Write a brief account on the origin of mitochondria and chloroplast.

Mitochondria and chloroplasts, essential organelles in eukaryotic cells, likely originated from free-living bacteria through the endosymbiotic theory.

The endosymbiotic theory is the scientific explanation that proposes how certain organelles, specifically mitochondria and chloroplasts, originated within eukaryotic cells. It suggests that these organelles were once independent, free-living bacteria that were engulfed by a primitive eukaryotic cell. Instead of being digested, these bacteria formed a mutually beneficial relationship with the host cell, eventually becoming integrated into the cell as permanent organelles. This theory is supported by evidence such as the presence of their own DNA, similarities to bacterial ribosomes, and the double membrane structure of these organelles.

The concept of endosymbiotic theory was first introduced by Russian botanist Konstantin Mereschkowski in 1905, who proposed that chloroplasts originated from symbiotic relationships between eukaryotic cells and free-living cyanobacteria. This idea was further explored by Ivan Wallin in the 1920s, who suggested that mitochondria also came from bacteria. However, it was Lynn Margulis, American biologist who, in 1967, provided a comprehensive modern version of the endosymbiotic theory. Her research offered compelling evidence that both mitochondria and chloroplasts originated from such symbiotic relationships, bringing the theory to broader scientific attention and significantly advancing our understanding of cell evolution.

Origin of Mitochondria

Mitochondria are often referred to as the powerhouses of the cell because they are responsible for producing ATP, the energy currency of the cell, through a process known as oxidative phosphorylation. The endosymbiotic theory suggests that mitochondria originated from an ancient aerobic (oxygen-using) bacterium, probably an ancestor of today's α-proteobacteria. A long time ago, a simple eukaryotic cell, which did not yet have complex organelles but could engulf other cells or particles, took in one of these bacteria. Instead of digesting it, the cell and the bacterium formed a mutually beneficial relationship. 

The host cell provided protection and nutrients to the bacterium, while the bacterium supplied the host cell with ATP, generated more efficiently in the oxygen-rich environment of early Earth. Over millions of years, this partnership deepened. The bacterium transferred much of its genetic material to the host cell's nucleus in a process known as horizontal gene transfer, which reduced the bacterium's independence and resulted in its evolution into the mitochondrion.

Origin of Chloroplasts

Chloroplasts are the organelles responsible for photosynthesis in plants and algae. They convert sunlight into chemical energy, producing oxygen and glucose, which are vital for life on Earth. The endosymbiotic theory suggests that chloroplasts originated from a free-living photosynthetic bacterium, most likely a cyanobacterium. Cyanobacteria are a group of bacteria capable of photosynthesis, and they were among the first organisms to produce oxygen as a byproduct of this process, contributing to the oxygenation of Earth's atmosphere.

An early eukaryotic cell, possibly one that already contained mitochondria, engulfed a cyanobacterium. Like the precursor to mitochondria, this cyanobacterium was not digested but instead entered into a symbiotic relationship with its host. The cyanobacterium provided the host cell with the ability to photosynthesize, creating a significant survival advantage in environments with abundant sunlight. Over time, the cyanobacterium transferred many of its genes to the host cell's nucleus, reducing its autonomy and evolving into the chloroplast.

Evidence Supporting Endosymbiotic Theory

The endosymbiotic theory is supported by extensive evidence from various fields of biology, including genetics, biochemistry and cell biology.

01. Genetic Evidence:

One of the most compelling pieces of evidence for the endosymbiotic theory is the presence of circular DNA in mitochondria and chloroplasts, which closely resembles the DNA found in bacteria. Unlike the linear DNA found in the nucleus of eukaryotic cells, the DNA in these organelles is circular and contains genes that are strikingly similar to those of their bacterial ancestors. For example, mitochondrial DNA is most closely related to the DNA of α-proteobacteria, while chloroplast DNA is similar to that of cyanobacteria.

02. Ribosomal Similarities:

Mitochondria and chloroplasts have their own ribosomes, the molecular machines responsible for protein synthesis. These ribosomes are more similar in size and structure to bacterial ribosomes (70S) than to the ribosomes found in the cytoplasm of eukaryotic cells (80S). This similarity suggests a prokaryotic origin for these organelles.

03. Double Membrane Structure:

Both mitochondria and chloroplasts are surrounded by a double membrane. The inner membrane is believed to be derived from the original membrane of the engulfed bacterium, while the outer membrane likely comes from the host cell's membrane, acquired during the engulfment process. This double membrane structure is consistent with the theory that these organelles were once independent bacteria.

04. Independent Replication:

Mitochondria and chloroplasts replicate independently of the cell's nucleus, using a process that resembles binary fission, the method by which bacteria reproduce. This mode of replication supports the idea that these organelles retain some of the characteristics of their free-living bacterial ancestors.


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SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2



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