USA Biology Olympiad (USABO) Practice Exam

According to the pressure-flow hypothesis of phloem transport, how does solute move?

1. From a lower concentration in the source to a higher concentration in the sink

2. From a higher concentration in the source to a lower concentration in the sink

3. At equal concentrations throughout

4. Only when energy is supplied

The correct answer is: From a higher concentration in the source to a lower concentration in the sink

The pressure-flow hypothesis explains how phloem transport operates from the source, where photosynthesis occurs or where nutrients are stored, to the sink, where these solutes are utilized or stored for later use. In this model, solute moves from an area of higher concentration at the source to an area of lower concentration at the sink. As solutes accumulate at the source, they create a gradient that drives water movement into the phloem through osmosis. This influx of water increases turgor pressure within the phloem sieve tubes, generating a pressure gradient that facilitates the movement of phloem sap toward areas with lower pressure, typically the sinks. This movement is driven primarily by differences in osmotic pressure, meaning that energy expenditure in the form of ATP is not necessary for the flow of solutes once the initial nutrient loading occurs at the source. Therefore, this model effectively captures the dynamic nature of phloem transport, underscoring how solutes and water work together to maintain the flow from sources to sinks in plants.