By the help of a microscopes, there is nothing so small as to escape our inquiry; hence there is a new visible world discovered to the understanding.”
Robert Hooke – discoverer of plant cells
Now that we have taken a tour of all the Kingdoms of Creation, it is time for us to settle down and get to know some of these amazing creatures a bit better, to enter their homes and have a cup of cocoa (or, you know, rotting logs if that’s their thing). On this visit to see our friends, we are going to learn more about what all creatures are made of: cells. Cells (as I am sure you recall because you are all wonderful students) are the basic building blocks of creatures. When you look at a bacteria or archaea (under a microscope or in a picture), you are looking at that creature’s one and only cell. Whereas when you look at a zebra, there are many cells of different kinds, none of which you can see with the unaided eye.
Although nearly all cells are vanishingly small, cells do come in different sizes, from the tiniest bacteria to a slime mold as big as a dinner plate! Cells in your body come in many shapes, like the oval red blood cells that carry oxygen through your body and the long skinny muscle cells that help you move. Some of the longest animal cells are the 12 m (40 ft) long nerve cells in large animals like colossal squids and giraffes, while perhaps the heaviest is the egg of an ostrich, which can weigh up to 1.5 kg (3 lb).
Cells come in variety of shapes as well. The Kingdoms of Archaea and Bacteria are, of course, home to different creatures, but as they are all single-celled, they do look quite similar. Walking through one of their towns (What, you don’t think bacteria have towns? Why do you think they are called bacteria colonies?), you would find creatures that look like long pills, a short string of pearls, springs, fluffy (but rather lumpy) pillows, or giant-eyed worms with tails (those are Tetanus, if you want to check them out). Many of them have little hair-like fringes all around them or long tails that whip around as they move.
The Kingdom of Protists, home to mostly single-celled creatures as well, contains one of the widest variety of cell shapes. Can you think why that might be? Since protists are all the creatures that don’t belong anywhere else, they tend to be dissimilar from one another, in their cells as in other ways. There are plant-like ones that are shaped like almonds or grooved saddles, animal-like ones that extend and retract “pseudo-arms” (fake arms) as they creep forward looking for food, and fungus-like slime molds whose enormous cells spread like creeping fingers as it goes about its work of decomposition.
As we start to move into more familiar creatures, we start to see more uniformity in cell shapes. Plant cells all have rigid walls that are often rectangular or hexagonal (six-sided). Fungi cells have a stiff wall as well but tend to be more round or blobby in shape. Animal cells are also more regular, but because the creatures are more complex they have more types: blocks of skin cells, red blood cells like a thumb-print cookie or a donut, long, skinny muscle cells, squishy fat cells, nerve cells with tails, and others. All of these types of cells help the creatures to do the jobs that they were created to do, adding to the wonderful variety of the world.