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Prokaryotes, protistans, plants Updated: |
Topics for March 2
Bacteria
Simple structure, complex chemistry
Population growth: by fission
Types of bacteria
Viruses and prions
Protista
Slime molds, protozoans, algae
Bacteria: simple or complex?
Bacteria are prokaryotes: they are characterized by a simple cell architecture, lacking distinctive cell organelles.
Bacteria are very diverse: there are many different species, each with its own distinctive ecology and physiology.
Together, the bacteria possess very diverse ecological and physiological capacities.
Major types of bacteria
Archaebacteria: methanogens, halophiles, thermophiles. (unique metabolism and genetics: unlike other bacteria).
Photoautotrophic eubacteria: cyanobacteria and purple and green sulfur bacteria
Chemoautotrophic bacteria: oxidize ammonia, sulfides, reduced iron, etc.
Chemoheterotrophic bacteria: decompose organic matter. A few are pathogens.
The archaebacteria ("beginning")
Methanogens: "methane makers".
Live in swamps, animal guts, etc, where there is no oxygen
Halophiles: "salt lovers"
Live near deep-sea hydrothermal vents.
Thermophiles: "heat lovers"
Live in highly acidic soils, hot springs, coal mine wastes.
Eubacteria ("regular bacteria")
Photoautotrophic bacteria: photosynthetic
Cyanobacteria (some can fix N2 gas as a source of nitrogen)
Chemoautotrophic bacteria: oxidize reduced chemicals such as ammonia, sulfide, or ferrous iron.
Chemoheterotrophic bacteria: oxidize organic matter.
Bacteria: good or bad?
The good guys:
Most bacteria are ecological recyclers, and decompose organic matter, recycle reduced chemicals, etc.
Some do useful tasks for us: Lactobacillus that assist with making pickles, buttermilk, yogurt, etc.
Bacteria: good or bad?
The bad guys:
Some bacteria cause disease. Examples:
Clostridium tetanus: a spore forming bacterium that causes tetanus.
Borrelia burgdorferi. Carried from mammal to mammal by ticks, it causes Lyme disease.
Some bacteria produce toxins. Example:
Clostridium botulinum produces toxins which cause botulism.
Bacteria growth: by fission
Bacteria divide by fission. As part of the process, their "chromosome" is replicated and the two copies are separated into the daughter cells.
Bacteria do have sex (sort of):
Bacteria can conjugate, and pass plasmid DNA from one cell to another.
Significance of plasmid transfer
Some plasmids carry genetic information which confers resistance to particular antibiotics.
Plasmids may be transferred from non-pathogenic bacteria to pathogenic bacteria.
The result: antibiotic resistance can be transferred to pathogenic bacteria.
Viruses: What are they?
Viruses are noncellular infectious agents that are characterized by:
1. a viral particle consists of a protein coat wrapped around a nucleic acid core (genetic material).
2. a virus cannot reproduce itself. (It can replicate only if it gains access to a host cell.)
Viruses: how they replicate and spread.
The protein coat of a virus has a recognition system for finding the appropriate host cell.
Upon contact, the virus nucleic acid enters the host cell.
After entry, the virus nucleic acid "takes over": the cell machinery is diverted to making virus particles, including copies of the nucleic acid and protein for the coat.
DNA and RNA viruses.
DNA Viruses have double stranded DNA. Examples: Herpes viruses: Pox viruses (small pox, cowpox): Papovaviruses (warts)
RNA Viruses have single stranded RNA. Examples: Retroviruses (HIV, some cancers); Enteroviruses (polio, hepatitis A); Paramyxoviruses (measles, mumps)
Viral multiplication cycle
1. Attachment to suitable host cell.
2. Penetration of viral nucleic acid into cell.
3. Replication of viral nucleic acid and viral protein synthesis inside cell.
4. Assembly of new viral particles.
5. Release of more viral particles from infected cell.
Antibiotics and disease.
Antibiotics are effective against specific bacteria. (However, with the evolution of resistance and its spread by plasmids, antibiotics are becoming less effective.)
Antibiotics are not effective against viruses: only the immune system will cope. (Vaccinations are effective because they provoke the immune system to react in advance.)
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)
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 and a diploid phase.
The haploid phase is dominant in the simpler non-vascular plants.
The diploid phase 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.
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.)
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