Animal Biology - GRE

The prostate gland is used to release fluid that adds to the fluidity of semen, and makes the semen alkaline in nature. This alkalinity helps counteract the acidity of the vaginal tract, and prolong the lifespan of the sperm.

Sperm are produced in the testes and are stored in the epididymis prior to release. The hormones involved in spermatogenesis are testosterone, produced by Leydig cells in the testes, and follicle-stimulating hormone, produced by the anterior pituitary.

The prostate gland is used to release fluid that adds to the fluidity of semen, and makes the semen alkaline in nature. This alkalinity helps counteract the acidity of the vaginal tract, and prolong the lifespan of the sperm.

Sperm are produced in the testes and are stored in the epididymis prior to release. The hormones involved in spermatogenesis are testosterone, produced by Leydig cells in the testes, and follicle-stimulating hormone, produced by the anterior pituitary.

Many people have the misconception that arteries only carry oxygenated blood that has been pumped out fo the heart. The truth is that arteries are responsible for carrying all blood away from the heart, whether it be oxygenated or deoxygenated. For example, the aorta is an artery that carries oxygenated blood from the heart to the tissues, however, the pulmonary arteries carry deoxygenated blood from the heart to the lungs. Any vessel that travels away from the heart is classified as either an artery or an arteriole, regardless of the blood it contains.

Blood vessels can be constricted or dilated in order to adjust blood pressure and reroute blood to areas in need of nutrients and oxygen. This constriction is done by smooth muscle, which is primarily found wrapped around arterioles.

Arteries also have a thick lining of smooth muscle, but are generally too large in diameter to be useful in directing blood flow. Capillaries have no smooth muscle and cannot be used to direct blood. Venules may have small layers of smooth muscle, but are not nearly as effective as arterioles.

Action potentials are spontaneously conducted so that the heart can pump automatically, without necessary stimulation from the central nervous system. These spontaneous action potentials are created by a group of cardiac cells called the sinoatrial node. Because it determines the heart rate, the sinoatrial node is considered the pacemaker of the heart.

After generation by the sinoatrial node, action potentials will cause the atria to contract and travel to the atrioventricular node. The atrioventricular node introduces a delay, which prevents the ventricles from contracting during atrial systole, which could push blood backward from the ventricle to the atrium. The atria relax and the signal is passed from the atrioventricular node to the bundle of His in the atrioventricular septum before spreading to the ventricles and causing ventricular systole.

Antibodies are produced by the adaptive immune system and to antigens presented by a pathogen. The antibody-antigen complex is then detected by cytotoxic T-cells, which destroy the infected cell. The antibodies are created by B-lymphocytes, which can differentiate when exposed to a specific pathogen. Differentiated B-lymphocytes known as plasma cells are responsible for the mass production of certain antibodies.

Macrophages are derived from monocytes and serve as phagocytes in the innate immune response. Basophils are also part of the innate immune response and, along with mast cells, produce histamine to initiate the inflammatory response.

Acquired immunity is developed in the body following a previously encountered infection. The innate immunity is, by definition, always present in the body and is used to attack all general forms of infection.

The granulocyte cells are considered part of innate immunity and help mediate the immune response against foreign pathogens. These cells include basophils, neutrophils, eosinophils, mast cells, and macrophages (which are differentiated from monocytes).

The adaptive immune response involves the production of antibodies against specific target antigens. Plasma cells are an integral part of the adaptive response and secrete large volumes of antibodies in response to a secondary infection by a previously encountered pathogen. T-cells and B-cells are also part of the adaptive response.

The five types of leukocytes are: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Three of these (neutrophils, eosinophils, and basophils) contain granules, tiny sacs containing enzymes which can lyse microorganisms. The other two leukocytes (lymphocytes and monocytes) do not contain these granules.

There are many types of white blood cells with distinct functions in the body. Neutrophils and monocytes are phagocytes that engulf bacteria. B-lymphocytes produce one type of antibody. Basophils are the least common of the leukocytes, and release histamine during inflammation.

Before a B-lymphocyte can proliferate and differentiate, it must present its antigen to a helper T-cell. If the helper T-cell recognizes the antigen as foreign, it will activate the B-lymphocytes, and cause them to differentiate into plasma cells and memory B-cells.

T and B cells are both part of the adaptive immune response. This means that they are typically made to handle very specific pathogens that may be encountered in the body. Before they are released into circulation, both must undergo negative selection, which makes sure that they do not respond to natural peptides that they encounter in the body. Failure of this step to take place can result in autoimmune diseases.

Many of the modulations of the immune response are controlled by helper T cells. In order to change the type of antibody secreted by a B cell, helper T cells need to interact with them via a number of cytokines. The types of cytokines secreted by helper T cells will tell the B cells which antibodies to start secreting.

The main stages of an action potential are depolarization, hyperpolarization, and repolarization. The resting membrane potential of the cell is roughly –65mV. During depolarization the neuron initiates the action potential by opening voltage-gated sodium channels. This allows an influx of sodium ions, which raises to membrane potential to roughly 50mV. Sodium channels are quick to react to the action potential stimulus, but voltage-gated potassium channels are slower. After the depolarization, the potassium channels open, allowing for a rapid efflux of potassium ions. This causes the membrane potential to rapidly drop, so much so that it becomes more negative than the resting potential. This drop below resting potential is known as hyperpolarization. Repolarization then occurs by action of the sodium-potassium pump, which uses ATP to reestablish the resting potential by removing sodium and importing potassium.

The absolute refractory period occurs when the initial gating mechanism of the sodium channels is activated, making them impervious to stimuli. In contract, the relative refractory period is closely linked to hyperpolarization and describes the period during which the cell can be stimulated, but only if the stimulus is large enough to overcome the hyperpolarized environment and reach threshold.

The action potential is composed of key changes in voltage for the neuronal cell body. The resting voltage in the cell is negative, due to the action of the sodium-potassium pump. When an action potential reaches the cell, voltage-gated sodium channels open, and sodium ions rush into the cell. This raises the voltage inside the cell in a process called depolarization.

As the voltage in the cell rises, the sodium channels begin to close, and voltage-gated potassium channels begin to open. As potassium ions exit the cell, the voltage drops back down to negative once again. This process is called repolarization. It takes a bit longer for the potassium channels to close, which causes a temporary hyperpolarization of the cell; however, once they close and the cell will eventually return to the initial negative resting potential by action of the sodium-potassium pump.

There is a larger number of voltage-gated calcium channels near the synaptic cleft on the pre-synaptic neuron. As an action potential approaches the synaptic cleft, these voltage-gated calcium channels open and allow for a rapid influx of calcium. The sudden influx of calcium ions into the cell causes a release of neurotransmitters into the synaptic cleft.

Sodium and potassium play key roles in establishing the resting membrane potential and propagating the action potential, but do not actually stimulate the release of the neurotransmitter.

Neurotransmitters cross the synaptic cleft to cause a change in the excitability of the downstream neuron. Acetylcholine is an excitatory neurotransmitter matching the description in the question stem. GABA is an inhibitory neurotransmitter and serotonin is responsible for feelings of happiness. Cortisol and epinephrine are hormones, not neurotransmitters, thus they are released into the bloodstream, not the synaptic cleft.

Repolarization is one of the last steps of an action potential, where the cell potential of the neuron is made to be negative in value once again. This step is accomplished by the opening of voltage-gated potassium channels, which allows for potassium to exit the neuron.

Action potentials are unique in that they are a one-way transmission of impulses throughout the nervous system. Action potentials will always be the same amplitude for a given neuron, regardless of the stimulus which caused it; however, the stimulus must be sufficient enough to cross the threshold, or the action potential will not occur. It is because of this feature that action potentials are said to be "all or nothing" in nature.

The correct answer is carbohydrates.

Salivary amylase can only digest carbohydrates. Proteases further along in the digestive pathway breakdown proteins, while lipases digest fats. Amino acids are the product of digested proteins.

Pepsinogen is released by chief cells into the stomach lumen. In the presence of hydrochloric acid (secreted by parietal cells), this inactive enzyme will be cleaved, creating pepsin. Pepsin is a protease responsible for breaking down proteins that enter the stomach.

Carbohydrates are digested by amylase enzymes, lipids are digested by lipases, and nucleic acids are digested by nucleases.