Systems Physiology - AP Biology

The hormone described in the question is luteinizing hormone (LH). If fertilization occurs, the LH levels will decrease and human chorionic gonadotropin (hCG) is released from the placenta to simulate LH function.

Hydrochloric acid, produced by the parietal cells of the stomach, activates the zymogen pepsinogen.

Schwann cells produce myelin for neurons of the peripheral nervous system, while oligodendrocytes produce myelin for neurons of the central nervous system.

Absolute refractory period occurs because of the inactivation of sodium channels. Since the sodium channels are inactivated, the neuron can’t depolarize and initiate another action potential. Relative refractory period occurs due to the slow inactivation of potassium channels. The voltage-gated potassium channels take a longer time to inactivate, which causes the cell to hyperpolarize. The cell becomes more negative than resting membrane potential as positive potassium ions exit the cell down their gradient. Since the cell has a more negative membrane potential, a larger stimulus is required to reach the threshold.

The absolute refractory period results from the initial gating of voltage-gated sodium channels. This initial mechanism ensures that the channel is incapable of opening, even when stimulated. The channel then shifts to a different gating mechanisms, which can be opened by a large enough electrical stimulus.

An electrical stimulus causes voltage-gated sodium channels in a neuron to open. Sodium then travels down its concentration gradient through the channels, into the cell. With the movement of sodium into the cell, the cell depolarizes (its membrane potential becomes more positive). The gradient that drives depolarization is established by the sodium-potassium pump, which causes two primary effects: the resting membrane potential is negative and there is a large concentration of sodium outside of the cell. When sodium channels open, sodium ions flow down both the electrical gradient formed by the negative membrane potential and the chemical gradient formed by ion concentrations.

Graded potentials reflect stimulus strength because they do not propagate via saltatory conduction (as action potentials do), and thus decrease in amplitude down the axon. All other answers listed are characteristics of action potentials.

Once the cell reaches threshold, an action potential is fired. Sodium, a positive ion, enters the cell, and causes the charge on the membrane to rise, or depolarize. After a certain point, sodium gates close. Potassium, another positive ion, then leaves the cell, and the charge on the membrane decreases. As the potassium ions exit, the membrane potential plunges even lower than the resting potential, causing it to become hyperpolarized. At this point, the sodium/potassium pump works to repolarize the cell to return to the resting membrane potential.

1. The cell reaches threshold

2. Sodium gates open, and sodium floods into the cell

3. The cell depolarizes

4. Sodium gates close

5. Potassium gates open and potassium leaves the cell

6. The cell hyperpolarizes

7. Potassium gates close

8. Na/K pump repolarizes the cell during refractory period

Major histocompatibility complex (MHC) class I molecules are found on virtually all cells in the body. They function in routine immune monitoring through presentation of short peptide fragments derived from degradation of intracellular proteins contained within the cell. The T-cell receptor on cytotoxic T-cells interacts with MHC class I, and if a foreign pathogen or peptide is presented, the cytotoxic T-cell becomes activated to kill infected cells. The same system also functions for detection of potential cancer cells.

The immune system is very adaptive. The body has many antibodies that will each recognize different antigens. If an antibody binds to an antigen, the antibody will be copied so that the body can quickly recognize the threat if it is exposed to the antigen a second time. This process is known as the adaptive immune response.

When an antigen is presented for a second time, antibodies to the antigen are released. These antibodies bind to the antigen, labelling it for attack by immune cells and preventing it from interacting the membrane proteins on the host cells.

Antibodies are proteins created by the immune system in order to neutralize foreign objects. An antibody would not be classified as an enzyme because it does not catalyze chemical reactions. When a foreign pathogen enters the body, it will have foreign receptors on its surface. These foreign receptors are known as antigens. When a pathogen is destroyed, immune cells can carry a sample of the antigen to the T-cells for identification. The T-cells help activate B-cells that will synthesize an antibody against the particular antigen. The selected B-cells differentiate into plasma cells and secrete antibody proteins into the blood, which bind the antigens and label the pathogen as foreign. This label attracts other immune cells to attack and destroy the pathogen.

The ability to form any of the three germ layers is known as pluripotency. Totipotent cells, such as the zygote, are able to form an entire organism, multipotent cells are able to form any cell within the same germ layer lineage, and progenitor cells are cells closer to differentiation, often found in adult organisms.

Antibodies are continuously made in the body in different shapes and forms. They are then sent into the blood stream to test for the presence of compatible antigens. Each antibody can only bind to one antigen, and each antigen can only bind to one antibody. Think of them like a codon-anticodon pair; there is only one possibility for them to form a perfectly complementary pair. Once the correct antibody binds to an antigen, they are tagged and used to stimulate production of more antibodies. The antibodies are only capable of binding and tagging the antigens. Cytotoxic T-cells are then able to recognize antibody binding patterns and actually destroy the infected cell.

From the passage, the MHC I molecule's blueprint is on chromosome 6. Therefore, the DNA must have been transcribed in the nucleus then translated in the ribosome. These ribosomes are on the endoplasmic reticulum. While in the endoplasmic reticulum, some of these proteins are degraded and react with a particular MHC I molecule.

According to the passage, after the MHC II molecules are fully synthesized in the endoplasmic reticulum, they are transported to the endosome. Foreign molecules are transported into the cell where they are degraded in the endosome as well. From there, the degraded pathogen interact with the MHC II molecule.

According to the passage, MHC I and cytotoxic t-cells are responsible for identifying the body's own protein. During organ transplant, foreign MHC I molecules are presented into the body. The host's cytotoxic T-cells will recognize these as foreign and will attack.

White blood cells is the correct answer here. After pathogen has entered the body, the antibodies to combat it are created by B cells that are a part of the white blood cells in the human body.

The spleen plays a key role in the immune system as one of the blood filtration centers of the body. It is also involved as a lymphatic organ, allowing filtrates from the blood to be distributed to the body via lymph.

Humoral, or B-cell, immunity is associated with the formation of antibodies. Plasma cells are B-lymphocytes that have been differentiated with the help of a helper T-cell. They release antibodies, which are created to respond to a specific pathogen in the body.

Cytotoxic T-cells are also activates by help T-cells, but are involved in cell-mediated immunity rather than humoral immunity. They target infected cells based on antibody tagging. Monocytes are a part of the innate immune response and are not involved in antibody interactions. They primarily differentiate into macrophages, which engage in phagocytosis of pathogens.

Basophils are the least common leukocyte found in the body, but play a key role in the inflammatory response. They contain histamine, which is a potent vasodilator. Upon release, histamine will increase blood flow to infected areas. Mast cells are another immune cell that is involved in histamine release, but are generally localized to various regions of the body rather than found in circulation.

Basophils, mast cells, eosinophils, and neutrophils are all considered granulocytes and are essential cells in the innate immune response. Plasma cells are differentiated B-lymphocytes that are responsible for mass-producing antibodies to a specific antigen.