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Symbiosis is defined as the co-evolved intimate relationship between members of different species. It comes in many flavours, including the one-sided and abusive parasitic relationship and the more reasonable mutualism where both parties benefit from the relationship. A third type, commensalism, is a relationship where one species benefits from the other one-sidedly, but without harming it.


Parasites are highly specialised organisms that derive food from their host, at its expense. Examples include the tapeworm (Taenia) as well as lice (Pediculus), both body and hair lice.

Tapeworms reside in the small intestine where they can conveniently tap into the host's nutrients, and so interfere with the normal absorption of the host's nutrients into its bloodstream, thereby depriving the host to potentially dangerous levels of malnutrition and other side effects such as anaemia and fatigue.

Lice live on the body, such as pubic lice or on eyelashes, or the scalp in the case of head lice, and feed on the host's blood. Sensitivity to lice saliva causes itchiness, and some lice can be vectors (carriers) of dangerous infectious agents such as Epidemic typhus and Trench fever.

Tapeworms such as Taenia solium, the pork tapeworm, can be more than a couple meters long, and has a specialised head that helps it attach to the small intestine of the host. It is hermaphroditic as its segments each have full reproductive systems. These segments of its body are called proglottids.


Malaria is transmitted via a vector which carries it without being affected, before passing it onto the final host of the parasite. In this case it is carried by the mosquito which transfers it via its bite in saliva. As such, the mosquito is the secondary host while the human is the primary host. The primary host is where the parasite actually reaches maturity and reproduces.

Symptoms upon infection include fever, headache, vomiting and fatigue.

The reproductive cells of the parasite (gametocytes) fuse inside the mosquito which then delivers the sporozoites into the host via the bite. The Plasmodium falciparum sporozoites get taken up by the lymphatic system in a human, and later they pass through the liver where they asexually reproduce, and then travel to red blood cells before releasing their gametocytes again.

Their reproduction in the liver doesn't bring any symptoms, but once they invade red blood cells and destroy them to invade more red blood cells, fever occurs in waves as new parasites move from the liver cells into red blood cells.

Red blood cells can become sticky due to so-called adhesion knobs on their surface. The parasite can easily hide inside the red blood cells to evade the host's immune system, as well as in the liver.


Green plants evolved their ability to photosynthesise by endosymbiosis with cyanoabcteria. This means plant cells joined up with photosynthesising bacterial cells. These became today's chloroplasts which contain the photosynthesising chemicals in green plants.

This mutually beneficial arrangement can be seen in parallel with mitochondria which have now become eukaryotic cells' powerhouses. This interdependent relationship sees both species relying on each other and benefiting in different ways. At the point in the very distant past when this relationship became established, early eukaryotic cells were much larger and more flexible than their prokaryotic counterparts, so engaged in engulfing other cells.

As it became more efficient to not destroy these cells for food, but rather allow them to reside within the host cell and produce lots of ATP the host cell could also use, the mutualistic relationship developed.

Evidence for this model includes the mitochondrial ribosomes being the bacterial type (70S) rather than the 80S type found elsewhere in the cell. Moreover, the DNA in mitochondria is coding and lacks the vast number of repeats typical of the genome of the host cell.

mDNA (mitochondrial DNA) has smaller genes without introns, while the chromosomes it forms are circular - the same as bacteria.

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