Ribosomes were discovered by Palade in 1955.

Ribosomes are small, dense organelles, about 20 nm in diameter, present in great numbers in the cell. Most are attached to the surface of RER but they can occur free in the cytoplasm.

Ribosomes are made from a combination of rRNA and protein. Eukaryotic ribosomes are composed of an equal quantity of RNA and protein. Therefore they are also called as ribonucleoproteins.

Ribosomes are involved in protein synthesis. They assemble amino acids in the right order to produce new proteins. The ribosome uses the code on messenger RNA (mRNA) to put amino acid together in chains to form specific proteins.

Generally, proteins that are to be used inside the cell are made on free ribosomes, while those that are to be secreted out of the cell are made on ribosomes that are bound to ER membranes.

New ribosomes are formed in the nucleolus.

Eukaryotic ribosome is 80S (60S + 40S). Mg controls this attachment.

A group of ribosomes attached to the same mRNA are called polysomes.

Nucleolus is a factory of ribosomes while ribosomes are the factory of protein synthesis.


The Golgi apparatus/Golgi complex was discovered by Comillo Golgi in 1898. It is found in eukaryotic cells.

Golgi apparatus consists of stacks of flattened, membrane bound sacs or flattened cavities or vesicles called Cisternae (5 – 8).

The whole organelle is a shifting, flexible structure; vesicles are constantly being added at one side and lost from the other. Generally, vesicles fuse with the forming face (the one nearest to the nucleus) and leave from the maturing face (the one nearest to the cell surface membrane).

Golgi complex has proteins, carbohydrates, glycoproteins and some enzymes. The Golgi complex appears to be involved with the synthesis and modification of proteins, lipids and carbohydrates.

Major functions of Golgi complex are formation of conjugated molecules and Secretions.

Proteins made on the ribosomes attached to ER are packaged into the vesicles by the ER. Some of the vesicles join with the Golgi complex, and the proteins they contain are modified before they are secreted out of the cell.

Golgi apparatus + Golgi vesicles = Golgi complex

Secretions are products formed in the cells on ribosomes and then pass to the outside through endoplasmic reticulum and Golgi apparatus.

In plants Golgi apparatus is involved in the synthesis of cell wall.


Mitochondria are important organelles of eukaryotic cells. They manufacture and supply energy to the cell. Therefore they are also called powerhouse of the cell.

Mitochondria are particularly abundant in metabolically active cells, tissues such as muscle and tissues involved in active transport.

The size (0.5-1.5 µn wide) and number of mitochondria varies and depend upon the physiological activity of the cell.

Under compound microscope mitochondria may be rod shaped, vesicles or filaments. In Electron Microscope, Mitochondria has two membranes, outer smooth and inner with cristae.

The inner surface of cristae in the mitochondrial matrix has small knob like structures called F1 particles. These are involved in ATP formation.

Mitochondrial matrix contains enzymes, coenzymes, and organic and inorganic salts.

Mitochondria also contain DNA and ribosomes.

Mitochondrial matrix helps in metabolic processes like Krebs’s cycle, aerobic respiration and fatty acid metabolism etc. Their main function is to make ATP via the process of aerobic respiration. ATP diffuses into the cell and provides instant chemical energy.

Mitochondria have a double membrane; the outer membrane is smooth while the inner one is folded. This arrangement gives as large internal surface area on which the complex reactions of aerobic respiration can take place.

Mitochondrion is a self-replicating organelle.


Centrioles are present in animal cells, some microorganisms and lower plants. They are absent in higher plants.

Centrioles are short bundles of filaments, set at right angles to each other. They are found in a clear area of cytoplasm known as the centrosome.

In a cross section, each centriole consists of a cylindrical array of 9 microtubules. Each microtubule has 3 tubules. The tubules are composed of special protein called Tubulin. The chemical composition and structure is similar to that of cilia and flagella.

Their function is the formation of spindle (that guides the chromosomes during cell division). In addition to spindle formation, the centrioles act as the center of formation for the whole cytoskeleton and they are known as microtubule organizing centers.