Protista: Definition, Characteristics, Reproduction, Examples, Classification

Protist meaning: Protista is one of the five kingdoms of eukaryotic microorganisms that include a large group of single-celled and simple two or few-celled organisms. They are autotrophic (like algae, performing photosynthesis) or heterotrophic (like amoebas, which consume other organisms). Protists are seen in aquatic environments and can reproduce either sexually or asexually. They are known as a link between prokaryotes and more complex eukaryotic organisms, such as plants, animals, and fungi. Kingdom Protista is a topic of the Biological Classification in Biology.

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What Are Protists?

Protista as a kingdom originated in the 19th century when scientists required an outlet for placing organisms that possess some features of plants and some features of animals. Protists are primary producers, decomposers and mutualistic organisms, which are essential components of ecosystems so these organisms offer important information on the phylogenic evolution of eukaryotic organisms.

General Characteristics of Kingdom Protista

The following are the characteristics of Protista:

Cell Structure

• Protists are further classified by possession of eukaryotic cells, meaning, their cells contain membrane-bound organelles and nuclei which are packed with genetic material.

• This separates them from the prokaryotic organisms such as bacteria.

• A protist cell is complex and it has a nucleus, in which there is the protist’s DNA, and other quantities such as mitochondria, the Golgi apparatus, and chloroplasts depending on the protist.

• It is through these organelles that protists can execute other various metabolic activities that makeup part of their nature.

Modes Of Nutrition

Autotrophic:

• Features of some protists include the ability to perform autotrophic nutrition, which implies the ability to produce food on their own through photosynthesis.

• Most of these protists have chloroplasts or other related structures to capture sunlight and convert it into other energy compounds such as glucose.

Heterotrophic:

• Most of the protists are heterotrophic and therefore they acquire their food by engulfing food particles or other organisms.

• They may exercise this through phagocytosis, in which a solid particle is surrounded and then digested.

Mixotrophic:

• Some complex protists are characterised by mixotrophic nutrition which is the alternation of autotrophic and heterotrophic nutrition based on certain conditions.

• For instance, they may grow photoautotrophically utilising sunlight for their energy requirements, but if light is not available, or nutrients are limited, they will change to photoheterotrophic growth.

Reproduction

Asexual reproduction

• Binary fission: They reproduce asexually though in binary fission where a parent cell splits into two pin-point-like genetically similar daughter cells.

• This process is often exhibited by the unicellular protists and it is useful in increasing the numbers of the population provided that the prevailing conditions are suitable.

• Budding: Some protists reproduce asexually by budding in which a part of the parent organism grows out of the body as a rounded outgrowth or as a bud.

• This bud is often shed to allow for the formation of new colonies or the dispersal of the individual quaking dog.

Sexual reproduction

• Some of the protists reproduce asexually through the method known as conjugation in which two organisms are connected temporarily to exchange genetic material.

• This exchange is responsible for differences in the genetic makeup of offspring which increases their chances of survival when the environmental conditions change.

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Classification of Kingdom Protista

The types of protists can be classified into:-

Plant-Like Protists (Algae)

• Protista examples: Diatoms, Euglena, Red Algae, Green Algae, Brown Algae

• Algae are plant-like organisms that are mainly found in water habitats, this includes both, fresh and saltwater habitats.

• It has a high diversity involving both size and shape; they range from single-celled organisms and measured in microns to large seaweed-like organisms.

• Algae contain diverse photosynthetic pigments and they include chlorophyll a, chlorophyll b, carotenoids and phycobilins.

• These pigments allow them to absorb light energy and conduct photosynthesis in all weather and water depths.

• Algae are also an important part of ecosystems as they are primary producers hence aiding in the formation of oxygen and cycling of nutrients.

• They are also of economic significance as food (e.g., seaweed, particularly nori to Japan), vitamins and minerals (e.g., spirulina), biopharmaceuticals (e.g., agar from red algae) and other sectors such as biogas, carbon dioxide absorption and sewage farming because of the algae’s fast growth and high biomass properties.

Animal-Like Protists (Protozoa)

• Protista examples: Amoeba, Paramecium, Trypanosoma, Plasmodium

Movement mechanisms

• Cilia: Cilia in protozoa like Paramecium are used in protists' locomotion. These are short, hair-like structures which move in a wave-like manner to move the organism in water.

• Flagella: Note that some protozoa like Trypanosoma are motile structures that possess flagella. Flagella are more extended whip-like structures that either oscillate or vibrate to permit the farming of the harbouring liquid media.

• Pseudopodia: The Amoeba and other similar protozoa have an organellum known as pseudopodia which is a change in shape involving temporary protrusions of the cell membrane plus its cortex. These extensions lie in the direction of movement, with the help of which the protozoa creep and engulf food particles.

Pathogenic Protozoa and Diseases

• Trypanosoma: Many diseases are caused by insect-borne parasites such as African sleeping sickness and Chagas disease.

• Plasmodium: Causes malaria, people get infected through the bites of infected mosquitoes.

• Amoeba: Certain types, for example, Entamoeba histolytica, are known to be capable of causing amoebic dysentery which is an intestinal sickness.

Fungus-like protists (Slime Molds and Water Molds)

Life cycle stages

• Slime Molds: Start as single-celled, acellular amoeba and grow into a multinucleated, motile and feeding organism known as a plasmodium. They then form reproductive structures that release spores,” often used to identify the plant kingdom or dominate the plant kingdom.

• Water Molds (Oomycetes): Reproduction is through biflagellate zoospores for motility and to infect host and sporangia that release spores in the environment.

Ecological roles

• Slime Molds: These are responsible for breaking down organic matter in the forests and may be predatory having the capability of consuming bacteria as well as fungi.

• Water Molds: Disease agents of plants (for example, being responsible for potato’s late blight) and have a part in the recycling of nutrients in aquatic ecosystems as the saprophytes.

Examples and Importance

• Slime Molds (Myxomycetes): The species are: Physarum polycephalum and Fuligo septica as examples that are researched for their unusual actions and as signs of ecological conditions.

• Water Molds (Oomycetes): Some of the Phytophthora diseases affecting crops are potato late blight, Phytophthora infestans and other diseases affecting fish and amphibians are Saprolegnia species.

Economic Importance of Kingdom Protists

The economic importance of protists is as follows:-

Agriculture

• Protists help in the decomposition of organic matter hence making the soil fertile by unlocking nutrients for the growth of plants.

• It also improves the structure of the soils and assists in the cycling of nutrients therefore improving agricultural production.

Medical Field

• Completing the group of primitive antibiotic-producing organisms are some of the slime moulds and water moulds.

• These compounds are analyzed for their prospect of use in the pharma related to bacterial infection.

Research Models

• Protists are therefore important organisms to use in biomedical and genetic research studies, this is because of their evolutionary features and easy-to-manipulate genetic systems.

• These are used in studying the mechanisms of diseases, functions of cells and ways of testing drugs.

Industrial Uses

• Some types of algae for instance microalgae and diatoms are used in the production of biofuels because of their richness in lipids.

• They can be grown in industrialised big tanks called bioreactors to give biodiesel and bioethanol for use as sources of energy.

Food Industry (Algae)

• They are used in the food industry in the form of Nutraceuticals such as spirulina and as food incidental additives.

• They are packed with protein, vitamins, and antioxidants in, hence have so much value in the diet of human beings and their general well-being.

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