All cells in living things can be classified in one of two basic cell types: prokaryotic and eukaryotic. Prokaryotic cells tend to be small, primitive, and independent of other cells and encompass bacteria, blue-green algae, and archae. Eukaryotic cells are larger, more complex types found in all other plants, animals, fungi, and protists. The different eukaryotic cell types are organized by structure and function, some of which exist independently of one another and others joined to other cells of common specialized function. The human body contains hundreds of different kinds of cells, many of which have unique functional structures.
A cell is the smallest part of a living thing and is the setting for all the chemical processes that are necessary to life. Prokaryotic cells are the most simple cell forms, having no nucleus or organelles. These cells have no chromosomes; circular plasmids contain the genetic material. Some prokaryotic cells do not need oxygen to carry out life processes.
Eukaryotic cells have a defined nucleus containing genetic material organized in linear chromosomes. They have various specialized organelles that carry out the processes and energy transfers necessary to sustaining life. Animal, plant, fungi, and protist cells are very different from each other. Fungi may be one-celled or multi-celled, have more than one nucleus, and have unsealed cell walls that allow cytoplasm to flow freely between cells.
Plant cells have several types of cells that perform multiple functions. The least specialized plant cell is the parenchyma cell, which metabolizes and store food. Collenchyma and sclerenchyma cells help support the plant.
Xylem cells conduct water while phloem conducts food. Epidermal cells cover the leaves, stems, and roots like a skin. Spongy leaf cells absorb light and make food for the plant. Root hairs are found on the roots and have a large surface area for absorbing water.
Human and animal cell types vary considerably depending on function and location. Some cells exist independently of other cells. White blood cells are an example. Other cells, such as skin cells, are firmly attached to other skin cells and work together to perform various functions. Many cells have specialized parts that perform operations no other cell type performs. Rod cells, for example, have photopigments that respond to light and transmit information to nerve cells that relay information to the brain, allowing a human to see.
Cells in humans and animals vary greatly in size. The smallest cells in the human body are sperm cells and the granule cell of the cerebellum. Some neurons, or nerve cells, may grow as long as several feet long and extend from the limbs to the spinal cord to the brain.