Structure and Life Processes of Rhizopus (Bread Mold)

 Objectives       

This  blog post provides readers with the following objectives. The reader will be able to:

o   Describe the structure of Rhizopus

o   Explain nutrition and reproduction in Rhizopus.

Rhizopus (Bread Mold)

Kingdom: Fungi

Phylum: Zygomycota

Habitat:  It lives on decaying organic matter. They are mostly found on moist bread or rotting fruit, kept under warm and humid conditions. 

This image is credited to D G Mackean www.biology-resources.com

Structure of Rhizopus

Rhizopus is eukaryotic, multinucleated and non-photosynthetic organism.

The body is made of fine thread-like structures called hyphae (singular: hypha). 

The hyphae branch into a complicated network known as the mycelium.

The cell wall of the hypha consists of chitin (complex protein and carbohydrates). 

The hypha contains a thin layer of cytoplasm which is multinucleated (several nuclei). The center of the tube of hypha is occupied by vacuole. Hyphae lack cross walls or septa, and therefore described as non-septate or aseptate.

There are three types of specialized hyphae:

a.  Stolons: are unbranched hyphae, which grow horizontally on the substrate.

b. Rhizoids: are root-like structures. They anchor the fungus into its substrate, release digestive enzymes, and absorb nutrients for the fungus.

c. Sporangiophores: are the upright hyphae which bear round reproductive structures called sporangia (singular: sporangium)

Life Processes of Rhizopus

Nutrition in Rhizopus

The mode of nutrition is saprophytic or saprotrophic, meaning is not capable of synthesizing its own food and it depend on dead and decaying organic matter. The rhizoids secrete hydrolytic enzymes onto food substrate to break down complex molecules into simpler and soluble substances. The end products are absorbed into the body by diffusion. This method of digestion is described as extracellular digestion.

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Reproduction in Rhizopus

Rhizopus is able to reproduce both asexually and sexually (conjugation)

1. Asexual reproduction: it occurs when environmental conditions are favorable. The sporangiospores or spores are produced inside a spherical structure called sporangium. Sporangium absorb water from atmosphere, swell and burst to release or liberate the spores. The spores are dispersed by wind and can grow into new mycelia when settle on suitable substrate (such as damp bread).

2,  Conjugation: It involves opposite mating (positive and negative) strains coming into contact. These strains grow towards each other and meet by developing protrusion called progametangia (singular: progametangium). The tips of the progametangia swells and develops into gametangia. The gametangia become separated from the rest of the hyphae by forming a cross wall called suspensor. The walls between the two touching gametangium dissolve allowing the two multinucleate protoplasts to mix. The nuclei fuse in pairs forming several diploid nuclei and later develop into a zygospore. A black thick-walled, forms around the zygospore. The zygospore can withstand adverse conditions. The wall of the zygospore cracks and produces a young hypha with sporangium containing spores.

Adaptation

1.  Rhizoids for anchorage and nutrients absorption.

2.  Sporangiophore which support sporangium.

3.  Spores or sporangiospores for asexual reproduction and colonizing new habitat.

4.  Sporangium which supports and protect the spores.

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