Prokaryotes can form relationships with other organisms. It can be mutualistic (both benefit), commensal (one benefits), or parasitic.

Prokaryotes can form close relationships with other organisms. Usually, bacteria live on or in other organisms – their host. These symbiotic relationships can be classified based on whether the host is helped, harmed, or not affected by the microorganisms. Such relationships are respectively classified as mutualistic, parasitic, and commensal.

In mutualistic interactions, both species benefit from the interaction. For example, lichens have a mutualistic relationship with algae. As these symbionts grow together, the glucose produced by the algae provides nourishment for both organisms. In contrast, the physical structure of the lichen protects the algae and makes certain nutrients in the atmosphere more available to the algae.

Mutualism. Lichen is a fungus that has symbiotic photosynthetic algae living inside its cells.

The second type of symbiotic relationship is called commensalism, one organism benefits while the other organism neither benefits nor suffers from the interaction—for example, the birds nesting in trees. The presence of the nest does not harm the tree, but the birds benefit significantly by providing shelter to its egg.

Commensalism. This is an example of a commensal relationship, in which the bird benefits, while the tree neither benefits nor is harmed. 

Parasitism is a type of symbiotic relationship between organisms in which the parasite benefits while it harms the host. A parasite is an organism that lives in or on another living organism (the host) and gets nutrients from it. For example, a tapeworm is a parasite that causes diseases in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed. The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is eating. An obligate parasite is a parasite that cannot reproduce or grow without a host.

Parasitism. This diagram shows the life cycle of a tapeworm (Taenia solium), a human worm parasite. 

Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Official Guide B/B Section Passage 3 Question 12

Practice Exam 1 B/B Section Question 44

Key Points

• The symbiotic relationships can be classified based on whether the host is helped (mutualistic), harmed (parasitic) or not affected by the microorganisms (commensal).

• In mutualism, the host and the other organism benefit from their interaction.

• In commensalism, one organism benefits while the host neither benefits nor suffers from the interaction.

• In parasitism, the parasite benefits while it harms the host; a parasite is an organism that lives in or on another living organism (the host) and gets nutrients from it.

Key Terms

symbionts: either of two organisms that live in symbiosis with one another

prokaryotes: an organism whose cell (or cells) are characterized by the absence of a nucleus or any other membrane-bound organelles

mutualistic: describes the interaction between two or more species where each species has a net benefit

symbiotic: close and long-term biological interaction between two different biological organisms

Lichens: is a symbiotic partnership of two separate organisms, a fungus and an algae

algae: a group of photosynthetic eukaryotic organisms

commensalism: an interaction when members of one species gain benefits while those of the other species neither benefit nor are harmed

parasitism: a type of symbiotic relationship between organisms in which the parasite benefits while it hurts the host.

obligate parasite: a parasite that cannot reproduce and grow outside of its host organism