3.2Unit 3: Practical Skills in Biology I

Using a light microscope and cell ultrastructure — IAL Edexcel Modular Revision Notes & Topical Questions

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3.2 Using a Light Microscope and Cell Ultrastructure — IAL EDEXCEL MODULAR notes: Prepare an animal-cell slide, observe it with a light microscope, make a labelled drawing, calculate cell size and relate the image to electron microscope ultrastructure.Thin specimen — place a smal…

Dehydration
Unclear image
Cross-contamination
Limitations
Title and magnification
Label lines

Part 1 of 8Preparing an animal-cell slide

Thin specimen

Thin specimen place a small liquid sample or thin smear of animal cells on a clean slide, so light passes through and cells do not overlap.

Stain

Stain add only the amount of stain the practical requires to improve contrast.

Coverslip

Coverslip lower it gently to limit air bubbles without pressing so hard that cells are distorted.

Dehydration

Dehydration a drop of water beneath the coverslip stops the thin specimen drying out.

Safety

Safety handle slides carefully because broken glass cuts skin, follow laboratory rules for human cell samples, dispose of biological material appropriately and protect skin from stains.

Part 2 of 8Finding and focusing the cells

Secure the slide on the stage and select the lowest-power objective lens, which gives a wide field of view and avoids the lens touching the coverslip.

Use the coarse focus to find the specimen and centre a group of cells.

Adjust the light so boundaries and internal structures can be distinguished.

Rotate to a higher-power objective only after the specimen is centred.

Sharpen the image with the fine focus.

Unclear image

Unclear image return to low power, check the specimen is thin enough for light to pass through and refocus.

Cross-contamination

Cross-contamination foreign cells on the slide can confuse the image.

Part 3 of 8What is visible in an animal cell

Light microscope

Light microscope a typical animal cell shows the cell-surface membrane, cytoplasm and a stained nucleus.
An animal-cell drawing must not include a cellulose cell wall or chloroplasts.
Small organelles cannot be resolved with an ordinary light microscope even though they are present.

Limitations

Limitations structures look different in different planes of section, and preparation of the specimen may alter cell structure.

Part 4 of 8Making a labelled biological drawing

Title and magnification

Title and magnification give the drawing a clear title and record the magnification used.

Outline

Outline use a sharp pencil on plain white paper with large, clean, single lines, correct proportions and no shading or sketching.

Content

Content draw only structures that can be seen, and fill as much of the page as possible.

Label lines

Label lines ruled, straight, not crossing, with no arrowheads, ending on the structure and kept to one side of the drawing.

Plan drawing

Plan drawing it shows the arrangement of tissues at low power and never includes individual cells.

Part 5 of 8Magnification and cell size

Magnification

Magnification image size ÷ actual size, which rearranges to actual size = image size ÷ magnification.

Image and actual size must be in the same units before dividing, and 1 mm = 1000 µm.
Total magnification equals eyepiece magnification multiplied by objective magnification.

Eyepiece graticule

Eyepiece graticule an engraved ruler in the eyepiece with no fixed units.
It is calibrated against a stage micrometer separately for each objective lens.
The size of one graticule unit equals the stage micrometer length divided by the number of graticule divisions covering it.

Answers

Answers state the unit and use suitable significant figures.

Part 6 of 8Animal-cell ultrastructure

Animal-cell ultrastructure

Nucleus

Nucleus it contains chromosomes and is surrounded by a double membrane with pores for exchange with the cytoplasm.

Rough endoplasmic reticulum

Rough endoplasmic reticulum it carries ribosomes and helps make proteins.

Golgi apparatus

Golgi apparatus it modifies and packages proteins into vesicles.

Mitochondria

Mitochondria they carry out aerobic respiration and have inner membranes folded into cristae, which increase the area for reactions.

Ribosomes

Ribosomes they assemble proteins, and cytoplasmic ribosomes in eukaryotes are 80S whereas mitochondria contain smaller 70S ribosomes.

Absent structures

Absent structures animal cells have a cell-surface membrane but no cellulose cell wall and no chloroplasts.

Internal membrane detail

Internal membrane detail it usually needs electron micrographs because the light microscope cannot resolve it.

Part 7 of 8Resolution and electron microscopy

Magnification makes an image larger, whereas resolution is the ability to distinguish two close points as separate.

Electron microscopes

Electron microscopes electrons have a shorter wavelength than visible light, so they reveal ultrastructure that stays blurred in light microscopy.

Specimens

Specimens electron micrographs are made from prepared non-living specimens and may show artefacts from preparation.

Part 8 of 8More from the specification

Prokaryotic versus Eukaryotic Cellular Organisation

Prokaryotic cells lack membrane-bound organelles and a true nucleus, possessing a single circular DNA chromosome within the nucleoid, 70S ribosomes, peptidoglycan cell walls,
- and accessory plasmids.

Distinctive Plant Cell Ultrastructure

Plant cells are distinguished by rigid cellulosic cell walls providing turgor resistance, a large permanent central vacuole regulating osmotic balance,
- and chloroplasts housing photosynthetic machinery.

Secretory Pathway of Extracellular Enzymes

Ribosomes on the rough endoplasmic reticulum translate precursor proteins,
which are folded and packaged into transport vesicles that fuse with the Golgi apparatus for post-translational modification and exocytosis.

Enzyme

A protein that acts as a biological catalyst.

Organism

A living thing, made of one or more organ systems.

Key terms

Term	Meaning
Magnification	How many times larger an image is than the actual specimen.
Resolution	The ability to distinguish two close points as separate.
Eyepiece graticule	An engraved scale in the eyepiece used to measure objects once calibrated.
Stage micrometer	A slide with an engraved scale used to calibrate the graticule.
Plan drawing	A low-power drawing of tissue arrangement without individual cells.
Cristae	Folds of the inner mitochondrial membrane.

Avoid these mistakes

Using high power first — always start on low power to find the specimen and protect the lens.

Drawing a cell wall or chloroplasts in an animal cell — animal cells have neither.

Calculating magnification with mixed units — convert mm to µm (multiply by 1000) first.

Confusing magnification with resolution — a larger image is not necessarily a clearer one.

Last updated: 18 October 2026