HiSET Science: Viruses, Bacteria, Protists, Fungi, Plants, and Animals

Viruses
Viruses are nonliving, infectious particles that act as parasites in living organisms. Viruses are acellular, which means that they lack cell structure. Viruses cannot reproduce outside of living cells. The structure of a virus is a nucleic acid genome, which may be either DNA or RNA, surrounded by a protective protein coat or capsid. In some viruses, the capsid may be surrounded by a lipid membrane or envelope. Viruses can contain up to 500 genes and have various shapes. They usually are too small to be seen without the aid of an electron microscope. Viruses can infect plants, animals, fungi, protists, and bacteria. Viruses can attack only specific types of cells that have specific receptors on their surfaces. Viruses do not divide or reproduce like living cells. Instead, they use the host cell they infect by 'reprogramming' it, using the nucleic acid genome, to make more copies of the virus. The host cell usually bursts to release these copies.


Bacteria
Bacteria are small, prokaryotic, single-celled organisms. Bacteria have a circular loop of DNA (plasmid) that is not contained within a nuclear membrane. Bacterial ribosomes are not bound to the endoplasmic reticulum, as in eukaryotes. A cell wall containing peptidoglycan surrounds the bacterial plasma membrane. Some bacteria such as pathogens are further encased in a gel-like, sticky layer called the capsule, which enhances their ability to cause disease. Bacteria can be autotrophs or heterotrophs. Some bacterial heterotrophs are saprophytes that function as decomposers in ecosystems. Many types of bacteria share commensal or mutualistic relationships with other organisms. Most bacteria reproduce asexually by binary fission. Two identical daughter cells are produced from one parent cell. Some bacteria can transfer genetic material to other bacteria through a process called conjugation, while some bacteria can incorporate DNA from the environment in a process called transformation.

Protists
Protists are small, eukaryotic, single-celled organisms. Although protists are small, they are much larger than prokaryotic bacteria. Protists have three general forms, which include plantlike protists, animal-like protists, and fungus-like protists. Plantlike protists are algae that contain chlorophyll and perform photosynthesis. Animal-like protists are protozoa with no cell walls that typically lack chlorophyll and are grouped by their method of locomotion, which may use flagella, cilia, or a different structure. Fungus-like protists, which do not have chitin in their cell walls, are generally grouped as either slime molds or water molds.
Protists may be autotrophic or heterotrophic. Autotrophic protists include many species of algae, while heterotrophic protists include parasitic, commensal, and mutualistic protozoa. Slime molds are heterotrophic fungus-like protists, which consume microorganisms. Some protists reproduce sexually, but most reproduce asexually by binary fission. Some reproduce asexually by spores while others reproduce by alternation of generations and require two hosts in their life cycle.

Fungi
Fungi are nonmotile organisms with eukaryotic cells and contain chitin in their cell walls. Most fungi are multicellular, but a few including yeast are unicellular. Fungi have multicellular filaments called hyphae that are grouped together into the mycelium. Fungi do not perform photosynthesis and are considered heterotrophs.
Fungi can be parasitic, mutualistic or free living. Free-living fungi include mushrooms and toadstools. Parasitic fungi include fungi responsible for ringworm and athlete's foot. Mycorrhizae are mutualistic fungi that live in or near plant roots increasing the roots' surface area of absorption. Almost all fungi reproduce asexually by spores, but most fungi also have a sexual phase in the production of spores. Some fungi reproduce by budding or fragmentation.

Plants
Plants are multicellular organisms with eukaryotic cells containing cellulose in their cell walls. Plant cells have chlorophyll and perform photosynthesis. Plants can be vascular or nonvascular. Vascular plants have true leaves, stems, and roots that contain xylem and phloem. Nonvascular plants lack true leaves, stems and roots and do not have any true vascular tissue but instead rely on diffusion and osmosis to transport most of materials or resources needed to survive.
Almost all plants are autotrophic, relying on photosynthesis for food. A small number do not have chlorophyll and are parasitic, but these are extremely rare.
Plants can reproduce sexually or asexually. Many plants reproduce by seeds produced in the fruits of the plants, while some plants reproduce by seeds on cones. One type of plant, ferns, reproduce by a different system that utilizes spores.
Some plants can even reproduce asexually by vegetative reproduction.

Structure, Organization, Modes of Nutrition, and Reproduction of Animals
Animals are multicellular organism with eukaryotic cells that do not have cell walls surrounding their plasma membranes. Animals have several possible structural body forms. Animals can be relatively simple in structure such as sponges, which do not have a nervous system. Other animals are more complex with cells organized into tissues, and tissues organized into organs, and organs even further organized into systems. Invertebrates such as arthropods, nematodes, and annelids have complex body systems. Vertebrates including fish, amphibians, reptiles, birds, and mammals are the most complex with detailed systems such as those with gills, air sacs, or lungs designed to exchange respiratory gases. All animals are heterotrophs and obtain their nutrition by consuming autotrophs or other heterotrophs. Most animals are motile, but some animals move their environment to bring food to them. All animals reproduce sexually at some point in their life cycle. Typically, this involves the union of a sperm and egg to produce a zygote.