Fluid Mosaic Model

Concept: The Fluid Mosaic


Concept: Spontaneous Assembly


Concept: Membrane Proteins Structure and Function


Concept: Membrane Proteins Function


Concept: Mosaic Structure Fits Function


Problem: Proteins in the plasma membrane are involved with all of the following except:


Problem: Which of the following does not influence membrane fluidity?


Fluid Mosaic Model Additional Practice Problems

Among the following, which amino acid is least likely to occur in the membrane crossing segments of a protein.

A. leucine (L)

B. leucine (L)

C. isoleucine (I)

D, phenylalanine (F)

E. aspartic acid (D)

Watch Solution

List 2 reasons why it is crucial that the cell membrane exhibit fluidity rather than be rigid.

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Which of these is a general feature of the lipid bilayer in all biological membranes?

A. Na+ and K+ ions readily diffuse across the bilayer.

B. Individual lipid molecules in one face (monolayer) of the bilayer readily diffuse (flip-flop) to the other monolayer.

C. Individual lipid molecules are free to diffuse laterally in the surface of the bilayer.

D. The bilayer is stabilized by covalent bonds between neighboring phospholipid molecules.

E. The polar head groups face inward toward the inside of the bilayer.

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Yeast strain X has ergosterol in its membranes, while yeast strain Z does not incorporate ergosterol into its membranes. These two yeast strains growing at 30ºC werew shifted to 20ºC. Based upon what you know about the function of cholesterol inanimal cell membranes, you would predict:

A. The membrane fluidity of strains X and Z would initially increase at the lower temperature.

B. At 20ºC, the membrane fluidity of strain Z would be greater than the membrane fluidity of strain X.

C. At 20ºC, the membrane fluidity of strain X would be greater than the membrane fluidity of strain Z.

D. A & B are correct.

E. A & C are correct.

Watch Solution

Why are membranes referred to as Fluid Mosaics? What does this mean at a molecular level?

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Which of the following statements are true? Select all correct answers. 

a. A plant that produces more monounsaturated fatty acids than polyunsaturated fatty acids in its membranes in winter has an increased resistance to freeze damage. 

b. Cell membranes in cold-tolerant winter wheat plants have a higher ratio of unsaturated fatty acids to saturated fatty acids than do cold-intolerant wheat varieties. 

c. Yeast cells that produce more unsaturated fatty acids than saturated fatty acids in response to cold have greater cold tolerance. 

d. Cell membranes in reindeer legs (near the hooves) are kept flexible because they have a large number of saturated fatty acids.


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It is said that ribonucleotides can diffuse through some types of liposomes. It is likely that the lipids present early in chemical evolution had short chains. Would liposomes formed from these types of lipids be more or less permeable to ribonucleotides than if early cells formed from long-chained lipids?

A) same permeability

B) less permeable

C) more permeable 

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When phospholipids are mixed with water their ____ interact with water and their ____ are repelled, forming a lipid bilayer. 

a. Hydrophobic heads, hydrophilic tails

b. Hydrophobic tails, hydrophilic heads

c. Hydrophilic heads, hydrophobic tails

d. Hydrophilic tails, hydrophobic heads

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A cell membrane is not: 

a. A fluid structure

b. A rigid structure

c. A mosaic structure

d. A bilayered structure

e. Composed of phospholipid molecules

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A phospholipid molecule: 

a. Has a hydrophilic head that is repelled by water

b. Has hydrophilic tails that are repelled by water

c. Has a hydrophilic head that is attracted to water

d. Has a hydrophobic head that is repelled by water

e. Has hydrophobic tails that are attracted to water

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The primary structural component of a cell membrane is: 

a. A cholesterol molecule

b. A protein molecule

c. A phospholipid molecule

d. An enzyme

e. A sugar molecule attached to a protein

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The molecules that convert extracellular signals into intracellular ones are

a.  neurotransmitters

b.  peptide hormones

c.  cell surface receptors

d.  growth factor proteins

e.  genes

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The Fluid Mosaic Model proposed by Singer and Nicolson in 1972 included that

a. The cell membrane was composed of lipids and proteins.

b. The cell membrane was composed of a phospholipid bilayer between two layers of globular proteins.

c. The cell membrane was composed of a phospholipid bilayer with globular proteins actually inserted into the bilayer.


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Cholesterol functions in the plasma membrane to

a. transport ions

b. serve as an energy molecule

c. regulate membrane fluidity

d. mediate steroid action


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The following are all functions of a typical plasma membrane except

a. transport of water and bulk material

b. selective transport of certain molecules and material

c. reception of information

d. expression of cellular identity

e. be permanent in composition

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On the outer surface of the plasma membrane there are marker molecules that identify the cell-type. Often these molecules are

a. ATP

b. amino acids

c. nucleotides

d. carbohydrate chains

e. inorganic ions

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The fluid nature of the membranes is attributed to a lateral movement of

a. protein channels

b. phospholipid molecules

c. antigen molecules

d. pumps such as the proton pump

e. the entire lipid bilayer

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Which of the following properties is not true of membrane phospholipids?

a.  The hydrophobic tails are oriented towards the interior.

b.  The hydrophilic heads are oriented towards the exterior.

c.  Once they are incorporated they remain in the membrane permanently.

d.   The bilayers made up of them are randomly interspersed with proteins.

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Membrane proteins are not very soluble in water, because they possess long stretches of non-polar amino acids that

a. are too long to interact with the water molecules

b. are hydrophobic

c. are transmembranal

d. are hydrophilic

e. serve as transport channels

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The plasma membrane is a thin sheet of lipid embedded with larger molecules

a. called proteins

b. called polymers

c. called nucleotides

d. of sodium and potassium ions

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The proteins of the plasma membrane are in large part responsible for the cell's ability to interact with its environment. They act as or are involved in all of the following except

a. channel

b. recognition

c. reception

d. transport

e. packaging (histones)

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When phospholipid molecules are placed into water, microspheres form spontaneously. These microspheres are called:

a. protons

b. archaebacteria

c. eukaryotes

d. coacervates

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