Question 1

Which experimental evidence first supported the idea that biological membranes are composed of a lipid bilayer?

Accepted Answer

Gorter and Grendel's extraction of lipids from red blood cells and calculation of surface area

Question 2

The image below (first image) shows a trilaminar structure observed by electron microscopy. What does this structure represent in the context of cell biology?

Accepted Answer

A lipid bilayer with associated proteins (unit membrane)

Question 3

Which of the following best describes the role of cholesterol in the plasma membrane at different temperatures?

Accepted Answer

It decreases membrane fluidity at high temperatures and increases it at low temperatures.

Question 4

A membrane is enriched in stearate (a saturated fatty acid). What effect does this have on the membrane's transition temperature ($$T_m$$)?

Accepted Answer

It raises $$T_m$$, making the membrane less fluid.

Question 5

Which of the following statements about lipid rafts is correct?

Accepted Answer

They are specialized membrane domains enriched in saturated fatty acids, cholesterol, and specific proteins.

Question 6

Which enzyme is responsible for the transverse movement ('flip-flop') of phospholipids across the bilayer?

Accepted Answer

Flippase (phospholipid translocase)

Question 7

A membrane protein is found to have seven hydrophobic transmembrane domains and hydrophilic regions on both sides of the membrane. Which type of protein is this most likely to be?

Accepted Answer

A transmembrane (integral) protein

Question 8

Which of the following best explains why different membranes within the same cell can have different lipid and protein compositions?

Accepted Answer

Different membranes have specialized functions requiring unique compositions.

Question 9

Which method allows for the visualization of membrane proteins by splitting the bilayer and observing the exposed interior surfaces?

Accepted Answer

Freeze-fracture electron microscopy

Question 10

A scientist uses SDS-PAGE to analyze membrane proteins. What is the main purpose of adding SDS to the protein sample?

Accepted Answer

To give all proteins a uniform negative charge

Question 11

Which of the following equations correctly represents the calculation Gorter and Grendel used to determine the number of lipid layers in the membrane?

Accepted Answer

$$	ext{Number of layers} = \frac{	ext{Surface area of extracted lipids}}{	ext{Surface area of cells}}$$

Question 12

Which property of fatty acids most directly decreases the transition temperature ($$T_m) of a $$membrane?

Accepted Answer

Increasing the number of double bonds (unsaturation)

Question 13

Which of the following is a real-world application of lipid rafts in cell biology?

Accepted Answer

They serve as sites for the assembly of signaling complexes during immune responses.

Question 14

A mutation causes a membrane protein to lose all hydrophobic amino acids in its transmembrane domain. What is the most likely consequence?

Accepted Answer

The protein will fail to insert into the membrane and remain in the cytosol.

Question 15

Which of the following best describes the effect of cholesterol on membrane fluidity at low temperatures?

Accepted Answer

Cholesterol increases membrane fluidity and prevents gelling.

Question 16

Which of the following is NOT a function of biological membranes?

Accepted Answer

Protein synthesis

Question 17

A patient has a genetic defect in flippase. Which of the following is a likely cellular consequence?

Accepted Answer

Impaired transverse movement of phospholipids, leading to loss of membrane asymmetry

Question 18

Which of the following best explains why the plasma membrane is described as a 'fluid mosaic'?

Accepted Answer

It consists of a dynamic bilayer of lipids with proteins embedded or associated, allowing lateral movement.

Structure and Function of Biological Membranes

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