Endosymbiosis is a relationship where one organism lives inside the other and both are benefited. It is thought that ancestral eukaryotic cells consumed aerobic bacteria and photosynthetic bacteria leading them to evolve into mitochondria and chloroplast respectively. Endosymbiotic theory stats that, the modern eukaryotic cells (mitochondria ) evolved in steps through inter-cooperation into cells from a nuclear line of descendants of chemoorganotrophic and phototrophic symbionts. It states that Mitochondria and Chloroplast were free-living bacteria that established residency in the primitive eukaryotic cells, eventually yielding the modern eukaryotic cell. Symbiosis is a specific type of relationship in which organisms from two different species live in a close, dependent relationship take advantages from each other.
On the basis of their relative autonomy and morphological resemblance to bacteria, it was suggested long ago that mitochondria and chloroplast were descendants of ancient prokaryotic organism. The theory postulate that an aerobic bacteria established residency within the cytoplasm of a primitive eukaryotic cell. This bacterium would represent the precursor of the present mitochondrion. In similar fashion, the endosymbiotic uptake of an oxygenic phototrophic prokaryote would have made the primitive eukaryotic photosynthesis. This phototrophic bacterium would then be considered the precursor of the present chloroplast.
Several pieces of evidence support this hypothesis:
i) Presence of DNA:
Mitochondria and Chloroplast DNA exists in closed circular form as it does in a prokaryotic cell. This DNA of the chloroplast is very similar to photosynthetic blue-green bacteria, while the mitochondrion DNA is very similar to the aerobic bacteria. Both organelles lack histones and introns like bacteria.
ii) Size of Ribosomes:
Ribosome exists either in a larger form (the 80s), typical of the cytoplasm of eukaryotic cells or smaller form (70s), unique to prokaryotes. Mitochondria and Chloroplast’s ribosome are the 70s in size, the same as those of prokaryotes.
iii) Inhibition by antibiotics:
Several antibiotics kill or inhibit bacteria by disrupting their 70s ribosomal function. The same antibiotics also inhibit ribosomal function in mitochondria and chloroplast. Like bacteria, mitochondria and chloroplast are sensitive to chloramphenicol, streptomycin etc.
iv) Evolutionary relationship:
Phylogenetic analyses using ribosomal RNA sequencing method, convincingly suggested that mitochondria and chloroplast are evolutionarily related to bacteria. The sequence comparison of mitochondria and chloroplast show that mitochondria came from bacterial line related to alpha-proteobacteria and chloroplast originated from cyanobacteria.
v) Same size:
Mitochondria and chloroplast are same in size as bacteria. Size of bacteria is commonly 0.1-10 micrometer, while the size of mitochondria and chloroplast is 0.5-10 micrometer and 1- 10 micrometer respectively.
Mitochondria and chloroplast are surrounded by two or more membranes like bacteria. Mitochondria have a double membrane that is a phospholipid bilayer. Chloroplast has three membranes outer membrane inner membrane, and thylakoid membrane. Gram-positive bacteria have a plasma membrane and a cell wall although gram-negative bacteria have an additional outer membrane.
vii) Enzyme secretion:
Mitochondria and chloroplast secrete several enzymes like bacteria. For example, mitochondria secrete monoamine oxidase, kynurenine hydroxylase, c-reductase fatty acid Co-A ligase, ATP synthase etc. Chloroplast enzymes include ATP synthase, NADP-malate dehydrogenase fructose-1, 6-bisphosphatase, phosphoribulokinase, glucose-6-phosphate dehydrogenase etc.
viii) Replication and protein synthesis:
Like bacteria, mitochondria and chloroplasts can replicate their genome and translate it to protein. Protein synthesis in mitochondria and chloroplast takes place by N-formyl methionyl tRNA that resembles with bacterial protein synthesis.
ix) Bacterial binary fission:
Mitochondria and chloroplast are divided by binary fission like bacteria. Like bacterial binary fission, mitochondria and chloroplast also replicate their genome and divide into two new organelles.
x) Electron transport chain:
Like bacterial electron transport chains occurred in the bacterial plasma membrane, mitochondria and chloroplast have also electron transport chain occurring in inner mitochondrial membrane and thylakoid membrane of chloroplast respectively.