What Are Eukaryotes?

The domain Eukarya represents a small but extremely diverse collection of organisms that share some major characteristics. All higher order plants and animals are multicellular eukaryotes, but there are many eukaryotic microorganisms (microbes) that can be single-celled or multicellular.

Eukaryotic cells all have a nucleus and other membrane-bound organelles. The nucleus is encased in a double layer lipid membrane. In order for all the chromosomes to fit within the nucleus, they exist as chromatin, a complex of deoxyribonucleic acid (DNA) and proteins that is highly condensed and tightly wrapped inside the nucleus of eukaryotes. The nuclear membrane contains pores that allow important molecules to be moved into and out of the nucleus. An organelle is a structure in a cell that has a specific task, such as a lysosome, mitochondrion, or Golgi apparatus.

The endoplasmic reticulum (ER) is a network of membranes that helps process molecules in a cell and transports cell materials. Sections of the ER that are embedded with ribosomes are called "rough ER" and are the sites of protein synthesis. Areas of the ER that do not contain ribosomes are called "smooth ER." Instead of synthesizing proteins, the smooth ER is associated with the production of fats and steroids. The Golgi apparatus is an organelle that attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules to their places inside or outside a cell. Transport occurs within a vesicle, a small, fluid-filled sac surrounded by a double layer membrane. Specialized vesicles include the lysosome, an organelle in the cytoplasm of eukaryotic cells that contains degradative enzymes enclosed in a membrane.

A mitochondrion (plural, mitochondria) is an organelle that changes energy from food into energy a cell can use. Mitochondria are enclosed in a double membrane and have many internal folds called cristae. They are found in large numbers throughout the cell. Plant cells contain both mitochondria and chloroplasts. A chloroplast is a membrane-bound organelle found in plants and some other organisms that captures energy from light and converts it into chemical energy. Mitochondria and chloroplasts contain their own DNA and are believed to have once been microbes themselves before they were incorporated by other cells in a process called endosymbiosis. Like bacteria, plant cells also have cell walls. Instead of peptidoglycan, plant cell walls are composed of the cross-linked polysaccharides cellulose, hemicellulose, and pectin.

Many types of eukaryotic cells have locomotor appendages. A locomotor appendage is an extension of the cytoplasm composed of microtubules and is used for motility, sensing the environment, and feeding. Locomotor appendages include flagella and cilia. Flagella of eukaryotic cells are tail-like appendages used for motility, but they differ from flagella of prokaryotic cells by the types and arrangements of the proteins that make up the structure. A cilium (plural, cilia) is a small, hairlike projection from cells that can be used for motility or sensing the environment. Eukaryotic cells also have a cytoskeleton made of microtubules, microfilaments, and intermediate filaments that give the cell structure.

Protists are highly diverse eukaryotic organisms that do not share any defining characteristics and are grouped together because they cannot be sorted into any other groups. These organisms can be unicellular or multicellular, and they all have a diverse array of life cycles, methods for locomotion, and unique cellular structures, and they exist at different levels of the food chain. An example of a protist is diatom, a type of single-celled photosynthetic plankton that lives in water and provides a source of food for many aquatic organisms. Another type of protist is red algae that are also photosynthetic, are found in fresh and marine water, and have microbial and multicellular forms. A euglena is another single-celled protist that lives in water, both fresh and marine. Most euglena are able to photosynthesize to produce their own food and are also able consume other organisms, so they are simultaneously autotrophs and heterotrophs. Amoeba are single-celled heterotrophs with amorphous shapes that occupy diverse habitats.

Protists are separated into three groups: plant-like protophyta, animal-like protozoa, and fungi-like slime molds. The protophyta contain mostly algae, a diverse group of photosynthetic organisms living in predominantly marine environments that contain chloroplasts and have cell walls like higher plants. There are unicellular and multicellular algae and all are photoautotrophs that use photosynthesis to make their own food from sunlight and carbon dioxide.

The protozoan (plural, protozoa) is a unicellular organism containing nuclei and membrane-bound organelles classified into the kingdom Protozoa. These organisms lack cell walls and need moist habitats to thrive. Protozoans can have diverse arrangements of cilia on their surface for movement and sensing their environment, or they can have one or more flagella. Protozoans such as amoebas can have a pseudopodium (plural, pseudopodia), a cytoplasmic extension that can be used for motility or for catching and ingesting prey. Some freshwater protozoa use contractile vacuoles in the cytoplasm to get rid of excess water in their cells. A vacuole is a membrane-bound organelle present in plant, fungal, protist, animal, and bacterial cells that functions to store solutions or materials. Some protozoa are intracellular parasites, such as Plasmodium falciparum, the organism that causes the human disease malaria. Slime molds are superficially fungi-like eukaryotic organisms that exist as free-living single-celled organisms that aggregate to form multicellular structures for reproduction. Slime molds are found on land, typically in association with plant detritus. They feed on bacteria, fungi, and decomposing plant matter. When food is abundant they survive as single-celled amorphous cells that resemble amoeba or they may fuse with other genetically similar individuals into a colonial mass called a plasmodium that can grow large enough to be visible. When food resources are less abundant, or environmental conditions are otherwise suboptimal, single-celled individuals will form plasmodia with reproductive structures that release spores.

Fungus is a term used to describe a broad kingdom of organisms from baker's yeast (Saccharomyces cerevisiae) to mushrooms such as Agaricus bisporus, or the common button mushroom. Fungi are used in the fermentation of foods like beer and wine, as leavening agents (substances that cause expansion through the release of gas) in bread, and directly as food (mushrooms). They are also medically important for producing antibiotics, most famously penicillin. Some fungi make mycotoxins, which are chemicals that are poisonous to animals and plants, and others produce hallucinogenic compounds like psilocybin.

Some fungi are responsible for human mycoses. Mycoses are fungal infections that cause a wide range of disease in humans, from superficial skin infections such as athlete's foot and ringworm, to infections involving the brain and other organs. Fungi can be unicellular or multicellular and are grouped together based on having a cell wall made of a polysaccharide called chitin. They are also heterotrophic, obtaining energy and carbon by consuming other organisms. Many fungi feed by secreting digestive enzymes into their environment and absorbing dissolved nutrients. For this reason they are considered principal decomposers of organic matter. Despite having cell walls like plants, fungi do not contain chlorophyll or carry out photosynthesis and are actually more closely related to animals.

Fungi can reproduce asexually or two different mating types can reproduce sexually. Asexual reproduction, such as fragmentation and budding, does not involve spores. In sexual reproduction, fungi make spores. A spore is a haploid reproductive structure of plants, algae, and fungi that divides by mitosis into a multicellular gametophyte. Spores can travel long distances or remain dormant for a long time. Some fungi are dimorphic, they have two different growth forms. Fungi that are dimorphic have unicellular yeast and multicellular life cycle stages depending on conditions such as temperature and moisture.

A helminth is a large, multicellular wormlike organism that is visible to the naked eye when fully mature. Helminth life cycles differ by type of organism, though tapeworms, flukes, and roundworms all have adult stages that may be visible, and larval or egg stages that are microbial. Some helminths are parasitic, meaning they require a host to live and feed on to survive. While feeding on a host, they also often block host nutrient absorption, causing disease. Some secrete chemicals that interfere with the immune response, allowing them to avoid immune detection and survive in the host for several years.

There are three types of helminths: the flatworms, the roundworms, and the Annelida, which are not parasitic and include large organisms like earthworms and leeches. Flatworms are also called platyhelminthes. A flatworm is flat and bilaterally symmetrical, meaning its left and right sides are mirror images. They are subdivided into several categories, many of which are important human parasites. One category of flatworms is found in the class Cestoda and are commonly called tapeworms. A cestode is a long, ribbon-like parasite that lacks a mouth or gut. It attaches to the lining of the digestive tract and absorbs nutrients directly through its skin. Tapeworm infection can be asymptomatic, or it can cause abdominal pain, diarrhea, nausea, and weight loss. Humans are at risk of being infected with a tapeworm by eating raw or undercooked meat or fish. A fluke is a trematode, an oval-shaped flatworm with a tough outer skin designed to withstand the harsh environment of the stomach.

Schistosomiasis, also called "snail fever," is a disease that is caused by being in contact with water contaminated with flatworms called schistosomes. The schistosomes infect snails that live in the water, and the snails release the worms into the water, which then enter human hosts through the skin. Children are often infected while playing in contaminated water. People who contract schistosomiasis experience abdominal pain and bloody diarrhea and can suffer from long-term issues such as liver and kidney failure and bladder cancer. A roundworm (also called a nemathelminth or nematode) is a long, thin worm with a bilaterally symmetrical body and a distinct head. The head has one or more rows of sharp teeth and a straw-like organ for sucking liquids. Unlike flatworms, roundworms have a tubular digestive tract and an anus for excretion of waste. Many roundworms cause serious human infections, such as trichinosis, a disease that leads to abdominal and muscle pain and inflammation. Humans get trichinosis by eating undercooked pork that contains eggs from the organism Trichinella.

The life cycle for all helminths follows a similar pattern. Adult helminths, such as flatworms, can be hermaphrodites (containing both male and female sexual organs), or have separate male and female sexes, such as roundworms. Hermaphrodite helminths reproduce sexually in processes where both partners can act as male or female. Helminth offspring are released as eggs. These eggs have a strong protective shell that enables them to survive in harsh environments and remain viable for several months. Some helminths can produce thousands of eggs at a time up to several times a day. Other helminths produce a smaller number of eggs, but each egg contains thousands of larvae. Most helminth eggs are microscopic, though some, such as tapeworms, release sacs of eggs that are visible to the unaided eye. The eggs are shed in the stool of the animal host and then enter the soil or water system and eventually get ingested by a new host. Once inside the new host the eggs hatch, and the larvae mature over the course of several weeks to months—and the cycle begins again.