The endomembrane system of the green algal flagellate, Gloeomonas kupfferi, is a highly ordered complex of endoplasmic reticulum sheets, 14–18 dictyosomes, vesicles and 8–12 peripheral vacuoles centered around a large nucleus. Each dictyosome is a highly polar cisternal stack with distinct cis and trans faces and an assortment of different-sized vesicles. The methylxanthine, caffeine, in concentrations of 1–2 mM, has been shown to be an effective disrupting agent of dictyosome integrity. For short treatments (1 h), caffeine causes an inhibition of typical trans-face vesiculation. Dictyosomes expand laterally and trans-face cisternal edges fuse. The resultant morphological products include exaggerated circular profiles and multidictyosomal masses. During longer treatment times (3–4 h), cis-face cisternae swell and ultimately disintegrate into large vesicles. Trans-face cisternae also vesiculate but do so into masses of tiny vesicles. The caffeine effects are reversible for up to 12 h, after which time cytological changes result in cell death. Phosphatase cytochemistry was also employed in this study to help reveal localized effects of caffeine. A putative explanation of the action of caffeine is presented.

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