Understanding External Digestion in Organisms

Different organisms have different ways of obtaining and processing food. Some digest food inside their bodies (internal digestion), while others release digestive enzymes outside their bodies to break down food material and then absorb the simpler substances. This latter process is known as external digestion or extracellular digestion.

Analyzing the Mode of Nutrition in Different Groups

Let's look at the options provided to identify the group where organisms break down food material outside their body and then absorb it:

• Option 1: Yeast, mushroom, bread mould
• Option 2: Mushroom, Amoeba, green plants
• Option 3: Caserta, tapeworm, lice
• Option 4: Paramecium, Amoeba, Caserta

We need to examine the mode of nutrition for each organism listed in these groups.

Organisms That Perform External Digestion

Organisms that break down food outside their body are typically saprophytes, meaning they feed on dead and decaying organic matter. Fungi are a prime example of organisms that use external digestion.

• Yeast: Yeast is a single-celled fungus. It obtains nutrients by secreting enzymes onto the food source (like sugar) and then absorbing the resulting simpler molecules. This is external digestion.
• Mushroom: Mushrooms are multi-cellular fungi. They grow on substrates like soil, logs, or decaying matter. They release digestive enzymes into the substrate, breaking down complex organic compounds, and then absorb the nutrients. This is external digestion.
• Bread mould: Bread mould (like Rhizopus) is also a type of fungus. It grows on bread and other food items, releasing enzymes to digest the food externally and then absorbing the broken-down substances. This is external digestion.

So, the first group (Yeast, mushroom, bread mould) consists entirely of fungi that perform external digestion.

Organisms That Perform Internal Digestion or Other Modes

Let's look at the other organisms mentioned in the options:

• Amoeba: Amoeba is a protozoan. It captures food particles and brings them inside its body within food vacuoles, where digestion occurs internally using enzymes. This is internal digestion (specifically phagocytosis followed by intracellular digestion).
• Green plants: Green plants are autotrophs. They produce their own food through photosynthesis, using sunlight, water, and carbon dioxide. They do not consume external food material or perform digestion in the typical sense, either internal or external.
• Caserta (Cuscuta): Cuscuta (dodder) is a parasitic plant. It lacks chlorophyll and absorbs nutrients directly from the host plant using special structures called haustoria. It does not perform external digestion of separate food material.
• Tapeworm: Tapeworms are parasitic animals. They live inside the host's digestive tract and absorb already digested food directly through their body surface. While they don't 'eat' in the conventional sense, they don't perform external digestion of complex food outside their body. Nutrients absorbed are already broken down by the host.
• Lice: Lice are parasitic insects. They feed on the blood of their host. They ingest food (blood) into their digestive system and digest it internally.
• Paramecium: Paramecium is a protozoan. It uses cilia to sweep food particles into a gullet, where they are enclosed in food vacuoles for internal digestion. This is internal digestion.

Let's summarize the modes of nutrition for the organisms in each option:

Based on this analysis, only the first group contains organisms (Yeast, mushroom, bread mould) that all perform external digestion.

Conclusion

The group of organisms where food material is broken down outside the body and then absorbed is Yeast, mushroom, and bread mould. These are all examples of fungi that exhibit saprophytic nutrition involving external breakdown of complex organic matter.

Revision Table: Modes of Nutrition

Additional Information on External Digestion

External digestion is a key characteristic of saprophytic organisms, particularly fungi. This process involves:

1. Secretion of digestive enzymes: The organism releases enzymes from its body onto the food source.
2. Breakdown of complex molecules: These enzymes break down complex organic compounds (like carbohydrates, proteins, fats) into simpler, soluble substances (like sugars, amino acids, fatty acids).
3. Absorption: The organism then absorbs these simpler, digested nutrients directly across its cell membrane or body surface.

This method allows saprophytes to obtain nutrients from non-living organic matter, playing a crucial role in decomposition and nutrient cycling in ecosystems.