Microscopy and observing cells: practical techniques
Cell biology • Cell structure
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Light microscope required practical (observing cells)
A standard required practical uses a light microscope to examine plant and animal cells. Slides may be provided or prepared from samples such as onion epidermis (plant) and cheek smear (animal); staining increases contrast so organelles become visible and a cover slip prevents drying and keeps the specimen flat. Method steps include placing the slide on the stage, beginning with the lowest‑power objective lens, focusing, increasing magnification as needed, and recording the magnification used when drawing observations . Drawings require clear, proportionate outlines, labels for identifiable structures, and annotation of magnification and scale. Observed differences between plant and animal cells (for example, cell wall and chloroplasts in plant cells and irregular shape of many animal cells) link structure to function and support comparisons between samples.
Preparing slides and staining
Plant tissue preparation involves removing a thin sheet of epidermis (for example from an onion) and mounting it flat on a slide, adding iodine stain to improve visibility of nuclei and cell walls, and lowering a cover slip to avoid air bubbles and compression artifacts. Animal cell sampling uses a gentle cheek scrape smeared on a slide and stained with methylene blue to reveal nuclei and cytoplasm; stains are irritants and require eye protection and careful handling . Staining increases contrast by binding to specific cell components, which causes faint structures to appear darker and therefore distinguishable. Excessive staining or poor mounting causes artefacts that mislead interpretation, so careful technique and consistent preparation reduce errors.
Drawing and interpreting microscope images
Microscope drawings must represent observed shapes and relative sizes accurately rather than photographic detail. Labels identify visible structures and a note of magnification or a scale bar provides size context. Comparison of drawings from different magnifications or tissues supports interpretation of adaptations, such as many chloroplasts in palisade cells for photosynthesis . Interpretation requires awareness of limitation: some organelles remain too small to resolve with a light microscope and appear as vague granules. Clear drawings and recorded magnification support valid comparisons and calculations of actual size from image measurements.
Magnification, units and calculations
Magnification expresses how many times larger an image is compared with the actual object and uses the formula magnification = image size ÷ actual size. Rearranged forms calculate actual size or image size when the other two variables are known. Consistent units (for example micrometres for cell dimensions) are essential when using these formulas . Cell lengths are usually measured in micrometres (µm). One micrometre equals 1 × 10−6 metres and 1 × 10−3 millimetres. Subcellular structures may require nanometre (nm) units when using high‑resolution electron microscopy .
Development of microscopy techniques
Simple lenses allow magnification of small organisms since the late 16th century, enabling the discovery that organisms consist of cells. Light‑microscope improvements extend magnification and contrast, but resolution remains limited by light wavelength. The invention of the transmission electron microscope (TEM) by Ernst Ruska and Max Knoll in 1931 and the later scanning electron microscope (SEM) extend the observable scale by using electron beams, enabling visualization of much smaller structures and new three‑dimensional surface detail . Historical development causes increases in both magnification and resolving power; improved sample preparation, staining and imaging techniques further refine the visible detail of cells and organelles.
Electron microscopy and understanding sub‑cellular structure
Electron microscopes replace visible light with electrons of far shorter wavelength, which reduces the minimum resolvable separation and therefore increases resolution. Typical light microscopes have a maximum resolution near 200 nm, while electron microscopes can resolve points as close as about 0.1 nm, enabling visualization of detailed internal structures such as mitochondrial cristae, chloroplast thylakoid arrangements, and the fine structure of membranes and ribosomes . Transmission electron microscopes (TEM) create flat, high‑resolution cross‑section images by transmitting electrons through thin slices, while scanning electron microscopes (SEM) scan an electron beam across sample surfaces to build three‑dimensional surface images. These capabilities produce direct evidence of subcellular organisation that light microscopy cannot resolve and so reshape understanding of cell ultrastructure and function.
Key notes
Important points to keep in mind