Saprozoic nutrition in protozoa: Which description best characterizes how nutrients are acquired under saprozoic feeding?

Introduction:
Protozoa display two broad feeding modes: holozoic (ingestion of solid food) and saprozoic (absorption of dissolved nutrients). Distinguishing these modes is fundamental for understanding trophic roles of protists in ecosystems and disease contexts.

Given Data / Assumptions:

• Saprozoic implies soluble, pre-digested organic matter.
• Transport occurs across membranes via diffusion, carriers, or pinocytosis.
• Food vacuoles are characteristic of phagocytosis (holozoic), not saprozoic uptake.

Concept / Approach:
In saprozoic feeding, the protozoan takes up dissolved molecules directly from the environment. Pinocytosis—“cell drinking”—creates small vesicles containing fluid and solutes; these differ from phagocytic vacuoles that enclose particles. Hence, the most accurate description is uptake of soluble nutrients by pinocytosis, forming pinocytotic vesicles for subsequent processing.

Step-by-Step Solution:
Identify the nutrient state: soluble molecules rather than particles. Match the mechanism: membrane invagination forming small vesicles (pinocytosis). Recognize processing: enzymes handle solutes within small vesicles or at the membrane. Select the option explicitly describing pinocytosis of soluble nutrients.

Verification / Alternative check:
Experimental tracers demonstrate uptake of dyes and small metabolites via pinocytotic pits in saprozoic protozoa, supporting the mechanism over phagocytosis.

Why Other Options Are Wrong:
Phagocytosis of solids – that is holozoic feeding.

Encystment…secretory vacuoles – unrelated to nutrient acquisition.

Extracellular digestion only – saprozoic organisms still absorb monomers internally.

Ingestion of living prey – again, holozoic behavior.

Common Pitfalls:
Mixing up “phago-” (solid) with “pino-” (liquid). Exam questions often hinge on this precise distinction.

Final Answer:
Pinocytosis of soluble nutrients forming pinocytotic vesicles.