Eukaryotes originated by symbiosis among prokaryotes
- Protists evolved to develop what prokaryotes donít have: a membrane enclosed nucleus, mitochondria, chloroplasts, endomembrane system, cytoskeleton, 9+2 flagella, and multiple chromosomes with linear DNA, diploid stage in life cycles, mitosis, meiosis and sex.
- Multicellular prokaryotes began being favored by natural selection:
- filaments began to develop in cyanobacteria
- complex bacterial communities, in which each species benefited from the metabolic specialties of another, began to evolve.
- Compartmentalization of different functions within single cells occurred.
- Compartments and organelles in eukaryotes may have evolved from infoldings in prokaryotes and endosymbiosis is responsible for the rest of the organelles developing (especially mitochondria and chloroplasts.)
- Endosymbiotic theory says that the origin of eukaryotes is from certain species of prokaryotes (notably photosynthetic and aerobic organisms) living within larger prokaryotes (this especially applies to chloroplasts and mitochondria once again).
- Eukaryotes also could have originated from archezoa---single celled prokaryotes (diplomonads) which may have developed a single haploid nucleus in a membrane envelope (without mitochondria and plastids) with simple cytoskeletons.
These protists ingest food, or have an animal-like mode of nutrition, and are heterotrophic. They ingest bacteria, other protists and detritus (dead organic matter). There are also symbiotic protozoa which are parasites that cause disease. Different types of protozoa include:
- Rhizopods: unicellular amoebas with pseudopodia for mobility and feeding that reproduce by mechanisms of cell division asexually. Most are free-living, but some are parastitic (eg entamoeba histolytica which causes dysentery in humans.)
- Actinopoda: Usually spherically symmetrical, feed with axopedia and have radiating pseudopodia supported by internal microtubules. They are components of plankton and there are two different varieties: heliozoans (which live in fresh water) and radiozoans (which are marine.)
- Foraminifera (forams): marine protists that have multi-chambered shells hardened by calcium carbonate. Strands of cytoplasm extend through the pores, allowing them to swim and feed.
- Apicomplexa: parasitic protists with complex life cycles in animal hosts, disseminating as tiny infectious cells called sporocytes and often requiring asexual and sexual stages during their life cycle.
- Zoomastigophora (Zooflagellates): this heterotrophic protist uses flagella for propulsion and absorbs molecules or consumes them by phagocytosis. They are mostly solitary, but some live in colonies, and can be free-living, symbiotic or parasitic.
- Ciliophora (ciliates): these protists use cilia for mobility and feeding. They mostly are solitary and live in fresh water. They have several unique characteristics, such as cytoplasmic organization and not genes directly providing hereditary information, and the presence of two types of nuclei, a large macronucleus and several tiny micronuclei. They generally reproduce by binary fission.
These protists (slime molds and water molds) are similar to fungi by concurrence---they have filamentous body structure, which is a morphological adaptation that increases exposure to the environment and allows for effective decomposition (its ecological role.) However, these protists are more similar to amoeboid protists than fungi in their cellular organization, reproduction and life cycles. The life cycle of funguslike protists is complex, giving it an adaptation to allow for survival in changing environments and transport to new food sources. Some examples of fungus-like protists are:
- Dinoflagelates: Components of pastures of phytoplankton, this is a type of miscroscopic algae floating near the surface of the sea. Dinoflagelate blooms are brownish-red because of xanthophylls and often release toxins. Some are symbionts or parasites.
- Bacillariophyta (diatoms): This type of algae is related to gold and brown algae; reproduces asexually and is contained in plankton.
- Chrysophyta (golden algae): This algae contains yellow and brown cartenoids and xanthophylls, is biflagellated, colonial, and survives by forming resistant cysts from which active cells emerge when conditions are favorable.
- Phaeophyta (brown algae): This algae is multicellular, colored by accessory pigments, possesses chloroplast structure homologous to golden algae and diatoms, and has anatomical and biological adaptations to rough tides. Also reproduces by alteration of generations from multicellular haploid to diploid form.
- Rhodophyta (red algae): This algae is red because of accessory pigment phycoerythrin, and inhabits warm coastal water while some can absorb blue and green wavelengths in deep water.
- Chlorophyta (green algae): This algae contains green chlorophyll, and can lie as plankton or occupy protozoa and invertebrates as photosynthetic symbionts that contribute to the food supply of the hosts.