The Microscope in Cell Studies (Cambridge (CIE) AS Biology): Flashcards
Exam code: 9700
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Define the term temporary mount.
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Define the term temporary mount.
A temporary mount is a slide prepared for short-term viewing, where the specimen is not permanently fixed or preserved.
Describe how to prepare a temporary wet mount of cellular material.
Place the specimen on a clean slide.
Add a drop of water (or a stain such as iodine).
Lower a coverslip slowly at an angle using a mounting needle.
Why is a stain such as iodine added when preparing cells?
It increases contrast, making otherwise transparent structures (e.g. the nucleus or starch grains) visible.
Why is the coverslip lowered slowly at an angle?
To avoid trapping air bubbles, which would obscure the specimen.
A stain such as iodine is added to increase the of transparent structures.
A stain such as iodine is added to increase the contrast of transparent structures.
State three rules for producing a good biological drawing.
Use clear, continuous lines with no shading.
Draw only what you see, in proportion.
Add a title, magnification and label lines drawn with a ruler.
Why must a specimen be thin for viewing with a light microscope?
So that light can pass through it, allowing the specimen to be seen clearly.
A biological drawing should use sharp, continuous lines with no .
A biological drawing should use sharp, continuous lines with no shading.
Define magnification.
Magnification is how many times larger an image is than the actual (real) size of the specimen.
State the equation linking magnification, image size and actual size.
Magnification = image size ÷ actual size
(M = I ÷ A)
A cell measures 50 µm across but appears 100 mm wide in a drawing. Calculate the magnification.
Convert to the same units: 100 mm = 100 000 µm.
Magnification = 100 000 ÷ 50 = ×2000
Why must image size and actual size be in the same units before calculating magnification?
Magnification is a ratio and has no units.
Mixing units (e.g. mm and µm) gives the wrong answer.
Magnification is calculated as image size divided by size.
Magnification is calculated as image size divided by actual size.
How many micrometres (µm) are there in one millimetre (mm)?
1000 µm.
What is an eyepiece graticule?
A small scale in the eyepiece with evenly spaced but arbitrary divisions, used to measure a specimen once it has been calibrated.
What is a stage micrometer?
A slide with an accurately engraved scale (often 1 mm split into 100 divisions of 10 µm), used to calibrate the eyepiece graticule.
Why must the eyepiece graticule be calibrated at each magnification?
Its divisions are arbitrary and represent a different real distance at each objective lens.
So the value of one division must be found using a stage micrometer.
Describe how to calibrate an eyepiece graticule.
Line up the eyepiece graticule scale with the stage micrometer scale.
Count how many stage-micrometer divisions line up with a number of eyepiece divisions.
Use the known size of a stage division to calculate the distance of one eyepiece division.
A stage micrometer is used to an eyepiece graticule.
A stage micrometer is used to calibrate an eyepiece graticule.
State the three units of length used in microscopy, from largest to smallest.
Millimetre (mm) → micrometre (µm) → nanometre (nm).
Each is 1000× smaller than the one before.
Define resolution.
Resolution is the ability to distinguish two points that are close together as separate.
Define magnification.
Magnification is how many times larger an image is than the actual size of the specimen.
Explain the difference between magnification and resolution.
Magnification = how much bigger the image is.
Resolution = the amount of detail — how close two points can be while still seen as separate.
Why does an electron microscope have a higher resolution than a light microscope?
Electrons have a much shorter wavelength than light.
This allows smaller, closer structures to be distinguished.
What is the difference between a transmission (TEM) and a scanning (SEM) electron microscope?
A TEM passes electrons through a thin specimen, giving a high-resolution 2D image of internal structure.
An SEM scans electrons across the surface, giving a 3D image of the surface at lower resolution.
True or False?
Increasing magnification always lets you see more detail.
False.
Beyond the limit of resolution, higher magnification just gives a bigger, blurred image with no extra detail.
The ability to distinguish two close points as separate is called .
The ability to distinguish two close points as separate is called resolution.
State the equation for calculating the actual size of a specimen.
Actual size = image size ÷ magnification
(A = I ÷ M)
A structure is magnified ×5000 and its image measures 40 mm. Calculate its actual size in µm.
Actual size = 40 mm ÷ 5000 = 0.008 mm.
0.008 mm × 1000 = 8 µm
How do you find the image size of a structure on a photomicrograph?
Measure it with a ruler in mm, then convert to µm or nm as needed.
Actual size is calculated as image size divided by .
Actual size is calculated as image size divided by magnification.
Convert 2.5 µm into nanometres.
2.5 µm × 1000 = 2500 nm.
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