How To View Cells Under A Microscope?
Cells and microscopes
- All living organisms are made up of cells.
- Cells are the smallest part of a living organism and are around 0.01 mm - 0.03 mm long.
- To look at a cell close up a microscope needs to be used.
- Many biological structures are too small to be seen with the naked eye
- Light microscopes allow tissues, cells, and sub-cellular structures to be seen and studied
- Light microscopes possess low magnification and resolution, but can view living specimens in their natural colours
- It is possible to prepare cheek cells and onion cells for viewing under a microscope in the school laboratory
Plant and animal cell slides
Let's take a look at how to observe cells under a microscope.
No prizes for guessing the first thing you'll need: a microscope. But don't worry if you don't have one of your own. Ask your school if they have one you can use. Then you'll need an onion. That's where you'll get your plant cells. Some food colouring - blue or green works best. A cotton bud. And a mouth. It's probably best to use your own.
So let's begin by looking at some plant cells. Cut a small chunk from your onion. Be careful with knives. Ask an adult if you need help. Peel a thin layer off that chunk and put it on your slide. This is what's called the epidermis. Using a drop of food colouring, stain the layer so you can see the cells. Pop a cover slip on the slide and then put the slide onto the microscope. Take a look! Check out that strong cell wall structure. It's solid so that it keeps a plant's shape and structure.
But this experiment isn't over. You can look at some animal cells too.
Grab a fresh slide and pop a drop of your food colouring on there. Take your cotton bud and rub the inside of your cheek. Told you you'd need a mouth. Transfer the cheek cells to the slide by rubbing the swab all over it. Drop the cover slip on the slide and stick it on the microscope. Go find those animal cells! Unlike the plant cells, animal cells are soft and fleshy. Animals don't need cells to keep their structure. That's what skeletons are for.
Apparatus
- an onion, a slide and cover slip, a cotton bud, some food colouring, disinfectant (solution), a plastic bag to put the cotton bud in and of course a microscope!
- light microscope
- 2 × glass slides
- 2 × cover slips
- freshly cut onion
- tweezers
- cotton bud
- 0.1 % methylene blue solution
- iodine solution
- paper towel
- mounted needle
Preparing an onion cell slide
- Step 1 - Carefully cut an onion in half (or ask an adult). Peel a thin layer of onion (the epidermis) off the cut onion.
- Step 2 - Place the layer of onion epidermis carefully on the glass slide, and cover with a cover slip.
- Step 3 - Stain the layer of onion with food colouring.
- Remove a thin layer of onion tissue from an onion
- Aim for the thin layer of tissue that can be found in between the layers of the onion; it should be possible to pull this layer of tissue away from a piece of onion using tweezers
- It is essential that the layer is only one cell thick so that light passes easily through the sample and so that individual cells can be clearly seen
- Add a drop of water to a clean microscope slide and gently place the onion tissue on the slide
- Add a small drop of iodine stain and gently lower the coverslip over the specimen using a mounted needle
- Lowering the coverslip slowly reduces the risk of squashing the sample and means that air bubbles are less likely to form
- Wipe away any excess water and stain using a paper towel
Preparing cheek cells
- Step 5 - Rub the tip of a new, clean soft-ended cotton bud against the side of your cheek. Dip the cotton bud in the disinfectant (solution), and dispose in a sealed plastic bag in the bin.
- Step 6 - Stain the slide by rubbing it with a cotton bud dipped in food colouring.
- Step 7 - Rub the cotton tip (with your cheek cells on it) against the food colouring.
- Take a toothpick, rub the inside of your cheek, put it on a slide, look at it
- You can spit onto a slide and see epithelial cells without doing any kind of stain.
Using the microscope
- Move the stage (the flat ledge the slide sits on) down to its lowest position.
- Place the glass slide onto the stage. Be careful pushing it under the clips that the cover slide doesn't move or crack.
- Select the lowest power objective lens.
- Turn the coarse focus knob slowly until you are able to see the cells.
- Turn the fine focus knob slowly until the cells are in focus and you can see them clearly.
- Repeat steps 1-5 using the higher power magnification to see the cells in more detail.
When using a high power microscope (also known as a compound microscope) it is best to start out with the lowest magnification, get your specimen in focus, and then move up to the higher magnifications one at a time. This is the easiest way to ensure that you will be able to focus in on your object quickly.
Magnification and field of view
- To see an object, the eye piece lens and the objective lens magnification are multiplied together to give the total magnification.
- Total magnification = eye piece lens magnification × objective lens magnification
- For example: 10 × 20 = 200
- Magnification = 200
- A light microscope has an eyepiece lens with 10x magnification and the objective lens is set to a power of 40x.
- Total magnification = power of eyepiece lens x power of objective lens
- Total magnification = 10 x 40
- Total magnification = 400x
- At 400x magnification you will be able to see bacteria, blood cells and protozoans swimming around.
- At 1000x magnification you will be able to see these same items, but you will be able to see them even closer up.
- Field of view is how much of your specimen or object you will be able to see through the microscope.
- At 40x magnification you will be able to see 5mm.
- At 100x magnification you will be able to see 2mm.
- At 400x magnification you will be able to see 0.45mm, or 450 microns.
- At 1000x magnification you will be able to see 0.180mm, or 180 microns.
Light microscopes and electron microscopes
- Light microscopes are limited to a maximum magnification of around 1500x, meaning that they can only be used to view whole cells and the largest cellular structures
- Light microscopes are generally only useful for viewing 2d structures
- For detailed examination of cells and cellular structures an electron microscope must be used
- Electron microscopes are capable of much higher magnification than light microscopes
- The light microscope can resolve details 0.2 μm apart
- Living cells are seen clearly in a phase-contrast or a differential-interference-contrast microscope
- Electron microscope
- Very large so can only be used in the science lab in which they are installed
- Vacuum needed (so specimens must be dead)
- Complicated sample preparation
- Over 500 000x magnification
- Light microscope
- Small and easy to carry
- No vacuum needed (so specimens can be living)
- Easy sample preparation
- Up to 2000x magnification
Staining and preparation
- Microscopy depends as much on techniques for preparing the specimen as on the performance of the microscope itself.
- Tissues are usually fixed and sectioned for microscopy
- Different components of the cell can be selectively stained
- Specific molecules can be located in cells by fluorescence microscopy
- Antibodies can be used to detect specific molecules
- Biological specimens require special preparation for the electron microscope
- Iodine solution is an irritant
- Risk of irritation to the eyes or skin
- Drench the area with water if in contact with iodine solution
- Wear eye protection
- Glass microscope slide and coverslip
- Risk of cuts to the skin if broken
- Care when using it to avoid breakages
- If breakage occurs, sweep up and dispose of broken glass carefully
- A cheek cell sample taken from a person with an infectious illness
- Risk of infection to others
- Do not perform sampling on a person with an infection of the throat or respiratory system
- Concentrated methylene blue
- Methylene blue is toxic if ingested
- Wear gloves / wash hands after use
- Do not allow children to handle or have access to methylene blue
