Learning objectives
Make temporary mounts and biological drawings, measure cells with an eyepiece graticule and stage micrometer, and calculate magnification and actual size
Compare light and electron microscopes, and distinguish magnification from resolution
Describe the ultrastructure of eukaryotic cells and the functions of organelles
Compare prokaryotic and eukaryotic cells
Outline the structure of viruses
Key definitions
Magnification
Image size ÷ actual size.
Resolution
The minimum distance between two points that can still be distinguished.
Prokaryote
A cell, such as a bacterium, with no nucleus and no other organelles surrounded by a double membrane.
Eyepiece graticule
A small scale in the eyepiece of a microscope; it must be calibrated against a stage micrometer before it is used to measure.
Stage micrometer
A slide carrying an accurate scale, usually 1 mm divided into 100 divisions of 10 µm each, used to calibrate the eyepiece graticule.
Tonoplast
The membrane that surrounds the large permanent vacuole of a plant cell.
Explanation
Light microscopes use visible light and are limited to about 200 nm resolution; electron microscopes use electrons with far shorter wavelength, resolving to below 1 nm (transmission EM; a scanning EM resolves a few nanometres), but require dead, dehydrated specimens. The magnification triangle (I = A × M) with unit conversion (1 mm = 1,000 μm = 1,000,000 nm) is a routine exam calculation.
Practical microscopy. To make a temporary mount, place a thin layer of cells, such as a strip of onion epidermis or a smear of cheek cells, in a drop of water or stain on a slide (iodine solution stains nuclei and starch grains; methylene blue stains the nuclei of animal cells), then lower a cover slip gently from one side with a mounted needle so that no air bubbles are trapped, and blot away any extra liquid. A biological drawing is made with a sharp pencil, using single, clear, continuous lines with no shading; it should fill at least half the space, keep the proportions seen, and have ruled label lines that touch the structure named, with the magnification or a scale stated. To measure a cell, calibrate the eyepiece graticule: line it up with the scale of a stage micrometer, whose divisions are 10 µm, and work out what one eyepiece division is worth. If 40 eyepiece divisions cover 25 stage divisions (250 µm), one eyepiece division is 250 ÷ 40 = 6.25 µm, so a cell spanning 16 eyepiece divisions is 100 µm long. The calibration must be repeated for each objective lens. The same formula, actual size = image size ÷ magnification, is used for photomicrographs and for electron micrographs: a transmission electron micrograph (TEM) shows a thin section through the inside of a cell, while a scanning electron micrograph (SEM) shows the surface in three dimensions.
Eukaryotic cells are bounded by a cell surface membrane and contain a nucleus (DNA, and a nucleolus that makes ribosomes) enclosed by a nuclear envelope of two membranes with pores through which mRNA leaves; rough ER (protein synthesis), smooth ER (lipids), Golgi (modifying and packaging), mitochondria (aerobic respiration, ATP), lysosomes (digestion), ribosomes (80S in the cytoplasm; mitochondria and chloroplasts contain 70S ribosomes and small circular DNA) and centrioles, which are built of microtubules and organise the spindle in animal cells. Some animal cells have cilia, which beat to move fluid or mucus over the cell surface, or microvilli, finger-like folds of the cell surface membrane that increase the surface area for absorption. Plant cells also have chloroplasts, a cellulose wall crossed by plasmodesmata (strands of cytoplasm that link neighbouring cells) and a large permanent vacuole enclosed by a membrane called the tonoplast. All cells use ATP made in respiration for energy-requiring processes such as active transport, movement and the synthesis of large molecules. A typical bacterium is a prokaryotic cell: unicellular, usually 1–5 µm in diameter, with circular DNA, often plasmids, 70S ribosomes, a peptidoglycan wall and no organelles surrounded by a double membrane (no nucleus, mitochondria or chloroplasts). Viruses are non-cellular: nucleic acid (DNA or RNA) in a protein capsid, sometimes with an outer envelope of phospholipids.
Formulae
Magnification = image size ÷ actual size
1 mm = 1,000 μm; 1 μm = 1,000 nm
Value of one eyepiece division = (number of stage micrometer divisions × 10 µm) ÷ number of eyepiece divisions they cover
Worked examples
q: A mitochondrion appears 25 mm long in an electron micrograph at ×10,000. Find its actual length in μm.
steps: Actual = image ÷ magnification = 25 mm ÷ 10,000 = 0.0025 mm
Convert: 0.0025 × 1,000 = 2.5 μm
In context
Antibiotics such as tetracycline target 70S ribosomes, sparing the host's 80S ribosomes — a direct use of the prokaryote–eukaryote difference.
Electron micrographs of secretory cells show extensive rough ER and Golgi, matching their function.
Common misconceptions
Confusing magnification (how many times larger the image is) with resolution (the smallest separation at which two points are still seen as separate).
Dividing an image size in mm by an actual size in μm — when calculating magnification both must be in the same unit.
Saying prokaryotes have no ribosomes — they have smaller 70S ribosomes.
Using an eyepiece graticule without calibrating it — its divisions have no fixed size, and the value of one division changes with every objective lens.
Study tips
Measure image size in mm, convert at the end.
Link each organelle to a function in one clause — that is how marks are allocated.
Summary
I = A × M; convert units.
Electron microscopes: higher resolution, dead specimens.
Organelle structure ↔ function; a bacterium is 1–5 µm across and has no organelles surrounded by a double membrane.
Calibrate the eyepiece graticule against a stage micrometer (10 µm divisions) for each objective lens.
Check your understanding
Question 1: Which organelle modifies and packages proteins?
1. Lysosome
2. Golgi apparatus
3. Rough ER
Question 2: Prokaryotic ribosomes are…
1. 70S
2. 80S
3. absent
Question 3: A stage micrometer is used to…
1. increase the resolution of the microscope
2. stain the specimen
3. calibrate the eyepiece graticule