Page 121 - Biology_F5

Biology for Advanced Level Secondary Schools

consists of only unicellular microscopic members lack sexual reproduction.
organisms (protozoans) while the latter is Asexual reproduction occurs by binary
the mixture of unicellular and multicellular fission (in which one nucleus divides),
organisms. multiple fission (in which many nuclei
divide into multiple daughter cells), or
Characteristics of protoctists budding (in which a new cell grows on
FOR ONLINE READING ONLY
(a) They are nucleated; that is, all are the surface of the mother cell). During
eukaryotes.
(b) Some protoctists are unicellular while all these types of asexual reproduction,
others are multicellular. an organism replicates its nucleus

(c) They lack tissue differentiation. and divides to form new organisms.
(d) They are adapted to both, aquatic and Sexual reproduction in protoctists is
terrestrial habitats. still primitive, given that it is mainly a
(e) They have various types of vesicles recombination of genetic material.
that perform different functions. For (i) Some protoctists are parasites, while
instance, contractile vacuole helps others are free-living. The free-living
protoctists to discharge excess water members are either autotrophs (such
taken by osmosis. They also have food as Spirogyra) or heterotrophs (such as
vacuole, as in Paramecium, which Amoeba proteus).
helps them in digestion of engulfed
food particles. 3.4.1 Classification of protoctists
(f) Their cell surfaces are diverse, ranging Classifying protoctists has been a
from just a plasma membrane, as in difficult task due to their high diversity.
Amoeba, to a stiffer surface, as in
Euglena to ensure the integrity of the Traditionally, protoctists were subdivided
cell. into several groups based on their
(g) Many protoctists are involved in similarities to higher kingdoms (Fungi,
endo-symbiotic relationship with Plantae and Animalia). This text will
other organisms. A good example deal with six selected phyla of Protoctista
is a radiolarian which harbors other namely; Rhizopoda, Zoomastigina,
photosynthetic protoctists. These Apicomplexa, Euglenophyta, Oomycota,
protoctists, through photosynthesis, and Chlorophyta.
synthesise food that is shared by
both, while the radiolarian in turn Phylum Rhizopoda
confers protection and provide some Rhizopoda is a broad group of amoeboid
metabolites to the other symbiont. protozoan placed in kingdom Protoctista.

(h) Both asexual and sexual types of The shape and organisation of pseudopodia
reproduction are common in the are among the main characteristics that
majority of protoctists, although some are used to classify members of phylum
Rhizopoda. This phylum comprises of

Form Five Student’s Book
114