Histidine has pKa values of 1.8 (α-carboxyl), 6.0 (imidazole side chain) and 9.2 (α-amino). What is the net charge of most histidine molecules at pH 7.4?
Pick one. The red pen does the rest.
The deciding lineA group is mostly protonated at a pH below its pKa and mostly deprotonated at a pH above it.
Why A: At pH 7.4, the carboxyl (pKa 1.8) is deprotonated, −1; the imidazole (pKa 6.0) is mostly deprotonated and neutral; the amino group (pKa 9.2) is still protonated, +1. Net charge: 0.
Why each choice fails
AHolds: −1 + 0 + 1 = 0.
BDirection reversed: +1 keeps the imidazole protonated, but pH 7.4 is above its pKa of 6.0, so most imidazole rings have lost that proton.
CDirection reversed twice: +2 also leaves the carboxyl protonated, but pH 7.4 is far above its pKa of 1.8.
DWrong comparison: −1 deprotonates the amino group, but pH 7.4 is below its pKa of 9.2, so it keeps its proton.
2,4-Dinitrophenol (DNP) is a lipid-soluble weak acid that carries H+ across the inner mitochondrial membrane. Shortly after DNP is added to cultured cells respiring aerobically, which change is expected?
Pick one. The red pen does the rest.
The deciding lineIn chemiosmotic coupling, electron transport pumps H+ out of the matrix and ATP synthase uses the returning H+ flow to make ATP; dissipating the gradient lets electron transport and O₂ use speed up while mitochondrial ATP synthesis falls.
Why D: DNP lets H+ leak back into the matrix without passing through ATP synthase, so the gradient collapses. Electron transport, no longer held back by the gradient, speeds up and uses more O₂, while ATP synthesis falls; the energy is released as heat.
Why each choice fails
ADirection reversed: DNP picks up H+ in the acidic intermembrane space and releases it in the matrix, so it carries H+ down the gradient and makes it shallower, not steeper; O₂ use then rises and ATP synthesis falls.
BLook-alike terms: O₂ use and ATP synthesis falling together describe an electron-transport inhibitor such as cyanide; DNP is an uncoupler, which leaves the chain working and makes it run faster.
COne step short: electron transport does speed up and pump more H+, but that H+ flows back into the matrix through DNP instead of through ATP synthase, so mitochondrial ATP synthesis falls.
DHolds: with H+ returning through DNP instead of ATP synthase, electron transport and O₂ use accelerate while ATP synthesis drops.
A protein with an isoelectric point (pI) of 5.0 is placed in a gel buffered at pH 8.0 and an electric field is applied. The protein will carry a:
Pick one. The red pen does the rest.
The deciding lineA protein is negatively charged when the pH is above its pI and positively charged when the pH is below its pI; in an electric field it migrates toward the electrode of opposite sign.
Why D: At pH 8.0, above the pI of 5.0, the protein is net negative and moves toward the positive electrode (the anode).
Why each choice fails
ADirection reversed: above the pI the protein is negative, not positive.
BOne step short: a net positive charge would move toward the negative electrode; both halves need to match.
CConditions ignored: net charge is zero only at the pI (pH 5.0), not at pH 8.0.
DHolds: pH > pI gives a net negative charge, drawn to the positive electrode.
If it isn’t helping, reply to your receipt email, or DM @redpenprep on Instagram within 30 days and get every dollar back. No questions asked. Email support
