How cells work
Your body is made of about 10 trillion cells. The largest human cells are about the diameter of a human hair, but most human cells are smaller and might be one tenth the diameter of a human hair.
Run your fingers through your hair now and look at a strand of hair. It is not very thick – maybe 100 microns in diameter (a micron is a millionth of a meter, so 100 microns is a tenth of a millimeter). A typical human cell might be a tenth of the diameter of your hair (10 microns). Look down at your little toe – it might be 2 or 3 billion cells or so, depending on how big you are. Imagine a whole house filled full of a baby peas. If the house is your little toe, the peas are the cells. That’s a lot of cells!
Bacteria are about the simplest cells that exist today. A bacteria is a single, self-contained, living cell. It is about one hundredth the size of a human cell (maybe a micron long, and a tenth of a micron wide), so it is invisible without a microscope. When you get an infection, the bacteria are swimming around your big cells like little rowboats next to a large ship.
Bacteria are a lot simpler than human cells. A bacterium consists of a outer wrapper called the cell membrane, and inside the membrane is a watery fluid called the cytoplasm. Cytoplasm might be 70 % water. The other 30 % is filled with proteins called enzymes that the cell has manufactured, along with smaller molecules like amino acids, glucose molecules and ATP. At the center of the cell is a ball of DNA (similar to a wadded up ball of string). If you were to stretch out this DNA into a single long strand, it would be incredibly long compared to the bacteria – about 1000 times longer!
Human cells are much more complex than bacteria – they contain a special nuclear membrane to protect the DNA, other membranes and structures like mitochondria and Golgi bodies, and a variety of other advanced features. However, the fundamental processes are the same in bacteria and human cells.