MCAT Biology : Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #921 : Biology

Type 1 diabetes is a well-understood autoimmune disease. Autoimmune diseases result from an immune system-mediated attack on one’s own body tissues. In normal development, an organ called the thymus introduces immune cells to the body’s normal proteins. This process is called negative selection, as those immune cells that recognize normal proteins are deleted. If cells evade this process, those that recognize normal proteins enter into circulation, where they can attack body tissues. The thymus is also important for activating T-cells that recognize foreign proteins.

As the figure below shows, immune cells typically originate in the bone marrow. Some immune cells, called T-cells, then go to the thymus for negative selection. Those that survive negative selection, enter into general circulation to fight infection. Other cells, called B-cells, directly enter general circulation from the bone marrow. It is a breakdown in this carefully orchestrated process that leads to autoimmune disease, such as type 1 diabetes.

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A doctor is studying blood collected from a sick patient. She centrifuges the blood sample to separate the hematocrit, the buffy coat, and the plasma. In which layer is she most likely to find most of the immune system cells?

Possible Answers:

Only in the plasma

In both the hematocrit and the plasma

Only in the buffy coat

In both the hematocrit and the buffy coat

Only in the hematocrit

Correct answer:

Only in the buffy coat

Explanation:

The buffy coat is so-named because it has the majority of white (immune) cellular material, while the hematocrit contains most of the red cell material (hemoglobin), and the plasma has the fluid and circulating proteins, such as antibodies.

Erythrocytes are found in the hematocrit, lymphocytes are found in the buffy coat, and clotting factors, albumin, and fluids are found in the plasma.

Example Question #922 : Biology

Which protein constitutes the largest percentage of plasma proteins and is responsible for transporting non-soluble substances through the body?

Possible Answers:

Albumin

Thrombin

Thromboxane

Plasminogen

Correct answer:

Albumin

Explanation:

Albumin is the protein that is involved in transporting a variety of products throughout the blood as well as the protein factors involved in blood clotting. Albumin makes up about 50% of the plasma protein distribution. It is responsible for binding insoluble molecules to transport them through the aqueous blood stream. Albumin is also the main driver of oncotic pressure in the arteries and veins to keep water from moving to the tissues via osmosis.

Plasminogen, thrombin, and thromboxane are all proteins involved in the clotting process and are also found in the blood.

Example Question #923 : Biology

An individual with hemophilia, most likely has a deficiency in which of the following proteins?

Possible Answers:

Renin

Fibrinogen

Vasopressin

Major histocompatibility complex proteins

Albumin

Correct answer:

Fibrinogen

Explanation:

Fibrinogen is converted to fibrin by the action of thrombin, and fibrin is the protein that actually forms a blood clot. An inability to properly form clots, such as is seen in hemophilia, could arise from a defect in fibrinogen, resulting in a problem with the clotting pathway.

Renin is an enzyme released by the kidney, which creates angiotensin to help raise blood pressure. Vasopressin is a posterior pituitary hormone that acts on the kidney to retain water. Albumin is a plasma protein that functions to maintain the oncotic pressure in the blood, as well as to transport fatty acids. Major histocompatibility complex (MHC) proteins are used to present antigens to the immune system.

Example Question #924 : Biology

Which of the following is not a component of blood plasma?

Possible Answers:

Albumin

Thrombin

Red blood cells

Water

Correct answer:

Red blood cells

Explanation:

The plasma portion of the blood is the extracellular matrix that suspends the erythrocytes and lymphocytes traveling through circulation. The plasma contains water, proteins (chiefly albumin), electrolytes, and clotting factors (such as thrombin). Whole blood contains the cells, as well as thx extracellular plasma. Blood serum refers to blood plasma that has had the clotting factors removed.

Example Question #32 : Hemoglobin, Blood Cells, And Blood Proteins

Mature erythrocytes contain which of the following organelles?

Possible Answers:

Golgi apparatus

Nucleus

None of these

Mitochondria

Correct answer:

None of these

Explanation:

Mature erythrocytes (red blood cells) do not contain a nucleus and lack most organelles. While immature, the erythrocyte contains a nucleus, but this gets extruded in order to make more room for hemoglobin to transport oxygen. It is important to understand the implications of not having any additional organelles; the red blood cell cannot derive energy from the citric acid cycle and cannot produce, transport, or exocytose proteins. This may seem like a problem, but it actually prevents the red blood cell from consuming the oxygen that it is transporting. If erythrocytes contained nuclei or mitochondria, they would metabolize the oxygen attached to hemoglobin, failing to deliver it to other tissues.

Example Question #925 : Biology

A mature erythrocyte primarily derives its energy from which of the following metabolic processes?

Possible Answers:

Glycogenolysis

Oxidative phosphorylation

Glycolysis

Lactic acid fermentation

Correct answer:

Glycolysis

Explanation:

The mature erythrocyte (red blood cell) contains no nucleus, mitochondria, Golgi apparatus, or endoplasmic reticulum. Consequently, the red blood cell can only derive energy from glycolysis, the anaerobic breakdown of glucose in the cytosol. Mitochondria would be required for any other metabolic process.

Example Question #34 : Hemoglobin, Blood Cells, And Blood Proteins

What enzyme is responsible for converting fibrinogen to fibrin, which helps platelets form clots in response to vessel injury?

Possible Answers:

Prothrombin

Platelet monoamine oxidase

Thrombin

Complement

Correct answer:

Thrombin

Explanation:

Thrombin, created from prothrombin after activation by coagulation factors from the liver, is responsible for turning fibrinogen to fibrin. Fibrin adheres to endothelial cells and platelets to allow clots to form in response to vessel damage. Complement helps attract immune cells to the site of injury. Platelet monoamine oxidase degrades amines, such as serotonin, in neuronal synapses.

Example Question #35 : Hemoglobin, Blood Cells, And Blood Proteins

Which of the following cell types does not possess a nucleus in its mature form?

Possible Answers:

Erythrocytes

Granulocytes

Neurons

Adipocytes

Correct answer:

Erythrocytes

Explanation:

Erythrocytes are chiefly responsible for delivering oxygen to cells throughout the body and do not possess nuclei in their mature form. The absence of a nucleus and mitochondria from mature erythrocytes prevents transported oxygen from being consumed by the carrier cell, instead of being delivered to the body.

Granulocytes are a division of white blood cells and frequently have lobed or multiple nuclei. Neurons contain nuclei in their somas and adipocytes contain peripheral nuclei, located close to the membrane.

Example Question #103 : Circulatory System

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

\(\displaystyle CO_{2} + H_{2}O \leftrightharpoons H^{+} + HCO_{3}^{-}\)

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.   

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride \(\displaystyle (NaCl)\) in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases. 

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more \(\displaystyle CO_{2}\) being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.  

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

How will excess intake of a carbonic anhydrase inhibitor affect the hemoglobin dissociation curve?

Possible Answers:

No change

Shifts to the right

Shifts to the left

Shifts up

Shifts down

Correct answer:

Shifts to the right

Explanation:

Carbonic anhydrase inhibitors will decrease the blood's pH. Conditions that will shift the hemoglobin dissociation curve to the right are increased temperature, increased 2,3-DPG, and decreased pH.

Example Question #1 : Other Circulatory Physiology

Which of the following statements about thermoregulation is incorrect?

Possible Answers:

One form of insulation used by the body is subcutaneous fat.

When someone is cold, they may eventually shiver to increase their heat production.

Sweat can cool someone down on a hot day, since sweat releases energy from the body to evaporate into the environment.

When someone is cold, they will experience vasodilation in their hands, allowing more blood flow to the hands to maintain uniform temperature through the whole body.

Correct answer:

When someone is cold, they will experience vasodilation in their hands, allowing more blood flow to the hands to maintain uniform temperature through the whole body.

Explanation:

When someone is cold they, will experience vasoconstriction in their hands, allowing less blood to flow to the hands. Their hands will feel colder, since less blood is reaching their hands. When the body is cold, it concentrates blood flow to the core. This maintains a constant, warm blood supply to the vital organs, as well as reduces the blood flow to extremities that have a greater surface area for heat loss to the environment.

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