Scientists — and people in general — love to divide things into categories. In some ways, life on Earth has done the same. Right now, scientists can divide cells into major categories — prokaryotes (or procaryotes; both spellings are okay) and eukaryotes.
Prokaryotes (PRO-kaer-ee-oats) are individualists. These organisms are small and single-celled. They might form into loose clumps of cells. But prokaryotes will never come together to take on different jobs within a single organism, such as a liver cell or a brain cell.
Eukaryotic cells are generally bigger — up to 10 times bigger, on average, than prokaryotes. Their cells also hold much more DNA than prokaryotic cells do. To hold up that big cell, eukaryotes have a cytoskeleton (Sy-toh-SKEL-eh-tun). Made from a network of protein threads, it forms a scaffold inside the cell to give it strength and help it move.
Keeping it simple
Prokaryotes make up two of the three big domains of life — those super kingdoms that scientists use to organize all living things. The domains of bacteria and archaea (Ar-KEY-uh) consist of prokaryotes only.
These single cells are small, and usually round or rod shaped. They might have one or more flagella (Fla-JEL-uh) — powered tails — hanging off the outside to move around. Prokaryotes often (but not always) have a cell wall for protection.
Inside, these cells throw together all they need to survive. But prokaryotes aren’t very organized. They let all their cell parts hang out together. Their DNA — the instruction manuals that tell these cells how to build everything they need — just floats around in the cells.
But don’t let the mess fool you. Prokaryotes are masterful survivors. Bacteria and archaea have learned to make meals of everything from sugars and sulfur, to gasoline and iron. They can get energy from sunlight or the chemicals spewed from deep-sea vents. Archaea in particular love extreme environments. They can be found in high-salt springs, rock crystals in caves or the acidic stomachs of other organisms. That means that prokaryotes are found on and in most places on Earth — including within our own bodies.
Eukaryotes keep it organized
Eukaryotes are the third domain of life. Animals, plants and fungi all fall under this umbrella, along with many other single-celled organisms, such as yeast. Prokaryotes might be able to eat almost anything, but these eukaryotes have other advantages.
These cells keep themselves tidy and organized. Eukaryotes tightly fold and pack their DNA into a nucleus — a pouch inside each cell. The cells have other pouches, too, called organelles. These neatly manage other cell functions. For example, one organelle is in charge of protein-making. Another disposes of trash.
Eukaryotic cells probably evolved from bacteria, and started out as hunters. They scooted around engulfing other, smaller cells. But some of those smaller cells didn’t get digested after they were eaten. Instead, they stuck around inside their larger host. These smaller cells now perform essential functions in eukaryotic cells.
Mitochondria (My-toh-KON-dree-uh) may have been one of these early victims. They now generate energy for eukaryotic cells. Chloroplasts (KLOR-oh-plasts) may have been another small prokaryote “eaten” by a eukaryote. These now hang out converting sunlight into energy inside plants and algae.
While some eukaryote are loners — like yeast cells or protists — others enjoy teamwork. They may band together into large conglomerations. These communities of cells often have the same DNA in each of their cells. Some of these cells, however, may use that DNA in different ways to perform special functions. One type of cell might control communication. Another might work on reproduction or digestion. The cell group then works as a team to pass on the organism’s DNA. These communities of cells evolved to become what are now known as plants, fungi and animals — including us.
acidic An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
archaea One of the three domains of life on Earth. This group consists of single-celled prokaryotes — organisms without a cell nucleus. Archaea are best known for living in extremely harsh environments, such as very salty water or highly acidic or hot places.
bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chloroplast A tiny structure in the cells of green algae and green plants that contain chlorophyll and creates glucose through photosynthesis.
crystal (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.
DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
domain (in biology) The highest taxonomic rank of organisms, above that of kingdom.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
eukaryote Any organism whose cells have a nucleus. Eukaryotes include all multicellular creatures (such as plants, animals and fungi) as well as certain types of single-celled microorganisms.
flagella (sing. flagellum) A thread-like structure that comes out of certain types of cells. The term derives from the Latin word for whip. And that's because the structures serve like oars to help these cells travel.
iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.
liver An organ of the body of animals with backbones that performs a number of important functions. It can store fat and sugar as energy, break down harmful substances for excretion by the body, and secrete bile, a greenish fluid released into the gut, where it helps digest fats and neutralize acids.
mitochondria (sing. mitochondrion) Structures in all cells (except bacteria and archaea) that break down nutrients, converting them into a form of energy known as ATP.
nucleus Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information.
organelle Specialized structures, such as mitochondria, found within a cell.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
prokaryote Any single-celled organism that does not have a nucleus or membrane-bound organelles.
protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.
protist A broad group of mostly single-celled organisms that are neither plants nor animals. Some, like algae, may appear plant-like. Those known as protozoans may appear animal-like. And still others appear fungi-like.
salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.
scaffold A framework erected to support something, such as people putting siding onto the exterior of a building.
sulfur A chemical element with an atomic number of sixteen. Sulfur, one of the most common elements in the universe, is an essential element for life. Because sulfur and its compounds can store a lot of energy, it is present in fertilizers and many industrial chemicals.
vent (n.) An opening through which gases or liquids can escape.
yeast One-celled fungi that can ferment carbohydrates (like sugars), producing carbon dioxide and alcohol. They also play a pivotal role in making many baked products rise.