How Things Work: Cells
Imagine the Empire State Building. Now fill it with lima beans. If your little finger was the size of the Empire State Building, each of those lima beans would represent a single cell. In fact, there are more cells in the human body than there would be lima beans if you filled all of New York City with them.
To use the textbook definition, the cell is the fundamental unit of life. If our cells malfunction we get sick, and if our cells stop working we die. There are essentially two types of cells: eukaryotes and prokaryotes.
Prokaryotic cells are simply membranous sacs with genetic material, such as DNA, floating around in a viscous soup of proteins, sugars, and enzymes. All prokaryotes are single-celled organisms, such as bacteria and the blue-green algae that make up pond scum. However, not all unicellular organisms are prokaryotic. Amoebas, for instance, are eukaryotic.
Sunflowers, hippopotami, fungi, and humans, as well as all other multicellular organisms, are also eukaryotic. Eukaryotic cells are much more complicated than single-celled bacteria. Each eukaryotic cell contains a number of different specialized subunits called organelles, or ?tiny organs.? All plant and animal cells have certain types of ?organs? in common. The nucleus, the ?brain? of the cell, contains the genetic material (DNA) that tells the other parts of the cell what to do. A double-layered membrane called the nuclear envelope surrounds the nucleus. Inside the nucleus is an area called the nucleoulus, where ribosomes are formed.
Ribosomes make proteins. Compared to the other organelles inside the cell, ribosomes are quite small. Ribosomes are one of the few structures found in both prokaryotic and eukaryotic cells. However, they are not technically organelles because they do not have membranes. Ribosomes can float around freely inside the cell, or they can stick to another organelle called the endoplasmic reticulum.
The endoplasmic reticulum, or ER, comprises a series of sacs and tubes with ribosomes stuck to the surface. The proteins made by the ribosomes go into the ER where they get ?processed.? The processed proteins go from the ER into a series of flattened sacs called the Golgi body. The Golgi body packages up the proteins and puts them into ?boxes? so that they can be shipped to their final destinations.
All these cellular processes require energy. The cell?s ?power plants? are called mitochondria. The mitochondria take broken-down food molecules and turn them into energy the cell can use.
In addition to all these structures, plant cells also have cell walls to give them rigidity, and chloroplasts filled with chlorophyll to allow them to make food from sunlight. Many cells have hairlike cilia or whiplike flagella that allow them to move around. Sperm cells are the only human cells that have flagella.
Everything that is or was alive is made up of cells ? even lima beans. But the cells that make up pond scum, lima beans, and humans are quite different. Plants and animals are made up of hundreds, often thousands, of different types of specialized cells; brain cells and kidney cells, for example, are quite different. It is this specialization of cells that leads to the diversity and
complexity of life.