Cells show great diversity in form and functions. Because of this, it was not easy to realize that all living organisms are made up of units sharing a common basic structure. Every unit is a cell. The other major issue for the discovering of the cell was the very small size they usually show.
1. Cell size
Cell size is measured in micrometers (µm). One micrometer, or micron, is one thousandth of a millimeter (10-3 millimeters), and one millionth of a meter (10-6 meters). A typical eukaryote cell is between 10 and 30 µm in size. This is true for the cells of a worm and for those of an elephant, but there are many more cells in the elephant. To be aware of how small the cells are, imagine a 1.70 meters tall person which is stretched to match the height of the Everest, which is about 8500 meters. The stretched giant cells of that person would measure only 1.3 centimeters, i.e., smaller than an euro cent coin (then, it would be a giant formed by a huge amount of euro cent coins).
However, there are eukaryote cells having uncommon dimensions (Figure 1). They can be very small, like sperm cells, whose head may be smaller than 4 µm in diameter, while others like the eggs of some birds and reptiles can be larger than 10 centimeters (thousands of microns) in their larger axis, but we should measure only the yolk, since the egg white is not part of the cell. An extreme example is the egg of ostriches. Some cells may have cytoplasmic extensions as large as several meters in length, such as the brain neurons of giraffes that innervate the more caudal part of the spinal cord. Smaller than eukaryote cells are prokaryote cells, which typically are around 1 to 2 µm in diameter, being Mycoplasma the smallest with 0.5 µm.
Most living organisms are unicellular, i.e., a single cell. Prokaryotes (bacteria and archaea) are the most abundant unicellular organisms. Unicellular eukaryote species are abundant too. Organisms that can be observed without microscopes are mostly multicellular, i.e., they are made up of many cells. Multicellular organisms are animals, plants, fungi and some algae. In general, larger multicellular organisms contain higher number of cells since they have a similar average cell size. Estimates of the total number of cells of an organism with similar size to humans may range from 1013 (1 followed by 13 zeros) to 1014 (1 followed by 14 zeros). To be aware of these numbers, it is estimated that the total number of cells in the human brain is about 86x109 neurons and of a mouse brain is about 15x109. The most abundant cells of the human body are red blood cells and glial/neuron cells of the nervous system.
Cell morphology is typically sketched as rounded, but this is probably the most uncommon shape (except for a few types of cells). Cell morphology in animal tissues is diverse, enormously diverse! It can range from rounded to star-like, from multi-lobed to filiform. Plant cells also show a wide diversity of forms, which is determined by the cell wall, being cuboidal and columnar shapes the most common shapes. See some examples in Figure 4.
Every living organism needs to perform many functions to maintain its integrity, and to grow and proliferate, which are carried out by many cell types working coordinately. These functions are extremely complex and diverse, going from those related to food digestion, detoxification, movement, reproduction, support, defense against pathogens, to those related to thinking, emotions or consciousness. All these functions are carried out by specialized cells, such as those of the gastrointestinal epithelium, liver, muscle, germ cells, bone, lymphocytes and neurons, respectively. Cells need a particular molecular framework, mainly based on proteins, to carry out their functions. In an organism, some functions may be carried out by only one cell type, but the cooperation of several cell types acting in a coordinated way is commonly needed.