Anatomy & Physiology For Dummies, 3rd Edition
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Although they're astoundingly varied, cells are also remarkably alike. All cells, at least all eukaryotic cells, are alike. Plants, animals, and fungi are eukaryotes (organisms made up of eukaryotic cells), and all their cells, in all their enormous complexity and variation, are fundamentally alike.

Yes, your skin cells, your kidney cells, and your bone cells are fundamentally similar to the leaf cells and root cells of a carrot; the cells of a mold, mushroom, or yeast; and the single cell of microorganisms called protists that live in water and soil.

Here's a simplistic description of a eukaryotic cell: It's a membrane-bound sac containing smaller but distinctive structures, called organelles ("little organs"), suspended in a gel-like matrix called the cytoplasm.

As their name suggests, organelles are functional subunits of a cell, as organs are functional subunits of an organism. One of the largest and most prominent organelles, the nucleus, controls a cell's functioning, similar to the way the nervous system controls an organism's functioning.

The term eukaryote is derived from the Greek term karyos, meaning "nut" or "kernel," which early biologists used to refer to the nucleus. The figure shows the general structure of a eukaryotic cell.

Illustration by Kathryn Born, MA

A cutaway view of a basic animal cell and its organelles.

The following table gives you an overview of the structures found within a eukaryotic cell.

Organelle Function
Organelles of Animal Cells (Including Humans)
Nucleus Controls the cell; houses the genetic material
Mitochondrion Cell "powerhouse"; site of cellular respiration
Endoplasmic reticulum Plays an important role in protein synthesis; participates in transporting cell products; involved in metabolizing (breaking down) fats as well as drugs
Ribosome Binds amino acids together under the direction of mRNA to make protein
Golgi apparatus Modifies proteins into functional form; "packages" cellular products in sacs called vesicles in which products can cross the cell membrane to exit the cell
Vacuoles Membrane-bound spaces in the cytoplasm that aid in endo- and exocytosis
Lysosomes Contain enzymes that break down harmful cell products and waste materials and actively transport them out of the cell

The organisms called bacteria (singular, bacterium) are made up of prokaryotic cells. Prokaryotic cells are very different from and much simpler than eukaryotic cells in their basic structure and organization. This difference between eukaryotic and prokaryotic organisms is the great divide in biology. At the cellular level, differences among animals, plants, fungi, and protists are almost negligible compared with the differences between these groups on the one hand and prokaryotes (bacteria) on the other.

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Erin Odya is an anatomy and physiology teacher at Carmel High School in Carmel, Indiana, one of Indiana’s top schools.

Maggie Norris is a freelance science writer living in the San Francisco Bay Area.

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