Final answer:
A flow unit in a biological system undergoes either a spontaneous process, occurring naturally, or a nonspontaneous process, requiring external energy. In plants, the two main bulk flow processes are transpiration and root pressure.
Step-by-step explanation:
As a flow unit moves through a process in a biological system, it generally undergoes one of two things: spontaneous processes or nonspontaneous processes. A spontaneous process occurs naturally under certain conditions without the need for an external energy input. For example, water naturally flowing downhill. In contrast, a nonspontaneous process requires energy from an external source to occur, like water being pumped uphill. In the context of plants, bulk flow is a critical process for the transport of fluids. The two main bulk flow processes are transpiration, which drives the upward movement of water from roots to the leaves through xylem, and root pressure, where water is pushed up from the roots due to water entering the roots from the soil.