9. Mitochondria isolated and placed in a buffered solution with a low pH start to produce
ATP. This is likely due to:
a. lower pH inside mitochondria
b. increased OH- concentration in the mitochondrial matrix
c. increased movement of H+ from the inter-membrane space to the matrix
d. decreased H+ in the inter-membrane space

Mitochondria placed in a low pH buffered solution produce ATP due to the establishment of a proton gradient mimicking natural chemiosmosis. The correct answer is option c: increased movement of H+ from the inter-membrane space to the matrix.​

Option Analysis

Low pH outside creates high H⁺ concentration, which enters the permeable outer membrane but not the inner membrane initially, accumulating in the intermembrane space. This reverses the normal gradient (high H⁺ in intermembrane space, low in matrix), driving protons through ATP synthase into the matrix to synthesize ATP without electron transport.​

• Option a (lower pH inside mitochondria): Incorrect, as low external pH raises intermembrane H⁺, not matrix pH; matrix remains relatively alkaline.​

• Option b (increased OH⁻ in matrix): Incorrect, as low pH means high H⁺/low OH⁻ outside; no mechanism increases matrix OH⁻.​

• Option c (correct): H⁺ moves from intermembrane space to matrix via ATP synthase, powering ATP production per chemiosmotic theory.​

• Option d (decreased H⁺ in intermembrane space): Incorrect, as low pH increases intermembrane H⁺, essential for the gradient.​

In mitochondrial ATP production, placing isolated mitochondria in a low pH buffered solution triggers ATP synthesis through chemiosmotic principles, as high external H⁺ creates a proton gradient across the inner membrane. This “acid bath” phenomenon, similar to classic experiments, demonstrates how pH gradients drive oxidative phosphorylation without respiration. For CSIR NET Life Sciences aspirants, understanding this clarifies chemiosmosis in isolated organelles.​

Chemiosmotic Mechanism

ATP synthase uses the proton motive force—ΔpH and Δψ—where H⁺ flows from intermembrane space (acidic) to matrix (alkaline), rotating the enzyme to phosphorylate ADP. Low pH buffer permeates the outer membrane, acidifying intermembrane space while matrix stays neutral, prompting H⁺ influx. This supports Peter Mitchell’s hypothesis, validated in mitochondria and chloroplasts.​

Exam Relevance

CSIR NET questions test this via options distinguishing gradient direction; option c aligns with H⁺ movement enabling ATP. Reverse pH (high external) fails ATP production, confirming gradient necessity. Key for topics like electron transport decoupling.​