The plasma membrane is involved in which activities?
(Choose one or more)
- import and export of nutrients and wastes
- cell recognition
- RNA interference
- cell signaling
- DNA replication and repair
- cell growth and motility
- Import and export of nutrients and wastes
- Cell recognition
- Cell signaling
- Cell growth and motility
(The cell membrane is indeed involved in cell signaling and recognition, growth and motility, and the import of nutrients and export of wastes.
The plasma membrane is not involved in DNA replication and repair or in the gene-silencing technique of RNA interference.)
Which characteristic describes the tails of phospholipids?
- coated with sugars
- hydrophobic
- stiff
- amphipathic
- hydrophilic
Hydrophobic
(The hydrocarbon tails of phospholipids tend to avoid contact with water, which helps drive the formation of the lipid bilayer.)
Which term correctly describes the entire phospholipid molecule?
- hydrophobic
- apathetic
- hydrophilic
- hydropathic
- amphipathic
Amphipathic
(Phospholipids contain both a hydrophilic and hydrophobic component and are therefore amphipathic. This property allows them to form bilayers in water, where the hydrophilic portions interact with the aqueous environment on either side of the membrane, while the hydrophobic portions are shielded from water in the bilayer’s interior.)
Why do phospholipids form bilayers in water?
- The hydrophilic head is insoluble in water.
- The hydrophobic head is attracted to water, while the hydrophilic tail shuns water.
- The hydrophobic head shuns water, while the hydrophilic tail is attracted to water.
- The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.
- The hydrophobic tail is attracted to water, while the hydrophilic head shuns water.
The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.
(The hydrophilic head can form electrostatic attractions and hydrogen bonds with water, while the hydrophobic tails are insoluble in water.)
In a lipid bilayer, where do lipids rapidly diffuse?
- in and out of the bilayer
- within the plane of one monolayer and back and forth between the monolayers
- within the plane of their own monolayer
- back and forth from one monolayer to the other in the bilayer
- not at all, because they remain in place within the bilayer
Within the plane of their own monolayer
(The lipid bilayer is a two-dimensional fluid in which phospholipids rapidly diffuse within the plane of their own monolayer.)
Which of the following would produce the most fluid lipid bilayer?
- phospholipids with fully saturated tails of 20 carbon atoms
- phospholipids with tails of 18 carbon atoms and two double bonds
- large amounts of cholesterol
- phospholipids with tails of 20 carbon atoms and two double bonds
- phospholipids with tails of 18 carbon atoms and two double bonds
Phospholipids with tails of 18 carbon atoms and two double bonds.
(A shorter chain length and double bonds both reduce the tendency of the phospholipid tails to interact with one another, thereby increasing the fluidity of the membrane.)
How does the inclusion of cholesterol affect animal cell membranes?
- It tends to make the lipid bilayer less fluid.
- It makes the lipid bilayer wider.
- It has little effect on the properties of the lipid bilayer.
- It makes the lipid bilayer more permeable.
- It tends to make the lipid bilayer more fluid.
It tends to make the lipid bilayer less fluid.
This stiffening makes the bilayer less flexible as well as less permeable.
In eukaryotic cells, phospholipids are synthesized by enzymes bound to which of the following?
- the cytosolic face of the plasma membrane
- the cytosolic face of the endoplasmic reticulum
- the inside of the endoplasmic reticulum
- the cytosolic face of the Golgi apparatus
- both monolayers of the endoplasmic reticulum
The cytosolic face of the endoplasmic reticulum.
(New phospholipids are added to the ER membrane asymmetrically. Some of the newly made phospholipids are subsequently moved from the cytosolic monolayer to the other half of the bilayer so that the membrane can grow evenly.)
One of the grand challenges in biology is understanding how the first cells formed on Earth. Since all cells are bound by a cell membrane, origin of life researchers are interested in modeling what the first membranes may have been like. What types of molecules might these researchers consider to be the original building blocks of cell membranes?
- hydrophobic molecules
- carbohydrate molecules
- hydrophilic molecules
- amphipathic molecules
Amphipathic molecules
(Amphipathic molecules, with both hydrophilic and hydrophobic regions, spontaneously form bilayers in aqueous solutions. A 2012 Chem. Soc. Rev. article discusses the different amphipathic molecules researchers use to model early cells.)
Which of the following would be most likely to disrupt lipid bilayer formation?
- addition of a phosphate to the end of the lipid tail
- addition of a methyl group to the end of the lipid tail
- addition of a hydroxyl group to the head group of the lipid
- addition of cholesterol to the membrane
Addition of a phosphate to the end of the lipid tail
(Addition of a negatively charged phosphate to the hydrophobic lipid tail would likely disrupt the formation of the lipid bilayer.)
Imagine you collected bacteria from the sediment in a frozen lake in Minnesota in January and compared the membranes to membranes from bacteria collected from a lake in Texas in June. Consider how the membranes would likely differ.
The membranes in bacteria from the Minnesota lake would most likely have which of the following?
- more unsaturated lipid tails than membranes in Texas bacteria
- more saturated lipid tails than membranes in Texas bacteria
- phospholipids with more negatively charged phosphate groups than membranes in Texas bacteria
- fewer lipid tails with cis double bonds than membranes in Texas bacteria
More unsaturated lipid tails than membranes in Texas bacteria.
(Unsaturated lipid tails with cis double bonds are kinked and pack less tightly than saturated lipids. The bacteria in a cold environment will have more of the unsaturated lipid tails to maintain fluidity even in cold temperatures.)
Which of the following is a function of proteins in the plasma membrane?
Choose one or more:
- generate the energy required for lipids to diffuse within the membrane
- allow specific ions to cross the plasma membrane, thereby controlling its electrical properties
- transport molecules across the membrane
- transmit extracellular signals to the cell interior
- serve as anchors to attach the cell to the extracellular matrix
- Allow specific ions to cross the plasma membrane, thereby controlling its electrical properties
- Transport molecules across the membrane
- Transmit extracellular signals to the cell interior
- Serve as anchors to attach the cell to the extracellular matrix
(Membrane proteins serve many functions. Some transport particular nutrients, metabolites, and ions across the lipid bilayer. Others anchor the membrane to macromolecules on either side. Still others function as receptors that detect chemical signals in the cell’s environment and relay them into the cell interior, or work as enzymes to catalyze specific reactions at the membrane. Each type of cell membrane contains a different set of proteins, reflecting the specialized functions of the particular membrane.)
In the α helices of transmembrane proteins, the hydrophobic side chains face which direction?
- the outside of the membrane-spanning helix
- the external or lumenal side of the membrane
- the inside of the membrane-spanning helix
- the cytosolic side of the membrane
The outside of the membrane-spanning helix
(This arrangement allows the exposed hydrophobic side chains of the α helix to interact with the hydrophobic tails of the lipid bilayer.)
Porin proteins—which form large, water-filled pores in mitochondrial and bacterial outer membranes—fold into β-barrel structures. The amino acids that face the outside of the barrel have what kind of side chains?
- hydrophobic
- charged
- hydrophilic
- polar
- amphipathic
Hydrophobic
(These hydrophobic side chains interact with the hydrophobic tails within the lipid bilayer. This arrangement allows the protein, which also contains hydrophilic amino acids and a hydrophilic peptide backbone, to penetrate the hydrophobic environment of the membrane.)
Which statements are true about the differences between phospholipids and detergents?
Choose one or more:
- Phospholipids are amphipathic, whereas detergents are hydrophobic.
- Phospholipids are hydrophobic, whereas detergents are amphipathic.
- Phospholipids form bilayers in water, whereas detergents tend to form micelles.
- Phospholipids have two hydrocarbon tails, whereas detergents have just one.
- Detergents are shaped like cones, whereas phospholipids are more cylindrical.
- Phospholipids form bilayers in water, whereas detergents tend to form micelles.
- Phospholipids have two hydrocarbon tails, whereas detergents have just one.
- Detergents are shaped like cones, whereas phospholipids are more cylindrical.
(Detergents differ from membrane phospholipids in that they have only a single hydrophobic tail. Because they have one tail, detergent molecules are shaped like cones; in water, this shape drives these amphipathic molecules to form small clusters called micelles, rather than forming a bilayer as do the phospholipids, which—with their two tails—are more cylindrical.)
The shape of a cell and the mechanical properties of its plasma membrane are determined by a meshwork of fibrous proteins called what?
- lamellipodium
- basal lamina
- cell cortex
- tight junction
- glycocalyx
Cell cortex
This meshwork of protein filaments is attached to the underside of the plasma membrane.
On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located?
- the underside
- both sides
- the extracellular side
- the cytosolic side
- the inside
The extracellular side
(The sugars on plasma membrane glycolipids, glycoproteins, and proteoglycans all face the cell exterior, where they form a carbohydrate layer or glycocalyx that coats the surface of the cell.)
How thick is the plasma membrane?
- 50 nm
- 50 meters
- 50 μm
- 50 mm
- 50 atoms
50 atoms
The plasma membrane is so thin it cannot be seen directly even with a light microscope.
What is a functionally specialized region of a cell membrane, typically characterized by the presence of specific proteins, called?
- membrane domain
- glycocalyx
- carbohydrate layer
- cell cortex
- sphingomyelin domain
Membrane domain
(Membrane domains are generated when cells restrict the movement of certain membrane proteins to localized areas within a cell membrane.)
Multipass transmembrane proteins can form pores across the lipid bilayer. The structure of one such channel is shown in the diagram.
In this figure, what do the areas shown in red represent?
- the hydrophobic side chains of the transmembrane α helices
- the hydrophilic side chains of the transmembrane α helices
- the hydrophobic side chains of the transmembrane β barrel
- the amphipathic side chains of the transmembrane α helices
- the hydrophobic lipid tails of the bilayer
- the hydrophilic side chains of the transmembrane β barrel
The hydrophilic side chains of the transmembrane α helices
(Small water-soluble molecules can pass through the water-filled pore formed by the hydrophilic side chains of the transmembrane helices, shown in red.)
How does the cortex of a typical animal cell differ from that of a mature red blood cell?
Choose one or more:
- It is richer in actin and in the motor protein myosin.
- It allows the cell to selectively take up material from the environment.
- It allows the cell to move.
- It is simpler than that of a red blood cell.
- It gives the animal cell a distinctive biconcave shape.
- It is richer in actin and in the motor protein myosin.
- It allows the cell to selectively take up material from the environment.
- It allows the cell to move.
(Whereas red blood cells need their cortex mainly to provide mechanical strength as they are pumped through blood vessels, other animal cells also use their cortex to selectively take up materials from their environment, to change their shape, and to move.)
When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol?
- It depends on whether the vesicle is coming from the endoplasmic reticulum or the Golgi apparatus.
- The blue monolayer will face the cytosol.
- Half the time the orange monolayer will face the cytosol, and half the time the blue monolayer will face the cytosol.
- It depends on the cargo the vesicle is carrying.
- The orange monolayer will face the cytosol.
The orange monolayer will face the cytosol.
(The cytosolic monolayer will always face the cytosol, whether the vesicle is moving between organelles or fusing with the plasma membrane.)
Animals exploit the phospholipid asymmetry of their plasma membrane to distinguish between live cells and dead ones. When animal cells undergo a form of programmed cell death called apoptosis, phosphatidylserine—a phospholipid that is normally confined to the cytosolic monolayer of the plasma membrane—rapidly translocates to the extracellular, outer monolayer. The presence of phosphatidylserine on the cell surface serves as a signal that helps direct the rapid removal of the dead cell.
How might a cell actively engineer this phospholipid redistribution?
- by boosting the activity of a flippase in the plasma membrane
- by inverting the existing plasma membrane
- by inactivating a scramblase in the plasma membrane
- by activating a scramblase and inactivating a flippase in the plasma membrane
- by inactivating both a flippase and a scramblase in the plasma membrane
By activating a scramblase and inactivating a flippase in the plasma membrane
(During programmed cell death (apoptosis), the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other is fully activated. This causes phosphatidylserine—initially deposited in the cytosolic monolayer—to become distributed to both halves of the bilayer. At the same time, the flippase that would normally transfer phosphatidylserine from the extracellular monolayer to the cytosolic monolayer is inactivated. Together, this causes phosphatidylserine to rapidly accumulate at the cell surface.)
True or False:
The phosphate group in a membrane phospholipid head always carries the negative charge.
True
