The eukaryotic cells are found in all plants, animal, fungi and protoctist. They are multicelled organism and reproduce in different ways (e.g. mitosis, meiosis). The surrounding cytoplasm contain many different membranous organelles, for example, fungi posses wall but are not photosynthetic; chloroplast is possessed by some protozoa but they have no wall; likewise some multicellular form don’t have organs and other differentiated tissue. (Clegg, 2001)
The aim of these experiment is to develop our knowledge and understanding on cells by knowing the significance of the use of cell and assess the importance of cell as an evidence in forensic cases (e.g. cells are used in DNA profiling); by being able distinguish between different cell types and their characteristic; using the microscope to examine different cell types and explaining the difference between them, especially in their size, colour and structure. This experiment also aims to build our confidence in using the microscope and improve our ability to calibrate a microscope to relevant sizes
Method
During this experiment, six different cell types (spermatozoa smear, squamous epithelium cells, pollen grain, diatoms mixed, bacillus subtilis, & human blood) were examined using a high power light microscope, which allows individual cells and some of their structure (e.g. nuclei,) to be seen. The advantage of this method is that it allows us to examine living cells and tissues, i.e. to know the size, colour, structure and the different between various cells. The disadvantage is that it has a limited resolution; that is it does not have the ability to distinguish between objects that are close together. This is a disadvantage because two different cells might be joined together and due to the limited resolution of the microscope, we might not be able to view them.
The microscope was first calibrated before the cells were examined, i.e. a stage micrometer was placed on the microscope stage, placing one side of the dark circle directly under the objective lens (x40 objective); when the microscope stage was at its highest point and was viewed through the eyepiece, the large focusing knob was use to move the stage downwards slowly until the line of the circle came to view, then the adjusted knob was use to clearly focus on the line and the stage micrometer was moved across until one could see the stage scale clearly (two scales was seen) , then, the stage micrometer was move across until the edge of the scale was in line with the eyepiece scale.
Finally the eyepiece (ES) units were counted and equals to 100 stage units.
The width of one ES unit was calculated using this equation
1ES unit = (No. Stage scale units (SS)/100) x 1000
No. Eyepiece scale units (ES)
To convert Stage scale to mm, divide by 100
To convert to µm multiply by 1000
The values was tabulated (the values calculated = width of 1ES units in µm)
The cells were then examine by securely placing the slides which contain the material on the stage; the stage was moved sideway until the visible material was directly underneath the objective lens, then the stage was then moved to its highest possible position, then looking through the eyepiece, the stage was then move downward slowly until the material on it was appearing through the eyepieces. The adjustment knob was then used to focus on the clearly on the material. Finally a member of staff was then asked to check if the correct structure was found. Then the cells were drawn using the instruction for drawing biological material, which is using a sharp HB pencil to draw and label on a clean A4 paper and give each diagram a title. Then a line is used to connect each label to the drawing, without shading.
The cells were measured by moving the slide until the left edge of the ocular scale was in line with the edge of the cell, then the number of ES unit equal the width of the cell was counted and the ES unit was converted into um i.e.
Width of cell = width in ES units x width of 1 ES unit in µm
(McCusekrs, 2007)
RESULTS
This equation was used in calculating the result.
1ES unit = (No. Stage scale units (SS)/100) x 1000
No. Eyepiece scale units (ES)
Using x40 magnification
1ES unit = 1/100 x 1000
4
= 2.5 µm
Using x100 magnification
1ES unit = 1/100 x 1000
10
=1µm
DIATOMS MIXED SPERMATOZOA SMEAR,
SQUAMOUS EPITHELIUM POLLEN GRAIN
BACILLUS SUBTILIS HUMAN BLOOD
Discussion
The aim of the experiment was to explain the different in cell structure and organization. From published information and my observation of the various cells, the difference and similarities between the six examined cells lies in their composition, looks and where they are found. For example, the spermatozoa have a unique shape compared to the other types of cells. It is about 0.05ml longs; consisting of a head, body and tail. The head contains the nucleus of genetic material from 23 chromosomes, which is covered by the ac cap. Mitochondrion which supplies energy is connected from the neck to the body. The tail contains protein fibers that contract on alternative sides, giving a characteristic wave-like movement, which drives the sperm through the seminal fluid. The head consist of the acrosomal vessel and nucleus; the neck has the mitochondria; while the tail has the plasma membrane and flagellum. (Martin, 2001)
It takes 72 days for one sperm cell to grow in the testes, and it requires a temperature which is three to five degrees below body temperature. An in-built thermostat in the scrotum keeps the sperm at the correct temperature while they are stored. Sperm contains small amount of fructose, ascorbic acid, cholesterol, critic acid, lactic acid, nitrogen, creatine, vitamins B12 and various salts and enzymes. An average sperm swims at the rate of 3mm (0.12inches) per minute. Some sperm swim better than others and they wave their tales more than 1000 times just to swim 1.25cm or half an inch. The reason why some sperms are better than the other is still a mystery to many fertility specialists. I think it might be because of the variation in the people’s lifestyles and diets. (Luke et al, 2006)
Sperm competition has being a driving force of sexual selection. This occurs in internal fertilized organism when more than one male ejaculate is simultaneously present in a female reproductive system. Some researcher suggested that greater sperm size improve the competitive ability of sperm, while other studies prove contradictory depending on the species. Also the roles of females in the evolution of sperm morphology have been point out to contribute to sperm competition. A research carried out by Luck et al (2006) shows the effect of species on sperm size. They investigated the male and female effects that influence sperm size in human. Female were mated with two males and the paternity outcome of both males were analyzed, after determining the sperm transfer and storage; they found a high value of the last male sperm precedence, which suggested that a strong interaction between rival sperm as occurred. Nevertheless, the data also showed a high frequency of removal of the sperm of the first male from the female reproductive tract prior to any interaction with the second male. This means that paternity depends mainly on the sperm that was successfully stored, and not on sperm sizes.
Spermatozoa cells are used in forensic investigation for cases of alleged rape. Although, it is not the most accurate method because it can not be used for an alleged rapist who has being sterilized or who has done vasectomy, instead of this method, sample of urine, semen, and blood could be used for DNA.
Pollen grain cells are also unique in their cell wall. The surface is divided into three strata: (i) an outer exine wall, which is the outer layer of a living pollen grain. It is thick and very resistant. It is composed of the sporopollenin (which is chemically stable and resists to all kind of environment) and the aperture (uninterrupted opening). (ii) An inner intine, which is the inner layer of a living pollen grain; it is multilayered and made up of cellulose. (iii)The pollen coast, which made up of lipids, protein, pigment and other compounds that fill the sculptured cavities of the pollen exine.
Pollen grain are classified according to the number and position of the aperture; the shape of the pollen as a whole; and the fine elaborate structure on the sexine. The patterns of pollen grain a characteristic of the species and are useful for scientist because pollen deposits can be examined with microscope, which could help in analyzing the geographical area of an incident. From observation, the pollen grain has a separate membrane and a variation in the size, colour and shape. (Clegg, 2000)
Diatoms are delicate unicellular organism that have yellow brown chloroplast that enable them to photosynthesize. Their cell wall is made of silica (SiO2), which refer to the frustule, and the organic material that coats the valves and the girdle. The frustule have two valves that fit within each other (one valve is smaller than the other and it varies in shape). A suture that allows two valves to move apart or toward one another is formed by the girdle element and valves. Diatom contains a nucleus, mitochondria, plastid, cytoplasm, protoplast and volutin. In forensic investigation, Diatoms could be used to know the original crime scene. For example, if a person has been transferred after being drowned. (Sadava, 1993)
Squamous epithelium cell are different in their cell structure because they are flat cell that have an irregular flattened shape. Their cell is made up of cytoplasm, plasma membrane and nucleus. The middle of the squamous epithelium cell has thick layer of connective tissue which contains blood and fat cells. The edge is made of uneven black ridged line and cell boundaries can not be made because the cells are flattened. (Dawson &Honeysett, 2001)
Human blood cell consists of red blood cells, plasma, platelets and white blood cells. The red blood cell has no nucleus, while the white blood has got. When a human blood cell is viewed from a light microscope, the cell membranes are visible. Each blood cell component can be separately viewed in details. In forensic investigation, human blood cells can be used for blood analysis as it carries genetic DNA. The use of blood cell might not be accurate in a case twin because they carry the same DNA, so another option might be fingerprint. (Clegg, 2000)
Conclusion
In conclusion, one can argue that the experiment was successful because the aim and objective were met, i.e. it shows the basic structures and features of cells, for example, the plasma membrane, cell walls, nucleus and etc. it also gives us an understanding of the cells function by allowing us to see what the cell comprises. In addition, it enables us examine the difference and similarities of cell types, e.g. nucleus is present in most cells, while other didn’t have one. In diatoms, the colour of the chloroplast is yellow-brown instead of green, which is the common source of energy for plant. The experiment was accurate because if it is performed under the same conditions, and procedure are followed, we will get the same measurement and result.
An individual could also argue that the experiment was not precise because a light microscope was used for all the cell, which has a limited resolution power. I believe that an electron microscope should have being used for some of cell because it has a higher magnification and resolving power than a light microscope, with the magnification up to about two million times when compared to about two thousand of a light microscope. It also allows us to see smaller object and their feature in details. The electron microscope uses electrostatic and electromagnetic lenses to control illumination and imaging on specimen, unlike the light microscope which uses glass lenses to focus on light and magnify small samples. A precise measurement would be made with an electron microscope because it is more likely to uncover some things that the light microscope has not shown. For example, joined cells might be shown as a single cell or element under a light microscope, but with an electron microscope, we are more likely to seen that two or more cells or element has joined together.
Before starting the experiment, a control sample should have being kept, but this was not the case. Therefore we do not have an evidence of the originals cells.
Reference:
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Clegg, C. J. (2000) Introduction to Advanced Biology. London: John Murray
David E. Sadava (1993) Cell Biology: Organelle Structure and Function. London: Jones and Bartlett
Dawson, B. & Honeysett, I. (2001) Biology for OCR A. edited by Bob McDuell London: Heinemann
Krommenhoek et al (1979) Biological Structures London: John Murray
Luck et al (2006) Genetica Journal: Male and female effect on sperm precedence in the giant sperm species Drosophila bifurca. Biomedical and Life sciences, 257-265. vol130; 3. Springer Netherlands
Martin, J. (2005) Biology 6th edn. Cambridge: Cambridge University Press McCuskers, S (2007) Forensic Investigation: Trace and Transfer lecture note. Canterbury: CCCU
Thorpe, O N. (1984) Cell Biology. Canada: John Wiley & Sons.
