Final answer:
When an arrow on the phase portrait points upwards, it indicates that both the pathogenic and probiotic bacteria population sizes will increase, representing exponential growth during the log phase.
Step-by-step explanation:
When an arrow on the phase portrait points straight up, it usually indicates that the population sizes of both the pathogenic and probiotic bacteria will increase over a short time period. This occurs during the log phase, also known as the exponential growth phase, where bacteria reproduce by prokaryotic fission and the population growth rate accelerates. The exponential growth pattern results in a J-shaped growth curve, demonstrating how population size increases more and more rapidly over time. Provided that nutrients are abundant and other growth conditions are ideal, both types of bacteria can display this kind of growth behavior. Hence, the correct answer is 1) The population sizes of both the pathogenic and probiotic bacteria will increase.