USA Biology Olympiad (USABO) Practice Exam

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How do arbuscules contribute to the symbiotic relationship between fungi and plants?

By increasing the surface area for nutrient exchange.
By providing defense against herbivores.
By regulating water loss in plant tissues.
By facilitating photosynthesis in fungi.

The correct answer is: By increasing the surface area for nutrient exchange.

Arbuscules are specialized structures formed by mycorrhizal fungi within the root cells of plants, and they play a crucial role in enhancing the mutualistic relationship between these organisms. The primary function of arbuscules is to increase the surface area for nutrient exchange between the fungus and the plant. This increased surface area allows for more efficient uptake of essential minerals and nutrients, particularly phosphorus, which is often limited in soil. The plant benefits from this enhanced nutrient availability, leading to improved growth and health. Meanwhile, the fungus receives carbohydrates and other organic compounds from the plant, which it uses as a source of energy. This reciprocal exchange of nutrients characterizes the synergistic nature of the symbiotic relationship, making the presence of arbuscules a critical factor for both parties' success.