Temporary Mount: Onion Peel MCQs Quiz | Class 9

This Class IX Science (Code 086) practical quiz covers Unit II: Practicals. The topic focuses on preparing a temporary mount of an onion peel, including skills in staining using Safranin, slide preparation techniques to avoid air bubbles, and identifying labeled diagram parts like the cell wall and nucleus. Test your knowledge on these practical skills, submit your answers to check your score, and download the solution PDF for revision.

Topic Overview

In this Class IX Science practical, students learn to prepare a temporary mount of an onion peel to observe plant cells under a compound microscope. The primary objective is to study the structural details of plant cells, specifically focusing on the cell wall, cytoplasm, nucleus, and vacuole. The experiment involves precise steps to ensure the peel is stained correctly and mounted without air bubbles for clear visibility.

Key Concepts

  • Staining: The stain used for plant cells (like onion peel) is Safranin. It stains the cell walls and nuclei a reddish-pink color, making them distinct against the cytoplasm. Animal cells (like cheek cells) are typically stained with Methylene Blue.
  • Mounting Medium: Glycerine is used while mounting the peel. Its primary function is to prevent the material from drying out (dehydration) and to provide a medium with a refractive index that improves clarity.
  • Observation: Under the microscope, onion cells appear as rectangular or brick-shaped cells arranged closely with no intercellular spaces. The distinct cell wall is visible on the outside, and a prominent nucleus is usually located at the periphery due to the large central vacuole.

Procedure Summary

The standard procedure involves peeling a thin layer from the concave side of a fleshy onion scale leaf. This peel is immediately placed in water to prevent folding and drying. It is then transferred to a slide, stained with a drop of Safranin, and excess stain is removed using blotting paper. A drop of glycerine is added, and a coverslip is gently lowered using a needle to avoid trapping air bubbles.

Important Precautions

Precaution Reason
Avoid Air Bubbles Bubbles appear as black circles with thick borders, obstructing the view of cells.
Do Not Over-stain Excess stain makes the slide too dark to distinguish cell organelles clearly.
Use Clean Slides Dust and fingerprints can introduce artifacts in the microscopic view.

Quick Revision Points

  • Onion Cell Shape: Rectangular.
  • Outermost Layer: Cell Wall (rigid).
  • Nucleus Position: Peripheral (side).
  • Prevent Drying: Use Glycerine.
  • Stain: Safranin.

Extra Practice Questions

1. Why is the peel taken from the concave side? (Answer: It is easier to get a thin, single layer.)
2. What happens if the coverslip is dropped suddenly? (Answer: Air bubbles get trapped.)
3. Can we use iodine instead of safranin? (Answer: Yes, iodine is also a stain for starch and plant tissues, but safranin is preferred for general cell structure.)
4. Why do we blot excess water/stain? (Answer: To prevent liquid from flowing onto the microscope stage.)
5. What magnification is usually used first? (Answer: Low power 10X, then High power 40X.)