9781422280300

Meet the Immune System

Inside Your Body, from Small to Smallest

Your body has trillions of cells. Each cell has a cell wall, or membrane, that contains a fluid called cytoplasm . Floating in the cytoplasm are organelles, smaller membrane-wrapped containers. One organelle, the nucleus, is the brains of the operation. It contains your DNA, which instructs the cell on how to make proteins. Mitochondria are the cell’s power plant. Golgi apparatus are in charge of packing up proteins for shipping elsewhere. Bacteria also live inside your body. Some

of them are pathogens, but some of them are helpful. Bacteria are simple single-celled creatures. They have a cell membrane and cytoplasm, but no organelles. Bacterial DNA isn’t contained in a nucleus; it just floats around in cytoplasm. Viruses are even simpler. In fact, they are so simple that scientists argue about whether viruses can be considered living creatures. A collection of genes wrapped in a container of protein, viruses can’t even reproduce by themselves. They have to invade a cell and use the cell’s reproductive capability to make more viruses. Proteins are large molecules that cells make by chaining together smaller molecules, called amino acids. Different combinations of amino acids make different proteins. Proteins form tiny chemical machines called enzymes, which performmuch of the work inside a cell. Molecules are collections of atoms, which are the fundamental unit of all matter, from bodies to boulders to stars.

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