Investigation 1 – Concept Day
Cellular Organization – Investigation 1
Note: In this Investigation, we wish to introduce the light microscope and prepare you to visualize cells in the Lab. While, as a LabLearner student, you probably will have used the elementary light microscopes on several occasions already, this is likely the first time you will use a binocular oil immersion microscope.
You will have time in the lab to become familiar with the oil immersion microscope. The first lab is therefore set up for you to simply view six different prepared slides. In each subsequent lab in this CELL, you will again use the oil immersion microscope. By the end of the CELL, you should become proficient using this valuable and exceedingly useful piece of equipment.
- This slide shows the “anatomy” of the binocular oil immersion microscope. LabLearner uses slightly different models of microscope depending on availability and new innovations. While your particular microscopes might look slightly different from the one pictured here, each and every part that is relevant is virtually the same on all of our “scopes”. Let’s consider the essential components of the microscope separately.
- Nosepiece: The nosepiece is the rotating disk to which the objective lenses are attached. It is turned in either direction to bring each objective lens into place, thus changing the magnification of the image. LabLearner middle school microscopes always come with four objective lenses attached to the nosepiece
- Stage: The stage is the flat, solid surface upon which the specimen slide is placed for examination. It moves up and down when the focus knobs are turned, which moves the slide closer or further from the objective lens. The microscope slide can be firmly affixed to the stage by placing it in the spring-loaded arm.
- Diaphragm: The diaphragm works much like the iris of your eye. By moving a lever on the diaphragm, the hole in it is opened and closed. This controls the amount of light that comes up through the light source and reaches the specimen. The diaphragm can be adjusted to virtually any opening between fully opened and closed. While manipulation of the diaphragm my improve contrast in some cases, it can be left wide open for nearly all purposes.
- Translation Knobs: The translation knobs precisely move the slide on the stage. This allows for examination of various areas of the specimen slide at any magnification. In less sophisticated microscopes, where the slide is affixed to the stage with two spring clips and there are no translation knobs, the slide must be moved with one’s thumb. This offers very little control in moving the slide, particularly at high magnifications.
- Coarse Focus Knob: There is a coarse and a fine focus knob on each side of the microscope. Both pairs of knobs do exactly the same thing and are meant to leave the user’s writing hand free to draw or make notes while the other hand focuses the instrument. The coarse focus knob is the larger, outer knob. It moves the stage up and down in much greater increments than the fine focus knob.
- Fine Focus Knob: This control is located inside the coarse focus knob. It moves the stage up and down very slightly. In practice, one first focuses with the coarse focus knob and then switches to the fine focus knob for perfect focus. The individual eyepieces may also be adjusted separately for the very best image possible.
- Light Intensity Knob: This knob is used to control the amount of light coming from the light source. It simply controls the electrical current reaching the bulb. In most models, the light intensity knob also is used to switch the power off.
- Eyepiece: Also called the ocular lens. There are two oculars on a biocular (or binocular) microscope. Because of differences between the left and right eye for many humans, the eyepieces can be focused independently. Some practice will yield exceptionally clear and bright images. Eyepieces typically have a magnification of x10, x15 or x20. LabLearner always uses x10 eyepieces.
- Objective lenses: LabLearner middle school microscopes come with four objective lenses (x4, x10, x40, and x100). Multiplying the power of the particular objective in use times the power of the eyepiece (x10 for LabLearner microscopes), gives the final magnification at which the specimen is viewed. Thus, each specimen can be view at x40, x100, x400 or x 1,000.
Note: All LabLearner microscopes use parfocal lenses. This means that, once a specimen is brought into focus with one objective in place, one can turn the nosepiece and switch to another objective lens and the specimen will be close to focused with that lens as well… only the fine focus knob will likely have to be used for perfect focus.
- This slide simply reviews how to determine total magnification.
Note: Calculating total magnification is something you already have experience with using the elementary LabLearner microscope in elementary CELLs.
Note: You are unlikely to have used an oil immersion lens in the past. This slide gives you a brief overview of how to use the lens and why it is useful in microscopy.
- The amount of light that enters a lens is an important determinant of image quality. The higher the power of an objective lens, the smaller an area of the specimen is seen.
Note: One can easily demonstrate this with any microscope by focusing with the x4 objective and then switching to the x10 and x40 while peering through the eyepiece.
- In other words, the more magnified the image, the less of the specimen is observed. As we increase the power of the objective lens, we are essentially sampling a smaller area of the specimen.
- This means that the lens is collecting less and less light as its power is increased. As one switches from the x4 to x10 to x40 objectives, the image becomes less bright. That is, less and less light is collected by the objective lens as its magnification power is increased.
Note: This is easily seen in doing the same test described above.
- As the magnification factor of the objective lens reaches higher values, such as x100, the low-level of light collected by the lens starts to have a significant impact on the image.
- An oil immersion x100 lenses helps improve the image it produces by increasing the amount of light it captures and sends up to the eyepiece for further magnification.
- It’s all about light refraction. As light travels from one medium to another, its path is refracted somewhat depending upon its wavelength and nature of the medium.
- Light traveling from air into glass or from glass back into air is refracted to a considerable extent. This occurs twice prior to reaching the objective lens; once when entering the slide and again when leaving the coverslip (shown in small inert at upper right).
- Immersion oil has similar refractive properties as glass and eliminates much of the refraction that normally would occur when light leaves the coverslip. Thus, more light is sent into the objective lens and a better, brighter image is produced.
- This slide initiates the discussion of cells. At this point, we highlight a major distinction between types of cells. On the left are examples of cells that exist as independent units. There are many more examples but those shown here include bacteria, protozoans (Amoeba in this case), and algae. Algae are plants and Amoebae are animals. Bacteria are prokaryotes.
- On the right, cells that only exist as components of tissue are shown. Obviously there are many other examples. Pictured here are cells of a plant leaf, cells in brain tissue, and cells in skeletal muscle.
- Yet another important distinction between types of cells is between prokaryotes and eukaryotes. This slide depicts a prokaryote; bacteria. Prokaryotes are notably different from eukaryotes (discussed on the next slide) in a number of respects:
- Prokaryotes have no distinct, membrane-bound nucleus. In prokaryotes, a DNA-containing chromosome is loose in the cytoplasm.
- Prokaryotes have no distinct, membrane-bound organelles, which are functionally distinct sub-compartments in the cytoplasm of eukaryotes.
- Prokaryotes are generally much smaller and harder to see in the microscope than eukaryotes.
Note: In Investigation 1 Lab, you will have a chance to view bacteria in two of your six “unknown” slides. Bacteria are prokaryotes. This slide includes an insert that shows the three basic morphologies of bacterial cells; spherical, rod, and spiral. You will see the first two examples in Lab.
- Most bacteria are harmless to humans and many are very useful. For example, Escherichia coli (one of the specimens you will view) lives in the human intestinal tract and is essential for our proper physiology. Bacteria are also extremely important decomposers in the food chain.
- Nonetheless, bacteria have also been a scourge in human history in terms of food spoilage and diseases ranging from the Black Death in the 14th century (Yersinia pestis) to the common pimples on teenagers’ faces (Propionibacterium acnes and Staphylococcus aureus).
This slide shows idealized examples of plant and animal cells. Both are eukaryotes. We will discuss the nature of the subcellular structures in later Investigations in this CELL. For now, notice that eukaryotes are distinctive from prokaryotes:
- Eukaryotes have a distinct, membrane-bound nucleus.
- Eukaryotes have distinct, membrane-bound organelles, which are functionally distinct sub-compartments in the cytoplasm.
- There are additional important differences between plant and animal cells that appear on this slide but we will discuss these differences in an upcoming Investigation.
- This final slide simply shows a good microscopic view of the six specimens that you will examine in Investigation 1 Lab.