Why do mitochondria have an inner membrane folded into cristae rather than a smooth inner membrane?
ACristae increase mitochondrial volume so more Krebs cycle enzymes fit in the matrix
BCristae increase the surface area of the inner membrane, accommodating more ATP synthase and ETC complexes
CCristae protect the mitochondrial DNA from reactive oxygen species
DCristae separate the Krebs cycle from glycolysis to prevent interference
The ETC complexes and ATP synthase are embedded in the inner membrane. More surface area means more of these complexes can be packed in, dramatically increasing the mitochondrion's ATP output capacity. This is the same design principle as intestinal villi or lung alveoli — folding is a common biological strategy for maximizing functional surface area within a fixed volume.
Question 2 True / False
A mitochondrion with its own DNA and ribosomes could survive and replicate independently if isolated from the cell.
TTrue
FFalse
Answer: False
Mitochondrial DNA encodes only ~13 proteins (in humans) — a tiny fraction of the ~1,500 proteins mitochondria require. The vast majority are encoded in the nuclear genome, synthesized by cytoplasmic ribosomes, and imported into the mitochondrion via translocase complexes. Mitochondria are fundamentally dependent on the cell's protein-import machinery and cannot replicate or function independently. Their genome is a vestige of their prokaryotic ancestor, substantially reduced over ~2 billion years of endosymbiosis.
Question 3 Short Answer
Why is the intermembrane space specifically important for ATP synthesis, rather than just any compartment outside the inner membrane?
Think about your answer, then reveal below.
Model answer: The ETC pumps protons from the matrix into the intermembrane space, creating a concentrated reservoir of H⁺. Because the intermembrane space is enclosed by the outer membrane, the protons cannot simply diffuse away — they are trapped at a high concentration. ATP synthase spans the inner membrane, and those trapped protons flow back through it into the matrix, driving ATP production.
ATP synthesis depends on maintaining a proton concentration difference (gradient) across the inner membrane. The intermembrane space acts as a holding chamber: the outer membrane is permeable to small ions (via porins) but the enclosed geometry keeps the proton-motive force intact long enough for ATP synthase to harvest it. If the outer membrane were absent, protons would equilibrate with the cytoplasm and the gradient would collapse.