Cell biology terminology The tonoplast is the membrane surrounding which compartment in a plant cell?

Introduction / Context:
Plant cells contain specialized compartments that do not exist or are less prominent in animal cells. A key organelle is the large central vacuole, which maintains turgor pressure, stores ions and metabolites, and participates in detoxification. The membrane that encloses this vacuole has a specific name that students must know to interpret plant physiology literature and microscopy images.

Given Data / Assumptions:

• The term to be identified is “tonoplast.”
• We are dealing with typical higher plant cells containing a prominent central vacuole.
• Each answer choice denotes a distinct cellular compartment or matrix.

Concept / Approach:
The tonoplast is the vacuolar membrane. It contains transporters and pumps (such as H+-ATPases and H+-pyrophosphatases) that acidify the vacuolar lumen and mediate ion homeostasis. Recognizing this term helps connect membrane transport, osmotic regulation, and storage functions unique to plant cells and some fungi.

Step-by-Step Solution:

Verification / Alternative check:
Textbooks and electron micrographs label the vacuolar membrane as tonoplast, with immunolocalization of tonoplast proteins (e.g., TIP aquaporins) confirming identity and function in water transport and osmotic balance.

Why Other Options Are Wrong:

• Cytoplasm: a fluid matrix without a single enclosing membrane.
• Nucleus: enclosed by the nuclear envelope, not the tonoplast.
• Mitochondria: have outer and inner mitochondrial membranes; not termed tonoplast.
• Chloroplast stroma: enclosed by chloroplast envelopes and thylakoid membranes.

Common Pitfalls:
Confusing tonoplast with “plasmalemma” (plasma membrane) or with the thylakoid membrane inside chloroplasts. Each has different protein composition and functions.

Final Answer:
Vacuole