Q2

Now that we have characterized the way the pH changes over time, we would like to understand the mechanism behind these changes. Below we propose several hypotheses which might explain the data. You should evaluate all these hypotheses and try to determine which of these explains ALL the data, which explains only SOME of the data, and which is not reasonable.

More than one possible mechanism may exist.
Thus, examine ALL answers (go from A to D)


A.The food vacuole membrane contains several proton pumps that allow the acidification of the food vacuole, once it has been formed.
B.The food vacuole membrane contains several proton pumps that allow the acidification of the food vacuole, once it has been formed. However, these proteins undergo proteolysis with time at low pH (between 7-10 minutes, as evident by Fig 1. Click HERE to see this figure again). As the number of active pumps decreases, the pH within the food vacuole gradually increases.
C.The food vacuole undergoes fusion with lysosomes shortly after it is formed. The membranes of these lysosomes contain proton pumps, which now transport protons (H⁺) into the enlarged food vacuole.This explains the initial decrease in the pH. As migration continues and digestion is completed, the lysosome membranes containing the proton pumps are pinched off the vacuole (and recycled). This results in pH increase in the vacuole.
D. The food vacuole membrane contains several hydroxyl (OH^-) pumps, that pump hydroxyl ions out of the vacuole, thus allowing the acidification of the vacuoles. However, these proteins undergo proteolysis as digestion progresses (i.e. their half life is very short). Therefore, gradually fewer pumps are still active, and so the pH within the food vacuole increases over time.

After examining ALL the options, continue.

Out of the four hypotheses suggested above, only two (B and C) were found to be plausible, since:

They explained ALL the observed data concerning the pH changes (initial acidification followed by pH increase), as opposed to hypothesis A, which explained only the initial acidification.
They suggested the existence of known cellular machinery, as opposed to hypothesis D, which suggested the existence of hydroxyl pumps.

Both hypotheses suggest that:

The acidification of the food vacuole is caused by the activity of proton pumps.
This activity is abolished after about 7-10 minutes (as is evident by the gradual pH increase shown in Figure 1).

The main differences

between the two hypotheses lie in:

The source of these proton pumps, which would influence the time course of events.
The mechanism of inactivation of the proton pumps.

Hypothesis B suggests that proton pumps are found in the food vacuole from the moment it is formed, whereas hypothesis C claims that the proton pumps are originally found in the membranes of lysosomes and become part of the food vacuole only after the lysosomes are fused to the food vacuoles, a process that happens shortly after the food vacuole formation.
Thus, according to hypothesis C there is a short interval at the beginning, when proton pumps are not found in the food vacuoles, whereas according to hypothesis B proton pumps are found in the food vacuole from the moment it is formed.

In order to test these hypotheses we have to determine:

Are there indeed proton pumps in the food vacuole?
If so, when are they present?

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