BIOL 1407 Instructor: Mr. Sanregret Review Sheet Chapter 27 prokaryotes 1) Prokaryotes (cells without true nuclei or membrane-bound organelles) were until recently all classified within a single kingdom, Monera. Since then biologists have realized that prokaryotes consist of two phylogenetically distinct domains: Bacteria and Archaea. 2) These two domains, together with the domain Eukarya, encompass all the living things on Earth. Eukarya consists of all the eukaryotic species on Earth: all animals, all plants, all fungi, and a variety of other kingdoms, including algae, protozoa, slime molds, etc. Archaea are actually more related to Eukarya than to Bacteria. 3) Most prokaryotes have a cell wall made of peptidoglycans, long sugars bound together with peptides. Cell wall structure varies among different taxonomic groups. Gram positive bacteria have a thicker cell wall than Gram negative bacteria. Gram negative bacteria have a second membrane surrounding their cell wall, which makes them more resistant to antibiotics that target the cell wall. Different bacteria types respond differently to different antibiotics due in part to cell wall structure. Gram negative bacteria tend to cause more diseases. 4) Many prokaryotes secrete a gelatinous capsule that allows the cells to stick to surfaces and other cells. The capsule also provides additional protection. Pili are also structures that facillitate attachment to surfaces (e.g. host membranes). 5) Many prokayotes are motile. They move with whip-like appendages called flagella. Flagella spin like propellers on a boats 6) The bacterial genome consists of a single loop of DNA. This single chromosome contains all of the genetic information essential for the cell’s life. Bacterial cells also have smaller loops of DNA called plasmids. These segments of DNA often contain bits of genetic information that are not absolutely essential (e.g., genes that confer antibiotic resistance, or genes that allow the digestion of a particular nutrient molecules). 7) Protein synthesis occurs at ribosomes, as in eukaryotes. Prokaryotic ribosomes are different in structure than eukaryotic ribosomes. Some antibiotics (e.g. tetracycline) kill bacteria by disrupting ribosome function. These antibiotics do not harm eukaryotes, like humans, because eukaryotic ribosomes are different in structure. 8) Prokaryotes do not have sexual reproduction, but they do recombine and vary their DNA in a number of ways: 1) transformation (picking up loose DNA from the environment); 2) conjugation (exchange of DNA between cells); 3) transduction (addition of DNA to prokaryote by a virus); 4) mutation (spontaneous change in DNA nucleotides). 9) “Growth” in prokaryotes normally refers to cell multiplication by binary fission, not growth in the size of an individual cell. 10) Within certain species, a bacterial cell contains what is called an “endospore.” An endospore is a replicated chromosome and a bit of cytoplasm surrounded by a wall within the cell. If the cell is destroyed under adverse conditions (e.g., lack of food, lack of water, toxic materials), the endospore may still survive in a quiescent state (no growth or development). Under favorable conditions, the endospore may begin functioning as a normal cell and resume growth and development. Endospores can sometimes withstand very harsh conditions or very long periods of stasis. 11) Phototrophs can convert light energy into organic chemical energy (food). Chemotrophs cannot use light energy and depend on finding energy-rich organic molecules in their environment. 12) Autotrophs have the ability to acquire and use carbon from carbon dioxide (CO 2). Heterotrophs must acquire carbon from non-gaseous sources, usually organic molecules (e.g. sugars, fats and proteins). 13) Most organisms are either chemoheterotrophs (dependent on pre-existing organic material for their energy and carbon needs) or photoautotrophs (capable of making their own food using sunlight and CO2 in the air). 14) Chemoheterotrophs include all animals (eat food), all fungi (absorb nutrients from the surface they grow on), a few plants that have evolved to be parasitic, and most prokaryotes. 15) Photoautotrophs include most plants, eukaryotic algae, and some prokaryotes (e.g. cyanobacteria or “blue-green algae”). 16) Some prokaryotes are chemoautotrophs. They are very much like photoautotrophs, where light energy organic chemical energy (food), but instead inorganic chemical energy organic chemical energy (food) A major example of chemoautotrophs are prokaryotes that live within species of worms near deep sea hydrothermal vents. These prokaryotes generate food molecules using energy from hydrogen sulfide and carbon from carbon dioxide in the water. Since the food is made within the worms’ bodies, they have evolved to have no digestive tract and are commonly referred to as “gutless worms.” 17) Some prokaryotes are photoheterotrophs. They are sometimes called lithotrophs, because they obtain carbon from minerals in the rocks they grow on, while acquiring energy from sunlight. Carnivorous plants (e.g. Venus fly trap, pitcher plant) are not photoheterotrophs, because although they acquire some nutrients from animal prey that most other plants acquire from the soil, they still get their carbon from CO2 in the atmosphere. 18) Prokaryotes have much greater diversity in metabolic pathways and flexibility in utilizing different potential nutrients than eukaryotes. Because of mutation and the other mechanisms by which prokayote DNA recombines, combined with a short generation time, prokaryotes can quickly adapt to a new nutrient source, and may utilize nutrient resource inaccessible to eukaryotes (e.g. cellulose, or crude oil). 19) Nitrogen is a necessary constituent of proteins and nucleic acids. Although N 2 makes up 79% of the Earth’s atmosphere, most organisms can only utilize nitrogen in other forms (nitrate, ammonia, amino acids, etc.). Nitrogen fixation is the conversion of N2 to non-gaseous chemical forms useful for making biological molecules. The only nitrogen fixing species are certain prokaryotes, the cyanobacteria. Without cyanobacteria the nitrogen containing molecules necessary for life would not be available. 20) Oxygen is very reactive. Unlike N2, O2 easily reacts and forms compounds “oxides” with other chemicals. This means that 1) O2 is hazardous. It is poisonous to organisms not adapted to its presence; 2) O2 is not stable and if not constantly generated (e.g. by photosynthetic organisms) it will disappear from the environment; 3) O2 may be used to breakdown food molecules for additional energy (i.e. aerobic respiration). 21) There are three relationships to oxygen among prokaryotes: 1) obligate aerobes (need O2 for aerobic respiration to survive); 2) facultative aerobes (can use O2 for respiration, but can also live without it); 3) obligate anaerobes (no aerobic respiration, and poisoned by O2; use either fermentation or anaerobic respiration for energy). 22) Almost all of the major forms of nutrition and metabolic pathways appeared within the first billion years of life (starting ~3.5 billion years ago). The first organisms were chemoheterotrophs that fed on the “primordial soup.” This food source would have become depleted over time, creating selective pressure for organisms that could make their own food (photosynthesis). 23) Photosynthesis appeared early. Once the O2-producing type of photosynthesis evolved, O2 would have built up in the environment as a waste product. As the concentration of oxygen in the atmosphere increased, the selective pressure for tolerance and utilization of oxygen would have also increased, which likely lead to the evolution of aerobic respiration.