Mitochondria and Chloroplasts Questions and Answers

Answer: A

Correct answer: cristae

The cristae are the infoldings of the inner mitochondrial membrane, and their number and depth track how much oxidative work the cell does - a cardiac muscle mitochondrion is packed with them. Thylakoids and grana belong to the chloroplast, which achieves the same end by a different geometry: stacked flattened sacs rather than folds.

Answer: B

Correct answer: matrix

The matrix is the compartment inside the inner membrane, and it holds the enzymes of the citric acid cycle and of fatty acid oxidation, along with the mitochondrial DNA and its ribosomes. The intermembrane space lies between the two membranes and is where protons accumulate; the stroma and thylakoid lumen are the corresponding chloroplast compartments.

Answer: C

Its enzymes are dissolved in the matrix, whereas the electron transport chain is embedded in the inner membrane beside them. Glycolysis, by contrast, happens in the cytosol before pyruvate ever reaches the mitochondrion. The three stages are thus spread across two separate compartments.

Answer: D

Embedding the carriers in a membrane is essential, because pumping protons across it is how the energy of the electrons is captured. A chain floating free in solution could store nothing. Compartmentation is therefore not incidental to respiration but essential to it.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the purpose. The rate of ATP production is limited by how much membrane is available to carry the respiratory complexes and the ATP synthase, so folding the inner membrane into cristae multiplies that area several times within the same volume. The outer membrane carries no such machinery and needs no such area.

Answer: B

Correct answer: a proton gradient across a membrane

Mitchell’s proposal was that the respiratory chain pumps protons out of the matrix, and that the electrochemical gradient so created - not any chemical intermediate - drives ATP synthesis as protons flow back through the ATP synthase. It was resisted for years because it required a *membrane* to be the coupling device rather than a molecule, and it earned the Nobel Prize in 1978.

Answer: C

Accumulating there makes the intermembrane space acidic relative to the matrix, and the resulting gradient stores the energy. The narrowness of the space helps the concentration build quickly. A wider space would dilute the protons and weaken the gradient.

Answer: D

The outer membrane is freely permeable to small molecules and ions through porins, so no gradient can be maintained across it. The whole gradient sits across the inner membrane, which is where the pumps and the synthase are.

Answer: A

The flow turns a rotor, and that mechanical rotation drives the chemistry at the catalytic head. It is one of the few molecular machines that genuinely rotates. Each full turn of the rotor yields three molecules of ATP.

Answer: B

Four protons per ATP over thirty molecules requires 120. The ratio is not a whole number in reality, which is one reason ATP yields per glucose are now quoted as approximate. Earlier textbook figures of 36 or 38 ATP per glucose were falsely precise.