State why a stain is added when preparing a temporary slide of plant tissue.
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To increase contrast between cell structures / to make specific structures visible
The Microscope In Cell Studies
Explain why the coverslip is lowered at an angle onto the specimen.
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To prevent air bubbles forming between slide and coverslip; air bubbles obscure the specimen / prevent clear focusing
State two features of a high-quality biological drawing of cells.
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Clean continuous single lines / no shading; correct proportions of cells; ruled label lines touching structures; only visible structures drawn (any 2)
Describe one difference between a plan diagram and a cell drawing.
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Plan diagram shows distribution of tissues only / no individual cells; cell drawing shows individual cells with cell walls and visible contents
A mitochondrion appears 25 mm long on an electron micrograph. The actual length is 5 µm. Calculate the magnification.
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Converting the image size to micrometres $$25\text{ mm}=25 000\text{ µm}$$ Finding the magnification $$M=\frac{\text{image size}}{\text{actual size}}=\frac{25 000}{5}$$ $$M=\times 5000$$
A chloroplast on a photomicrograph is 12 mm long at a magnification of ×3000. Calculate the actual length of the chloroplast in micrometres.
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Converting the image size to micrometres $$12\text{ mm}=12 000\text{ µm}$$ Finding the actual size $$A=\frac{\text{image size}}{M}=\frac{12 000}{3000}$$ $$A=4\text{ µm}$$
Explain why an eyepiece graticule must be calibrated using a stage micrometer before measuring cells.
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Eyepiece graticule has arbitrary divisions / no fixed real size; stage micrometer has divisions of known length; calibration converts eyepiece units into real units (µm)
A student finds that 80 eyepiece divisions line up with 20 stage micrometer divisions, where each stage division is 10 µm. Calculate the actual length represented by one eyepiece division.
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Finding the real length of the matched divisions $$20\times 10=200\text{ µm}$$ Finding the length of one eyepiece division $$\frac{200}{80}=2.5\text{ µm}$$
Suggest why the eyepiece graticule must be recalibrated after switching from the ×10 to the ×40 objective.
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Higher objective magnifies the specimen more / changes the field of view; one eyepiece division now represents a smaller real distance
Define the term resolution.
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The smallest distance between two points that can be distinguished / seen as separate
Distinguish between magnification and resolution.
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Magnification is how many times larger the image is than the actual specimen; resolution is the smallest distance between two points that can be seen as separate
Explain why an electron microscope can produce images of greater detail than a light microscope, and discuss one limitation of electron microscopy compared with light microscopy.
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Electrons have a much shorter wavelength than visible light; this gives higher resolution / allows smaller structures to be distinguished as separate points; useful magnification is therefore much greater (e.g. ×500 000 vs ×1500); ultrastructure such as ribosomes and membranes can be seen; limitation – specimens must be dead / in a vacuum / cannot view living cells; specimens require complex preparation / staining with heavy metals / risk of artefacts; images are black and white, not coloured (any 6)