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Difficulty: EasyProcedural Steps

To prepare a wet-mount slide of onion epidermal cells for observation under a compound light microscope, a technician must execute several precise steps. First, the technician uses forceps to peel a single, paper-thin layer of epidermal tissue from the inner curve of a fresh onion section. This thin layer is immediately placed flat on the center of a clean glass slide, ensuring the tissue does not fold over itself. Next, a single drop of iodine solution is applied directly onto the tissue; this stain binds to starch and proteins, making the cell walls and nuclei clearly visible. Finally, the technician holds a plastic coverslip at a forty-five-degree angle against the slide, touching the edge of the iodine drop, and slowly lowers it over the onion specimen to avoid trapping air bubbles.

According to the described procedure, which of the following steps must be performed immediately before the technician adds the drop of iodine stain to the onion specimen?

  1. Placing the peeled onion tissue flat on the center of the glass slideAnswer
  2. B
    Lowering the coverslip over the onion specimen at a forty-five-degree angle
  3. C
    Peeling a single, paper-thin layer of epidermal tissue with forceps
  4. D
    Adjusting the focus of the compound light microscope

Answer

Placing the peeled onion tissue flat on the center of the glass slide
The passage states that after peeling the onion tissue, the layer is 'immediately placed flat on the center of a clean glass slide.' The very next sentence states that 'Next, a single drop of iodine solution is applied.' Thus, placing the tissue on the slide is the action performed immediately prior to adding the stain.

Step-by-Step Solution

1
Locate the target step (adding the drop of iodine stain) in the text.
The text states: 'Next, a single drop of iodine solution is applied directly onto the tissue...'
Finding where the target action occurs in the text is necessary to identify what immediately precedes it.
2
Read the sentence immediately preceding the target step.
The text states: 'This thin layer is immediately placed flat on the center of a clean glass slide...'
This establishes the chronological sequence of events leading up to the target action.
3
Confirm that no other steps are performed between placing the tissue on the slide and applying the iodine solution.
The text moves directly from placing the tissue flat on the slide to applying the iodine stain, with no intermediate steps.
This ensures the selected step is the one performed immediately before the target action.

Key Concept

Identifying the sequential order and immediate prerequisites of steps in a technical or scientific procedure.
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Topics

Procedural Steps

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