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Difficulty: Very hardKingdom Fungi: Moulds, Yeasts, and Mushrooms

A biological experiment compares metabolic activities and cellular organization between the filamentous bread mould *Rhizopus stolonifer* grown on a starch-agar substrate and the unicellular yeast *Saccharomyces cerevisiae* cultured in a liquid glucose broth under anaerobic conditions. Which of the following statements correctly accounts for their extracellular digestive mechanisms and cellular structural adaptations?

  1. *Rhizopus stolonifer* secretes amylase externally via its hyphae to hydrolyze starch into soluble sugars for absorption, while *Saccharomyces cerevisiae* directly absorbs dissolved glucose across its chitinous cell wall to undergo intracellular fermentation.Answer
  2. B
    *Rhizopus stolonifer* engulfs solid starch particles holozoically via phagocytosis into food vacuoles, while *Saccharomyces cerevisiae* utilizes chloroplasts to photosynthesize glucose before fermentation.
  3. C
    *Rhizopus stolonifer* possesses a cell wall composed of peptidoglycan that actively transports intact starch polymers, while *Saccharomyces cerevisiae* lacks a cell wall to facilitate endocytosis of glucose.
  4. D
    *rhizopus stolonifer* secretes digestive enzymes internally through septate hyphae, whereas *saccharomyces Cerevisiae* absorbs glucose using a cellulose-reinforced cell membrane.

Answer

*Rhizopus stolonifer* secretes amylase externally via its hyphae to hydrolyze starch into soluble sugars for absorption, while *Saccharomyces cerevisiae* directly absorbs dissolved glucose across its chitinous cell wall to undergo intracellular fermentation.
The statement accurately reflects fungal saprophytism: *Rhizopus stolonifer* releases extracellular enzymes (e.g., amylase) from its coenocytic hyphae to break down complex starch into simple glucose molecules, which are then absorbed across its chitin cell wall. In contrast, *Saccharomyces cerevisiae* directly absorbs available glucose across its chitinous wall to conduct cytosolic glycolysis and alcoholic fermentation in anaerobic conditions.

Step-by-Step Solution

1
Analyze the mode of nutrition in Kingdom Fungi
All fungi are heterotrophic saprophytes or parasites that perform extracellular digestion by secreting enzymes (such as amylase or cellulase) onto complex organic substrates and absorbing the resulting soluble nutrients.
Fungi cannot ingest solid food particles (holozoic) nor photosynthesize (autotrophic).
2
Examine the cellular structural composition of fungal cell walls
The cell walls of both moulds (*Rhizopus*) and yeasts (*Saccharomyces*) are made of chitin, distinguishing them from bacterial peptidoglycan and plant cellulose.
Chitin provides structural rigidity while permitting osmosis and passive/active transport of simple dissolved monomers like glucose.
3
Evaluate the metabolic pathways of the two fungal archetypes in the given scenario
*Rhizopus stolonifer* hydrolyzes insoluble starch on agar into glucose prior to absorption. *Saccharomyces cerevisiae*, already in a simple glucose medium under anaerobic conditions, absorbs glucose directly and ferments it into ethanol and carbon dioxide.
Direct uptake of simple sugars bypasses the need for extracellular polysaccharide cleavage when monosaccharides are already available.

Key Concept

Saprophytic Extracellular Digestion and Chitinous Cellular Structure in Fungi
Estimated Time:2m 0s
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Topics

Kingdom Fungi: Moulds, Yeasts, and Mushrooms

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