Modes of Nutrition

All living organisms exhibit various modes of nutrition, which can be categorized into two major classes: autotrophic and heterotrophic nutrition.

Autotrophic Nutrition

Plants are known as autotrophs because they can manufacture their own food (organic compounds) from inorganic materials like water and carbon dioxide. Autotrophs generally utilize two modes of nutrition:

• Photoautotrophs: Organisms that obtain energy from sunlight to synthesize organic materials through photosynthesis. Examples include plants, green algae, red algae, and cyanobacteria.
• Chemoautotrophs: Organisms that derive energy from inorganic chemical processes. Chemoautotrophs thrive in environments where plants cannot survive, such as ocean depths or acidic hot springs. Examples include nitrogen-fixing bacteria (Nitrosomonas) and sulfur-oxidizing bacteria.

Heterotrophic Nutrition

This type of nutrition involves organisms that cannot produce their own food and rely on autotrophs for sustenance. These organisms are termed heterotrophs.

1. Holozoic: Organisms that consume complex organic materials in solid form, requiring ingestion, digestion, and absorption. Subcategories include:
   • Carnivores: Animals that feed on other animals.
   • Herbivores: Animals that feed on plants.
   • Omnivores: Animals that eat both plants and animals.
   • Scavengers: Animals that consume dead organisms.
2. Parasitic: Organisms (parasites) that feed on other living organisms (hosts). They can be classified as:
   • Ectoparasites: Parasites that live on the surface of their hosts.
   • Endoparasites: Parasites that reside inside their hosts, such as:
     • Tapeworm (Taenia solium): A flattened tape-like body that attaches to the intestinal lining of its host.
     • Guinea Worm (Dracunculus medinensis): An endoparasite found in subcutaneous tissues.
     • Roundworm (Ascaris lumbricoides): A common parasite found in the alimentary canal.
3. Symbiotic: This mode involves two organisms of different species (symbionts) living together for mutual benefit. Examples include:
   • Nitrogen-fixing bacteria in the root nodules of leguminous plants.
   • Algae and fungi in lichens, where algae provide food and fungi offer protection and absorb water.
   • Termites and protozoa in the gut, where protozoa help digest cellulose.
   • Sea anemones and hermit crabs, where the sea anemone provides protection and the hermit crab offers leftover food.
4. Saprophytic: Organisms (saprophytes) that feed on non-living organic matter, such as plant and animal remains. They secrete digestive enzymes to digest food (extracellular digestion). Examples include Rhizopus, mushrooms, and bacteria.

Carnivorous Plants

Carnivorous plants derive some or most of their nutrients from trapping and consuming animals or protozoans, typically insects and other arthropods. They often grow in nutrient-poor environments, such as acidic bogs.

• Pitfall Plants: These plants have pitcher-shaped traps. For example, Nepenthes contains a watery fluid secreted by glands. Insects attracted by sugary secretions fall into the pitcher and are digested by enzymes.
• Sundew (Drosera): The leaves of this plant have glandular hairs that trap and digest insects. The tentacles secrete digestive enzymes that cover and digest captured insects.