Biology 9700/11 — October/November 2019

Background Concept

Cytoplasm is the aqueous, gel-like substance that fills the cell and surrounds the organelles. In eukaryotic cells it refers to the cytosol (the fluid component) together with the dissolved substances, ions and enzymes it contains, but excluding the internal contents of membrane-bound organelles such as mitochondria, chloroplasts and the endoplasmic reticulum.

Plasmodesmata (singular: plasmodesma) are narrow channels, typically 20–40 nm wide, that traverse the cell walls of adjacent plant cells, allowing direct communication between them. Each plasmodesma has two key components:

1. A desmotubule — a tubule of endoplasmic reticulum running through the channel, continuous with the ER of both cells.
2. A ring of cytoplasm surrounding the desmotubule, continuous with the cytoplasm of the two neighbouring cells. This cytoplasmic sleeve is the main route for cell-to-cell transport of small molecules (symplastic transport).

Because the cytoplasm within a plasmodesma is continuous with the cytoplasm of the two adjacent cells, plasmodesmata form the basis of the symplast — the living, interconnected network of plant cell cytoplasm.

Understanding the Question

The command word is implicit, but the question is asking you to identify which of the four listed structures actually contains cytoplasm within it. Three of the four are membrane-bound compartments with their own internal environment distinct from cytoplasm; the fourth is a channel that, by definition, is filled with cytoplasm. The mark scheme credits option C.

Approach

The strategy is to consider each option and decide whether the interior of that structure is cytoplasm or some other internal environment:

• If the structure is a membrane-bound organelle, its interior is named (matrix, stroma, lumen) and is not cytoplasm.
• If the structure is a channel or gap through which cytoplasm flows, then it contains cytoplasm.

Step-by-Step Reasoning

• Option A — Chloroplasts: A chloroplast is a double-membrane-bound organelle. Its internal space is divided into the stroma (the fluid surrounding the thylakoids) and the thylakoid lumen. Neither of these is referred to as cytoplasm. ✗
• Option B — Mitochondria: A mitochondrion is a double-membrane-bound organelle. Its internal compartment is the matrix. This is not cytoplasm. ✗
• Option C — Plasmodesmata: A plasmodesma is a channel through the plant cell wall. Its central desmotubule is endoplasmic reticulum, and surrounding this is a sleeve of cytoplasm continuous with that of the two adjoining cells. ✓
• Option D — Smooth endoplasmic reticulum: The ER is itself a membrane system. Its lumen is not cytoplasm. The cytoplasm surrounds the ER; it is not contained within it. ✗

Therefore the only structure listed whose interior is cytoplasm is the plasmodesma, giving C.

Key Takeaways

• Cytoplasm refers to the cytosol and its contents outside membrane-bound organelles.
• The internal spaces of mitochondria (matrix), chloroplasts (stroma) and the ER (lumen) are not cytoplasm.
• Plasmodesmata are the exception here: their central channel is filled with cytoplasm, linking the cytoplasm of adjacent plant cells into the symplast.

Common Mistakes

• Confusing the lumen of the endoplasmic reticulum with cytoplasm — they are different compartments separated by the ER membrane.
• Assuming the chloroplast stroma or mitochondrial matrix is "cytoplasm-like" and counting them as correct — biological terminology is precise, and these have their own names.
• Not knowing what a plasmodesma is, and therefore guessing an organelle instead.

Things to Be Careful About

• The word cytoplasm in cell biology is a specific term; the mark scheme will not credit vague substitutes such as "fluid" or "inside".
• Plasmodesmata are found only in plants and some algae (organisms with cell walls); the equivalent in animal cells is a gap junction, but gap junctions are not listed here.
• "Plasmodesmata" (plural) and "plasmodesma" (singular) — do not confuse the two spellings.