Introduction to Kingdom Protista

Introduction

The Kingdom Protista is a group of eukaryotic organisms called protists. These organisms are very diverse as they do not have much in common when compared to each other. Protists are categorised as Kingdom Protista since they do not fit into any other kingdom of organisms. 

In general terms, protists are eukaryotes and do not belong to the category of plants, animals or fungi. Some protists are microscopic and unicellular. However, in some exceptional cases like seaweed and giant kelp, they can also be multicellular and large. Similarly, some protists like algae are photosynthetic while others are heterotrophic. Therefore, all protists show different properties and rarely show any similarities with the others. 

The members of Kingdom Protista are considered to have less complex structures than other eukaryotic organisms. 

Different Types of Protists 

As mentioned above, all protists are different. Based on their nature, they can be categorised into three broad categories. The three categories are explained below with examples of Kingdom Protista:

• Animal-like protists 

Protozoa or animal-like protists are known as the first eukaryotes on the planet. Therefore, being the early eukaryotes, they led to the development of all other animal life on the earth. However, they are not considered animals themselves. 

The word protozoa means “first animal”, and they are said to be an evolved version of bacteria. These organisms of Kingdom Protista are colonial or unicellular. They don’t have cell walls and most of them are motile. A protozoan cell contains mitochondria and digestive vacuoles. 

The animal-like protists are also heterotrophs. This means they do not produce their food through photosynthesis and find nutrients from the external environment. 

Examples: Amoebas, flagellates, apicomplexans, ciliates, sporozoa and rhizopoda.   

• Plant-like protists 

Algae or plant-like protists are found in aquatic habitats, which refer to the surfaces or areas that have water. They are called plant-like because they produce their food through the process of photosynthesis. They also contain chlorophyll and chloroplasts. Unlike animal-like protists, algae have a cell wall, which is made of cellulose. However, though they are called plant-like, they do not have any vascular tissues, leaves, roots or stems. 

A notable fact about algae is that they are a major producer of oxygen. They are known to be responsible for producing 50% of the oxygen for the entire planet. Some types of algae are unicellular like diatoms, while others are multicellular like seaweed. Seaweeds are known to be the largest forms of algae as well as the most complex ones. 

The cell of algae contains a nucleus and chloroplasts that are enclosed in membranes.   

Examples: Seaweeds (the big algae), chlorophyta (green algae), rhodophyta (red algae), phaeophyta (brown algae), xanthophyta (yellow-green algae), phytoplankton (the small algae), bacillariophyta and pyrrophyta (fire algae).

• Fungi-like protists 

Moulds or fungi-like protists are generally found in damp areas. They are also heterotrophs and take their nutrition from dead and decaying organisms. Moulds were classified as fungus-like because of the production of sporangia, a fungal structure responsible for the production of spores. However, later, they were considered to be different from fungi. 

Moulds reproduce using pores. They have a cell wall made of cellulose. Contrary to this, the cells of fungi have cell walls made of chitin. Unlike fungi, the fungi-like protists also do not have divisions between their cells. It is another feature that differentiates them from fungi. When we talk about nuclei, moulds have diploid nuclei as compared to fungi that have haploid nuclei. 

Due to such differences, the classification of moulds as fungi-like organisms is still under consideration by scientists. 

Examples: Slime moulds and water moulds. 

Characteristics of Protista 

Protists are a large category of eukaryotes. The total number of species in Kingdom Protista is between 60,000 and 200,000. 

Protists are both unicellular and multicellular. All protist cells have nuclear membranes with organelles bound within the membranes. There is a wide variety of protists, which leads to very diverse characteristics of each species. The size of protists ranges from 10 micrometres to several metres. 

The following are the characteristics of protists: 

• Nutrition 

The mode of nutrition of protists varies greatly and they get food in several ways. Some protists are autotrophic, which means they produce their food using methods like photosynthesis and chemosynthesis. Some of these protists, like Spirogyra which are photoautotrophs, have chloroplasts. 

Other protists are heterotrophic, which means they get their food from other organisms. There is another category of protists that get energy from both photosynthesis and external environments. They are known as mixotrophs.  

The modes of nutrition used by protists can be divided into three types:

1. Ingestive (used by animal-like protists)
2. Photosynthetic (used by plant-like protists)
3. Absorptive (used by fungus-like protists)

In the ingestive mode of nutrition, the protozoa simply swallow organisms like bacteria. For this, they extend their cell walls and membranes around the food and create a food vacuole from which they digest it. 

In photosynthesis, carbon dioxide and solar energy are used to create food. On the other hand, in absorptive nutrition, the protists get food by the process of diffusion. The nutrients in this method are absorbed by the cell membrane. This method is used by the protists to consume dead matter. 

• Reproduction 

Unlike other eukaryotes, protists have unique and complex life cycles. Based on the species and other factors such as environmental conditions (like sunlight), most protists go through different cycles of reproduction. Therefore, it can be sexual as well as asexual. The green algae or Spirogyra is known to reproduce by both forms of reproduction. 

• Habitat 

Regardless of the type of protists, most organisms from the Kingdom Protista tend to live in moist environments. Hence, they are generally found in places where there is water. The most common places are the ocean, puddles, lakes, marshes and even damp soil. 

Some protists are free-living organisms, which means they are found on their own. However, the others are known as symbionts, which means they live in or on other living organisms. The symbiont protists can also be found in or on human beings. 

Motility

Motility is the ability of an organism to move, using its bodily functions. Some protists are motile while some are not. In the case of motile protists, three modes of movement are adopted:

1. Pseudopods 
2. Cilia
3. Flagella 

The flagella or cilia are used for movement as the protists whip or beat them on the watery surfaces. The flagella of protists are very different from that of prokaryotic flagella. The ones that protists have are projections of cytoplasm, which are supported by microtubule bundles. 

In some cases, the protists use false feet, also known as pseudopods. They are fluid-like projections of cytoplasm but are not supported by any microtubules. They help protists move and consume food. 

Conclusion 

Protists are eukaryotes that do not belong to any category of animals, fungi or plants. They are microorganisms that have very diverse qualities and cannot even be related together. The reason that protists are a part of Kingdom Protista is that they are extremely diverse and therefore cannot be categorised with any other kingdom.