Mitochondrion is bounded by two unit membranes separated by perimitochondrial space (60 � 80 �). The outer membrane is specially permeable because of presence of integral proteins called porins. The inner membrane is selective permeable. The inner membrane is folded or convoluted to form mitochondrial crests. In animals these are called cristae and in plants these folding are called tubuli or microvili.
The matrix facing face is called �M� face and face towards perimitochondrial space is called �C� face. The �M� face have some small stalked particles called oxysomes or F1 particle or elementory particle or Fernandez � Moran Particles. Each particle is made up of base, stalk and head and is about 10nm in length. Number of oxysomes varies to 104 to 105 per mitochondrion and chemically they are made of phospholipid core and protein cortex. Oxysomes have ATPase enzyme molecule (Packer, 1967) and therefore, responsible for ATP synthesis. These elementary particles are also called F0 � F1 particles.

In the matrix 2�6 copies of naked, double stranded DNA (circular) and ribosome of 70 S type are present. It is rich in G-C ratio. Basic histone proteins are absent in mitochondrial DNA. The synthesis of ATP in mitochondria is called oxidative phosphorylation, which is O2 dependent and light independent. Cristae control dark respiration. F0 particles synthesize all the enzymes required to operate Kreb�s cycle. Inner membrane contains cytochrome.

(i) Mitochondria are called power house or storage batteries or ATP mills as these are sites of ATP formation.
(ii) Intermediate products of cell respiration are used in the formation of steroids, cytochromes, chlorophyll, etc.
(iii) These are also seat of some amino acid biosynthesis.
(iv) Mitochondria also regulate the calcium ion concentration inside the cell.
(v) Site of Krebs cycle and electron transport system.
(vi) Site of thermiogenesis.
(vii) Yolk nucleus (a mitochondrial cloud and golgi bodies) controls vitellogenesis.
(viii) Mitochondria of spermatid form nebenkern (middle piece) of sperm during spermiogenesis.
(ix) It is capable of producing its own DNA.
(x) Mitochondria release energy during respiration.
(xi) Mitochondria contain electron transport system.

# Plastids are semi-autonomous organelles having DNA, RNA, Ribosomes and double membrane envelope which store or synthesize various types of organic compounds as ATP and NADPH + H+ etc. These are largest cell organelles in plant cell.

# History
(i) Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).
(ii) A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein.
(iii) Park and Biggins (1964) gave the concept of quantasomes.
(iv) The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall.
(v) The term thylakoid was given by Menke (1962).
(vi) Fine structure was given by Mayer.

# Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts.

-Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and occur in the cells not exposed to sunlight e.g., seeds underground stems, roots, tubers, rhizomes etc. These are of three types.
(i) Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains.
(ii) Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g. castor endosperm, tube rose, etc.
(iii) Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

-Chromoplasts : Coloured plastids other than green are kown as chromoplasts. These are present in petals and fruits, imparting different colours (red, yellow, orange etc) for attracting insects and animals. These also carry on photosynthesis.
These may arise from the chloroplasts due to replacement of chlorophyll by other pigments e.g. tomato and chillies or from leucoplasts by the development of pigments.
All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin express different colours on different pH value. These are variously coloured e.g. in flowers. They give colour to petals and help in pollination. They are water soluble. They are found in cell sap.
Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into chromatophores.

- Chloroplast : Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments.

It is double membrane structure. Both membranes are smooth. The inner membrane is less permeable than outer but rich in proteins especially carrier proteins. Each membrane is 90 � 100 � thick. The inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into two parts.

1. Grana : Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel membranous sheets, called lamellae, which form a number of oval � shaped closed sacs, called thylakoids. Thylakoids are structural and functional elements of chloroplasts. These thylakoids contain all the requirements of light reactions e.g., pigments like chlorophyll, carotenoids, plastoquinone, plastocyanin, etc. that are involved in photosynthesis. Each thylakoid has an intrathylakoid space, called loculus (size 10-30�) bounded by a unit membrane. Along the inner side of thylakoid membrane, there are number of small rounded para-crystalline bodies, called quantasomes (a quantasome is the photosynthetic unit) which can trap a mole of quantum of light and can bring about photosynthetic act. Each quantasome contains about 230 chlorophyll molecules and 50 carotenoid molecules.
In eukaryotic plant cells, a number of thylakoids are superimposed like a pile of coins to form a granum. The number of thylakoids in a granum ranges from 10-100 (average number is 20-50). The number of grana per chloroplast also varies widely e.g., one granum per chloroplast in Euglena while there are 40-60 grana per chloroplast in spinach. The size of each granum varies from 0.2 � 0.6 in diameter. But in blue-green algae, the thylakoids are not organised to form granum.
Adjacent grana are interconnected by branched tubules, called stromal lamellae or Fret-channel or Fret membranes.

2. Stroma : It is transparent, proteinaceous and watery substance. Dark reaction of photosynthesis occurs in this portion. Stroma is almost filled with �Rubisco� (about 15% of total enzyme, protein) enzyme is accepted by this enzyme. CO2 assimilation results in carbohydrate formation. It has 20 � 60 copies of naked circular double stranded DNA. Each DNA copy is 40 in length, which can code for 125 amino acids. All plastids of a cell called as �Plastidome� (Dangeared 1920) in stroma. Amount of DNA per chloroplast is 10�15 g. Chloroplast genome has 145 kilobase pairs. It shows semiautonomous nature and ribosomes are of 70 S type.