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AS Level · Biology 3 (IAL) WBI13

Biological Techniques: Staining, Sectioning & Slide Preparation

Revise Biological Techniques: Staining, Sectioning & Slide Preparation for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I

Biological Techniques: Staining, Sectioning & Slide Preparation

🔬 Big idea: You cannot see a cell wall, a nucleus or a xylem vessel in an untreated piece of tissue — the material is thick and almost colourless. Sectioning makes light pass through it; staining makes the structures differ from one another. Every step exists to solve one of those two problems.

Summary — What This Topic Covers

Why sections must be thin, and how to cut them
What the common stains bind to and what colour they give
Preparing a wet mount and a squash without trapping air
Using a graticule and stage micrometer to measure what you see
Calculating magnification and actual size

1. Sectioning — Why Thin Matters

A light microscope works by shining light through the specimen. Tissue more than a few cells thick blocks the light and gives a dark, muddled image with several layers of cells superimposed.

Cut with a sharp blade in one smooth stroke — sawing tears the tissue
Support soft tissue between two pieces of pith, or in a rolled leaf, so it does not squash
Take the thinnest section you can and select the best one under the microscope — you are expected to cut several
Keep the section moist throughout, or the cells shrink and distort

Analogy

Holding a whole apple up to a lamp shows nothing. Cut a slice thin enough and the light comes through, and you can finally see how it is arranged inside.

2. Stains — What Binds to What

Stains work because different cell components have different chemistry, so a dye binds to some and not others. Learn the pairing, not just the colour.

STAIN BINDS TO RESULT ───────────────────────────────────────────────────────────────── Iodine in KI starch blue-black Methylene blue nucleic acids / nuclei blue Acetic orcein chromosomes dark red Toluidine blue O nuclei and cell walls blue / purple Phloroglucinol + HCl lignin red Sudan III / IV lipids red

Key Term

A differential stain colours different structures differently, so they can be told apart. This is why a stained section shows a nucleus clearly while an unstained one does not.

Common mistake

Saying a stain "makes it easier to see". Say what it binds to and what colour that structure becomes — that is where the mark is.

3. Wet Mounts and Squashes

The standard mount, in order:

1. place a drop of water (or stain) on a clean slide 2. add the section, flat, using forceps or a mounted needle 3. lower the coverslip SLOWLY from one edge, using a needle 4. draw off excess liquid with filter paper from the opposite edge

Why lower it slowly

Dropping the coverslip flat traps air bubbles. Under the microscope these are round, very dark-edged and often mistaken for cells — they are the most common thing students misidentify.

A squash (used for root tip mitosis) adds a step: after staining and gentle warming in acid, press down firmly on the coverslip through filter paper. This spreads the cells into a single layer so individual chromosomes can be seen.

The distinction examiners test

vertically