If you are reading this, then you are probably in an AP Biology class. By this point, you have probably already taken the AP Biology Exam as well. For those who have – I hope you did well! For those reading this at another time, possibly before the test – good luck! It’s a league of a test all in its own! What makes the AP Biology test so challenging is that biology is such a wide field of study. Virtually anything from the natural world can find its way into a question. As a student taking it, you have to be prepared to answer whatever is given to you! My science professor did a spectacular job of preparing my class for such a critical test. However, even the best laid plans can go astray. Point being, even though we were productive all year, my class didn’t achieve everything we were hoping. So, the last week leading up to the test was nothing but a “cramming phase”. This post, a tribute to our last study sessions, is a compilation of the various topics we covered in that time. Pay attention, because it’s about to get very random!
First up on our list of discussion topics is Developmental Biology. We spent most of our time going over a lot of terms and vocabulary that apply to this category of science. As a result, we did not spend much time going over individual steps or components in-depth. As far as embryonic development, we talked about some of the initial terminology used during early development. The blastula, for example, is a sphere full of cells. The embryo, as it develops further, will create what is known as a blastula pore. This is the first instance in which a fold occurs toward the center of the blastula sphere. The process of Gastrulation is when the tissue truly folds inward on itself. The end result is a ball with 3 layers. Each layer is important as it creates a different structure of the developing organism. The outermost layer is known as the ectoderm, and is responsible for skin and nervous system. The middle layer is the mesoderm, creating tissues, organs, muscles, and the skeleton. The endoderm is responsible for creating glands and inner organ linings. The interesting part of this is that most creatures, whether it be a fish or a chicken, develop in this three layer fashion.
The layers become important because they very based on the complexity of the creature. Humans, for example, are known as Deuterostomes. Under a category known as Triploblasts (meaning to have three layers in the embryo), we form a separate mouth that is distinct from our anus (the latter being formed from the blastula pore). A step below us, but still in the Triploblast category, are Protostomes. They have a unified mouth and anus, which is formed from the blastula pore. An example of such a creature would be a squid or jellyfish. Finally, less complex creatures are under a category known as Diploblasts (they only have two layers – the ectoderm and endoderm). To put it simply, think about cnidarians. Their primary feature is that they have radial symmetry.
The main point of learning about these embryonic features is to understand that many animals develop differently. We can use this knowledge to help further our research of cell development, as well as in experiments involving the moving/affecting of cells. When it comes to comparing the developmental biology of animals, there are two terms that come to mind: homologous and analogous. They both refer to structures that are found on animals. Homologous structures are things that animals have that come from a common ancestor. An example would be the forearm in the skeletal system of bats and humans. They are particularly similar in their structure and appearance. An analogous structure, on the other hand, is a feature with similar functions, but no common ancestor. The best example of this would be a bird wing and an insect wing. Both are used for flight, but they are developed drastically different.
One other term we learned about within Developmental Biology is apoptosis. This is when the organism’s body intentionally tells certain cells to die. An example would be human hands. In the embryo, there is a webbing that connects the fingers together. Apoptosis is the process that tells the web cells to die, removing the webbing, and giving us defined fingers. Another example is a tadpole’s tale. As the tadpole develops into a frog, the tail undergoes apoptosis. The cells are reabsorbed into the frog after they are killed.
Now on to the second piece of information that we covered during our final week. Understand that this information does not in any way, shape, or form relate to the previous topic or potentially the next one. Think of this post as a “grab bag” that is comprised of many smaller, separate potential posts. Next we discussed the Gibbs Free Energy equation. This equation is as follows:
The change in free energy = change in enthalpy – temperature x change in entropy.
This equation is used to find the total amount of energy that can be used to do work. Enthalpy is the measure of the total energy within any given situation/system. Entropy is the number of “combinations”/the energy that is already used in a system. Entropy is multiplied by temperature because as the temperature increases, the amount of energy used also increases.
This is a basic formula that, greatly simplified, shows:
usable energy = total energy – temperature x used energy.
While we are on the topic of equations, there is another equation that we learned that was almost guaranteed to show up on the AP Exam: the Chi Square Test. While this test is somewhat elaborate, it provides some useful information that determines whether a series of data fits/correlates to Mendel’s laws. His laws were that a monohybrid cross (cross breeding one trait) would have a 3:1 ratio of traits, while a dihybrid cross (cross breeding two traits) would have a 9:3:3:1 ratio of traits. Let’s use an example problem to understand the processes behind Chi Square:
Example: A plant heterozygous for color undergoes self pollination. There are 27 total offspring. 23 of the offspring are purple, and only 4 are white. Do a Chi Square Test to see if the results match a Mendelian Ratio.
The equation for a Chi Square Test is as follows:
Using such an equation is relatively simple! We know the observed numbers: 23 purple plants, and 4 white plants. Now, to find the expected, we have to see what a 3:1 ratio would look like with 27 plants. (This involves dividing 27 by 4, giving you about 6 plants, then multiplying by three to get 20.) This means our expected ratio should be 20:7. Is this close enough to fit Mendelian’s theory?
To find out, we have to plug in our data to the equation. We must do this twice, once for each phenotype of plant.
The purple plants would be the following: (23-20) squared / 20 = (9/20)
The white plants would be the following: (4-7) squared / 7 = (9/7)
Since you are finding the sum, you must add these two numbers together. After you simplify, you get 1.74. This is our chi squared value. Now we must see if the results match the 3:1 ratio.
To do this, we must compare it to a Chi Square Table. The table works off of degrees of freedom and probability. Degrees of freedom is found by taking the number of offspring minus 1 (purple and white make 2. So 2-1 = 1). Probability, as a rule of thumb, is accurate if the 0.05 value is used. Use the table below to find the point where there is 0.05 probability and 1 degree of freedom.
The number you found is 3.84. You know that you have found an accurate test group if your results from the chi square test fall below the probability number. As our number 1.74 is below 3.84, then we know we have a proper 3:1 ratio. Mendel’s laws work for this population!
And that concludes today’s programming! What you just saw were the three big topics that we talked about as last minute preparations for the AP Biology Exam. These concepts are good things to know in this field of science, I will say. I hope you learned something, and enjoyed this very informal blog post!