Two theories of membrane structure are discussed below. Unit Membrane Model Phospholipid molecules are the most abundant type of lipids in cell membranes. Each consists of a nonpolar, hydrophobic tail, and a polar, hydrophilic head. The phospholipid chains orient themselves to form a bilayer with the tails pointed inward and the hydrophilic heads lined up on both sides of the bilayer. An extended monomolecular layer of protein coats both the outside of the membrane and the inside of the bilayer, but the protein does not penetrate the bilayer. When thin slices of membranes were examined with an electron microscope, three layers were seen: two electron-dense layers, with one wider layer between them How does the Unit Membrane Model differ from the Fluid Mosaic Model? O A. The location of proteins differs in the two models. OB. The Unit Membrane Model has a monomolecular layer of protein on each surface, while the Fluid Mosaic Model has a bimolecular layer of protein on each surface. O C. The Unit Membrane Model has one layer of phospholipids, while the Fluid Mosaic Model has two layers. O D. The Unit Membrane Model contains dissolved protein, while the Fluid Mosaic Model is coated with a monomolecular layer of protein on each surface.

Biochemistry
9th Edition
ISBN:9781319114671
Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.
Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.
Chapter1: Biochemistry: An Evolving Science
Section: Chapter Questions
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Two theories of membrane structure are
discussed below.
Unit Membrane Model
Phospholipid molecules are the most abundant
type of lipids in cell membranes. Each consists of a
nonpolar, hydrophobic tail, and a polar, hydrophilic
head. The phospholipid chains orient themselves to
form a bilayer with the tails pointed inward and the
hydrophilic heads lined up on both sides of the
bilayer. An extended monomolecular layer of protein
coats both the outside of the membrane and the
inside of the bilayer, but the protein does not
penetrate the bilayer.
When thin slices of membranes were examined
with an electron microscope, three layers were seen:
two electron-dense layers, with one wider layer
between them.
It was believed that the electron-dense layers
represented proteins, and the lighter area
represented lipids.
Fluid Mosaic Model
The structure of the fluid mosaic membrane is a
bilayer of phospholipid molecules with proteins
inserted into the bilayer. Hydrophilic regions of both
the lipids and proteins are found on the outside
surface, while the hydrophobic regions of both types
of molecules are found on the inside surface.
Because lipids are not fixed rigidly in the membrane,
the membrane is capable of fluid movement.
The theory that the proteins penetrate the lipid
layer is supported when membranes are split along
the middle of the lipid layer. When the top half is
peeled back, small bumps can be seen on the
remaining surface. These are assumed to be
proteins.
Which of the following observations would invalidate the Unit
Membrane Model?
O A. When a thin section of membrane is observed using a
microscope at high magnification, two layers of
phospholipid heads are observed.
OB. Thermodynamic measurements indicate that the
phospholipid heads are exposed to water.
O C. When a membrane is frozen and then split from
surface to surface, proteins are observed within the
hydrocarbon chains.
O D. Thermodynamic measurements indicate that the
phospholipid tails are hidden inside the membrane,
away from exposure to water.
If the small bumps seen when half of the membrane is
peeled away were chemically shown to consist of the
lipid cholesterol, how would the Fluid Mosaic Model
have to be modified?
O A. The proteins would have to be embedded less than
halfway through the membrane.
O B. There could be no proteins in the membrane.
O C. The lipids would have to be embedded in the proteins.
O D. It would not necessarily have to be altered, but there
would be less evidence supporting it.
Which of the following is a weakness in the Unit Membrane
Model?
O A. The Unit Membrane Model does not define the cell's
size.
OB. The Unit Membrane Model does not suggest how the
exchange of nutrients and wastes occurs.
O C. The Unit Membrane Model does not act as a barrier
between the conter of the cell and the cell's external
environment.
O D. The hydrocarbon bilayer allows passage of polar
molecules through the membrane.
How does the Unit Membrane Model differ from the Fluid
Mosaic Model?
O A. The location of proteins differs in the two models.
OB. The Unit Membrane Model has a monomolecular layer
of protein on each surface, while the Fluid Mosaic
Model has a bimolecular layer of protein on each
surface.
O C. The Unit Membrane Model has one layer of
phospholipids, while the Fluid Mosaic Model has two
layers.
O D. The Unit Membrane Model contains dissolved protein,
while the Fluid Mosaic Model is coated with a
monomolecular layer of protein on each surface.
Transcribed Image Text:Two theories of membrane structure are discussed below. Unit Membrane Model Phospholipid molecules are the most abundant type of lipids in cell membranes. Each consists of a nonpolar, hydrophobic tail, and a polar, hydrophilic head. The phospholipid chains orient themselves to form a bilayer with the tails pointed inward and the hydrophilic heads lined up on both sides of the bilayer. An extended monomolecular layer of protein coats both the outside of the membrane and the inside of the bilayer, but the protein does not penetrate the bilayer. When thin slices of membranes were examined with an electron microscope, three layers were seen: two electron-dense layers, with one wider layer between them. It was believed that the electron-dense layers represented proteins, and the lighter area represented lipids. Fluid Mosaic Model The structure of the fluid mosaic membrane is a bilayer of phospholipid molecules with proteins inserted into the bilayer. Hydrophilic regions of both the lipids and proteins are found on the outside surface, while the hydrophobic regions of both types of molecules are found on the inside surface. Because lipids are not fixed rigidly in the membrane, the membrane is capable of fluid movement. The theory that the proteins penetrate the lipid layer is supported when membranes are split along the middle of the lipid layer. When the top half is peeled back, small bumps can be seen on the remaining surface. These are assumed to be proteins. Which of the following observations would invalidate the Unit Membrane Model? O A. When a thin section of membrane is observed using a microscope at high magnification, two layers of phospholipid heads are observed. OB. Thermodynamic measurements indicate that the phospholipid heads are exposed to water. O C. When a membrane is frozen and then split from surface to surface, proteins are observed within the hydrocarbon chains. O D. Thermodynamic measurements indicate that the phospholipid tails are hidden inside the membrane, away from exposure to water. If the small bumps seen when half of the membrane is peeled away were chemically shown to consist of the lipid cholesterol, how would the Fluid Mosaic Model have to be modified? O A. The proteins would have to be embedded less than halfway through the membrane. O B. There could be no proteins in the membrane. O C. The lipids would have to be embedded in the proteins. O D. It would not necessarily have to be altered, but there would be less evidence supporting it. Which of the following is a weakness in the Unit Membrane Model? O A. The Unit Membrane Model does not define the cell's size. OB. The Unit Membrane Model does not suggest how the exchange of nutrients and wastes occurs. O C. The Unit Membrane Model does not act as a barrier between the conter of the cell and the cell's external environment. O D. The hydrocarbon bilayer allows passage of polar molecules through the membrane. How does the Unit Membrane Model differ from the Fluid Mosaic Model? O A. The location of proteins differs in the two models. OB. The Unit Membrane Model has a monomolecular layer of protein on each surface, while the Fluid Mosaic Model has a bimolecular layer of protein on each surface. O C. The Unit Membrane Model has one layer of phospholipids, while the Fluid Mosaic Model has two layers. O D. The Unit Membrane Model contains dissolved protein, while the Fluid Mosaic Model is coated with a monomolecular layer of protein on each surface.
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