Characteristics of 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 organelle 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 crosslinked 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.
Eukaryotic Cell Structures
Characteristics of Fungi
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.
Organisms of the Kingdom Fungi
Types of Parasitic Worms
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 such as 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.
Life Cycle of Schistosoma Parasite
The life cycle for all helminths follows a similar pattern. Adult helminths can be hermaphrodites (containing both male and female sexual organs), such as flatworms, 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.