2. Protoctistas

BIOGEOSPHERE Bilingual Biology and Geology I.E.S. "J.S.Elcano (Sanlúcar Bda.)

Unit 10: The simplest living beings 1 2 3 4

2. Protoctists

The Protoctista Kingdom includes a large number of creatures than can not be classify in other kingdoms.

All the Protoctists have eukaryotic cells, but they can be:

- autotrophic or heterotrophic

- unicellular or multicellular (without true tissues)

We can differentiate:

- Protoctists that are similar to animals (protozoa)

- Protoctists that are similar to plants (algae)

This group is so diverse that this kingdom will probably be divided in the future.

2.1. Protozoa

These organisms are unicellular and heterotrophic.

They have eukaryotic animal cells.

They can only be seen under a microscope.

We can find them in the soil or in water (zooplankton). Other ones are parasites and produce diseases.

They have asexual reproduction. This can be by:

- Bipartition (dividing the cell mother in two)

- Sporulation (producing spores)

They can be classified into four groups, according to the way they move:

- Flagellates

They move using flagella.

E. g. Trypanosome. (It is responsible for sleeping sickness and spreads by Tse-tse fly)

- Ciliates

They move using cilia (they are organelles similar to flagella, but much shorter and very numerous)

E. g. Paramecium (swimmer)

and Vorticella (live join to the substrate)

- Rizopods

They move using pseudopodia (they are extensions of their cytoplasm, that they use to feed too)

E. g. Amoeba and Entamoeba

(It is responsible for dysentery)

- Sporozoans

They don’t move.

E. g. Plasmodium (It is responsible for malaria and spreads by Anopheles mosquito)

Animation: What are protozoa? (University of Manchester)

2.2. Algae

This group includes organisms that can be unicellular or multicellular. They have eukaryotic plants cells. All algae are autotrophic and do photosynthesis.

They can live joined to substrate (benthonic algae) or not (planktonic algae).

For a long time, they were included in the Plants Kingdom, but today they are in the Protoctists, because these organisms don’t have true tissues or true organs.

a) Multicellular algae

Most of them are marine, but some live in fresh water. The largest and most complex marine algae are called seaweed.

Multicellular algae do not have true tissues,

but they have structures similar to plant organs:

- Blades (false “leaves”)

- Stipe (false “stem”) sometimes with floats (the keep up the algae)

- Holdfast (false “roots” that hold them to the substrate, but do not absorb nutrients)

They reproduce by alternation of generations, that is in one generation they reproduce asexually and in the following generation, they reproduce sexually.

They are classified in three groups according to the photosynthetic pigments they have:

- Green algae

Their most important pigment is

chlorophyll (green pigment)

Most part of them live in fresh water.

There are unicellular and multicellular.

They live near the surface.

They seem to be the ancestors of plants.

- Brown algae

Their most important pigment

gives them brown colour.

They are marine and multicellular.

It is the group with the most

complex structure and largest forms.

They live in middle depth water.

- Red algae

Their most important pigment

gives them a red colour.

They are unicellular or multicellular, and marines.

They live attached to the substrate and at great depth in the ocean.

b) Unicellular algae

There are several groups of unicellular algae. They formed the phytoplankton.

They can form colonies (group of unicellular organisms that live joined together and look like a pluricellular organism, but without true tissues or division of work among the cells).