Which of the following organisms is an example of a saprotroph that obtains nutrition from dead and decaying organic matter?

Introduction / Context:
Living organisms can be classified by how they obtain food and energy. Some are autotrophs, like green plants, which make their own food. Others are heterotrophs, which depend on other organisms. Within heterotrophs, saprotrophs are a special group that feed on dead and decaying organic matter. This question asks you to identify which organism from the list is saprotrophic. Recognising this mode of nutrition is important in understanding nutrient cycling and decomposition in ecosystems.

Given Data / Assumptions:
– The concept tested is saprotrophic nutrition, where organisms digest dead organic matter outside their bodies and then absorb the nutrients.
– Options include Mushroom, Man, Pigeon, and Algae.
– We assume basic school level knowledge that fungi are saprotrophs and that most animals are not saprotrophs.
– The task is to pick the organism that best fits the saprotrophic lifestyle in common examples.

Concept / Approach:
Saprotrophs (also called saprophytes in older texts) secrete digestive enzymes onto dead plant or animal material, break it down externally, and then absorb the simpler substances. Most fungi, including many mushrooms, follow this mode of nutrition. They grow on rotting wood, leaf litter, or dead organic matter and play a major role in decomposition. Man and pigeon are both animals that ingest food and digest it internally; they are heterotrophs but not saprotrophs. Algae are typically autotrophs that perform photosynthesis and do not mainly live on dead organic matter in the saprotrophic sense. Therefore, mushroom, which is a fungal fruiting body, is the correct example of a saprotroph among the options.

Step-by-Step Solution:
Step 1: Recall that saprotrophic organisms feed on dead and decaying organic matter using external digestion. Step 2: Identify fungi, especially mushrooms growing on rotting logs or soil rich in decaying material, as classic examples of saprotrophs. Step 3: Recognise that man and pigeon are animals that take in food and digest it inside the body, which is a different mode of nutrition. Step 4: Remember that most algae are photosynthetic autotrophs, using sunlight to make food rather than decomposing dead matter. Step 5: Conclude that mushroom is the correct saprotrophic organism in this list.

Verification / Alternative check:
School biology textbooks on nutrition in plants and animals usually mention that fungi, including mushrooms and bread mould, are saprotrophs. They are shown growing on stale bread, fallen leaves, or dead wood, and the text explains how they release enzymes to digest complex organic matter outside their bodies. The same chapters describe humans and birds as organisms that take in prepared food and digest it internally, and algae as photosynthetic producers. This consistent description across sources verifies that mushrooms exemplify saprotrophic nutrition.

Why Other Options Are Wrong:
Man: Humans are heterotrophs that ingest food and digest it internally in the digestive system; they are not saprotrophs because they do not feed on dead organic matter using external digestion in the ecological sense.
Pigeon: Pigeons are birds that feed on grains and other food items and digest them internally; they are not decomposers and therefore are not saprotrophs.
Algae: Most algae are autotrophic organisms that perform photosynthesis in aquatic environments; they are producers rather than decomposers, so this option does not fit the saprotroph definition.

Common Pitfalls:
A common mistake is to assume that any organism that sometimes eats dead material is saprotrophic, which is not correct. The key features of saprotrophs are external digestion and a primary ecological role in decomposition. Another pitfall is confusion between saprotrophs and parasites, which live on a living host. To avoid these errors, remember that mushrooms on a rotting log and mould on stale bread are textbook examples of saprotrophic fungi that break down dead matter and recycle nutrients back into the ecosystem.

Final Answer:
The organism that shows saprotrophic nutrition by feeding on dead and decaying organic matter is the Mushroom.