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AP Biology 2014 Prokaryotes From an evolutionary viewpoint bacteria (unicellular organisms) are extremely successful. They evolved from a protobiont and have been around since the beginning of life on earth. They can thrive in any environment and are incredibly diverse, and exist in massive numbers. It is their structural, functional, and genetic adaptations that have allowed prokaryotes to be so successful. A. Structural Adaptations in Prokaryotes 1. Come in 3 main shapes: Spheres (cocci), rod-shaped (bacillus), spiral shaped (spirochaetes). Different shapes (phenotypes) allows for selection. 2. Possess a tough cell wall. It provides protection, and keeps the cell’s shape. It prevents bursting which allows them to inhabit hypotonic environments. 3. A Capsule Some bacteria have a sticky polysaccharide capsule that covers the cell wall. This allows them to stick or anchor to a surface such as the intestinal lining of animals; consequently they don’t get washed away as food moves through the intestines. 4. Fimbrae or Pilli These are hair-like projections that allow bacteria to attach to a host or to other bacterial cells to form a colony. Like a capsule, these adaptations are significant because they prevent bacteria from being moved from their environment. They also allow bacteria in close proximity to exchange plasmids (transformation) 5. Flagella Some bacteria have flagella for propulsion. These bacteria show “taxis” which is purposeful movement towards a stimulus such as light, oxygen, or food. B. Internal Organization of Prokaryotes Prokaryotes do not exhibit cellular compartmentalization; that is they do not have membrane-bound organelles. However, many bacterial species still carry out complex functions like aerobic respiration. The enzymes of glycolysis, the Krebs cycle and electron transport chains are organized along the plasma membrane. Some aerobic bacteria have infoldings of their plasma membrane like the cristae of mitochondria. Some photosynthetic bacteria also have infoldings of their plasma membrane that resemble the grana of chloroplasts. Some bacteria have a basic organization to their metabolic machinery which is important because it allows for more efficient respiration and photosynthesis. C. Genomic Organization The prokaryotic genome is a single, circular chromosome (DNA molecule) that is not associated with histone proteins, and there is no nuclear envelope. Consequently, bacterial DNA is easy and fast to replicate, transcribe, and translate. This means fast reproduction and bacteria can make proteins and enzymes very quickly in order to adjust to changing environmental conditions, like a new food source. Ribosomes are smaller than eukaryotic ribosomes. Plasmids These are small circular pieces of DNA that operate independently of the main bacterial chromosome. They are a very significant adaptation because they allow for the rapid transfer of useful genes from one bacterium to another. Plasmids often carry the genes required for the production of pili (bacterial sex) and antibiotic resistance, both of which increase fitness. The small size and organizational structure of the prokaryotic genome has contributed greatly to their success. They reproduce very quickly (binary fission occurs once every 1-3 hours). This allows for a high mutation rate which creates new genes for selection and adaptation to new environments. They are awesome at adapting to change, good for them, bad for us. See MRSA. D. Modes of Nutrition As a group they show great diversity in their modes of nutrition and method of attaining energy. As a group bacteria can live anywhere because the have the ability to get energy and carbon in a number of different ways. 1. Photoautotrophs These use light as an energy source and CO2 as a source of carbon (like plants and certain algae). Cyanobacteria, also called blue-green algae are critical to the biosphere. 2. Chemoautotrophs Use inorganic chemicals like hydrogen sulfide and methane as an energy source and CO2 as a source of carbon. 3. Photoheterotrophs Use light as a source of energy and organic compounds for a carbon source. 4. Chemoheterotrophs Energy source and carbon source are organic compounds such as sugars, and proteins. E. Metabolism in Prokaryotes Prokaryotic species include: Obligate aerobes: these require oxygen to survive. Facultative anaerobes: can survive with or without oxygen. Obligate anaerobes: these are poisoned by oxygen. These are adaptations that allow the exploitation of a variety of environments thus increasing their fitness as a group. Nitrogen Fixing Bacteria An extremely important group of organisms. They have the ability to take gaseous nitrogen in the air and “fix” it (via metabolism) into inorganic nitrates (NO2, NO3). They are the only organisms on earth to do this! These nitrates can then be incorporated into other organisms and used to build organic molecules that contain nitrogen (amino acids, nucleotides etc). Nitrogen fixing bacteria live in symbiotic relationships with the root systems of plants. They get food from the plant and provide usable nitrogen to the plant and hence every heterotroph, including ourselves, on earth. Prokaryotic Phylogeny Molecular systematics (DNA, RNA homologies) have identified new major clades of prokarotes. Ex. Clamydias and spirochaetes share a common ancestor, interestingly they both cause STD’s. Role of Prokaryotes in the Biosphere Bacteria are crucial in that they decompose dead organisms (detritus) and in doing so recycle chemical elements between the biotic and abiotic compartments of the ecosystem. Nitrogen fixing bacteria add usable nitrogen to the biotic component. Many bacteria live in symbiotic relationships with other organisms (bacteria in your gut) Other bacterial species are parasites or human pathogens. Bacteria cause about half of all human disease They have been weaponized (anthrax) Usefulness In biotechnology (restriction enzymes, and cloning genes) They are agents of bioremediation: used to breakdown oil spills and pollutants. Used to make large scale amounts of vitamins and hormones (insulin, growth hormone).