Submitting Essays for Regrading

When grading your essays, the TAs and I strive to make the score that you receive on your essay accurately indicate how well you have answered the question. At times it is possible that the score that you received on your essay does not correlate with how well you answer the question so this essay resubmission policy is designed to correct those rare occurances.

Rules for Submitting Essays

1. Answers written in pencil can not be submitted for regrading.

2. Do not turn in essays until you have carefully examined the answer keys (provided below or on bulletin board). Take a look at the answer key and carefully compare your answer to the key. Ask the following questions. Did your answer actually answer the question that was asked? Did your answer include all of the details shown on the answer key? Was your answer as well organized as the answer key?

3. I only want to look at tests where you feel there is a severe difference between the score you earned and how well you answered the question (that means 3 or more poingt). I do NOT want to see any 19/20s!

4. To resubmit an essay please type out a brief paragraph describing why you think that your answer deserved more points that it was awarded. Staple the essay to this sheet (make sure you put your name on it) and leave the sheet in a box that is located outside of my office in McClellan Hall (room 215) by 5:00 PM on Monday March 26.

Answer Key For First Midterm


Essay
Exponential population growth is a model of population growth that only occurs when the per capita growth rate is constant. In populations experiencing exponential growth, the population size keeps increasing over time at ever increasing rates. Thus, populations growing exponentially would keep increasing in size forever. Obviously, we do not see this pattern of growth in nature, so this must not be a realistic model of population growth.

The problem with exponential growth is that the per capita growth rate is not constant. In fact, in most species we expect that the per capita growth rate should be negatively density dependent. The per capita growth rate (r) can be calculated by subtracting the per capita death rate (d) from the per capita birth rate (r). Because both per capita birth rates and per capita death rates are density dependent, it is not surprising that per capita growth rate is also density dependent and thus not a constant.

The per capita birth rate is negatively density dependent. As population sizes increase there are more individuals competing for the same amount of food, so each individual gets less food. Because the number of babies that a female produces is related to the amount of resources she has, as each female has less food, they should have fewer births resulting in a lower per capita birth rate.

The per capita death rate is positively density dependent. Per capita death rates increase with population size for three possible reasons. First, as population size increases competition causes each individual to get less food which increases their chance of dying of starvation. Second, predation rates could be higher in larger populations if predators were attracted to areas of high prey density or if prey were easier to catch when population sizes were high (maybe there are not enough hiding places to go around). Finally, mortality caused by disease could be higher in more dense populations because it is easier to spread disease throughout the population when individuals are in close contact.

Because per capita birth rates decrease and per capita death rates increase as population size increases, the per capita growth rate decreases as population sizes increases which results in a non-exponential pattern of growth.

Essay
The process of natural selection causes traits that increase the survival, reproduction, and mating ability of organisms. Thus, long tails in peacocks have been molded by this process.

If we assume that the long tails of peacocks can be explained by the handicap principle, then the reason that females choose to mate with the males with the longest tails is that the long tails are a “handicap” to effective survival in the peacocks natural environment. Thus, males that have managed to survive with large tails must have genes that code for phenotypic characteristics that make them especially good at surviving (finding food, escaping predation , etc.). Thus, by choosing to mate with a male with the most extreme tail, a female hopes to benefit by passing on those good survival genes to their offspring so both her sons and her daughters should have higher survival.

For the process of natural selection to occur there must be variation in phenotype within a population, the variation must be heritable, and variation in traits must affect fitness. Assume that tail length has a genetic component and that there is variation in tail length within a population of peacock ancestors. If females are choosing males with the most extreme trait because of the handicap principle then males with longest tails should be chosen to mate more often and the offspring of females that mate with long tails should survive better. Thus, there is a fitness benefit associated with mating with a male with the longest tail.

If this is true, then over time the frequency distribution of tail length in males should move to the right (directional selection) so we see increases in both the maximum, minimum, and mean tail length. The introduction of new mutations that allow males to produce even longer tails should allow directional selection to continue so that the mean tail length of males increases. We would expect directional selection to continue until the mating benefit of having the longest tail was offset by the survival disadvantage of having an extremely long tail and the fitness advantage of having the longest tail disappears. At this point stabilizing slection would kick in and direction selection would cease.


Answer Key for Retest

Posted on the wall where I posted MC answers.

Answer Key for Second Midterm

Graph A.
As light intensity increases the rate of photosynthesis increases until the rate of photosynthesis reaches a plateau. Initially, increases in light intensity greatly increase the rate of photosynthesis but as light intensity increases, the rate of increase in photosynthesis decreases until it equals zero.

Graph B.
As the concentration of carbon dioxide increases the rate of photosynthesis increases until the rate of photosynthesis reaches a plateau. Initially, increases in carbon dioxide concentration greatly increase the rate of photosynthesis but as carbon dioxide levels increase, the rate of increase of photosynthesis decreases until it equals zero.

Graph C
As temperature increases the rate of photosynthesis increases until it reaches a maximum and then decreases until the rate of photosynthesis is zero.

Essay
The rate of photosynthesis can be limited by a variety of environmental factors including light intensity, atmospheric carbon dioxide concentration, temperature, and soil nutrient content.

Increasing light intensity results in an increase in photosynthetic rates until photosynthetic rates reach a maximum suggesting that light intensity can limit the rate of photosynthesis. Light intensity limits the rate of photosynthesis by limiting the production of ATP and NADPH that are required to power the Calvin Cycle.
When further increases in light intensity fail to increase photosynthetic rates then other factors such as atmospheric carbon dioxide concentration or the amount of RuBP carboxylase have become the rate limiting step. The fact that the same increase in light intensity results in diminishing increases in photosynthetic rate as light intensity increases suggests that multiple factors can be limiting at the same time. When light levels are low light appears to be most limiting, but as light intensity increases other factors in addition to light levels appear to limit photosynthetic rates.

Increasing atmospheric carbon dioxide concentrations results in an increase in photosynthetic rates until photosynthetic rates reach a maximum suggesting that the atmospheric concentration of carbon dioxide can also limit the rate of photosynthesis. Carbon dioxide limits the rate of photosynthesis because carbon dioxide is used in the Calvin Cycle to produce glucose.

When further increases in atmospheric carbon dioxide concentration fail to increase photosynthetic rates then other factors such as light availability or the amount of RuBP carboxylase have become the rate limiting factors. The fact that the same increase in carbon dioxide content results in diminishing increases in photosynthetic rate as light intensity increases suggests that multiple factors can be limiting at the same time. When carbon dioxide concentrations are low carbon dioxide concentration appears to be most limiting factor, but as carbon dioxide concentration increases other factors in addition to light levels appear to limit photosynthetic rates.

Essay 
Humans require energy to do biological work. The energy used to power biological work in humans come in the form of energy released by ATP. Energy in ATP is converted from energy stored in chemical bonds of glucose by the process of cellular respiration. Humans are capable of breaking down sugar to release ATP in both aerobic and anaerobic conditions so they use both aerobic and anaerobic respiration.

Aerobic respiration breaks down glucose in the presence of oxygen to release 38 ATP per glucose. Glycolysis, which occurs in the cytosol of the cell, breaks down glucose into two molecules of pyryuvate + H+ which requires the input of energy from 2 ATPs to release energy in 4 ATPs. Hydrogen ions are picked up by the hydrogen carrier NADH and then released to react with oxygen to form water. The pyruvate moves into the mitochondria where is is broken down to produce acetyl Co A and CO2. The Acetyl Co A enters the Krebs cycle where one molecule of ATP is release for each Acetyl CoA that enters the cycle. An excited electron undergoes electron transport which produces a large number of ATP. The ATPs are produced by chemiosmosis which is powered by a hydrogen ion concentration gradient that is produced when energy released by electron transport is used to actively transport H+. Aerobic respiration is a very effective way of converting glucose into ATP.

When oxygen is not available to remove the H+ from the system then organisms must use an anaerobic pathway. Glucose is still broken down by glycolysis with the same net gain of 2 ATP per glucose. In order to keep glycolysis going forward the pyruvate and H+ react to form acetaldehyde which is converted to ethanol (alcohol fementation). Alcohol fermentation is not as advantageous to the cells because alcohol can be poisonous to cells and potential energy in pyruvate can not be released. Humans need to use anaerobic respiration when they are so active that they are using oxygen faster than their system can supply it such that cells momentarily become anoxic environments.