USA Biology Olympiad (USABO) Practice Exam

What do mycorrhizal fungi primarily obtain from plants in their symbiotic relationship?

1. Carbohydrates.

2. Nitrogen compounds.

3. Water molecules.

4. Carbon dioxide.

The correct answer is: Carbohydrates.

Mycorrhizal fungi primarily obtain carbohydrates from plants in their symbiotic relationship, making them essential partners in the plant ecosystem. The relationship between mycorrhizal fungi and plants is a type of mutualism; in exchange for carbohydrates produced through photosynthesis, which the fungi cannot produce themselves, the fungi enhance water and nutrient uptake for the plant, particularly phosphorous and nitrogen. The carbohydrates serve as a primary energy source for the fungi, enabling them to grow and spread their hyphal networks, which can significantly increase the surface area for nutrient absorption. This benefit is crucial for the plant, especially in nutrient-poor soils, as mycorrhizal associations can enhance the overall health and growth of plants. In contrast, while some mycorrhizal fungi may assist in nitrogen uptake, they are not primarily acquiring nitrogen compounds, nor do they obtain water in a form that is typical for plants. Additionally, carbon dioxide, being a product of respiration rather than a resource obtained from plants, is not relevant in this context. Understanding the significance of this symbiotic relationship highlights the vital roles that both plants and mycorrhizal fungi play in ecosystems.