Biology

Kingdom Protista

3,937 words, 9 sections

Protista collects everything eukaryotic that is not plainly an animal, a plant or a fungus: single-celled and colonial forms that swim, crawl, photosynthesise, or digest whatever reaches them from outside. It carries 84 distinct questions across 45 official papers between 2000 and 2024, set by all six boards that examine Biology, which works out at fewer than two questions a paper, with 432 more on the chapter in the practice bank at a mean difficulty of 1.73. The PMDC syllabus table names no Protista chapter and the papers do not agree with that omission, so what follows rests on measured examination evidence rather than on a learning outcome that does not exist. In the textbooks the material runs as one section headed PROTISTS AND FUNGI, from the protists into the fungi and the four-division table that closes it, and that is where a student hunting for this chapter will find it. Mastery here means placing an unfamiliar organism from two clues visible down a microscope, running the malaria cycle from vector back to vector, and naming the disease or the product attached to any organism handed over.

All protozoa are unicellular, so the whole organism is one mass of cytoplasm holding every structure a typical cell would have: they eat, move, hold their internal conditions steady and reproduce inside that one cell, with no tissue organisation, which is what separates them from the Metazoa. Most take food in by endocytosis, and vacuoles do the internal work, food vacuoles digesting engulfed material, contractile vacuoles pumping out the water that enters by osmosis, storage vacuoles holding waste aside.

The groups are defined by how they move, and there are three locomotory organs plus one absence. Amoebae lack flagella and move on pseudopodia, temporary cytoplasmic projections that also serve to feed. Foraminifera and actinopods move on pseudopodia too and build a shell or test with pores, calcareous in the foraminifera and siliceous in the actinopods. Zooflagellates have whip-like anterior flagella and feed by ingesting or absorbing. Ciliates such as Paramecium carry thousands of short hairs whose coordinated beat lets them reverse and turn, and they alone have two kinds of nucleus, a diploid micronucleus for the sexual process and a polyploid macronucleus running metabolism. Apicomplexans have no locomotory structure at all, they flex, they form spores, and many split their life between two hosts.

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Introduction

1. Protozoa: one cell, and four ways of movingAll protozoa are unicellular, so the whole organism is one mass of cytoplasm holding every structure a typical cell would have: they eat, move, hold their inter