Question

recall that ph expresses the concentration of protons (h⁺) in a solution. most proteins denature - meaning that they lose tertiary structure and unfold - at ph values below 5 or over 10. protein denaturation is life - threatening, so most cells maintain an intracellular ph of 7.0 - 7.6. how is it possible, then, that some bacteria can live in your stomach, where the ph is 1.0, or in extremely basic habitats where the surrounding water has a ph of 12 or even 13? choose the single best answer.
their membrane proteins do not have tertiary structures, so are not susceptible to denaturing based on low or high ph.
the structure of their cell membranes strictly regulates the transport of protons into and out of the cell.
denaturing occurs at low and high ph because the presence of many or few protons disrupts the bonds and interactions that stabilize protein folding.

Explanation:

Most proteins denature at extreme pH values. Bacteria in extreme pH environments can survive because their cell - membrane structure regulates proton transport, maintaining an appropriate intracellular pH for protein stability. The first option is incorrect as membrane proteins do have tertiary structures. The third option is a general statement about denaturing and not an explanation for bacterial survival.

Answer:

The structure of their cell membranes strictly regulates the transport of protons into and out of the cell.