What Are Cells?

In 1655, an English scientist named Robert Hooke used a rudimentary microscope to study the pores in cork; he named those pores cells. Advances in microscope technology continued through the 1800s, producing sufficiently high-resolution images to enable scientists to identify and label specific structures within cells. In 1839, German botanist Matthias Schleiden and German biologist Theodor Schwann suggested a complete cell theory, asserting that the fundamental unit in all animals was a cell with an identifiable nucleus, an internal membrane-bound structure. While these discoveries seemed miraculous in the 1800s, today's electron microscopes are far more powerful and able to focus on details as small as 50 picometers (one picometer equals 10^–12 m). Advances in microscopy helped provide evidence to support the tenets of cell theory: cells are the basic unit of life, all organisms are composed of cells, and cells are derived from previously existing cells.

Cells perform all the basic functions of an organism. Cells take in and process the energy from food and carry the genetic information that controls reproduction, growth, and energy generation of metabolism. All cells also have a cell membrane with a lipid bilayer structure that helps the cell control its interactions with its environment. The cellular structure of living organisms is one of the unifying features of biology, as all organisms share the same basic format.

Cells are made up of molecules, structures that are composed of elements such as hydrogen, oxygen, and carbon bonded together. The most common molecule in cells is water, but cells also have larger biological molecules such as proteins, lipids, and carbohydrates. Specialized cells may also contain minerals that aid in the structure or function of the cells. For example, bone cells have high concentrations of calcium, and red blood cells contain iron, which aids in the delivery of oxygen to cells throughout an animal body. Plant cells also use molecules of various minerals in their functions. For example, potassium and magnesium are vital for the process of photosynthesis (using the energy of sunlight to form glucose). Without nitrogen, plant cells develop poorly, with stunted growth and yellow leaves.

The nature and function of each cell is determined by its DNA. Some organisms are composed of only one cell that performs all the functions of the organism, including growth, metabolic functions, and reproduction. In organisms composed of multiple cells, some functions of the organism may be controlled by specialized cells or groups of cells. For example, while all cells in a human have the capacity to reproduce themselves, the specialized sperm and egg cells are the cells that function to reproduce the entire organism. Similarly, specialized immune cells are responsible for detecting and eliminating disease-causing pathogens.