Mitochondria:

The mitochondria are filamentous or granular cytoplasmic organelles of all aerobic cells of higher animals and plants. They are also found in micro organisms including Algae, Protozoa and Fungi. They were first observed by Kolliker in 1850 as granular structures in the striated muscles. The name ‘mitochondria’ was given to them by Benda (1897-98). Various steps of glycolysis in mitochondria was discovered by two German biochemists Embden and Meyerhof. Embden got the Nobel Prize in 1922. Sir Hans Adolph Krebs, in 1937 found out various reactions of citric acid cycle. Kennedy and Lehninger (1948-50) showed that Citric acid cycle, oxidative phosphorylation and fatty acid oxidation took place inthe mitochondria. The number of mitochondria in a cell depends on the type and functional state of the cell. Certain cells contain large number of mitochondriae.g., eggs of sea urchin contain 140,000-150,000 mitochondria. Oocytes of  amphibians contain 300,000 mitochondria. Liver cells of rat contain only 500-1600 mitocho dria. Some algal cells may contain only one mitochondrion.The mitochondria may be filamentous or granular in shape. They vary in size from 0.5 µm to 2.0 µm. Due to their minute nature they can not be seen under light microscope. Each mitochondrion is bound by two highly specialized membranes.
The outer membrane is smooth. It is separated from the inner membrane by a 6-8 nm wide space. The inner membrane is highly convoluted, forming a series of inflodings known as cristae.
Thus mitochondria are double membrane envelopes. The inner membrane divides the mitochondrial space into two distinct chambers. The outer compartment is the peri-mitochondrial space. It is found between outer and inner membranes. The inner compartment is the matrix space. It is filled with a dense gel like substance called mitochondrial matrix. The matrix contains lipids, proteins and circular DNA molecules. The outer and inner membranes, intermembrane space and mitochondrial matrix contain several enzymes. Hence the mitochondria perform several important functions such as oxidation, dehydrogenation, oxidative phosphorylation and respiratory chain of the cell. Since mitochondaria play a key role in the oxidation of carbohydrates and fats, they are considered as the actual respiratory organs of the cells. During such biological oxidations large amount of energy is released. The energy is utilized by the mitochondria for synthesis of the energy rich compound known as adenosine tri phosphate or ATP. Due to this function, the mitochondria are also known as “power houses” of the cell. In animal cells mitochondria produce 95 % of ATP molecules.

Ribosomes:

The ribosomes are small dense, rounded and granular particles. They contain ribonucleoprotein. They occur either freely in the matrix of the mitochondria, chloroplast and cytoplasm or remain attached with the membrane of the endoplasmic reticulum and nucleus. The ribosomes were described by G.E.Palade in 1952. The name ‘ribosome’ was coined by R. B. Roberts in 1958. The ribosomes occur in both prokaryotic and eukaryotic cells. In the cells in which active protein synthesis takes place, the ribosomes remain attached with the membranes of the endoplasmic reticulum. The cells where such active synthesis happens are pancreatic cells, hepatic cells, osteoblasts, serous cells of submaxillary gland, chief cells of the glandular stomach, thyroid cells and mammary gland cells. The ribosomes are spheroid structures with a diameter of 150 to 250 Å. Each ribosome is composed of two subunits. One subunit is large in size and has a dome like shape. The other ribosomal subunit is smaller in size and it occurs above the larer subunit forming a cap-like structure. The ribosomes are chemically composed of RNA and proteins. The ribosomal RNA (rRNA) play a central role in the process of protein synthesis.The ribosomal proteins enhance the catalytic function of the rRNA. The functioning of rRNA is under genetic control.