Introduction to Kingdom Protista
The Kingdom Protista includes an incredible diversity of different types of organisms, including algae, protozoans, and slime molds. No one even knows how many species there are, though estimates range between 65,000 to 200,000. All protists are eukaryotes, complex cells with nuclear membranes and organelles like mitochondria and chloroplasts. They can be either unicellular or multicellular, and in this group we find the first inkling of what is to come in evolutionary history, the union of eukaryotic cells into a colonial organism, where various cell types perform certain tasks, communicate with one another, and together function like a multicellular organism.
Some protists are autotrophs, a photosynthetic group of phyla referred to as the algae. Autotrophs manufacture their own energy by photosynthesis or chemosynthesis. Algae use various combinations of the major chlorophyll pigments, chlorophyll a, b, and c, mixed with a wide array of other pigments that give some of them very distinctive colors.
Some protists are heterotrophs, a group of phyla called the protozoa. Heterotrophs get their energy by consuming other organisms. Protists reproduce asexually by binary fission, and a few species are capable of sexual reproduction. Many have very complex life cycles.
Protists are so small that they do not need any special organs to exchange gases or excrete wastes. They rely on simple diffusion, the passive movement of materials from an area of high concentration to an area of low concentration, to move gases and waste materials in and out of the cell. Diffusion results from the random motion of molecules (black and white marble analogy). This is a two-edged sword. They don�t need to invest energy in complex respiratory or excretory tissue. On the other hand, diffusion only works if you�re really small, so most protists are limited to being small single cells. Their small size is also due to the inability of cilia or flagella to provide enough energy to move a large cell through the water.
Protists eat by phagocytosis - they engulf their food in their cell membrane, and pinch off a section of membrane to form a hollow space inside the cell. This hollow space, now enclosed by membranes, is called a vacuole. Vacuoles are handy little structures. Protists also use them to store water, enzymes, and waste products. Paramecium and many other protists have a complex type called a contractile vacuole, which drains the cell of waste products and squirts them outside the cell.
All protists are aquatic. Many protists can move through the water by means of flagella, or cilia, or pseudopodia (= false feet). Cilia and flagella are tiny movable hairs. Motile cells usually have one or two long flagella, or numerous shorter cilia. The internal structure of cilia and flagella is basically the same. All of the characteristics that this group shares are primitive traits, a perilous thing to base any classification on, because convergent evolution may be responsible for these superficial similarities. So the concept of the Kingdom Protista has been justly criticized as a �taxonomic grab bag� for a whole bunch of primitive organisms only distantly related to one another.
Protists are mainly defined by what they are not - they are not bacteria or fungi, they are not plants or animals. Protists gave rise to all other plants and animals. But where did protists themselves come from? The earliest protists we can recognize in the fossil record date back to about 1 billion, 200 million years ago. We do not know how the various groups of protists are related to one another. We assume they arose from certain groups of bacteria, but which groups and when are still investigating. Different phyla of protists are so unlike one another, many probably evolved independently from completely different groups of bacteria. Lynn Margulis recognizes nearly 50 different phyla of protists, or Protoctista, as this kingdom is sometimes called. We will take a more conservative approach, and focus on nine important phyla of protists.
Some protists are autotrophs, a photosynthetic group of phyla referred to as the algae. Autotrophs manufacture their own energy by photosynthesis or chemosynthesis. Algae use various combinations of the major chlorophyll pigments, chlorophyll a, b, and c, mixed with a wide array of other pigments that give some of them very distinctive colors.
Some protists are heterotrophs, a group of phyla called the protozoa. Heterotrophs get their energy by consuming other organisms. Protists reproduce asexually by binary fission, and a few species are capable of sexual reproduction. Many have very complex life cycles.
Protists are so small that they do not need any special organs to exchange gases or excrete wastes. They rely on simple diffusion, the passive movement of materials from an area of high concentration to an area of low concentration, to move gases and waste materials in and out of the cell. Diffusion results from the random motion of molecules (black and white marble analogy). This is a two-edged sword. They don�t need to invest energy in complex respiratory or excretory tissue. On the other hand, diffusion only works if you�re really small, so most protists are limited to being small single cells. Their small size is also due to the inability of cilia or flagella to provide enough energy to move a large cell through the water.
Protists eat by phagocytosis - they engulf their food in their cell membrane, and pinch off a section of membrane to form a hollow space inside the cell. This hollow space, now enclosed by membranes, is called a vacuole. Vacuoles are handy little structures. Protists also use them to store water, enzymes, and waste products. Paramecium and many other protists have a complex type called a contractile vacuole, which drains the cell of waste products and squirts them outside the cell.
All protists are aquatic. Many protists can move through the water by means of flagella, or cilia, or pseudopodia (= false feet). Cilia and flagella are tiny movable hairs. Motile cells usually have one or two long flagella, or numerous shorter cilia. The internal structure of cilia and flagella is basically the same. All of the characteristics that this group shares are primitive traits, a perilous thing to base any classification on, because convergent evolution may be responsible for these superficial similarities. So the concept of the Kingdom Protista has been justly criticized as a �taxonomic grab bag� for a whole bunch of primitive organisms only distantly related to one another.
Protists are mainly defined by what they are not - they are not bacteria or fungi, they are not plants or animals. Protists gave rise to all other plants and animals. But where did protists themselves come from? The earliest protists we can recognize in the fossil record date back to about 1 billion, 200 million years ago. We do not know how the various groups of protists are related to one another. We assume they arose from certain groups of bacteria, but which groups and when are still investigating. Different phyla of protists are so unlike one another, many probably evolved independently from completely different groups of bacteria. Lynn Margulis recognizes nearly 50 different phyla of protists, or Protoctista, as this kingdom is sometimes called. We will take a more conservative approach, and focus on nine important phyla of protists.
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