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4.3.5 Energy Storage and Metabolism
As energy resource Euglena builds carbohydrates, fats and oils, that are produced
exterior to the chloroplasts and are then stored in so-called paramylum 14 grains,
and in lipid, crystalloid and pyrenoid bodies.
Paramylon is isomeric with starch, but has additional β −1, 3-glucose linkages.
Paramylon occurs as membrane bound granule in the cytosol, the surrounding
membrane is somewhat distinctive since most storage grains are not directly bound
by a membrane. The paramylon granule also is unique among carbohydrate storage
products in plant groups because of its high crystallinity. Microfibrils of 4 nm
cross-section traverse the paramylon grain, composed of rectangular segments and
wedges (see Fig. 4.18), in an overall concentric pattern. ([108])
Figure 4.18: Electron micrograph of a freeze-etched paramylum granule.
The fracture plane has revealed the concentric layers of the inner material.
The arrow heads indicate where possible wedging or segmentation of the
inner material has occurred. The outer layer (remains of the unit membrane)
is indicated by the arrows. Scale bar is 500 nm. Image adapted from [109].
If there is not enough light to feed on, Euglena can turn for some time to a
heterotroph organism. It then uses energy rich compounds synthesized by other organisms.
Chloroplasts dissolve into proplastids and Euglena takes up its food in an
osmotrophic way through the cell surface. Additional to the uptake of solubilized
organic substances Euglena can transfect detritus and bacteria at the invagination
of the pellicle.
The numerous proplastids (with a size of 1−2µm in diameter) can regenerate
14 Paramylum is not detectable by the classical iodine-starch test. As iodine is not very soluble
in water, it is dissolved in water in the presence of potassium iodide - this makes a linear triiodide
ion complex which is soluble. The iodine molecule then slips inside of the amylose coil (if it is
not present the color of the solution stays orange or yellow) and the liquid changes to intense
blue-black color. ([110])
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