co2 and water . it is made up of a large no of proteins that play an important role in producing ATP  and also helps in transfer of metabolites across the membrane . the inner membrane has infoldings called the cristae that increase the surface area for the complexes and proyeins that aid in the production of ATP 
3. matrix - matrix is complex mixture of enzymes that r important for the synthesis of of ATP molecules . special mitochondrial ribosomes , tRNAs  and the mitochondrial DNA.  it has o2 , co2 and other recyclable intermediates 

main function is to produce energy for various parts of the body . it also helps in building of certain parts of  blood and hormones like estrogens and testosterone . 
                            hope it helps u . pls mark it as best ans if u like it

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A mitochondria contains outer and inner phospholipid bilayer and proteins.There are 5 distinct part of mitochondria they r as follows:1)the outer membrane,2)inner membrane,3)intermediate membrane,4)cristae space,5)matrix
outer membrane:this contain large no. of proteins called porins
intermediate membrane:contains only one protein i.e,cytochrome c.
inner membrane:it contains 151 polypeptides and perform different functions,it also contain 1/5 of the total protein present in mitochondria,also it has an unusual phospholipid called cardiolipin.
cristae space:it expands the area of mitochondria and allows production of ATP in large nos.,also these are studd with F1 particles called oxysomes.
Matrix:It contain 2/3 of the total protein of mitochondria,it helps in the production of ATP,it also has many enzymes,ribosomes,tRNA,and copies of mitochondrial DNA genomes.

Its function is to give energy to the cell by producing ATP,also mitochondria are often called "POWERHOUSE" of the cell.