Observing the cell
The discovery of the cell has always depended on the quality of the instrument used to look at it. Think of a school science fair: a pond-water slide looks like nothing under a magnifying glass, but under a good microscope it is full of life. The better the microscope, the more internal structures can be distinguished; and the better the sample is prepared, the more faithfully what we see reflects what really happens inside the living cell.
Light and electron microscopy
When you look at a drop of pond water in your biology lab you can see paramecia swimming about, but if you try to focus on their membrane or their internal organelles, the image shows nothing more than a transparent mass with no detail: what differences between light and beams of electrons explain why we need two such different types of microscope?
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.
The light microscope and its magnification
When you look at an onion root tip slide using different objective lenses in the microscope, the size at which you perceive the cells changes dramatically: in what way do the objective and eyepiece lenses together determine the final size at which you observe the sample?
Explain the difference between magnification and resolving power of a microscope and explain why greatly magnifying an image without improving resolution (so-called…
Explain the difference between magnification and resolving power of a microscope and explain why greatly magnifying an image without improving resolution (so-called "empty magnification") does not allow further details to be distinguished.
Cell size and surface-area-to-volume ratio
Why is a bacterium a few micrometers wide, and why can't a single cell grow as big as a basketball? The answer is geometry: as a cell grows, its volume (which needs supplies) increases faster than its surface (where supplies enter).
Resolving power
When you zoom way in on a low-quality photo on your phone, the image becomes huge but the details do not increase and everything turns blurry: why is the magnification of an optical instrument of no use if it does not improve the minimum distance at which it can distinguish two nearby points?
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.