Concept:
Mitochondria are double-membrane-bound organelles essential for aerobic cellular respiration, often referred to as the "powerhouses of the cell." Their structural framework consists of an outer membrane and a highly folded inner membrane. These two membranes divide the organelle into two distinct fluid-filled compartments:
• The outer compartment, known as the intermembrane space, located between the outer and inner membranes.
• The inner compartment, fully bounded and enclosed by the inner membrane.
Step 1: Identify the compartment enclosed by the inner membrane
The internal space enclosed directly by the highly convoluted inner mitochondrial membrane (which forms folds called cristae) is filled with a dense, homogeneous, gel-like proteinaceous fluid. This central core compartment is scientifically defined as the
matrix.
The matrix contains the circular mitochondrial DNA, ribosomes ($70\text{S}$), RNA molecules, and all the essential enzymes required for the Krebs cycle (Tricarboxylic Acid Cycle) and amino acid synthesis. Thus, the inner membrane encloses the matrix.
Step 2: Analysis and elimination of incorrect options
• Option (1) Aqueous humor: This is a clear, watery fluid filling the anterior chamber of the vertebrate eye between the cornea and the lens. It has no structural relationship with cellular organelles.
• Option (3) Cytosol: This is the liquid component of the cytoplasm surrounding all the organelles inside a cell. Mitochondria float within the cytosol; the cytosol is not inside the mitochondrion.
• Option (4) Mucus: This is a slippery, viscous secretion produced by mucous membranes and goblet cells in multicellular organisms to protect epithelial linings. It is an extracellular product, completely unrelated to sub-cellular membrane anatomy.
Therefore, option (2) is uniquely correct.
