OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    Cell structure: (c) the use of staining in light microscopy

    This point requires learners to understand why stains are used in light microscopy and to know named examples.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 3 of 10
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    This point requires learners to understand why stains are used in light microscopy and to know named examples. Staining increases contrast because many biological specimens are transparent and have little natural colour, so without a stain cell structures are hard to distinguish. Different stains bind to different components: iodine solution stains starch granules blue-black and is used to make plant cell contents visible; methylene blue stains nuclei and other acidic components blue in animal cells; eosin can be used as a counterstain. Learners should be able to select the appropriate stain for a named structure and explain the resulting colour change. MCQ items may ask why a stain is needed, which stain is suitable for a particular specimen, or what colour a structure appears after staining.

    What earns the marks

    • Stains increase contrast so that transparent or colourless cell structures become visible.
    • Iodine solution stains starch blue-black and is used with plant tissue.
    • Methylene blue stains nuclei and other acidic components blue in animal cells.
    • The choice of stain depends on the structure or molecule to be highlighted.

    Common mistakes and how to put them right

    • Learners think stains magnify the specimen.

      Put it right: Stains increase contrast, while magnification is provided by the lenses.

    • Learners assume one stain works for all structures.

      Put it right: Different stains bind to different components.

    • Learners confuse the colour produced by iodine on starch.

      Put it right: Starch stains blue-black.

    Exam tips

    • Link each named stain to the structure it highlights and the colour produced.
    • When asked why a stain is used, focus on contrast and visibility rather than magnification.
    • Check whether the specimen is plant or animal before choosing a stain.

    Try the lesson

    The start of the Cell structure: (c) the use of staining in light microscopy lesson, made from checked teaching notes for this specification point.

    Have a go first

    What is the main benefit of using a stain when examining an unstained specimen under a light microscope?

    Listen

    This point requires learners to understand why stains are used in light microscopy and to know named examples. Staining increases contrast because many biological specimens are transparent and have little natural colour, so without a stain cell structures are hard to distinguish. Different stains bind to different components: iodine solution stains starch granules blue-black and is used to make plant cell contents visible; methylene blue stains nuclei and other acidic components blue in animal cells; eosin can be used as a counterstain. Learners should be able to select the appropriate stain for a named structure and explain the resulting colour change. MCQ items may ask why a stain is needed, which stain is suitable for a particular specimen, or what colour a structure appears after staining. A strong answer earns marks for these points. Stains increase contrast so that transparent or colourless cell structures become visible. Iodine solution stains starch blue-black and is used with plant tissue. Methylene blue stains nuclei and other acidic components blue in animal cells. The choice of stain depends on the structure or molecule to be highlighted. A common slip: Learners think stains magnify the specimen. Instead, stains increase contrast, while magnification is provided by the lenses. Exam tip: Link each named stain to the structure it highlights and the colour produced.

    See it

    A top-down flowchart. A transparent or colourless specimen leads to the stain increasing contrast, which leads to structures becoming visible. From the stain node three branches lead to iodine solution, methylene blue and eosin as a counterstain. Iodine solution leads to starch granules appearing blue-black and to plant cell contents being visible. Methylene blue leads to nuclei and other acidic components appearing blue and to animal cells.
    Shows why stains are used in light microscopy and which named stains highlight which structures and colours.

    Key words

    Say what each word means before you turn the card.

    Purpose of staining

    Try answering aloud or in your head before turning the card.

    Named stains and their targets

    Try answering aloud or in your head before turning the card.

    Colour changes observed

    Try answering aloud or in your head before turning the card.

    Contrast versus magnification

    Try answering aloud or in your head before turning the card.

    See every definition
    Purpose of staining
    Staining increases contrast because many biological specimens are transparent and have little natural colour, so without a stain cell structures are hard to distinguish.
    Named stains and their targets
    Iodine solution stains starch granules and is used to make plant cell contents visible, while methylene blue stains nuclei and other acidic components in animal cells.
    Colour changes observed
    Iodine solution stains starch blue-black, and methylene blue stains nuclei and other acidic components blue.
    Contrast versus magnification
    Stains increase contrast, while magnification is provided by the lenses.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A transparent or colourless specimen leads to the stain increasing contrast, which leads to structures becoming visible. From the stain node three branches lead to iodine solution, methylene blue and eosin as a counterstain. Iodine solution leads to starch granules appearing blue-black and to plant cell contents being visible. Methylene blue leads to nuclei and other acidic components appearing blue and to animal cells.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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