Mitochondria—Early Evidence for Osmotic Activity Historically, mitochondria were recognized as osmotically active organelles. Which observation best supports this conclusion about their membranes?

Introduction / Context:
Before the molecular era, classical experiments assessed mitochondrial integrity by observing osmotic swelling and contraction. These behaviors indicated that mitochondrial membranes act as selectively permeable barriers, a hallmark of osmotically active compartments.

Given Data / Assumptions:

• Isolated mitochondria respond to changes in external osmolarity.
• Swelling occurs in hypotonic media; shrinkage occurs in hypertonic media.
• These responses require intact membranes.

Concept / Approach:
Semi permeable membranes allow water to move in response to solute gradients. Mitochondria exhibit changes in volume when exposed to different osmotic conditions, demonstrating the presence and function of such membranes long before detailed lipid–protein compositions were known.

Step-by-Step Solution:

Verification / Alternative check:
Electron micrographs show swollen mitochondria under hypotonic conditions; functional assays tie membrane permeability to oxidative phosphorylation competence.

Why Other Options Are Wrong:

• Statements about exact tonicity (hypo/hyper) relative to cytoplasm are context-dependent and were not the basis of the historical inference.
• Symbiotic origin is evolutionary evidence, not osmotic evidence.
• Claiming no barriers contradicts observed osmotic behavior.

Common Pitfalls:
Equating osmotic activity with specific tonicity values rather than membrane selectivity and water movement.

Final Answer:
Mitochondria are bounded by a semipermeable membrane capable of osmotic swelling/shrinkage