7.33
Visão Geral dos Protistas
Os protistas são microrganismos eucarióticos diversos que não possuem os tecidos especializados das plantas e dos animais, nem as paredes celulares qu…
Os protistas são microrganismos eucarióticos unicelulares, coloniais ou multicelulares encontrados em vários ambientes aquáticos e úmidos.
Eles desempenham papéis ecológicos essenciais, como produtores primários, decompositores e parasitas.
Os protistas são agrupados como semelhantes a animais, plantas ou fungos com base em como eles se movem, se alimentam e se reproduzem
Sua diversidade evolutiva os coloca em diferentes supergrupos Eukarya, como Archaeplastida, o clado SAR, Excavata e Amoebozoa. Algumas formas semelhantes a protistas também ocorrem em Opisthokonta, lar de fungos e animais.
Os protistas podem se reproduzir assexuada ou sexualmente. Alguns deles até exibem alternância de gerações ou formação de cistos para sobrevivência.
Eles usam estruturas como cílios, flagelos ou pseudópodes para movimento, vacúolos contráteis para osmorregulação e vacúolos alimentares para digestão.
Muitos protistas possuem mitocôndrias e cloroplastos, fornecendo fortes evidências que apóiam a teoria da endossimbiose, que afirma que as mitocôndrias e os cloroplastos evoluíram de células procarióticas engolfadas.
Em vez de serem digeridas, essas células engolfadas formaram uma relação simbiótica mutualística, eventualmente evoluindo para organelas permanentes.
View the full transcript and gain access to JoVE Core videos
Q1: What are the main characteristics that define protists?
Protists are unicellular, colonial, or multicellular eukaryotic microorganisms found in aquatic and moist environments. They lack specialized tissues like plants and animals, and lack the chitinous cell walls of fungi. Their early divergence within Eukarya resulted in remarkable structural, functional, and ecological diversity across multiple supergroups.
Q2: How do protists move and regulate their internal environment?
Protists use diverse locomotion structures including cilia, flagella, or pseudopodia for movement. Contractile vacuoles regulate osmosis in freshwater species, maintaining water balance. Some protists like dinoflagellates possess thecal plates for protection, while diatoms have silica-based cell walls that aid in defense and carbon sequestration.
Q3: What nutritional strategies do protists employ?
Protists use autotrophic, heterotrophic, or mixotrophic nutrition. Autotrophic protists like algae photosynthesize and serve as primary producers. Heterotrophic protists ingest or absorb organic matter. Mixotrophs like Euglena switch between photosynthesis and heterotrophy in response to light or nutrient availability, enhancing ecological flexibility.
Q4: How do protists reproduce and maintain genetic diversity?
Protists reproduce asexually via binary fission, budding, or multiple fission. Sexual reproduction, as seen in Paramecium, involves conjugation where cells exchange genetic material before division. Some protists like Laminaria exhibit alternation of generations, cycling between haploid and diploid stages to enhance genetic diversity and survival.
Q5: What does the endosymbiotic theory explain about protist organelles?
The endosymbiotic theory explains that early eukaryotes engulfed bacteria around 1.5 to 2 billion years ago, leading to organelle formation. Mitochondria arose from ancestral proteobacteria through primary endosymbiosis, while chloroplasts originated from cyanobacteria. Understanding these origins supports the three domain system of life framework.
Q6: What ecological roles do protists play in aquatic ecosystems?
Protists play key roles as primary producers, decomposers, and parasites in nutrient cycling and food webs. Photosynthetic protists contribute to oxygen production and carbon sequestration, while decomposers recycle organic matter. They serve as essential prey for larger organisms and sustain aquatic ecosystems through their diverse metabolic functions.
Q7: Which protists are medically significant and why?
Several protists cause serious human diseases. Plasmodium, transmitted by Anopheles mosquitoes, causes malaria. Trypanosoma induces African sleeping sickness, and Giardia leads to gastrointestinal infections. These pathogenic protists highlight the medical importance of understanding protist biology and their transmission routes for disease prevention and treatment.