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Cytoplasm

The Eukaryotic Cell

The cytoplasm and most organelles and inclusions of fungal cytoplasm are typical of eukaryotic organisms.

NUCLEI: Nuclei are always present in living cells. In the fungi nuclei are 1 - 3 µm, somewhat smaller than most other eukaryotic organisms where they range from 3 - 10 µm. The appearance is similar, though the amount of DNA is generally less, even allowing for the haploid state of most nuclei. Nuclei may move through the cytoplasm of most filamentous fungi, even through the dolipore septum of the Basidiomycota. LINK


Mitochondrium.

MITOCHONDRIA: The mitochondria of fungi are clearly recognisable. They have a double bilayer membrane and contain complex internal membranes. They differ from other eukaryotic organisms in that the mitochondria are commonly elongate, oriented along the hyphal axis. The membranes are organised as parallel lamellae usually oriented along the long axis. This orientation is particularly common in older regions of the hypha where vacuoles comprise a large proportion of each compartment, and the cytoplasm is between the vacuole and the wall. The morphology of mitochondria in yeast cells may differ. Giant, branched mitochondria have been observed in yeasts, and intermediate forms occur in cells transforming from yeast-like to filamentous growth.

CYTOPLASM: The cytoplasm is typical is all respects of a eukaryotic cell. Of particular interest is the presence of plasmids. These have been characterised in yeasts. LINK As many as one hundred plasmids are found in yeast cells. Plasmids are also found in filamentous fungi, where some are associated with disease virulence. LINK

MEMBRANES: The organelles are associated with an internal membrane system, in some cases linked in with parts of the membrane. This seems particularly important for movement of materials about the cell, from the site of formation to the site of delivery. The components include vesicles and tubules.

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Tubules, Vesicles and Vacuoles

Vacuoles are essential for cell function in fungi. Fungi are characterised by the presence of spherical to tubular vacuoles. Tubular vacuoles are located longitudinally in the hyphae, they pass through pores of the septa, interconnecting parts of the thallus. The tubules may form a network within a cell. The structure and distribution of tubular vacuoles differs between fungi, especially between filamentous and yeast-like fungi, and change with maturity of the fungus.

The discovery of tubules is relatively recent. Previously, use of chemical fixative caused tubules to fragment into vesicles. Use of fluorescent markers and microscopic examination of living hyphae have enabled the tubules to be observed and their dynamism followed. Tubules are most concentrated in the hyphal tip. It appears that tubules swell forming localised vesicles. The vesicles are moved rapidly along the tubule, through pores of the septum, and to or from hyphal tips. Each compartment may have many tubules operating, at different speeds, and in both directions simultaneously.

Movement of tubules and associated vesicles is controlled by the cytoskeleton. Vacuoles in filamentous fungi utilise microtubules and their motor proteins for movement. Yeasts appear to rely more on actin cables for movement.

In older hyphae, the principle component of cellular compartments appears to be the vacuole. The vacuoles appear to have several functions. Recent evidence suggests that the vacuoles may have considerable activity and dynamism. They may be involved with recycling, storage and transport. Vacuoles appear to store enzymes, other macromolecules, lipids, mineral nutrients such as polyphosphate, and toxins. The control of pH and ion homeostasis in compartments appears to rest within the vacuole. Thus vacuoles play a critical and complex role in fungi.

Because vacuoles and the tubular membrane system are linked, it has been postulated that fungi manufacture growth requirements in vacuoles, and deliver growth requirements to other parts using the tubular system. In growth, wall components may be manufactured in vacuoles and delivered to the tip, where the wall is laid down. The wall components would include wall precursors, and enzymes necessary for wall growth. At the same time, enzymes for extracellular digestion would be delivered from the endoplasmic reticulum to the tip, and the results of digestion transported from the tip to sites of storage.

OTHER ORGANELLES: A number of organelles appear in specific taxa. The largest division of fungi are the Ascomycota. Filamentous forms always have an inclusion called the Woronin body that is associated with the simple pore of the septum. Woronin bodies are spherical electron dense bodies approximately 0.1 to 1 mm usually seen surrounded by an electron light halo in EM preparations. The contents are probably protein, surrounded by a membrane. While their function is unclear, Woronin bodies appear to block pores of the septum following damage to compartments.

Finally, some vesicles called lomasomes are located between the plasmamembrane and the wall. The function of lomasomes is unclear. Because they are always located in zones of active hyphal elongation, a role in polymer synthesis and deposition has been suggested.

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Anaerobic Fungi

HYDROGENOSOMES: In some obligately anaerobic organisms, including fungi, an organelle is observed which appears to produce hydrogenase and pyruvate oxidoreductase. The enzymes function in the anaerobic conversion of organic carbon to energy. The organelle is called a hydrogenosome. Hydrogenosomes have been observed in the zoosporic fungi found in the rumen of herbivores.

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Polarity

Fungal compartments have a decided differentiation from one end to the other. LINK The differences at the tip are most pronounced, where walls are thinner and less well differentiated, and structures associated with delivery of materials for elongation are most obvious and active. In addition, the Ca++ concentration in the cytoplasm is higher at the tip. Electrical fields are associated with physical structures and chemical signals. While they differ along the hyphal tip, an applied field may alter rates and direction of tip growth. The overall evidence indicates the electrical fields are a consequence of tip regulation rather than the cause. Finally, turgor pressure appears to be associated with hyphal elongation, indicating that the movement of ions and water into the cell is highly regulated, and part of the explanation of polarity.

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Conclusion

Fungi have a typical eukaryotic structure, which is modified to enable a filamentous lifestyle. With few exceptions, fungi contain structures which are similar to those found in other eukaryotic organisms, or which have similar functions. The structures that differentiate fungi from other eukaryotic microbes commonly increase the effective functioning of an elongate organism.

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References

Gow NAR & Gadd GM 1995 The Growing Fungus. Chapman Hall

Richards A, Veses & Gow NAR 2010 Vacuole dynamics in fungi. Fungal Biology Reviews doi:10.1016/j.fbr.2010.04.002

Weber RWS 2002 Vacuoles and the fungal lifestyle. Mycologist 16: 10 - 20.

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