Which of the following organisms cannot exist freely on its own?

Step 1: Analyze the question We need the organism that cannot live freely on its own, meaning it depends on a host. That is a parasite.

Step 2: Review each option

• A. Chlamydomonas is a unicellular green alga. It lives freely in freshwater ponds and photosynthesizes. It is free-living.
• B. Amoeba is a unicellular protozoan found in ponds and moist soil. It feeds on its own by phagocytosis. It is free-living. (Entamoeba histolytica is parasitic, but plain Amoeba is not.)
• C. Paramecium is a ciliate protozoan that lives in freshwater and feeds on bacteria. It is free-living.
• D. Plasmodium is the protozoan parasite that causes malaria. It lives inside human red blood cells and liver cells, and also in the female Anopheles mosquito. It cannot complete its life cycle without these hosts, so it cannot exist freely on its own.

Step 3: Conclusion Plasmodium is an obligate parasite, so it is the one that cannot exist freely.

Answer: D. Plasmodium

Exam tip: In Biology, "cannot exist freely" or "depends on a host" points to a parasite. Remember that Plasmodium needs two hosts: the Anopheles mosquito (vector) and humans. Chlamydomonas, Amoeba and Paramecium are the classic free-living microorganisms in WASSCE questions.