7.1 PROTISTS – The Evolutionary Relationship

Introduction

Protists are a varied group of organisms. They all have eukaryotic cells, which are similar to those found in fungi, animals, and plants. But this is what sets them apart from bacteria, which are prokaryotes. Eukaryotic cells have nuclei and other parts enclosed in membranes, like mitochondria and plastids. Biologists have different opinions on what exactly protists are. Some think of the protist kingdom broadly, including both those that are heterotrophs (like protozoa, slime molds, and water molds) and those that are autotrophs (like algae).

Definition

Protists: Protists are a varied group of organisms that have eukaryotic cells, similar to those found in fungi, animals, and plants.

In simple words: Protists are a diverse group of eukaryotic organisms that are not classified as plants, animals, or fungi.

Core Concept

• Protists are a varied group of organisms with eukaryotic cells.
• Their cells are similar to those found in fungi, animals, and plants.
• This sets them apart from bacteria, which are prokaryotes.
• Eukaryotic cells have nuclei and other parts enclosed in membranes, like mitochondria and plastids.
• Biologists have different opinions on what exactly protists are.
• Some think of the protist kingdom broadly, including:
• Those that are heterotrophs (like protozoa, slime molds, and water molds).
• Those that are autotrophs (like algae).

Detailed Explanation

Polyphyletic Origin – Where They Come From

Definition: The term "polyphyletic" refers to a grouping of organisms that do not share a common evolutionary ancestor and may have evolved independently from different ancestral sources and are not closely related in terms of their evolutionary history.

Key Points:

• Protists don't all come from the same ancestor.
• In fact, they have many different origins.
• We put any eukaryotic creature that is not a fungus, animal, or plant into the protist category just for convenience.
• If we sorted them according to more specific shared characteristics, there might be many different categories within Protista, maybe as many as twenty.

The Remarkable Diversity of Protists

Size:

• Protists come in many sizes.
• Some are so small you need a microscope to see them, like protozoa.
• Others, like giant kelp (a type of brown algae), can be as long as 60 meters (almost 200 feet).
• While most protists are single-celled, some form groups of cells (colonies), and a few have many nuclei in one cell (coenocytic).
• But unlike animals, fungi, and plants, even the multicellular protists have pretty simple body structures without specialized tissues.

Nutrition:

• Protists have various ways of getting their nutrients.
• Some, like algae, have chlorophyll and can make their own food through photosynthesis, similar to plants.
• Others, such as water molds, absorb their food, much like fungi.
• Some, like protozoa and slime molds, are more like animals because they eat food from other living things.

Habitat Diversity and Mode of Life:

• Protists are very adaptable and exhibit a wide range of habitat diversity.
• Most protists live in water, either in oceans or freshwater, and they are part of the plankton community.
• Protists can also live in damp soil, leaf litter, and decomposing organic matter, where the presence of moisture supports their survival and reproduction.
• Many protists are free living, while others establish symbiotic association with various organisms.
• These relationships range from mutualism, where both parties benefit fairly equally, to parasitism, where one organism relies on the other for its metabolic needs, either living on or inside it.
• Certain protists act as parasites, infecting animals or plants, while others engage in symbiotic associations where they have mutually beneficial partnerships with other organisms.
• Some protists are extremophiles, and can live in extreme environments such as snow, hot springs and acidic ponds.

Reproduction:

• Protists have diverse ways of reproducing.
• Protists can reproduce asexually without involving mating or sexually with both meiosis and syngamy (the union of gametes).
• Unlike animals, they usually lack multicellular sex organs, nor do they form embryos.

Locomotion:

• Most protists are motile at some stage of their life cycle and have various means of moving around (locomotion).
• They can move by extending cell protrusions (a process called amoeboid motion), by using hair-like structures called cilia to create waves, or by propelling themselves with whip-like structures called flagella.
• Many protists use a mix of two or more of these locomotion methods, such as using both flagella and amoeboid motion simultaneously.

Summary Table of Protist Diversity

In simple words:

Protists are diverse eukaryotes with various shapes and functions, not fitting into the categories of plants, animals, or fungi; they are mostly single-celled but can also form colonies and live in water, moist environments, or as parasites.

Intermediate Level

Polyphyletic Origin in Detail

Diversity of Protists in Detail

Nutrition in Detail

Mode of Life in Detail

Reproduction in Detail

Locomotion in Detail

Important Points

• 1. Protists are a varied group of organisms. They all have eukaryotic cells, which are similar to those found in fungi, animals, and plants.
• 2. This is what sets them apart from bacteria, which are prokaryotes. Eukaryotic cells have nuclei and other parts enclosed in membranes, like mitochondria and plastids.
• 3. Biologists have different opinions on what exactly protists are. Some think of the protist kingdom broadly, including both those that are heterotrophs (like protozoa, slime molds, and water molds) and those that are autotrophs (like algae).
• 4. Polyphyletic Origin – Where They Come From:
• - The term "polyphyletic" refers to a grouping of organisms that do not share a common evolutionary ancestor and may have evolved independently from different ancestral sources and are not closely related in terms of their evolutionary history.
• - Protists don't all come from the same ancestor. In fact, they have many different origins.
• - We put any eukaryotic creature that is not a fungus, animal, or plant into the protist category just for convenience.
• - If we sorted them according to more specific shared characteristics, there might be many different categories within Protista, maybe as many as twenty.
• 5. The Remarkable Diversity of Protists:
• - Size: Protists come in many sizes. Some are so small you need a microscope to see them, like protozoa. Others, like giant kelp (a type of brown algae), can be as long as 60 meters (almost 200 feet). While most protists are single-celled, some form groups of cells (colonies), and a few have many nuclei in one cell (coenocytic). But unlike animals, fungi, and plants, even the multicellular protists have pretty simple body structures without specialized tissues.
• - Nutrition: Protists have various ways of getting their nutrients. Some, like algae, have chlorophyll and can make their own food through photosynthesis, similar to plants. Others, such as water molds, absorb their food, much like fungi. Some, like protozoa and slime molds, are more like animals because they eat food from other living things.
• - Habitat Diversity and Mode of Life: Protists are very adaptable and exhibit a wide range of habitat diversity. Most protists live in water, either in oceans or freshwater, and they are part of the plankton community. Protists can also live in damp soil, leaf litter, and decomposing organic matter, where the presence of moisture supports their survival and reproduction. Many protists are free living, while others establish symbiotic association with various organisms. These relationships range from mutualism, where both parties benefit fairly equally, to parasitism, where one organism relies on the other for its metabolic needs, either living on or inside it. Certain protists act as parasites, infecting animals or plants, while others engage in symbiotic associations where they have mutually beneficial partnerships with other organisms. Some protists are extremophiles, and can live in extreme environments such as snow, hot springs and acidic ponds.
• - Reproduction: Protists have diverse ways of reproducing. Protists can reproduce asexually without involving mating or sexually with both meiosis and syngamy (the union of gametes). Unlike animals, they usually lack multicellular sex organs, nor do they form embryos.
• - Locomotion: Most protists are motile at some stage of their life cycle and have various means of moving around (locomotion). They can move by extending cell protrusions (a process called amoeboid motion), by using hair-like structures called cilia to create waves, or by propelling themselves with whip-like structures called flagella. Many protists use a mix of two or more of these locomotion methods, such as using both flagella and amoeboid motion simultaneously.

Remember

• - Protists = diverse eukaryotes.
• - Not plants, animals, or fungi.
• - Polyphyletic = different origins.
• - Size = microscopic to 60 meters.
• - Nutrition = autotrophic or heterotrophic.
• - Habitat = water, damp soil, leaf litter, decomposing organic matter, extreme environments.
• - Mode of life = free-living, symbiotic, parasitic, extremophiles.
• - Reproduction = asexual or sexual.
• - Locomotion = amoeboid motion, cilia, flagella, or a mix.

Common Mistakes

• 1. Thinking protists are a single group – Protists are polyphyletic; they have different origins.
• 2. Thinking all protists are unicellular – Some protists are multicellular (e.g., giant kelp).
• 3. Thinking all protists are heterotrophic – Some protists are autotrophic (e.g., algae).
• 4. Thinking all protists are harmful – Many protists are beneficial or harmless.
• 5. Confusing protists with bacteria – Protists are eukaryotes; bacteria are prokaryotes.
• 6. Thinking protists have specialized tissues – Even multicellular protists have simple body structures without specialized tissues.
• 7. Confusing the modes of life – Free-living, symbiotic, parasitic, extremophiles.
• 8. Thinking all protists move the same way – Protists move by amoeboid motion, cilia, flagella, or a mix.
• 9. Forgetting that protists can be coenocytic – Coenocytic means many nuclei in one cell.
• 10. Thinking protists form embryos – Protists do not form embryos.
• 11. Confusing symbiosis and parasitism – Symbiosis = living together; Parasitism = one benefits at the expense of the other.
• 12. Thinking protists are only found in water – Protists are also found in damp soil, leaf litter, decomposing organic matter, and extreme environments.

Quick Revision

• Protists are a varied group of organisms. They all have eukaryotic cells, which are similar to those found in fungi, animals, and plants.
• This is what sets them apart from bacteria, which are prokaryotes. Eukaryotic cells have nuclei and other parts enclosed in membranes, like mitochondria and plastids.
• Biologists have different opinions on what exactly protists are. Some think of the protist kingdom broadly, including both those that are heterotrophs (like protozoa, slime molds, and water molds) and those that are autotrophs (like algae).
• Polyphyletic Origin – Where They Come From:
• The term "polyphyletic" refers to a grouping of organisms that do not share a common evolutionary ancestor and may have evolved independently from different ancestral sources and are not closely related in terms of their evolutionary history.
• Protists don't all come from the same ancestor. In fact, they have many different origins.
• We put any eukaryotic creature that is not a fungus, animal, or plant into the protist category just for convenience.
• If we sorted them according to more specific shared characteristics, there might be many different categories within Protista, maybe as many as twenty.
• The Remarkable Diversity of Protists:
• Size: Protists come in many sizes. Some are so small you need a microscope to see them, like protozoa. Others, like giant kelp (a type of brown algae), can be as long as 60 meters (almost 200 feet). While most protists are single-celled, some form groups of cells (colonies), and a few have many nuclei in one cell (coenocytic). But unlike animals, fungi, and plants, even the multicellular protists have pretty simple body structures without specialized tissues.
• Nutrition: Protists have various ways of getting their nutrients. Some, like algae, have chlorophyll and can make their own food through photosynthesis, similar to plants. Others, such as water molds, absorb their food, much like fungi. Some, like protozoa and slime molds, are more like animals because they eat food from other living things.
• Habitat Diversity and Mode of Life: Protists are very adaptable and exhibit a wide range of habitat diversity. Most protists live in water, either in oceans or freshwater, and they are part of the plankton community. Protists can also live in damp soil, leaf litter, and decomposing organic matter, where the presence of moisture supports their survival and reproduction. Many protists are free living, while others establish symbiotic association with various organisms. These relationships range from mutualism, where both parties benefit fairly equally, to parasitism, where one organism relies on the other for its metabolic needs, either living on or inside it. Certain protists act as parasites, infecting animals or plants, while others engage in symbiotic associations where they have mutually beneficial partnerships with other organisms. Some protists are extremophiles, and can live in extreme environments such as snow, hot springs and acidic ponds.
• Reproduction: Protists have diverse ways of reproducing. Protists can reproduce asexually without involving mating or sexually with both meiosis and syngamy (the union of gametes). Unlike animals, they usually lack multicellular sex organs, nor do they form embryos.
• Locomotion: Most protists are motile at some stage of their life cycle and have various means of moving around (locomotion). They can move by extending cell protrusions (a process called amoeboid motion), by using hair-like structures called cilia to create waves, or by propelling themselves with whip-like structures called flagella. Many protists use a mix of two or more of these locomotion methods, such as using both flagella and amoeboid motion simultaneously.
• Protists are diverse eukaryotes with various shapes and functions, not fitting into the categories of plants, animals, or fungi; they are mostly single-celled but can also form colonies and live in water, moist environments, or as parasites.

60-Second Revision

• Protists = diverse eukaryotes.
• Not plants, animals, or fungi.
• Polyphyletic = different origins.
• Size = microscopic to 60 meters.
• Nutrition = autotrophic or heterotrophic.
• Habitat = water, damp soil, leaf litter, decomposing organic matter, extreme environments.
• Mode of life = free-living, symbiotic, parasitic, extremophiles.
• Reproduction = asexual or sexual.
• Locomotion = amoeboid motion, cilia, flagella, or a mix.
• Warning: Protists are not a single group; they are polyphyletic. Not all protists are unicellular. Not all protists are heterotrophic. Protists are eukaryotes, not prokaryotes. Even multicellular protists have simple body structures. Protists do not form embryos.