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Protistans, plants Updated: |
Topics for March 4
Prions: infectious proteins
Protista
Slime molds, protozoans, algae
Evolutionary trends in plants
Mosses and Ferns
Prions: infectious protein particles.
Prions are small proteins linked to a few rare infectious diseases.
Prions have no nucleic acid: they are proteins.
Creutzfeldt-Jakob disease is linked to prions. This disease is characterized by degeneration of the central nervous system. It has been linked to "mad-cow" disease.
Protista: unicellular eukaryotes
The simplest eukaryotes: they have a unicellular grade of construction.
Eukaryote characteristics: cell organelles, including:
nucleus, large ribosomes, endoplasmic reticulum, Golgi bodies, mitochondria
The protista includes a very diverse array of organisms.
Examples of protista
Slime molds: Unicellular amoeba-like organisms which sometimes aggregate into a reproductive colony. (see figure 18.15)
Amoeboid protozoans. Examples: Amoeba proteus, Giardia sp., Entamoeba histolytica
Ciliated protozoans. Example: Paramecium
Sporozoans. Parasitic protozoa that require a final host for a complete life cycle. Example: Plasmodium (Malaria)
Slime molds: solitary or colonial? (video 51561-->)
Slime molds exist most of the time as solitary amoeboid cells.
Amoeboid cells may aggregate to form a colony. The colony produces a fruiting body with spores.
Slime molds are of interest because they bridge between solitary cells and multicellular organisms.
Malaria: Caused by Plasmodium
(1) A person can be infected if bitten by a mosquito which has been infected by Plasmodium.
(2) The life stage (sporoszoite) that infects humans first lodges in the liver. The sporozoites reproduce in the liver and release merozoites that invade red blood cells.
Malaria (cont.)
(3) Some merozoites develop into male or female gametophytes, which are released to the blood stream. If a mosquito bites an infected person, the gametophytes infect the mosquito.
(4) In the mosquito, the gametophytes reproduce sexually, to form zygotes. The zygotes develop into sporozoites, completing the Plasmodium life cycle.
Malaria (cont.)
Parasite life cycles often involve several different life stages, each adapted to a particular host species.
The infection modifies mosquito behavior: infected mosquitoes bite repeatedly, thereby spreading the parasite.
Persons with and HbS allele are much more resistant, because infected red blood cells "self-destruct".
Photosynthetic protista
Diatoms: abundant species in freshwater and the oceans, that build glass cell walls.
Dinoflagellates: some species are responsible for red tides when they are very abundant. (Their toxins can be dangerous.)
Brown algae: examples are the large kelp that develop in shallow seas. These species show some tissue specialization.
Green algae: possible ancestors of the higher plants.
Green algae share a number of biochemical traits with higher plants, including the chemical form of chlorophyll.
Like many other protista, green algae are capable of sexual reproduction. Example: Chlamydomonas. Note that the dominant life phase is the haploid phase. Most of the time, haploid cells reproduce asexually.
Life cycle of plants.
Plants alternate between a haploid phase (the gametophyte) and a diploid phase (the sprorophyte).
The gametophyte is dominant in the simpler non-vascular plants.
The sporophyte is dominant in higher plants. (see figure 19.2).
Evolutionary trends in plants.
1. The haploid phase is dominant in simpler plants; the diploid phase is dominant in higher plants.
2. Simpler plants are very dependent on liquid water; higher plants are less dependent on liquid water.
3. Higher plants produce seeds (a life stage adapted to dispersal).
The bryophytes: Simple land plants.
Examples of bryophytes: mosses, liverworts, hornworts.
Mosses are partially independent of liquid water. They can grow in damp terrestrial environments.
A critical life stage is dependent on liquid water: sperm must swim to the egg producing structures.
Life cycle of a moss. (figure 19.4).
The most conspicuous stage of moss is the gametophyte: A haploid stage.
Male gametophytes produce sperm, which must swim to the female gametophyte.
Female gametophytes produce eggs, which are retained in special structures on the female gametophyte.
The fertilized egg is a zygote, dependent on the female gametophyte.
Life cycle of a moss (cont.)
The zygote develops as a multicellular sporophyte, but remains dependent on the female gametophyte.
Meiosis takes place in a specialized structure on the sporophyte (the sporangium), producing spores (not gametes!).
The spores divide and produce male or female gametophytes.
The sporophyte of mosses: a dependent stage.
The sporophyte phase of mosses remains attached to the female gametophyte.
In contrast to higher plants, the sporophyte of mosses has a brief existence (as part of sexual reproduction).
The most conspicuous phase of mosses is the gametophyte phase.
Adaptation to life on land.
Mosses are able to grow on land, not in the water (algae grow only in the water).
Adaptations to land include:
1. A waxy cuticle that reduces evaporation.
2. A cellular jacket around the cells that produce sperm and egg (protecting them).
3. A large gametophyte (haploid) stage that supports the sporophyte stage.
What keeps mosses tied to damp habitats?
Mosses produce sperm that are dependent on liquid water. With out the presence of liquid water, the sperm are unable to reach the egg.
Because of this feature of the life cycle, mosses are tied to habitats in which there is some liquid water at least part of the time. (Rain drops or water film are sufficient.)
Seedless vascular plants.
The seedless vascular plants have an independent long-lived sporophyte stage. The gametophyte stage is smaller, but independent.
The sporophyte possess a complex of vascular tissue: vascular tissue permits larger size because water can be translocated from the soil to the plant.
Seedless vascular plants: a diverse array of plant species.
Ferns: the best known seedless vascular plants.
Lycophytes: Living species are small and inconspicuous, but their ancestors were the dominant plants of the Carboniferous Period.
Horsetails: Only a few surviving species, but like the Lycophytes, they were once dominant land plants.
Life cycle of a fern (see figure 19.8)
The life cycle of a fern includes a free-living gametophyte stage. It is small and inconspicuous, and lacks vascular tissue.
The zygote begins life attached to the gametophyte, but soon develops into a large and independent sporophyte. The sporophyte has vascular tissue and may attain a very large size.
The gametophyte of ferns.
The gametophyte is haploid.
Both sperm and eggs are produced on the same plant (by mitosis!)
The gametophyte begins life with the germination of a haploid spore. The spores are an effective dispersal phase of ferns.
The sporophyte of ferns.
The large and familiar phase of ferns is the sporophyte. It is the diploid phase.
The sporophyte has vascular tissue, and can conduct water from the soil to other parts of the plant.
The sporophyte produces haploid spores by meiosis. ( Spores are the dusty brown material on the underside of the "leaf".)
Mitosis and meiosis in the life cycle of a fern.
A haploid spore germinates and begins to divide by mitosis to form the small multicellular gametophyte stage.
The gametophyte stage produces gametes (by mitosis) which fuse to form a zygote.
The zygote divides by mitosis to form the large multicellular sporophyte stage.
Mitosis and meiosis in Ferns (continued).
The sporophyte produces haploid spores by meiosis. The spores are an effective dispersal phase in the life cycle of the fern.
Note that meiosis produces spores, not gametes. A spores germinates and grows into an independent gametophyte stage. (Meiosis does not produce gametes in these plants!)
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