Cells are really important: all living things are made of them. They are often one of the first things you learn about when you start secondary school, and feature prominently at GCSE and A-Level too. And they are fascinating – and hugely varied – things.
If you follow me on twitter, you’ll know that last week was very exciting, because I finally got the camera which fits into my microscope up and running, allowing me to view what I can see through my microscope on my computer screen. Which means I can take pictures for a blog post!
Eukaryotic cells are plant and animal cells, with a cell membrane, cytoplasm, and – crucially – a nucleus. These are the ones you can easily look at under a microscope at home.
Animal cells have:
A cell membrane, which controls what enters and leaves the cell and gives the cell shape.- Cytoplasm, a jelly-like substance which is where chemical reactions (which keep the cell alive!) happen.
- A nucleus, which contains DNA, controls what the cell does, and contains the instructions to make new cells.
You can image animal cells from your cheek easily under a microscope. Using a cotton bud, swab the inside of your cheek, and then wipe the cotton bud very gently onto a clean microscope slide. To be able to see the cells, you need to stain them. Methylene blue is traditionally used, which you can buy from aquatic shops (it’s used for treating fungal infections in fish, among many other uses); Twitter users assure me you can use food colouring too, although I haven’t tried this. A cover slip is next, which serves the dual purpose of protecting the lens of the microscope, and making the sample an even thickness. (It’s worth putting the cover slip on which the slide on a paper towel, to catch any drips that spill out.)
Human cheek cells at x100 magnification.
Human cheek cells as x400 magnification.
Human cheek cell at x400 magnification. This one looks exactly like the diagram I was taught to draw when I was 11. Can you spot all the different parts?
A pair of human cheek cells next to each other at x400 magnification. Notice how they are not identical, but are similar and show all the same parts.
In addition to this, plant cells have:
A cell wall, which provides the cell – and in turn, the plant – with structure and support. (Remember, plants don’t have bones!)- A permanent vacuole, containing cell sap (made of water, amino acids, glucose, and minerals).
- Chloroplasts (in the bits above ground), which contain chlorophyll, a green pigment which absorbs light for photosynthesis.
Onion skin cells (the thin film between layers of an onion) are some of the easiest to image, using iodine as the stain. These don’t have chloroplasts, which makes sense when you think about where onion bulbs grow, half submerged in soil and with all but the outer most papery layer hidden from the light.
Onion skin cells at x40 magnification. See how they are much more oblong in shape than the human cheek cells, and they stack together like bricks in a wall.
Onion skin cells at x100 magnification.
Onion skin cells at x100 magnification. Which parts of a plant cell can you identify? Which parts are missing? Why might this be?
Onion skin cells at x400 magnification. How can you tell that the onion skin cells are bigger than the human cheek cells?
Other organelles in eukaryotic cells include:
- Mitochondria, which carry out respiration.
- Ribosomes, which are the site of protein synthesis.
Prokaryotic cells are much smaller, covering things such as bacteria. There is huge variety among prokaryotic cells, but the key difference is that they have no nucleus. Their DNA is a single loop in the cytoplasm, and there are sometimes smaller rings called plasmids. It is much harder to image prokaryotic cells because they are so small.
Prokaryotic cells, take with oil immersion microscopy (which means the magnification is even higher than any of the other pictures here!). Gram staining can be used to distinguish between gram-positive and gram-negative bacteria.