MCAT Biology : Lipids and Metabolism

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #339 : Organic Chemistry, Biochemistry, And Metabolism

The process of glycolysis is used by all cells of the body to turn glucose into ATP for cellular energy. When stores of glucose are low, however, the body can break down a form of stored glucose in the liver to increase glucose reserves. The supply of glycogen is limited, and eventually the body must break down free fatty acids (FFAs) through a process called beta-oxidation.

Which organ in the body cannot perform beta-oxidation, thus requiring the use of ketone bodies when stores of glucose are depleted?

Possible Answers:

Heart

Muscle

Brain

Liver

Correct answer:

Brain

Explanation:

The brain is unable to perform beta-oxidation of free fatty acids in the event of a prolonged fasting state. It is important to know this aspect of metabolism. In a fasting state, the liver beta-oxidizes free fatty acids into ketone bodies for the brain to use. Additionally, when energy demands are high, muscles can break down fat for additional ATP.

Unlike other organs in the body, the heart relies almost entirely on beta-oxidation for its energy needs. 

Example Question #340 : Organic Chemistry, Biochemistry, And Metabolism

The process of glycolysis is used by all cells of the body to turn glucose into ATP for cellular energy. When stores of glucose are low, however, the body can break down a form of stored glucose in the liver to increase glucose reserves. The supply of glycogen is limited, and eventually the body must break down free fatty acids (FFAs) through a process called beta-oxidation.

What is the end-product of beta-oxidation?

Possible Answers:

Pyruvate

Acetyl-CoA

Acetoacetate

Glucose

Correct answer:

Acetyl-CoA

Explanation:

Free fatty acids are chains of acetyl-CoA molecules linked together. When a free fatty acid undergoes beta-oxidation, it is returned to its component parts of acetyl-CoA. It is important to know that free fatty acids cannot be used to make glucose; they can only be fed into the Krebs cycle.

Example Question #1723 : Mcat Biological Sciences

In which of the following places does the breakdown phase of beta-oxidation occur?

Possible Answers:

Mitochondrial intermembrane space

Cytosol

Endoplasmic reticulum

Mitochondrial matrix

Nucleus

Correct answer:

Mitochondrial matrix

Explanation:

Beta-oxidation is the metabolization of fatty acids to generate acetyl CoA, which can be used in the Krebs cycle. This process always occurs in the mitochondrial matrix.

Example Question #1 : Lipids And Metabolism

Which of the following describes a beta oxidation reaction?

Possible Answers:

Fatty acid is converted to acetyl-CoA

Glycogen is converted to glucose

Protein is converted to alpha-keto acid

Glucose is converted to glycogen

Acetyl-CoA is converted to fatty acid

Correct answer:

Fatty acid is converted to acetyl-CoA

Explanation:

Beta oxidation is the process by which fatty acid molecules are broken down in the mitochondria to produce acetyl-coA, which can then enter the citric acid (Krebs) cycle. The correct transition from reactant to product for beta oxidation is fatty acid to acetyl-CoA.

Example Question #342 : Organic Chemistry, Biochemistry, And Metabolism

Fatty acids and cholesterol are stored in tissues as __________ and __________, respectively.

Possible Answers:

sphingolipids . . . cholesteryl esters

triacylglycerols . . . high-density lipoprotein (HDL)

eicosanoids . . . triacylglycerols 

cholesteryl esters . . . ketone bodies 

triacylglycerols . . . cholesteryl esters

Correct answer:

triacylglycerols . . . cholesteryl esters

Explanation:

Fatty acids are stored as triacylglycerols in adipose tissue, while cholesterol is stored as cholesteryl esters in a number of different tissues. Both fatty acids and cholesterol are hydrophobic molecules, which is why they are stored as lipid droplets within their respective tissues. 

Example Question #1731 : Mcat Biological Sciences

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Which of the following hormones utilizes cholesterol as a precursor? 

I. Cortisol

II. Aldosterone

III. Mineralocorticoid 

Possible Answers:

II only

I, II and III 

I only

None of these

III only

Correct answer:

I, II and III 

Explanation:

Both cortisol and aldosterone are synthesized in the adrenal cortex with cholesterol as the precursor. Mineralocorticoid refers aldosterone, which is also secreted by the adrenal cortex. All of these hormones are steroidal, which means they are derived from cholesterol. Other steroid hormones are the sex hormones.  

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