Kingdom Classification - Protista

Characteristics

• Unicellular: Most organisms in this kingdom are single-celled.
• Eukaryotic: They possess a true nucleus and other membrane-bound organelles.
• Nutrition: Can be autotrophic (photosynthetic), heterotrophic (ingestive or absorptive), or mixotrophic.
• Locomotion: May have flagella, cilia, or pseudopods for movement.
• Habitat: Primarily aquatic, found in freshwater, saltwater, and moist terrestrial environments.

Amoeba

Amoeba is a common example of a protist, known for its changing shape.

Basic Structure

• Irregular Shape: Lacks a fixed shape, constantly changing due to the formation of pseudopods.
• Cell Membrane: The outer boundary that encloses the cytoplasm.
• Cytoplasm: Divided into two parts:
  • Ectoplasm: The clear, outer, non-granular layer.
  • Endoplasm: The inner, granular, more fluid layer containing organelles.
• Nucleus: A single, large, dense, and usually spherical structure that controls cell activities.
• Contractile Vacuole: Regulates water balance (osmoregulation) by expelling excess water.
• Food Vacuoles: Formed during feeding, containing ingested food particles.

Life Processes

• Nutrition (Holozoic):
  1. Ingestion: Amoeba engulfs food particles (e.g., bacteria, algae) using pseudopods, forming a food cup that becomes a food vacuole.
  2. Digestion: Lysosomes containing digestive enzymes fuse with the food vacuole, breaking down complex food into simpler substances.
  3. Absorption: Digested food diffuses into the cytoplasm.
  4. Assimilation: Absorbed food is utilized for energy, growth, and repair.
  5. Egestion: Undigested waste is expelled from the cell at any point on the surface.
• Locomotion: Moves by forming temporary finger-like projections called pseudopods (false feet). The cytoplasm flows into these pseudopods, pulling the cell forward.
• Respiration: Occurs through the general body surface. Oxygen dissolved in water diffuses into the cytoplasm, and carbon dioxide diffuses out.
• Excretion: Waste products (e.g., ammonia) are removed by diffusion through the cell membrane. The contractile vacuole also helps in expelling excess water and some metabolic wastes.
• Reproduction:
  • Binary Fission: Under favorable conditions, the nucleus divides, followed by the division of the cytoplasm, resulting in two identical daughter Amoebae.
  • Multiple Fission (Encystment): Under unfavorable conditions, Amoeba withdraws its pseudopods, becomes spherical, and secretes a protective three-layered cyst wall around itself. Inside the cyst, the nucleus divides repeatedly, forming many small daughter nuclei. Each nucleus then develops into a small Amoeba (pseudopodiospores). When favorable conditions return, the cyst wall breaks, releasing the young Amoebae.