The Cell: Building Block of Life

All living things are composed of cells, from a bacterium which may be made up of a single cell to human beings with trillions. Growth, metabolism, and reproduction are among the essential functions that cells perform. Biologists compare cells to tiny factories, as they are constantly bringing in new materials, manufacturing what the organism requires, and disposing of waste materials. [1]

Two main types of cells

Prokaryotic

A type of simple organism such as a bacterium is an example of a prokaryotic cell. Prokaryotic cells lack a membrane-bound nucleus. Instead the organism's DNA is housed in the nucleoid. Prokaryotic are much less complex structurally than eukaryotic cells. Prokaryotes are also much smaller and exist in huge numbers with the environment. Prokaryotes are found in soil, water, air, in human intestines, and et cetera.

Eukaryotic

Eukaryotic cells, found in animals, fungi, plants, and protists, are complex cells with a membrane-enclosed nucleus containing DNA. Eukaryotic cells are made up of specialized compartments called organelles, listed below.

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Organelles of eukaryotic cells

All cells have an outer membrane called a plasma membrane which encloses a substance called cytoplasm and the cell's genetic material. Eukaryotic cells have a number of other structures known as organelles.


Numbered diagram of a eukaryotic cell.
  • A centriole, (depicted as #13 in the diagram at left), is short cylinder-shaped structure, important in cell division.
  • Cytoplasm is the contents of the cell between the nucleus and the plasma membrane. Cytoplasm contains all the structures and compartments within the cell except the nucleus. See #11 in the diagram.
  • Endoplasmic reticulum [distinguished as "smooth" (see diagram #8 at left) and "rough" (see diagram #5 at left)] helps move materials through the cytoplasm.
  • The Golgi apparatus (or "Golgi body") is a complex stack of flattened membrane sacs that package products of the cell. See #6 in the diagram at left.
  • Lysosomes break down food or foreign particles. The lysosomes in white blood cells can destroy bacteria. See #12 in the diagram at left.
  • Mitochondria, (see #9 on the diagram), are tiny sausage-shaped organelles that produce energy by converting chemical energy in food into a form of energy the cell can use. Biologists call the mitochondrion "the powerhouse of the cell." The more energy a cell demands, the more mitochondria present in that cell. For example, muscle cells require much more energy than skin cells, thus more mitochondria will be present in muscle cells. (Note that in plant and algae cells, chloroplasts specialize in energy production, converting solar energy into nutrients through photosynthesis.)
  • Located inside the cell's nucleus, the nucleolus is made up of RNA and protein and plays a role in the formation of ribosomes. See #1 on the diagram.
  • Nucleus is like the brain of the cell. It is the largest organelle and contains chromosomes, which in turn contain the cell's DNA. The nucleus stores genetic information for the ribosomes to use. See #2 in the diagram.
  • The plasma membrane separates the cell from the surrounding environment. It regulates what is permitted to enter and exit the cell and communicates with neighboring cells. See #14 on the diagram.
  • Ribosomes are small circular structures, the most numerous organelles in the cell. Ribosomes are the site of protein synthesis. See #3 on the diagram.
  • Vesicles, (#4 above) and vacuoles (#10 above) are membranous sacs. Vesicles are smaller than vacuoles.

Cells, Tissues, Organs, Organ Systems, Organisms

A collection of cells form tissues. Tissues often form organs, an organ joins two or more kinds of tissue to perform specific functions in an organ system. For example, the human heart contains muscle tissue, nerve tissue, and connective tissue. The heart, along with blood vessels and lymph vessels, form the circulatory system, an organ system.

An organ system, found in more complex animals, is a group of organs that carries out a major activity. For instance, the human digestive system, which breaks down food into a form that cells can use, includes the mouth, esophagus, stomach, small intestine, large intestine, gallbladder, liver, and pancreas.

Other human systems include: respiratory, circulatory, nervous, urinary, reproductive, endocrine, skeletal, muscular. In an organism that has organ systems, all the systems together compose the body.


Fun Fact: The human body contains more red blood cells than any other type of cell.

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