discussion responses

Week 7-Topic 2
Emily Pearce posted Mar 18, 2020 6:01 PM


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Differentiate between food chains and food webs and recognize the importance of each: A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. In a food chain, each organism occupies a different trophic level, defined by how many energy transfers separate it from the basic input of the chain. Food webs consist of many interconnected food chains and are more realistic representation of consumption relationships in ecosystems. Energy transfer between trophic levels is inefficient with a typical efficiency around 10%. This inefficiency limits the length of food chains.

Explain how photosynthetic and chemosynthetic organisms acquire food in a food web: Ecosystems depend upon the ability of some organisms to convert inorganic compounds into food that other organisms can then exploit. The majority of life on the planet is based on a food chain which revolves around sunlight, as plants make food via photosynthesis. However, in environments where there is no sunlight and thus no plants, organisms instead rely on primary production through a process called chemosynthesis, which runs on chemical energy. Together, photosynthesis and chemosynthesis fuel all life on Earth. Photosynthesis occurs in plants and some bacteria, wherever there is sufficient sunlight on land, in shallow water, even inside and below clear ice. All photosynthetic organisms use solar energy to turn carbon dioxide and water into sugar and oxygen. Chemosynthesis occurs in bacteria and other organisms and involves the use of energy released by inorganic chemical reactions to produce food. All chemosynthetic organisms use energy released by chemical reactions to make a sugar, but different species use different pathways. For example, at hydrothermal vents, vent bacteria oxidize hydrogen sulfide, add carbon dioxide and oxygen, and produce sugar, sulfur, and water.

Discuss the biogeochemical cycles of water, carbon, nitrogen, phosphorus, and sulfur: Carbon is found in all organic macromolecules and is also a key component of fossil fuels. Nitrogen is needed for our DNA, RNA, and proteins and is critical to human agriculture. Phosphorus is a key component of DNA and RNA and is one of the main ingredients, along with nitrogen, in artificial fertilizers used in agriculture. Sulfur is key to protein structure and is released to the atmosphere by the burning of fossil fuels. These cycles don’t happen in isolation, and the water cycle is a particularly important driver of other biogeochemical cycles. For example, the movement of water is critical for the leaching of nitrogen and phosphate into rivers, lakes, and oceans. The ocean is also a major reservoir for carbon. Though each element or compound takes its own route, all of these key chemical nutrients cycle through the biosphere, moving between the biotic-living and abiotic-nonliving worlds and from one living organism to another.


“Food Chains & Food Webs.” Ecology, Khan Academy, www.khanacademy.org/science/biology/ecology/intro-to-ecosystems/a/food-chains-food-webs.

“What Is the Difference between Photosynthesis and Chemosynthesis?” Ocean Exploration Facts: NOAA Office of Ocean Exploration and Research, US Department of Commerce/National Oceanic and Atmospheric Administration, 21 Sept. 2012, oceanexplorer.noaa.gov/facts/photochemo.html.

Week 7: Topic 1
Emily Pearce posted Mar 18, 2020 5:49 PM


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Exponential growth occurs very rapidly in populations with unlimited natural resources. The best example of exponential growth is seen in bacteria. Bacteria are prokaryotes that reproduce by prokaryotic fission. This division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an unlimited supply of nutrients, after an hour there will be one round of division, resulting in 2000 organisms. In another hour, each of the 2000 organisms will double, producing 4000; after the third hour, there should be 8000 bacteria in the flask; and so on. The important concept of exponential growth is that the population growth rate, the number of organisms added in each reproductive generation is accelerating, increasing at a greater and greater rate. After 1 day and 24 of these cycles, the population would have increased from 1000 to more than 16 billion.
Logistic growth of a population size occurs when resources are limited, thereby setting a maximum number an environment can support. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time. Examples in wild populations include sheep and harbor seals. In both, the population size exceeds the carrying capacity for short periods of time and then falls below the carrying capacity afterwards. This fluctuation in population size continues to occur as the population oscillates around its carrying capacity.


“Environmental Limits to Population Growth.” Boundless Biology, Lumen, courses.lumenlearning.com/boundless-biology/chapter/environmental-limits-to-population-growth/.

Exponential Growth & Logistic Growth. Khan Academy, www.khanacademy.org/science/biology/ecology/population-growth-and-regulation/a/exponential-logistic-growth.

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