ڤنرڠن
Cells: the building blocks of life
All living organisms are made of cells. A cell is the smallest unit of an organism that can carry out the processes of life, and it is far too small to see clearly without help. Some organisms, such as humans, oak trees and mushrooms, are made of millions or billions of cells working together. These are multicellular organisms. Other organisms consist of a single cell that carries out every life process by itself. These are unicellular (single-celled) organisms.
Microorganisms are living things so small that a microscope is needed to see them. Microorganisms are typically single-celled. Examples include bacteria, many types of algae such as Chlorella, and protozoa such as Amoeba and Paramecium. Yeast, a type of fungus, is also single-celled. A few microorganisms are multicellular, which is why the word typically is used, but the large majority of those met at this level are one cell each.
Cells are measured in micrometres (symbol µm). One micrometre is one thousandth of a millimetre, so 1 mm = 1000 µm. A typical animal cell is about 10 to 30 µm across, and many bacteria are only 1 to 5 µm long. The human eye can see objects down to roughly 0.1 mm (100 µm), so individual cells are usually invisible without magnification.
The invention of the microscope allowed scientists to discover cells and to confirm that every organism examined is built from them. Using a microscope correctly is therefore a core skill for studying living things.
Parts of the light microscope
A light microscope uses a beam of light and glass lenses to produce an enlarged image of a thin specimen. Each part has a specific job.
| Part | Function |
|---|---|
| Eyepiece lens | The lens looked through; magnifies the image (commonly ×10) |
| Objective lenses | A set of lenses of different strengths (commonly ×4, ×10, ×40) on a rotating nosepiece |
| Nosepiece | Rotates to bring a chosen objective lens over the slide |
| Stage | The flat platform that supports the slide |
| Stage clips | Hold the slide in position |
| Light source (or mirror) | Provides light that passes up through the specimen |
| Coarse focus knob | Moves the stage or lens a large distance to bring the image roughly into focus |
| Fine focus knob | Moves it a very small distance to sharpen the image |
| Arm | The curved part used, with the base, for carrying the microscope |
| Base | The heavy foot that keeps the microscope stable |
Light passes from the source through a hole in the stage, through the thin specimen, up through the objective lens and then the eyepiece lens into the eye. Because light has to pass through the specimen, the specimen must be thin and at least partly transparent.
Setting up and focusing the microscope
A consistent method protects the lenses and the slide and gives a sharp image.
- Carry the microscope with two hands, one holding the arm and the other supporting the base.
- Place it on a flat, stable surface and switch on the light.
- Rotate the nosepiece so that the lowest-power objective lens (the shortest one, usually ×4) is over the stage.
- Place the slide on the stage, with the specimen over the hole, and secure it with the stage clips.
- Looking from the side, use the coarse focus knob to bring the objective lens close to the slide without touching it.
- Looking through the eyepiece, turn the coarse focus knob so the lens and slide move apart until the image appears, then sharpen it with the fine focus knob.
- Move the slide so the part of interest is in the centre of the view. To see more detail, rotate to the next objective lens and refocus using the fine focus knob only.
Starting with the lowest power makes the specimen easy to find because the field of view is wide. Raising the power narrows the field of view and makes the image darker, so the specimen must already be centred. Using the coarse focus knob at high power risks driving the lens into the slide, cracking the coverslip and scratching the lens.
After use, return to the lowest-power objective, remove the slide, switch off the light and store the microscope covered. Lenses should be cleaned only with lens tissue.
Preparing a temporary slide
A temporary slide (or wet mount) is a quick preparation that lasts for the duration of the observation. The specimen must be thin enough for light to pass through, so a single layer of cells is ideal.
Plant cells (onion epidermis)
- Cut an onion and peel off a thin, transparent layer of skin from the inside of a fleshy leaf.
- Lay the piece flat on a clean slide using forceps.
- Add one drop of iodine solution (a stain).
- Lower a coverslip at an angle using a mounted needle, so that air bubbles are not trapped.
- Blot away any excess liquid with filter paper.
Animal cells (cheek cells)
- Gently scrape the inside of the cheek with a clean cotton bud or sterile spatula.
- Smear the cells onto a drop of water or stain on a slide.
- Lower a coverslip as above. A stain such as methylene blue makes the cells visible. Used cotton buds and slides must be disposed of or disinfected as directed.
Why stain? Cells are mostly colourless and transparent, so their parts are hard to see. A stain is absorbed more strongly by some structures, such as the nucleus, which then stand out. Iodine stains the nucleus of onion cells yellow-brown.
Why a coverslip? It holds the specimen flat, keeps it in a thin even layer, protects the objective lens from the liquid, and reduces drying out. Under the microscope, onion cells appear as regular, brick-shaped cells fitting closely together, each with a visible cell wall and nucleus. Cheek cells appear as irregular, rounded, flattened cells with a visible nucleus and no cell wall.
Calculating magnification
Magnification states how many times larger an image appears than the real specimen. It has no unit and is written with a multiplication sign, for example ×400.
Total magnification of the microscope
> total magnification = eyepiece lens magnification × objective lens magnification
For example, a ×10 eyepiece with a ×40 objective gives ×400 total magnification. The lower and higher settings of the same microscope give different values: ×10 eyepiece with ×4 objective gives ×40, and with ×10 objective gives ×100.
Magnification of a drawing or photograph
> magnification = image size ÷ actual size
The formula can be rearranged:
> image size = magnification × actual size
> actual size = image size ÷ magnification
A useful memory aid is a triangle with image size on top and magnification and actual size at the bottom. Cover the quantity wanted to see the calculation.
Units must match. Measure the image with a ruler in millimetres. Cells are so small that actual size is better given in micrometres, so convert the image from mm to µm by multiplying by 1000 before dividing by the magnification. Image size is measured along the same line in the drawing as the actual size would be measured on the specimen, usually the length or width.
Magnification makes an image bigger, but it does not guarantee detail. Resolution is the ability to show two close points as separate. A light microscope has a resolution limit set by the wavelength of light, so beyond a certain point increasing magnification only makes the image bigger and blurrier.
Drawing what is seen
A biological drawing is a scientific record, not a work of art. It must show what is actually seen, accurately and clearly.
- Use a sharp pencil and unlined paper, and make the drawing large enough to fill about half the page.
- Draw clear, continuous single lines. Do not shade or colour.
- Draw only a few representative cells, not the entire field of view, and show their true shapes and relative proportions.
- Draw label lines with a ruler. Lines must not cross and must touch the part they label. Write labels horizontally, to the side of the drawing.
- Give the drawing a title that names the specimen, and state the total magnification used, for example "Onion epidermal cells, ×100".
- To record a scale, measure the drawing with a ruler and calculate magnification using image size ÷ actual size.
Plant cells should show the cell wall, the nucleus and, if visible, the cytoplasm and a large vacuole. Animal cheek cells show a cell membrane boundary, cytoplasm and a nucleus. Because the same structures can look different depending on the stain and the focus, only features that can actually be seen should be drawn.
