Proton pumps embedded in the cell membrane use ATP to move hydrogen ions across the membrane. Which of the following best descri
bes a function of the proton pump? A. It decreases the pH inside the cell to prevent toxicity. B. It generates a voltage across the membrane as a way to store energy. C. It helps maintain homeostasis by absorbing electrical charges. D. It increases the fluidity of the cell membrane as energy is used.
Although protons resemble other positive ions such as Na+ and K+ in their movement across membranes, in some respects they are unique. Hydrogen atoms are by far the most abundant type of atom in living organisms; they are plentiful not only in all carbon-containing biological molecules, but also in the water molecules that surround them. The protons in water are highly mobile, flickering through the hydrogen-bonded network of water molecules by rapidly dissociating from one water molecule to associate with its neighbor, Protons are thought to move across a protein pump embedded in a lipid bilayer in a similar way: they transfer from one amino acid side chain to another, following a special channel through the protein.
