In plant cell ultrastructure, the closely stacked, flattened sacs of thylakoid membranes found only inside chloroplasts are collectively known by which term?

Introduction / Context:
Plant cells contain unique organelles that are not found in animal cells, and one of the most important of these is the chloroplast. Inside chloroplasts, the arrangement of membranes into stacks is crucial for capturing light energy in photosynthesis. This question tests whether you can correctly name the closely stacked, flattened sacs that are characteristic of chloroplast internal structure.

Given Data / Assumptions:

• We are dealing with plant cells, not animal cells.
• The structures are closely stacked flattened sacs inside chloroplasts.
• These sacs are involved in photosynthesis.
• Options include grana, stroma, cristae, Golgi cisternae, and vacuoles.

Concept / Approach:
Chloroplasts have an internal membrane system made of thylakoids. Thylakoid membranes contain chlorophyll and other pigments that capture light. These thylakoids are often arranged in stacks, and each stack is called a granum (plural: grana). The stroma is the fluid filled space surrounding the thylakoids. Cristae are folds of the inner mitochondrial membrane and belong to mitochondria, not chloroplasts. Golgi cisternae are flattened sacs in the Golgi apparatus. Vacuoles are membrane bound sacs for storage. Therefore, the closely stacked flattened sacs in chloroplasts are properly called grana.

Step-by-Step Solution:
Step 1: Identify that the question is about plant specific structures in chloroplasts. Step 2: Recall that chloroplasts contain thylakoid membranes which are often arranged into stacks. Step 3: Remember that each stack of thylakoid discs is called a granum, with the plural form grana. Step 4: Recognise that the stroma refers to the fluid matrix surrounding the thylakoids, not the stacks themselves. Step 5: Distinguish cristae as folds of mitochondrial inner membrane, not part of chloroplast structure. Step 6: Note that Golgi cisternae and vacuoles are other organelles and do not match the chloroplast description. Step 7: Conclude that the correct term for the stacked flattened sacs in plant chloroplasts is grana.

Verification / Alternative check:
Electron micrographs and diagrams of chloroplasts in textbooks show disc like thylakoids arranged in stacks labelled as grana. Labels also identify the surrounding stroma and the envelope membranes. Separate diagrams of mitochondria show cristae, confirming that cristae belong to mitochondria, not chloroplasts. Diagrams of the Golgi apparatus show flattened stacks, but these are not restricted to plants and are not inside chloroplasts. This visual evidence supports grana as the correct answer.

Why Other Options Are Wrong:
Stroma: The fluid matrix of the chloroplast, not the closely stacked sacs. Cristae: Folds of the inner membrane of mitochondria, unrelated to chloroplast thylakoids. Golgi cisternae: Flattened sacs of the Golgi apparatus, present in both plant and animal cells, not specifically in chloroplasts. Vacuoles: Large storage organelles, often central in plant cells, but not stacked thylakoid sacs.

Common Pitfalls:
Because both chloroplasts and mitochondria have internal membranes, students sometimes mix up terms like grana and cristae. A helpful memory aid is that grana and green both start with g, and chloroplasts are green due to chlorophyll. Cristae, in contrast, are associated with mitochondria and energy production through respiration. Keeping these associations straight helps you avoid confusion on exams.

Final Answer:
In plant chloroplasts, the closely stacked flattened sacs are called grana.