In the school lab, Yaritza puts a drop of water from the Malecón under the microscope and sees tiny organisms swimming. When she tries to focus on their membrane or on what is inside them, she sees only a transparent mass with no detail. What does it take to see more? The answer depends on the instrument and on how the sample is prepared.
Optical and electron microscopy
There are two main families of microscopes. They differ in the "light" they use to illuminate the sample, and everything else depends on that: what you can see, in how much detail, and whether the cell can be alive.
With the optical microscope you can see cells, nuclei, and chloroplasts. With the electron microscope you can see small organelles such as mitochondria and ribosomes, and also viruses.
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Distinguish between light microscopy and electron microscopy: what each uses to "illuminate" the sample, what type of structures each technique allows us to observe, and…
Distinguish between light microscopy and electron microscopy: what each uses to "illuminate" the sample, what type of structures each technique allows us to observe, and whether it is possible to observe living cells with them.
A research team wants to check whether bacteria in a sample are infected by bacteriophages. Design the study: choose the appropriate microscopy technique, justify the…
A research team wants to check whether bacteria in a sample are infected by bacteriophages. Design the study: choose the appropriate microscopy technique, justify the choice over alternatives and describe what the team would expect to observe if infection were occurring.
When you look at an onion preparation and change objectives, the cells look bigger and bigger. How are the lenses combined to give the final size of the image?
Each part of the microscope has its function:
Part
Function
Eyepiece
Lens you look through; magnifies the image (normally $10\times$)
Objectives (on the nosepiece)
Lenses that form the first enlarged image
Stage
Surface where the preparation is placed
Coarse focus knob
Quick focusing, with large movement
Fine focus knob
Fine focusing, with small movement
Light source
Illuminates the sample from below
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Name the basic parts of a light microscope (eyepiece, objective lenses, stage, coarse and fine focus knobs, and light source) and state the function of each.
Name the basic parts of a light microscope (eyepiece, objective lenses, stage, coarse and fine focus knobs, and light source) and state the function of each.
Explain the difference between magnification and resolving power of a microscope and reason why greatly magnifying an image without improving resolution (so-called…
Explain the difference between magnification and resolving power of a microscope and reason why greatly magnifying an image without improving resolution (so-called "empty magnification") does not allow further details to be distinguished.
If you enlarge a poor-quality photo on your phone, it gets huge but no new details appear. Microscopes work the same way. What really limits what you can see?
In practice, the electron microscope has limitations: it is very expensive and bulky, it needs specialized staff, it only gives black-and-white images, and it requires dead, dehydrated samples prepared through a long process. That is why the optical one is used in high schools and in many laboratories.
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The diagram shows a scale of sizes with the resolution limits of the human eye, the light microscope (LM) and the transmission electron microscope (TEM). State which…
The diagram shows a scale of sizes with the resolution limits of the human eye, the light microscope (LM) and the transmission electron microscope (TEM). State which instrument you would need to observe in detail: a) a flu virus; b) a bacterium; c) a ribosome; d) a skin cell. Justify each choice based on the diagram.
The electron microscope reveals organelles and viruses that the light microscope cannot show. Explain which property of the electron beam gives it a higher resolving…
The electron microscope reveals organelles and viruses that the light microscope cannot show. Explain which property of the electron beam gives it a higher resolving power and point out two practical limitations of electron microscopy compared to light microscopy.
A sample is not put under the microscope as it is. If it is thick, light does not pass through; and if the structures are transparent, they are barely distinguishable. That is why it is prepared.
There are two good reasons to stain a sample: cells are almost transparent and staining gives contrast; and each dye binds to different structures, so it lets you tell apart, for example, the nucleus from the cytoplasm. In the school lab, methylene blue and Lugol's solution (iodine) are used.
For the electron microscope the process is longer: the sample is fixed (its structures are preserved), dehydrated, embedded in resin, cut into ultrathin slices, and contrasted with heavy metals.
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Most biological samples are stained before being observed under a light microscope. Explain two reasons for staining a sample and name a stain frequently used in school…
Most biological samples are stained before being observed under a light microscope. Explain two reasons for staining a sample and name a stain frequently used in school laboratories.
A biomedical laboratory needs to observe in detail the ultrastructural alterations of mitochondrial cristae and RER-bound ribosomes in biopsies from patients with a…
A biomedical laboratory needs to observe in detail the ultrastructural alterations of mitochondrial cristae and RER-bound ribosomes in biopsies from patients with a myopathy. A technician suggests fixing the tissue with formaldehyde, staining it with methylene blue, and observing it under an optical microscope using a 100× oil immersion objective and visible light (average wavelength of 550 nm).
Quantitatively justify, using the concept of resolution limit and Abbe's equation, why this proposal is unfeasible for resolving cristae and ribosomes (20-30 nm). Then, design the correct technical protocol: specify which exact microscopy modality they must use and justify why it is essential to cut the sample into ultrathin sections (less than 100 nm) and stain it with heavy metal salts (such as osmium or uranium) instead of organic dyes.
Go back to the beginning: Yaritza couldn't see details because the optical microscope does not resolve very small structures, and because an unstained sample gives hardly any contrast.
Let's practice! 3 medium exercises in this section