Characteristics and Structure of the Rhizopus.
Characteristics and Structure of the Rhizopus.
Rhizopus is a fungus that lives on dead and decaying organic matter.
It survives in a warm, humid environment with organic matter as its substrate.
Rhizopus is whitish when it starts to grow and develops into a pale greyish mass as it matures, eventually becoming black.
It is the common mould on bread, kenkey and fruits.
Biological classification
Kingdom fungi
Reasons:
- They lack chlorophyll.
- Presence of hyphae, which form mycelium
- They possess no true roots, stems and leaves
- Presence of cell wall made of chitin
- They are eukaryotic.
Phylum Zygomycota
Reasons:
- Possess gametangia which form zygospores.
- Presence of a sporangium on a sporangiophore.
- They are non-septate (their hyphae have no cross wall)
Habitat of Rhizopus stolonifer
- Moist bread
- Fermented kenkey.
- Rotten fruits and vegetables.
Structure of Rhizopus.
The basic unit of Rhizopus is a microscopic thread-like structure called a hypha.- The hyphae form an interwoven mass called mycelium (plural – mycelia).
- Mycelium is the vegetative part of a fungus.
- There are three main parts of the mycelium.
These are stolon, rhizoid and sporangiophore.
Stolon:
- This is a hypha that grows horizontally on the substrate
Rhizoid:
- This is a root-like hypha which is more branched than the other two types and grows from the stolon into the substrate.
- Rhizoid supports the fungus on the substrate
- Secretes digestive enzymes
- Also, absorbs nutrients from the substrate.
Sporangiophore:
- This is an upright and unbranched hypha.
- It bears a swollen end called a sporangium (plural = sporangia).
- Sporangia contain spores, which are used in asexual reproduction.
Adaptive features of Rhizopus.
- Presence of sporangia, which produce spores and protect them.
- Possesses a sporangiophore which holds sporangia in position for the dispersal of spores.
- Mycelium for vegetative growth.
- Also, spores are present for asexual reproduction.
- Presence of rhizoid for absorption of nutrients, anchorage and secretion of digestive enzymes.
- Presence of a stolon for vegetative growth.
Mode of reproduction.
- Sexual reproduction by gametangia is a means of surviving adverse conditions.
- Asexual reproduction by spores in the sporangia.
Asexual reproduction in Rhizopus
- Asexual reproduction is by spores produced in the sporangium.
- In this process, a vertical hypha called a sporangiophore
- The tip of the sporangiophore then enlarges into a sporangium.
- During the development of the sporangium, dense cytoplasm containing several nuclei migrates to the periphery and develops into spores.
- The central portion contains a vacuole and is called the columella.
- Young sporangium is white or colourless.
- As the sporangium matures, it becomes black and dries.
- The walls crack and release the dry, powdery spores into the air.
- Spores land on suitable substrates such as moist bread or fermented kenkey.
Under favourable conditions, such as the presence of moisture, a spore develops into a hypha, and a hypha later develops into mycelium
Sexual reproduction
- Sexual reproduction in Rhizopus is called conjugation.
- Conjugation in Rhizopus involves the union of two different strains of mycelia (plus (+) and minus (-) strains).
- When the hyphae of (+) and (-) strains come together, they develop short swollen side branches called Progametangia (progametangium, which is singular)
- When the progametangia develop and meet a cross-wall, it develops between their tips.
- Each tip then develops into a gametangium (plural – gametangia) with a stalk called a Suspensor.
- Each gametangium contains several haploid nuclei.
- Later, the cross-wall breaks down.
- The gametangia fuse and their contents mix.
- Their nuclei pair up as + and -, which fuse to form many diploid nuclei.
- The fused gametangia become a zygospore.
- The zygospore grows and develops a thick, black and resistant wall, after which it is disconnected from the parent hyphae.
- The zygospore can withstand adverse conditions.
In favourable conditions, the zygospore undergoes meiosis, germinates and develops into a haploid hypha which is either (+) or (-).
Factors that enhance growth in Rhizopus.
- Suitable temperature for enzymatic activities.
- Moisture activates enzymes to digest food.
- Presence of dead organic matter/nutrients for the growth of mycelium.
Adaptations of Rhizopus to dispersal.
- Spores are light in weight.
- Numerous spores to ensure survival.
- Thickened walls of spores for protection against unfavourable environmental conditions.
Mode of nutrition.
- Rhizopus has no chlorophyll and undergoes a saprophytic mode of nutrition.
- The hyphae penetrate the substrate
- Secretes digestive enzymes on the food to digest it extracellularly, converting it into a soluble form.
- The end products are absorbed into hyphae by diffusion.
Movement in Rhizopus
- The hyphae grow and extend through the substrate.
- This is a form of growth movement.
- Although the spores do not move, they are dispersed through the air or water.
Respiration in Rhizopus
- They are primarily aerobic, but some species can switch to anaerobic respiration in low oxygen environments.
Sensitivity:
- Rhizopus responds to environmental stimuli like nutrient availability, light, and humidity.
- For example, the growth rate of Rhizopus slows down in low temperatures.
Growth:
- Rhizopus grows rapidly through spore germination and hyphal elongation.
Economic importance of Rhizopus
- Contaminates food or causes food spoilage, which leads to post-harvest losses.
- Causes decay or decomposition
- Improves soil fertility
- Causes diseases in plants.
- Some Rhizopus species are used in industrial fermentation processes, and in the production of traditional foods in some cultures (e.g. Rhizopus oligosporus is used in the production of tempeh).
- Natural recycling of wastes from the ecosystem.
- Spores of Rhizopus can contribute to indoor air quality problems and trigger allergic reactions.
- Some species of Rhizopus are used in enzyme production and in the production of organic acids (e.g. lactic acid, fumaric acid).
Questions For Revision.
1. State four parts of Rhizopus (bread mould) and their functions.
2. Name two habitats of Rhizopus.
3. Explain briefly how Rhizopus can be cultured in the laboratory.
4. Describe how Rhizopus obtain food.
5. State the effect of the presence of the following on Rhizopus.
- High humidity
- Temperature
- Dead organic matter
6. Briefly explain how Rhizopus recycle the soil nutrients by decomposition.
