Cell Structure and Organisation: Question 8
Syllabus 2.1
A cell diagram is labelled with a thin layer that forms the outer boundary of the cytoplasm in every plant, animal and bacterial cell. A caption beside the label states that this structure controls which substances can enter and leave the cell.
Which structure is being described?
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Worked solution
Step 1: Identify the key clues in the stem
Two clues are given: the structure (1) forms the outer boundary of the cytoplasm in plant, animal AND bacterial cells, and (2) controls which substances can enter and leave the cell.
Step 2: Match the clues to a structure
The cell membrane surrounds the cytoplasm of every cell, whether plant, animal or bacterial, and it controls the movement of substances into and out of the cell. This matches both clues exactly.
Step 3: Rule out structures found in only some cell types
The cell wall is only present in plant and bacterial cells (not animal cells), so it cannot be the structure described as present in all three cell types. This confirms the answer must be the cell membrane, which every cell type has.
Why the other options are wrong
- B (cell wall): not present in animal cells, so it cannot be the structure common to all three cell types; it is also freely permeable, allowing water and dissolved substances through, rather than controlling exactly what enters and leaves.
- C (nucleus): controls the cell’s activities and holds the genetic material, but it does not form the outer boundary of the cytoplasm, and it is absent from bacterial cells entirely.
- D (cytoplasm): the stem describes a layer forming the boundary of the cytoplasm, not the cytoplasm itself.
Final answer
The structure being described is the cell membrane, option A.