This document summarizes key aspects of protists, including their origins, classification, cellular structure, homeostasis, reproduction, symbiotic relationships, and parasitic life cycles. It notes that protists are a diverse group that likely arose from multiple ancestral archaea over 1.5 billion years ago. Their unicellular organization allows for complex internal differentiation and specialized structures/organelles despite their small size. Both asexual and sexual reproduction occur among protists. Many protists engage in symbiotic relationships with other organisms, particularly parasitism which can involve multi-host life cycles.

1. By
SYED SHAKEEL SHAH
Department of Zoology
University of Narowal

2. Introduction
 Ancient members of the Archaea were the first living
organisms on this planet.
 The Archaea gave rise to the kingdom Protista (also called
Protoctista) about 1.5 billion years ago.
 The endosymbiont hypothesis is one of a number of
explanations of how this could have occurred.
 Most scientists agree that the protists probably arose from
more than one ancestral Archaean group.

3.  Depending on the classification system, zoologists
recognize between 7 and 45 phyla of protists.
 These phyla represent numerous evolutionary lineages.
 Groups of organisms believed to have had separate origins
are said to be polyphyletic.
 Some protists are plantlike because they are primarily
autotrophic (they produce their own food).
 Others are animal-like because they are primarily
heterotrophic (they feed on other organisms).

4. LIFE WITHIN A SINGLE PLASMA
MEMBRANE
 Protozoa display unicellular (cytoplasmic) organization, which
does not necessarily imply that they are simple organisms.
Often, they are more complex than any particular cell in higher
organisms.
 In some protozoan phyla, individuals group to form colonies,
associations of individuals that are not dependent on one
another for most functions.
 Protozoan colonies, however, can become complex, with some
individuals becoming so specialized that differentiating between
a colony and a multicellular organism becomes difficult.

5. MAINTAINING HOMEOSTASIS
 A regular arrangement of microtubules, called the
pellicle, underlies the plasma membrane of many
protozoa.
 The pellicle is rigid enough to maintain the shape of the
protozoan, but it is also flexible.
 The cytoplasm of a protozoan is differentiated into two
regions.
 The portion of the cytoplasm just beneath the pellicle is
called ectoplasm. It is relatively clear and firm.
 The inner cytoplasm, called endoplasm, is usually
granular and more fluid.
 Most marine protozoa have solute concentrations similar
to that of their environments.

6.  Freshwater protozoa, however, must regulate the water and
solute concentrations of their cytoplasm.
 Water enters freshwater protozoa by osmosis because of higher
solute concentrations in the protozoan than in the
environment.
 Contractile vacuoles remove this excess water.
 In some protozoa, contractile vacuoles form by the coalescence
of smaller vacuoles.
 In others, the vacuoles are permanent organelles that collecting
tubules radiating into the cytoplasm fill.
 Contracting microfilaments have been implicated in the
emptying of contractile vacuoles.

7.  Most protozoa absorb dissolved nutrients by active transport or
ingest whole or particulate food through endocytosis.
 Some protozoa ingest food in a specialized region analogous to a
mouth, called the cytopharynx.
 Digestion and transport of food occurs in food vacuoles that
form during endocytosis.
 Enzymes and acidity changes mediate digestion.
 Food vacuoles fuse with enzyme-containing lysosomes and
circulate through the cytoplasm, distributing the products of
digestion.
 After digestion is complete, the vacuoles are called egestion
vacuoles.

8.  They release their contents by exocytosis, sometimes at a
specialized region of the plasma membrane or pellicle
called the cytopyge.
 Because protozoa are small, they have a large surface area in
proportion to their volume.
 This high surface-area to-volume ratio facilitates two other
maintenance functions: gas exchange and excretion.
 Gas exchange involves acquiring oxygen for cellular
respiration and eliminating the carbon dioxide produced as
a by-product.
 Excretion is the elimination of the nitrogenous by-
products of protein metabolism, which, in protozoa, are
mostly ammonia.
 Both gas exchange and excretion occur by diffusion across
the plasma membrane.

10. REPRODUCTION
 Both asexual and sexual reproduction occur among the
protozoa.
 One of the simplest and most common forms of asexual
reproduction is binary fission.
 In binary fission, mitosis produces two nuclei that are
distributed into two similar-sized individuals when the
cytoplasm divides.
 During cytokinesis, some organelles duplicate to ensure
that each new protozoan has the needed organelles to
continue life.
 Depending on the group of protozoa, cytokinesis may be
longitudinal or transverse.

11.  During budding, mitosis is followed by the incorporation
of one nucleus into a cytoplasmic mass that is much
smaller than the parent cell.
 Multiple fission or schizogony occurs when a large
number of daughter cells form from the division of a single
protozoan.
 Schizogony begins with multiple mitotic divisions in a
mature individual.
 When a certain number of nuclei have been produced,
cytoplasmic division results in the separation of each
nucleus into a new cell.

12.  Sexual reproduction requires gamete formation and the
subsequent fusion of gametes to form a zygote.
 In most protozoa, the sexually mature individual is
haploid.
 Gametes are produced by mitosis, and meiosis follows
union of the gametes.
 Ciliated protozoa are an exception to this pattern.

14. Symbiotic Lifestyles
 Symbiosis is an intimate association between two organisms.
 For many protozoa, these interactions involve a form of
symbiosis called parasitism, in which one organism lives in or
on a second organism, called a host.
 The host is harmed but usually survives, at least long enough for
the parasite to complete one or more life cycles.
 The relationships between a parasite and its host(s) are often
complex.
 Some parasites have life histories involving multiple hosts.

15.  The definitive host harbors the sexual stages of the
parasite.
 The sexual stages may produce offspring that enter another
host, called an intermediate host, where they reproduce
asexually.
 Some life cycles require more than one intermediate host
and more than one immature stage.
 For the life cycle to be complete, the final, asexual stage
must have access to a definitive host.
 Commensalism is symbiotic relationship in which one
member of the relationship benefits, and the second
member is neither benefited nor harmed.
 Mutualism is a symbiotic relationship in which both
species benefit.

16. Question…?