Cell and Light Microscope

Aim

In this assignment I am going to carry out a practical examination on a plant (onion) and an animal (cheek) cell under a light microscope. The stain methods I am going to use are iodine for my onion cell and methylene blue for my cheek cell.

Introduction

All animals and plants are made up of cells. Animal and plant cells are made from millions of tiny cells, so small that we are unable to see them with our naked eye. Animals and plants have many features in common within a cell such as a cell membrane, cytoplasm, mitochondria, ribosomes and the nucleus. Plant cells also have a cell wall, chloroplasts and a vacuole. We use microscopes to observe cells.

I am going to carry out my practical examination using a light microscope to produce a magnified image of each cell. There are many features of a light microscope such as: * They use light * They have a lower resolution than electrons microscopes * They have a maximum resolution of about 0.2 micrometres * The maximum useful magnification of a light microscope is about x1500 * They are able to view live specimen

I am using iodine to stain the plant (onion) cell because iodine reacts with the starch within a cell, causing the starch to instantly turn a partially black colour and the rest to turn a dark yellow colour. This will allow me to see the cell through a light microscope. I am using methylene blue to stain the animal (cheek) cell because the blue indicator turns colourless indicating live cells. Methylene blue reacts with the nuclei creating a more visible picture and I will be able to see the cells more clearly.

Safety Precautions

* I need to make sure the area I am using to conduct my experiment is clean and tidy * I need to make sure my hair is tied back * I need to make sure I have all my apparatus available to conduct my practical experiment * I need to ensure proper handling and care is taken when handling my equipment to prevent any injury to myself or possibly damaging the apparatus

Hypothesis

I think I will clearly be able to see both an animal (cheek) and a plant (onion) cell. I think it will be possible to distinguish the cell membrane, cytoplasm and the nucleus within both cells. Within the plant (onion) cell I think I will also be able to see the cell wall, vacuole and chloroplasts.

Apparatus

* Microscope * Slide * Cover slip * Iodine * Methylene blue * Pipette * Scalpel * Tweezers * Gloves * Lens cleaning tissue * Cotton wool bud * Onion sample

Method

1. Tidy desk and make sure surrounding area is clean.

2. Set up microscope and gather all my necessary materials.

3. Make sure my slide, cover slip and pipette are all clean and dust free.

4. Place slide on a clean, dry, dust free surface.

5. Prepare each slide with specimen. For my onion (plant) cell, I will take a small portion of the onion sample and remove the thin skin from inside the onion ring by using my pair of tweezers, placing it onto my slide. For my check (animal) cell, I will gently scrape the inside lining of my cheek with a cotton bud. I will then rub it onto the slide.

6. Add a droplet of stain indicator onto each specimen. I am using iodine for my onion (plant) cell and methylene blue for my cheek (animal) cell. 7. Slowly place my cover slip on top of indicator. Gently place one edge of cover slip down before lowering the rest.

8. Gently secure slide and cover slip combination on the microscope stage within the stage clips.

9. Start with lowest magnification.

10. While looking through the ocular focus downward with the coarse focus knob until I can see my specimen and fine focus until I can see specimen clearly.

11. Increase magnification and repeat.

On the following pages I have drawn pictures of what I saw through the light microscope.

Conclusion

In conducting this experiment with a light microscope I found out it was better to place my slide under low power to locate the cells, then observe them more closely under high power.

I found the animal (cheek) cells were more difficult to find and get into focus. I think this was because they are much smaller and more disperse than the plant (onion) cell.

I could clearly see the similarities between a plant (onion) and an animal (cheek) cell. Their organelles were clearly visible under magnification. I was also able to distinguish the cell wall, chloroplasts and vacuole within the plant (onion) cell.

There is also a noticeable difference in the shapes of an animal (cheek) and a plant (onion) cell. The animal cell is more round whereas plant cells are more rectangular. This is because animal cells have no cell wall ant therefore their shape is irregular.

This supports my hypothesis because I predicted it would be possible to clearly see a plant (onion) and an animal (cheek) cell under a light microscope. I was also able to clearly distinguish the individual organelles within each cell. I could see the cytoplasm, cell membrane and nucleus in both the animal (cheek) cell and plant (onion) cell. I was also able to distinguish the cell wall, vacuole and chloroplasts within my plant (onion) cell.

Evaluation

My experiment was successful using a light microscope to produce a magnified image of specimen.
From conducting a practical experiment on both an animal (cheek) and a plant (onion) cell, I can clearly see both cells have similar organelles. This shows I have been successful in carrying out my method to the proper procedures.

Although I could clearly see each individual cell, I also noticed air bubbles when looking through my eye piece onto the animal (cheek) specimen slide. I think this has happened when I was placing my cover slip onto the indicator.
If I was to repeat the same experiment again I would:

- take extra care in placing my cover slip down more gently to prevent air bubbles.

- I would repeat results for more accuracy.

-I would like to see the difference in resolution using an electron microscope. Electron microscopes have a higher resolution than light microscopes and therefore would give a more detailed image.