Microscopy & Drawing Scientific Diagrams

Purpose of Staining: Many biological structures are transparent or have low contrast, making them difficult to distinguish under a light microscope. Stains are chemical dyes that bind to specific cellular components, increasing contrast and allowing for the identification of organelles or storage molecules.

Iodine Staining: Iodine in potassium iodide solution is a common stain used to detect starch grains within plant cells. In the presence of starch, the light orange-brown iodine solution undergoes a chemical reaction to turn a deep blue-black color, highlighting the storage polysaccharide.

Application Technique: Stains should be applied sparingly, often just a single drop, to avoid over-saturating the specimen. When applying a coverslip, it should be lowered at an angle to prevent the formation of air bubbles, which can appear as dark-ringed 'artefacts' that obscure the view.

Line Work and Shading: Biological drawings must consist of clear, continuous, single lines drawn with a sharp HB pencil. Shading and sketching are strictly prohibited in scientific diagrams, as they can be misinterpreted as actual cellular textures or structures.

Labeling Standards: Labels must be connected to structures by straight lines drawn with a ruler, and these lines should never cross each other. Label lines should not have arrowheads or crosses at the ends, and all text should ideally be written on one side of the drawing for clarity.

Proportional Accuracy: The drawing must be a large, well-defined representation that accurately reflects the proportions of the specimen observed. It is vital to draw only what is actually visible through the lens, rather than what the observer expects to see based on textbook diagrams.

The Magnification Formula: The relationship between the observed image and the actual specimen is defined by the formula $M = I / A$, where $M$ is magnification, $I$ is the size of the image (or drawing), and $A$ is the actual size of the object.

Unit Consistency: Before performing any calculation, all measurements must be converted into the same units. It is standard practice to convert larger units (like millimeters) into smaller units (like micrometers) to maintain precision and avoid decimal errors.

Conversion Factors: To convert from millimeters (mm) to micrometers ($\mu m$), multiply by $1000$. To convert from micrometers ($\mu m$) to nanometers (nm), multiply by $1000$ again. Conversely, divide by $1000$ when moving from smaller to larger units.

Unit Conversion Errors: A frequent mistake is forgetting to convert measurements to the same units before using the $M = I / A$ formula. Always check if the question provides the actual size in $\mu m$ and the image size in $mm$; convert the $mm$ to $\mu m$ by multiplying by $1000$ first.

Drawing Artefacts: Students often lose marks by drawing air bubbles or dust particles (artefacts) instead of cell structures. Ensure that any structure drawn is consistent across multiple cells to verify it is a genuine biological feature.

Label Line Precision: Ensure label lines touch the exact structure being identified without overlapping other lines. Examiners look for 'clean' diagrams where the labels are clearly separated from the drawing itself, usually aligned vertically on the right-hand side.