Chapter 5: Eukaryotic Microbes – Structure & Diversity

Eukaryotic cell envelopes feature a plasma membrane enriched in sterols and sphingolipids, and when present, a cell wall composed of chemically simpler polysaccharides like cellulose or chitin, rather than bacterial peptidoglycan. The dynamic cytoplasm contains the cytosol and an essential cytoskeleton organized into three types of interconnected filaments: actin filaments, microtubules, and intermediate filaments (the latter of which are absent in plants and fungi). Internal membrane systems manage synthesis and transport; the endoplasmic reticulum (RER and SER) is the primary site for protein and lipid synthesis, and it passes materials to the Golgi apparatus (or dictyosome) for further modification, packaging, and ultimate secretion. Misfolded proteins are managed by a built-in quality assurance system where they are tagged with ubiquitin and degraded by the 26S proteasome. Eukaryotic cells uniquely bring in external materials through various types of endocytosis—including phagocytosis, pinocytosis, and receptor-mediated pathways—which deliver substances to lysosomes or equivalent vacuoles for intracellular digestion via hydrolytic enzymes. Genetic information is housed in the visually prominent, double-membrane nucleus, where DNA is structured as chromatin coiled around histones to form nucleosomes. The nucleus also contains the nucleolus, responsible for synthesizing rRNA necessary for the larger 80S ribosomes. Energy conservation is carried out by double-membrane organelles believed to be derived from endosymbiotic bacteria: aerobic mitochondria generate ATP via oxidative phosphorylation using O2, while some anaerobic protists possess hydrogenosomes, which produce ATP by fermentation, yielding hydrogen gas. Photosynthetic protists feature chloroplasts, where light reactions take place on internal thylakoid membranes and carbon fixation occurs in the stroma. Motility is achieved using longer flagella or shorter, more numerous cilia, both sharing the characteristic 9 + 2 microtubule arrangement (axoneme) and utilizing the motor protein dynein for movement.