A Level Biology Practice Exam

What is the role of a counterstain in microscopy?

1. To eliminate unwanted colors in a sample

2. To apply a second stain that contrasts with the first

3. To enhance the resolution of a microscope

4. To prepare the sample for imaging without staining

The correct answer is: To apply a second stain that contrasts with the first

The role of a counterstain in microscopy is to apply a second stain that contrasts with the first. This is particularly important in techniques such as histology, where multiple components of a tissue or cell sample may not be easily distinguishable using a single stain alone. By using a counterstain, certain structures within the sample can be highlighted, providing contrast that enhances the visibility of specific features against the background. For example, in a Gram stain procedure used in microbiology, the initial stain differentiates between Gram-positive and Gram-negative bacteria based on their cell wall composition, while the counterstain allows the contrast necessary to visualize the different types distinctly. This technique aids in identifying and analyzing the morphology of cells or tissues under the microscope, thus greatly improving the interpretive capabilities of the observer. The specific colors or types of stains used can also assist in identifying particular cellular components, making the use of a counterstain critical in many microscopic analyses.