Which of the following relationship involves only one organism

Analyze the question: We need the relationship in which only ONE organism is involved. The others involve two different organisms living together.

Explain the concepts:

• Saprophytism: An organism (e.g., fungi like mushrooms, bread mould, some bacteria) feeds on dead and decaying organic matter. It takes nutrients from dead material, not from another living partner, so only one living organism is involved.
• Commensalism: Two organisms live together. One benefits (e.g., an orchid growing on a tree) and the other is neither harmed nor helped.
• Parasitism: Two organisms: the parasite benefits and the host is harmed (e.g., tapeworm in humans, Plasmodium in humans).
• Symbiosis: A close association between two different organisms, where both usually benefit (e.g., nitrogen-fixing Rhizobium in legume root nodules).

Step-by-step solution:

1. Commensalism needs two organisms (a benefiting one and a host/neutral one), so it is not the answer.
2. Parasitism needs a parasite and a host, so it is not the answer.
3. Symbiosis needs two partners living together, so it is not the answer.
4. Saprophytism involves one living organism feeding on dead organic matter (not a living partner). This fits the question.

Answer: A. Saprophytism.

Exam tip: Remember that "relationship" words like commensalism, parasitism, and symbiosis all need a pair of living organisms. Saprophytes feed on dead matter, so there is no living partner. Think: Sapro = rotten/dead.