Major organelles of the cell

Major organelles of the cell

 Nucleus, Ribosomes, Golgi complex, and chloroplast their discovery, structure, and functions

In this article, we shall discuss some major organelles ( Nucleus, Ribosomes, Golgi complex, and chloroplast) of the cell.

                                                  1.   Nucleus

                                                     Discovery of Nucleus

It is a double membrane organelle of the cell. Nucleus was described by Franz Baure in 1804. A comprehension concept was presented by a Scottish botanist Robert Brown in 1831. Robert Brown was examining orchids under a microscope when he observed an opaque area which he called an areola or nucleus, in the cell of the flower’s outer layer.


  The nucleus is the brain of the eukaryotic cell, i.e. all the functions of the cell is regulated by the nucleus. It is absent in prokaryotic cells (bacteria and cyanobacteria). Like mitochondria, it has two membranes (outer and inner). The membrane of the nucleus is called the nuclear membrane or nuclear envelope. Pores are present in nuclear envelop through which substances can travel in or out of the nucleus.

The whole material present inside the nucleus is called nucleoplasm. The fluid present in nucleoplasm is known as “nucleosol”. The most prominent part of the nucleus, i.e. Nucleolus also exists in the nucleoplasm. A nucleolus is granular material and non-membranous. Some cells contain more than one nucleolus (plural nuclei).

The nucleolus of the cell consists of rRNA, DNA, and ribosomal protein. A eukaryotic cell lacks nucleolus and does not have the ability to produce protein. As the two ribosomal subunits leave the nucleus through pores, they associate to form a functional ribosome. The main components of the nucleolus are the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC).

Transcription of the ribosomal DNA occurs in FC. The DFC contains fibrillarin protein which plays an important role in ribosomal RNA processing. Inside nucleoplasm genetic material also exists. During the non-dividing phase of the cell, it appears as a thread-like structure called chromatin. Genetic material appears in visible form in the dividing phase of the cell.

Nuclear pores are present in variations in different cells. In undifferentiated cells (egg cells) they are present in abundance. It is because more transport becomes resulting the production of more protein, so that the cell may grow properly having a large number of pores. In specialized or differentiated cells (red blood cells) pores are present in smaller numbers.


Because the nucleus of a cell contains genetic code, which determines the amino acid sequence of proteins critical for day-to-day function, it serves as the information center of the cell. Information in DNA is transcribed, into a range of mRNA molecules, each of which encodes the information for one protein. The mRNAs after activation exit the nucleus and come into the cytoplasm, where they are translated, serving as templates for the synthesis of specific proteins.

                                     2.   Ribosomes

                                       Discovery of ribosomes

In 1955, George E. Palade discovered ribosomes and described them as small particles inside the cytoplasm that are associated with the endoplasmic reticulum membrane. They are also known as Palade’s organelle.


Ribosomes are non-membranous organelles that are assembled in the nucleolus. They are also called protein factories because they play a prominent role in protein synthesis. Both types of cells i.e. prokaryotic as well as eukaryotic have ribosomes. Their size is larger in eukaryotic cells as compared to prokaryotic cells. Two subunits (larger and smaller) associate to form ribosomes.

In the eukaryotic cell, the sedimentation rate of larger and smaller subunits are 60S and 40S respectively, while in prokaryotic they are 50S and 30S. Ribosomes can be found floating on the cytoplasm and are attached to the rough endoplasmic reticulum (RER).


The most prominent function of ribosomes is protein synthesis. The mRNA moves to the cytoplasm through nuclear pores. The smaller subunit of the ribosome recognizes mRNA and attaches to it, then the larger subunit attaches with mRNA and stars protein synthesis, as mRNA contains genetic code.

  Polysome: Some cells require a large amount of energy, in such a condition many ribosomes collectively attached to a single mRNA and start protein synthesis. This combination of ribosomes is called a polysome.

                                                          3. Golgi Complex

                                                           Discovery of Golgi Complex

This organelle was discovered by an Italian physician Camilo Golgi (1898) while investigating the nervous system. The first name given to this organelle was “internal reticular apparatus” by him.


It is a complete set of interconnected flattened stacks with associated vesicles. Golgi complex or Golgi apparatus are membrane-bounded (single cell) organelle. Flattened stacks are called “cisternae”. Each cisterna carries an enzyme meant to modify protein that travels through it. Golgi complex has two faces (1) forming face and (2) maturating face.

  1. Forming Face: It is the outer face and has a convex shape. Forming the face is associated with the rough endoplasmic reticulum (RER).
  2. Maturating Face: It is the inner face and has a concave shape. The maturating face is associated with the plasma membrane.


The major function of the Golgi complex is secretion. In many cells, synthetic products from rough endoplasmic reticulum are transferred to the Golgi complex, from where they pass from the cell through the plasma membrane by pinocytosis. In endocrine cells, it assists in hormone secretion. Vacuoles and vesicles which are the main components of the Golgi apparatus become with protein-lipoid material storage.

These stored substances help in secretary action. In lactating mammary gland of mice,  produced protein droplets that are related to the Golgi apparatus. These droplets generally open onto the cell surface by fusion of their enclosing membrane with the plasma membrane.

                                                      3.  Chloroplast

                                                      Discovery Of Chloroplast

The origin of chloroplast was first described by the Russian biologist KonstantinMereschkowski in 1905. These are absent in animal cells and exist in plants, algae, and amoeboid pauinella chromatophora.

                                                    Structure of Chloroplast

It has an outer and inner membrane with an intermediate empty space in between. Inside chloroplast stacks of thylakoids, called grana, as well as dense fluid, the stroma is present. Thylakoids contain green pigment, the chlorophyll that is for photosynthesis in plants.

Certain plants contain an additional set of membranous tubules called peripheral reticulum which originates from the inner membrane of the chloroplast. Small vesicles bud off from the inner membrane of the chloroplast and combine to form the tubules of the peripheral reticulum.


  • Chloroplast are the sites of photosynthesis in plants, which consists of light-dependent and light-independent reactions to harness the solar energy and convert it into chemical energy
  • Chloroplasts are the source of the cellular sensor.
  • The chloroplast, nucleus, cell membrane, and ER are vital organelles for pathogen defense.
  • Produces ATP, by the process of photosynthesis.
  • Produces NADPH along molecular oxygen i.e. photolysis of water.

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