Biochemistry : Sterols and Steroids

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #182 : Biochemistry

Which of the following would be characteristic of a steroid hormone? 

Possible Answers:

Binding to receptors on the outside of the cell membrane

Effects are short term and quickly reversed

Binding to cytosolic or nuclear receptors

Not requiring any proteins to transport them to target cell in the bloodstream

Correct answer:

Binding to cytosolic or nuclear receptors

Explanation:

Steroid hormones are derivatives of cholesterol and therefore can pass through the plasma membrane of cells since they are lipophilic. Steroid hormones generally don't bind on the surface of plasma membranes, but instead to receptors inside the cell itself. Since they are nonpolar, they are carried by proteins in the blood. 

Example Question #2 : Sterols And Steroids

A steroid molecule consists of how many fused carbon rings?

Possible Answers:

2

5

4

6

3

Correct answer:

4

Explanation:

Steroids are derivatives of the molecule cyclopentanoperhydrophenanthrene which is composed of 4 fused carbon rings. There are three hexane rings and one pentane ring in the structure of steroids.

Example Question #3 : Sterols And Steroids

Cholesterol has a structure most similar to which of these?

Possible Answers:

Carbohydrate

Glycolipid

Phospholipid

Free fatty acid

Steroid

Correct answer:

Steroid

Explanation:

Although it is one of the three main types of membrane lipids (along with phospholipids and glycolipids) cholesterol's structure is quite different.  It is a steroid, and so it is made from four linked hydrocarbon rings, including three six-membered rings and one five-membered ring.

Example Question #4 : Sterols And Steroids

Which of the following is not made from cholesterol?

Possible Answers:

Insulin

Estrodiol

Testosterone

Progesterone

Cortisol

Correct answer:

Insulin

Explanation:

Cholesterol is a very important molecule for a number of reasons. One function of cholesterol is to regulate membrane fluidity in the plasma membrane. Another very important role that cholesterol plays is as the precursor to a number of steroid hormones including cortisol, testosterone, estradiol, and progesterone. Insulin, however, is a peptide hormone made in the pancreas.

Example Question #5 : Sterols And Steroids

All of the carbons in cholesterol are derived from carbons in what molecule?

Possible Answers:

Acetoacetate

Glucose

Mevalonate

Glycerol

Acetyl-CoA

Correct answer:

Acetyl-CoA

Explanation:

Cholesterol is a molecule with 27 carbon atoms. In order to synthesize this steroid, a three step process must be completed. First, isopentenyl pyrophosphate is synthesized. Second, six isopentenyl pyrophosphates form squalene. Third, squalene cyclizes and the product of this is converted to cholesterol. The building blocks that began this whole process were carbons from acetyl-CoA.

Example Question #6 : Sterols And Steroids

The first of three steps of cholesterol synthesis occurs in the cytoplasm. Where do the following two steps occur, respectively?

Possible Answers:

Cytoplasm

Endoplasmic reticulum; cytoplasm

Endoplasmic reticulum

Endoplasmic reticulum; Golgi apparatus

Cytoplasm; endoplasmic reticulum

Correct answer:

Endoplasmic reticulum

Explanation:

The fist step of cholesterol synthesis is the synthesis of isopentenyl pyrophosphate from acetyl-CoA molecules, and this occurs in the cytoplasm. The second step of cholesterol synthesis is six isopentenyl pyrophosphate molecules converging to form squalene. This occurs in the endoplasmic reticulum. The third step of cholesterol synthesis is the cyclization of squalene and the final conversion of the molecule to cholesterol. This occurs in the endoplasmic reticulum.

Example Question #7 : Sterols And Steroids

Which step in cholesterol synthesis is the committed step?

Possible Answers:

Synthesis of squalene from isopentenyl pyrophosphate units

Cyclization of squalene

Isomerization of isopentenyl pyrophosphate to dimethyallyl pyrophosphate

Synthesis of mevalonate by HMG-CoA reductase

Condensation of two farnesyl pyrophosphates by squalene synthase 

Correct answer:

Synthesis of mevalonate by HMG-CoA reductase

Explanation:

Once acetyl-CoA molecules have been converted to 3-hydroxy-3-methylglutary-CoA (HMG-CoA), reduction to mevalonate occurs. This synthesis is an irreversible step in the pathway to create cholesterol, and therefore it is the committed step in cholesterol synthesis.

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