D. Oak bark.
The correct answer is D. Oak bark.
Robert Hooke was the first person to see cells through a light microscope in 1665. This was possible due to the invention and development of microscopy. Prior to this time, microscopic structures had not been seen before and so scientists did not really understand what living organisms were made of.
Hooke actually saw cork cells but really what he noticed was the cell walls since the living cells were actually dead.
The scientist van Leeuwenhoek also saw cells through a microscope in 1676 but realized that the structures he was seeing were alive. He coined the term animalcules to describe these organisms. The first cells he saw were protozoa, and then in later years, he saw microbes (bacteria).
These discoveries led to the development of the cell theory which recognized that all life forms were made of cells. Another tenet of this theory was that a cell could only arise from a pre-existing cell.
Over the years the light microscope became more advanced and there was the invention of the electron microscope. Transmission electron microscopes allowed people to see fine details inside cells and scanning electron microscopes enables people to visualize details on the surfaces of cells.
The invention of the microscope
Scientists only realized that living organisms were made of small units after the microscope was invented. The first microscopes used light and optics in order to magnify specimens.
It was not until the microscope was invented that cells could even be seen by people since they are very tiny. An animal cell is on average only about 20µm in size, which is smaller than what humans can see with the naked eye.
In later years, researchers were able to develop microscopes that used electrons instead of light for visualizing structures. The electron microscope was a 20th-century invention which led to many new discoveries being made about cells at the subcellular level.
Scientists could actually now see all of the organelles inside the cells and could even determine what the inside elements of these little structures looked like.
Electron microscopes use electrons to make an image that is then visible on a screen. These are very expensive and large microscopes which magnify a lot more than a light microscope.
A scanning electron microscope (SEM) allows visualization of surfaces while a transmission electron microscope (TEM) enables the inside of structures to be seen.
Type of microscopy used determines how much specimen preparation is needed. Single cells may not need to be stained with a light microscope, depending on what is being examined. However, in many cases cells are stained.
This can be important, especially in medicine. For instance, if your doctor orders a blood cell count, then special stains are applied to your blood so that the different white blood cells can be distinguished under a microscope. These can then be counted to determine if you have an infection.
Electron microscopy requires complex preparation procedures. A scanning electron microscope views the surfaces of objects so preparation is not too complex.
The specimen is cleaned, fixed, dried and then it is attached to a holder and then coated with gold dust. The gold is used because it is a very good conductive material for the electrons.
Preparing a specimen for transmission electron microscopy is more complex and time-consuming. This is because extremely thin sections are needed.
With this type of microscope, the aim is to look inside the specimen. Thus, a diamond knife is used to cut very thin sections of material for viewing under the microscope.
Microscopes have different degrees of resolution, and certainly, those with a higher magnification have a greater resolution.
The resolution is defined as the smallest distance between two places on a specimen that can be distinguished. An electron microscope has a greater resolution than a light microscope since the magnifying power is much higher.
Organelles that can be seen in cells using a light microscope include chloroplasts and the nucleus, and sometimes mitochondria. Ribosomes are structures that are too small to be seen using light microscopy.
In fact, ribosomes and the intricate structures of cell walls and cell membranes can only be seen using electron microscopy. A light microscope can show that a cell wall or membrane is present, but it cannot show the finer details of these structures.
Robert Hooke was the first person to see cells in plant tissue. He was looking at cork cells through the microscope and what he actually saw was the cell walls. He called these brick structures cells and postulated that these were the units of life.
While Robert Hooke was noticing plant cells van Leeuwenhoek was noticing animal cells. Plants do not move and have cell walls, so the cells that Hooke saw looked like little bricks.
In contrast to Hooke, van Leuwenhoek noticed that the cells he was looking at actually moved. These were probably protists or small organisms like rotifers that he was actually seeing. His description of them as animalcules suggests that he noticed that they moved and were thus in a sense like tiny animals.
While Hooke was actually seeing dead cells, Van Leeuwenhoek was seeing live cells. This is because cork cells are non-living, yet they were still visible because of the hard cell walls.
A major problem with being able to see structures inside cells was not only the limited magnification of light microscopy but also the fact that many structures were colorless and could not be seen as separate from the cytoplasm.
The development of various staining techniques greatly facilitated the resolution of organelles inside of cells. There was a disadvantage to this though, in that there was some distortion of tissues.
This has always been a concern of scientists since aberrations or artifacts from staining can cause misinterpretation of what a person is seeing.
- B Alberts, A Johnson, J Lewis, et al. (2002). Molecular Biology of the Cell, 4th edition. New York: USA, Garland Science Publishers.
- ED Hansen (2018). Zoology. Retrieved from Encyclopedia Britannica.
- Editors of Encyclopedia Britannica (2018). Antonie van Leeuwenhoek. Retrieved from Encyclopedia Britannica.
- Editors of Encyclopedia Britannica (2018). Robert Hooke. Retrieved from Encyclopedia Britannica.
- DC Joy, BJ Ford, S Bradbury (2018). Electron microscope. Retrieved from Encyclopedia Britannica.