23.2 Characteristics of Protists

Protist Cell Structure and Organization

Protists are eukaryotic organisms that don't fit neatly into the plant, animal, or fungal kingdoms. They're incredibly diverse, ranging from single-celled amoebae to giant multicellular kelp. Understanding their characteristics means understanding how flexible eukaryotic life can be.

Structural features of protist cells

Every protist cell is eukaryotic, meaning it has a true nucleus and membrane-bound organelles. Beyond that shared trait, protist cells vary enormously in how they're organized.

Cellular organization falls into three broad categories:

• Unicellular: A single cell carries out all life functions (amoebae, diatoms)
• Colonial: Individual cells live together in a group but with limited specialization (Volvox, a hollow sphere of cells where only a few are reproductive)
• Multicellular: Cells differentiate into distinct types with specific roles (kelp, which can grow over 30 meters long and has blade, stipe, and holdfast structures)

Key organelles found across protist groups:

• Mitochondria for aerobic respiration and ATP production
• Chloroplasts in photosynthetic species like Euglena and green algae
• Endoplasmic reticulum and Golgi apparatus for synthesizing and transporting proteins and lipids
• Contractile vacuoles for osmoregulation, especially in freshwater species like Paramecium, where water constantly flows in by osmosis and must be pumped out

Specialized structures give protists their remarkable adaptations:

• The pellicle is a flexible protein-rich outer covering found in organisms like Euglena. Unlike a rigid cell wall, it allows the cell to change shape while still maintaining structural support.
• Locomotion structures include cilia (short, hair-like projections covering Paramecium), flagella (longer whip-like structures on Euglena), and pseudopodia (temporary cytoplasmic extensions used by Amoeba to creep along surfaces).
• Extrusomes are membrane-bound organelles that can discharge their contents outside the cell. Trichocysts in Paramecium fire needle-like projections for defense, while nematocysts in some dinoflagellates help with prey capture.

Protist diversity and classification

Because protists are so varied, they're often grouped informally by how they obtain nutrition or how they move:

• Protozoans are animal-like protists. They're typically motile and heterotrophic, feeding on bacteria, other protists, or organic debris. Examples include Amoeba, Paramecium, and Plasmodium (the malaria parasite).
• Algae are plant-like protists that carry out photosynthesis. They range from unicellular diatoms to massive multicellular kelp forests. Algae produce a significant portion of Earth's oxygen.

These categories aren't formal taxonomic groups. They're convenient labels, and some organisms blur the lines (like Euglena, which photosynthesizes but can also feed heterotrophically).

Symbiosis plays a major role in protist ecology. Zooxanthellae are photosynthetic dinoflagellates that live inside coral tissue, providing the coral with sugars from photosynthesis in exchange for shelter and nutrients. When corals lose their zooxanthellae due to environmental stress, coral bleaching occurs.

Endosymbiotic theory explains how protists acquired some of their organelles. According to this theory, mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Over time, these engulfed organisms became permanent organelles. Key evidence includes the fact that both mitochondria and chloroplasts have their own double membranes and circular DNA, similar to bacteria.

Protist Metabolism and Life Cycles

Metabolic strategies in protists

Protists use three main strategies to obtain energy, and these strategies reflect how adaptable they are to different environments.

• Phototrophy: These protists use light energy to drive photosynthesis, just like plants. They contain chloroplasts or other chlorophyll-bearing structures. Examples include euglenoids, dinoflagellates, and green algae.
• Heterotrophy: These protists obtain organic compounds from their environment. Two common mechanisms are:
  • Phagocytosis: The cell engulfs and digests food particles or whole organisms. Amoeba surrounds prey with pseudopodia and pulls it into a food vacuole.
  • Pinocytosis: The cell takes in dissolved nutrients from the surrounding fluid through tiny vesicles. Paramecium uses this alongside its oral groove for feeding.
• Mixotrophy: Some protists can switch between phototrophy and heterotrophy depending on conditions. When light is available, they photosynthesize; when it's scarce, they feed heterotrophically. Certain dinoflagellates and chrysophytes (golden algae) use this flexible strategy.

Life cycle patterns of protists

Protist reproduction ranges from straightforward cell division to complex multi-stage life cycles.

Asexual reproduction produces genetically identical offspring:

1. Binary fission: The parent cell divides equally into two daughter cells. This is the most common method (Amoeba, Paramecium).
2. Multiple fission: The parent cell's nucleus divides several times before the cytoplasm splits, producing many daughter cells at once. Plasmodium uses this inside red blood cells during a malaria infection.
3. Budding: A small outgrowth forms on the parent and eventually separates as a new individual. (Note: Hydra is sometimes listed as an example, but Hydra is actually a cnidarian animal, not a protist. Budding does occur in some protists like certain yeasts and suctorians.)
4. Fragmentation: The organism breaks into pieces, each capable of growing into a complete new individual. This occurs in some multicellular algae.

Sexual reproduction introduces genetic variation:

1. Conjugation: Two cells temporarily fuse and exchange genetic material through a cytoplasmic bridge, then separate. Paramecium does this, and both cells end up with new genetic combinations.
2. Syngamy: Two gametes permanently fuse to form a diploid zygote. Volvox and Chlamydomonas reproduce this way.
3. Meiosis: The diploid zygote undergoes reduction division to produce haploid offspring. In Plasmodium, meiosis occurs shortly after zygote formation.

Life cycle variations differ in which ploidy stage dominates:

Some protists have especially complex life cycles involving multiple developmental stages like cysts (dormant, resistant forms), spores, and gametes. Foraminifera and apicomplexans like Plasmodium cycle through several distinct body forms, often in different host organisms or environments.