Lecture 03: Energy, Life and Transformations: Suggested Exam Questions
Q: According to the first law of thermodynamics energy is neither created nor destroyed. If this is so, why can't we just re-use and recycle energy?
A: Once energy dissipates into the environment, at the ambient temperature of the environment, it no longer has an ability to do work (2nd law of thermodynamics).
Potential exam question #1:
Explain why global vegetarianism would lead to more efficient land-use, using the 2nd law of thermodynamics.
The 2nd law of thermodynamics tells us that all transformations of energy from one form to another require a net increase in entropy. That means animals feeding on vegetation can not possibly convert all of the chemical energy stored in the organic molecules of the vegetation to energy stored in their meat. In fact, most animals can only convert 5 - 20% of the energy in their feed to energy stored in the chemistry of their own bodies. Thus, eating plants directly cuts down on the space required to produce a given unit of energy for human consumption.
Potential exam question #2:
A. Explain the phrase: "The chemicals of life are reduced".
B. We think that early Earth had a reducing environment, in contrast to the oxidizing environment that we have now. How could this have affected the ways in which living organisms stored their chemical energy?
A. The carbon atoms in organic molecules tend to be reduced, meaning they have gained electrons. This is a low entropy state - meaning it takes energy to make carbon atoms behave this way.
Energy is stored in concentrated form when the compound is in reduced state. Reduction reactions require energy, which the plants or autotrophs obtain from sunlight. When these high energy carbohydrates (in reduced form) are oxidized during respiration, energy is released. This energy is used to drive life processes.
B. The organisms that live in the oxidizing environment of modern Earth store energy in the form of reduced chemical compounds (e.g., sugars) because the oxidation of these reduced compounds by the environment releases energy (e.g., converting sugar to CO2). In a reducing environment, the same relationship is reversed; oxidized chemical compounds release energy when reduced by the environment, and thus we expect that oxidized species would have been stored by living organisms as energy sources.
Potential exam question #3:
One of your classmates has another of her newest and greatest ideas. She tells you, "I've got a solution for renewable power generation from the ocean." She hands you a sheet of paper with a schematic on it. Her idea involves a very tall apparatus that will sit inside the ocean astride layers of water at different temperatures. "That's ridiculous," a friend of hers comments. "The 2nd law of thermodynamics says you can't run a boat off of the heat in the ocean, so you wouldn't be able to generate power either." Is this idea unworkable on those grounds or not? Why?
One of the implications of the second law is that energy flows from high temperature to low temperature. The reason why you "can't run a boat off of the heat in the ocean" is because FLOW of heat produces work and not just the heat alone. In OTEC (Ocean Thermal Energy Conversion), work is done in a thermodynamic cycle which uses hot water from the ocean surface and cold water from deep below, to evaporate and condense the working fluid (a fluid with very low boiling point - like ammonia). When the working fluid is evaporated and condensed, heat is transferred from one substance to another (flow of heat). This changes the phase of the substance from gas to liquid and vice versa. Since pressurized gas has higher entropy, it will rotate a turbine, losing its heat content in the process and hence produce electricity.
Check this out for a better understanding of how OTEC works: http://www.youtube.com/watch?v=x59MptHscxY
Potential exam question #4:
Describe the relationship between light intensity and primary production.
Under low light intensities, the rate of photosynthesis in plants varies in direct proportion to irradiance (the amount of light striking a surface). Greater light intensities have the effect of increasing the rate of photosynthesis and, thereby, primary production. However, this effect tapers off at the "saturation point." This point describes the level of light intensity above which the rate of photosynthesis no longer responds to increasing light intensity. As light intensity is increased, the rate photosynthesis increases. The point at which primary production due to photosynthesis equals the loss due to respiration is called Compensation Point. beyond this point, photosynthesis dominates respiration as light intensity increases.
Potential exam question #5:
Why do living things require a continuous flow of energy? Touch on the concepts of the 1st and 2nd laws of thermodynamics and entropy in your response.
According to the 1st law of thermodynamics, energy is neither created nor destroyed - only transformed. However, the 2nd law of thermodynamics tells us that energy forms differ in the amount of disorder, or entropy, associated with the energy and that energy always flows in the direction from lower-entropy to higher-entropy forms in chemical, physical, and biological reactions that occur spontaneously. In addition, in each of these transformations, some of the capacity of the energy to do work is used up.
Living things must exist out of equilibrium with their environment (at a lower entropy chemically) in order to do work (growth, maintenance, reproduction). Maintaining this lower state of entropy requires a continuous flow of energy since the transformation of energy for work by organisms inevitably involves losses according to the 2nd law of thermodynamics.
Potential Exam question #6
Explain the processes that heat the atmosphere.
Short-wave radiation from the sun heats the earth's surface. This then radiates long-wave radiation out into the atmosphere thereby heating the atmosphere. The atmosphere is heated from BELOW. Latent heat from evapotranspiration of terrestrial plants is also releasing heat into the atmosphere. The evapotranspiration is the equivalent of the plants "sweating" the plant is cooled by the atmosphere absorbing that heat. This process of evapotranspiration and warming is also repeated on the oceans.