Test 2 Ch 11 Flashcards
fluid mosaic model
made up of lots of pieces such as phospholipids, proteins, etc.. that are fluid and move around
major membrane components (from least to most abundant)
-Lipids (made up of phospholipids)
-Protein
-Carbohydrates (only found outside of membrane)
-Cholesterol
lipids are
amphipathic
amphipathic
Molecule with both hydrophobic
and hydrophilic properties
Structure of lipid
They are hydrophilic
- They have a glycerol backbone
-Phosphate group attached to 3rd carbon (charged)
Types of lipids
Types of lipids vary by
chemical structure attached to
phosphate (charged or polar)
-Choline=phosphotidylcholine
-Serine=phosphotidylserine
- Others: ethanolamine, inositol
Glycolipids have sugar attached directly to glycerol insteaf of phosphate
Structure of lipid
Hydrophobic
2 hydrocarbon (fatty acid) tails
-Usually one is saturated, and one has one unsaturated carbon bond
formation of a spherical lipid bilayer is
spontaneous
-In a planar phospholipid bilayer, hydrophobic tails (white layer) are exposed to water along the edges which is energetically unfavorable.
The formation of a sealed compartment shields hydrophobic tails forming an enclosed structure(called liposomes) from water which is energetically favorable
-Also, the formation of a spear is the lowest free energy form so thats why they occur spontaneously
liposomes
lipid spheres, spontaneously formed in water
3 characteristics of a lipid bilayer
- Fluidity
- Asymmetry
- Selective Permeability
Describe the fluidity of lipids
Lateral diffusion, rotation and flexion are continuous
Flip-flop: moving phospholipid from one bilayer to the other bilayer is rare.
lateral diffusion
across one side of the bilayer
flip-flop (rarely occurs)
any phospholipid moves from one space to another it is energetically unfavorable
Factors that influence fluidity
- Length of hydrocarbon tail
its 14-24 carbons long
Shorter is more fluid
-Degree of saturation of tail
More unsaturated (double bond) is more fluid
More saturated (only single bonds) is less fluid
-Temperature
Higher temperature is more fluid
Some bacteria are able to adjust the length of the hydrocarbon tail and the degree of saturation to maintain constant fluidity at different temperatures
In animal cells ONLY, cholesterol keeps degree of fluidity constant
Cholesterol
Short, rigid amphipathic lipid in animal cells, interacts with lipid tails, acts as fluidity buffer
- Acts as a buffer that prevents cholesterol from binding too close together.
Importance of membrane fluidity
-Allows proteins to diffuse and interact with each other
-Distribution of proteins and lipids from point of insertion
-Ensure even distribution of proteins and lipids to daughter cells
- If packed too tightly it becomes fragile if its too far then it can be pulled apart more easily
What characteristics of lipid tails would you expect to find in a bacterium living in the Yellowstone hot springs?
A. Long and more saturated
B. Long and less saturated
C. Short and more saturated
D. Short and less saturated
A. Long and more saturated to combat the extreme warm temperature it does the opposite
Lipid bilayer Asymmetry
If you were to look at the lipid bilayer there’s a different composition of phospholipids on either sides
Theres also the extracellular side is the Non cytosolic face/side and the intracellular side is the cytosolic side since its in contact with cytosol.
carbohydrates are only added to
the noncytosolic side/face, the extracellular side
lipid movement from ER to Plasma Membrane
Synthesis in the ER:
Lipids (phospholipids, cholesterol, sphingolipids) are synthesized in the smooth endoplasmic reticulum (ER).
Packaging into Vesicles:
After synthesis, lipids are packed into vesicles that bud off from the ER. This step is aided by COPII proteins that help form the vesicles.
Transport to the Golgi:
The vesicles travel to the Golgi apparatus via the cytoskeleton, using motor proteins (like dynein and kinesin) for movement.
Sorting in the Golgi:
In the Golgi, lipids may undergo modifications (like glycosylation) and are sorted into vesicles that will be sent to specific locations, including the plasma membrane.
Vesicle Transport to the Plasma Membrane:
Lipid-containing vesicles travel along microtubules to the plasma membrane, assisted by motor proteins (like kinesin).
Vesicle Fusion with Plasma Membrane:
The vesicle fuses with the plasma membrane through SNARE proteins, incorporating the lipids into the membrane.
Incorporation into the Plasma Membrane:
After fusion, the lipids become part of the plasma membrane, contributing to its structure and function.
Key Points to Remember:
COPII proteins: Help in vesicle formation from the ER.
Motor proteins (dynein/kinesin): Transport vesicles along the cytoskeleton.
SNARE proteins: Facilitate vesicle fusion with the plasma membrane.
The process ensures that lipids reach the plasma membrane, maintaining membrane structure and function.
Generation of lipid asymmetry
- All phospholipids (PL) added to membranes on cytosolic face of ER. Scramblase moves random PL to non-cytosolic face to balance two sides of membrane.
- PL delivered from ER to Golgi, Flippases move selected types of PL to cytosolic face.
- Carbohydrates only added to PL in ER and Golgi lumen, thus only found on non-cytosolic face.
- PL travel via vesicles from ER to Golgi to plasma membrane maintaining two faces. Non-cytosolic face is in the lumen of the ER and Golgi and is extracellular on the plasma membrane.
In selective permeability
-Small, Nonpolar Molecules (Green Arrow - Easily Permeable):
Examples: O₂, CO₂, N₂, and steroid hormones
These molecules can diffuse freely through the membrane because they are nonpolar and can dissolve in the hydrophobic bilayer core
-Small, Uncharged Polar Molecules (Blue Arrow - Partially Permeable)
Examples: H₂O, ethanol, glycerol
These molecules can pass through the membrane, but at a slower rate compared to nonpolar molecules.
-Larger Uncharged Polar Molecules (Red Arrow - Poorly Permeable)
Examples: Amino acids, glucose, nucleosides
These molecules struggle to pass through the membrane because they are larger and hydrophilic.
-Ions (Yellow Arrow - Impermeable)
Examples: H⁺, Na⁺, K⁺, Ca²⁺, Cl⁻, Mg²⁺, HCO₃⁻
Ions cannot pass through the lipid bilayer on their own due to their charge; they require transport proteins (channels or carriers).
3 characteristics of the lipid bilayer
- Fluidity
- Lateral movement within one face
- Applies to lipids and proteins - Asymmetry
-Different lipid types on cytosolic and non-cytosolic faces
- Sugars only on non-cytosolic face - Selective Permeability
- Only small hydrophobic molecules diffuse freely through membrane
Membrane proteins
2nd most abundant component of
membranes
50% of dry mass of membrane
Protein Functions in membranes:
- Transporters and ion channels
such as glucose transporter
- Anchors (some proteins act as anchors to the cytoskeleton)
such as integrins (helps with stability)
-Receptors
such as Insulin receptor
-Enzymes
such as adenylyl cyclase (involved in cell signaling)
4 ways proteins associate with the membrane is
transmembrane: These proteins span the entire lipid bilayer. They have hydrophobic regions that interact with the membrane’s interior and hydrophilic regions that extend into the aqueous environment.
Example: Ion channels, receptors (e.g., GPCRs)
monolayer-associated: These proteins are anchored to only one side of the lipid bilayer. They have a hydrophobic region embedded in one leaflet of the bilayer, while the rest of the protein extends into the cytosol.
Example: Some enzymes and signaling proteins
lipid-linked: These proteins are covalently attached to a lipid molecule, which is embedded in the bilayer.
The lipid anchor helps tether the protein to the membrane, but the protein itself does not directly interact with the bilayer.
Example: GPI-anchored proteins
protein-attached: These proteins do not interact with the membrane directly but bind to transmembrane or lipid-linked proteins via noncovalent interactions.
They can easily be removed without disrupting the bilayer.
Example: Some cytoskeletal proteins, signaling molecules
integral vs peripheral membrane proteins
Integral Membrane Proteins (Embedded in the membrane)
Definition: These proteins are permanently attached to the membrane and require detergents to remove them.
Types:
Transmembrane proteins → Fully span the membrane (e.g., ion channels, receptors).
Monolayer-associated proteins → Embedded in only one leaflet of the bilayer.
Lipid-linked proteins → Covalently attached to a lipid that is part of the membrane.
Peripheral proteins
Definition: These proteins are not embedded in the membrane; instead, they attach to integral proteins or the lipid bilayer through noncovalent interactions (e.g., hydrogen bonds, ionic interactions).
Type:
Protein-attached proteins → Bind indirectly to the membrane via interactions with integral proteins.
integral
require detergent to remove them
peripheral
require only heat, pH change, or salt change to remove membrane
How detergents help remove integral membrane proteins from the lipid bilayer by solubilizing them.
Membrane Protein in Lipid Bilayer (Left Side)
The green structure represents an integral membrane protein spanning the bilayer.
These proteins are embedded in the membrane due to their hydrophobic regions, which interact with the hydrophobic lipid tails.
Detergent Molecules (Middle)
Detergents are amphipathic molecules (they have both hydrophilic heads and hydrophobic tails).
When added, detergent monomers form micelles (small clusters where hydrophobic tails face inward and hydrophilic heads face outward).
Disrupting the Membrane (Right Side)
The detergent molecules interact with the hydrophobic regions of both the lipid bilayer and membrane protein.
This forms a water-soluble protein-lipid-detergent complex, effectively pulling the protein out of the membrane.
The remaining lipids also mix with detergent molecules, forming lipid-detergent micelles.
Key Takeaways:
Detergents help solubilize integral membrane proteins, allowing them to be studied in isolation.
They work by disrupting hydrophobic interactions, breaking apart the lipid bilayer.
This is useful for biochemical studies of membrane proteins, such as structural analysis or functional assays.
If you treat an intact cell membrane with a mild acid and pellet the cells in a centrifuge, the liquid portion (supernatant) contains proteins. These proteins were associated with the membrane
as____________
A. integral proteins on the extracellular face
B. peripheral proteins on the cytosolic face
C. peripheral proteins on the extracellular face
D. integral proteins on the cytosolic face
E. integral protein passing through the membrane and
exposed to both the cytosolic and extracellular face.
C. peripheral proteins on the extracellular face
conformations of transmembrane proteins
alpha helix and beta barrel
alpha helix
Key Features:
Peptide bonds are polar, but hydrogen bonding within the helix hides them from the hydrophobic membrane interior.
-Hydrophobic side chains interact with the lipid bilayer, stabilizing the protein.
-The α-helix is the most common transmembrane structure.
Amphipathic Helices:
One side polar (hydrophilic), one side non-polar (hydrophobic).
Every few amino acids are hydrophobic, allowing interaction with membrane lipids.
Function: Multiple amphipathic helices can form channels or pores for molecule transport.
B barrel
Key Features:
-Made of β-sheets that fold into a cylindrical shape.
-Forms a hollow pore or channel across the membrane.
-Hydrophobic outer surface interacts with the lipid bilayer.
-Hydrophilic inner surface allows passage of polar molecules.
Function:
Found in bacteria, mitochondria, and chloroplasts.
Acts as porins for selective molecule transport.
Comparison to α-Helix:
More rigid than α-helices.
Larger pores, ideal for passive diffusion.
The structure of the polypeptide of a
transmembrane protein where it
crosses the membrane must be:
A. globular
B. ⍺ helix
C. beta barrel
D. A or B
E. B or C
E. B or C it has to form either alpha or beta which are both secondary structures
Experiment Showing Membrane Protein Movement
Overview:
Frye & Edidin (1970) experiment demonstrated that membrane proteins move within the lipid bilayer, supporting the fluid mosaic model.
Experimental Steps:
Labeling Proteins:
Mouse cell: Membrane proteins labeled with rhodamine (red).
Human cell: Membrane proteins labeled with fluorescein (blue).
Cell Fusion:
Mouse and human cells were fused to create a hybrid cell.
At time = 0 min, red and blue proteins remained in separate halves of the hybrid cell membrane.
Incubation at 37°C:
After 40 minutes, red and blue proteins became evenly mixed, proving that membrane proteins move freely within the bilayer.
Key Takeaways:
✔ Membrane proteins are fluid and can move laterally.
✔ Supports the fluid mosaic model of biological membranes.
✔ Protein movement depends on temperature and membrane composition.
limiting protein movement
- Attach to Interior Cell Cortex – Proteins anchor to the cytoskeleton inside the cell, restricting movement.
- Attach to Extracellular Matrix – Proteins tether to external molecules, limiting diffusion.
- Attach to Adjacent Cell Proteins – Proteins interact with neighbors, keeping them in place.
- Tight Junctions Block Mobility – Tight junctions create barriers, preventing protein movement across regions. They are junctions between the cells that don’t let anything pass
This keeps proteins in the right locations for proper function!
cell cortex
Network of proteins on cytoplasmic side of membrane making framework
cell cortex
Just below the cell membrane consists of sceptrin and actin
Functions:
- Strength
-Change cell shape during movement
- Many transmembrane proteins attached to cell cortex
Spectrin (gives red blood cells their concave shape) and actin
- Long thin flexible rod-shaped protein
- Gives RBC concave shape
- Spectrin mutations
-RBC break in capillaries
-Extreme anemia
Carbohydrates
-3rd most abundant component of membranes
*All carbohydrates are added to lipids or membrane
proteins in lumen of ER and Golgi, thus only on non-cytosolic side of membrane and they move to the plasma membrane and end up extracellular
*Total carbohydrates= glycocalyx
*Function:
*Protects
*Lubricates: absorbs water to protect from mechanical damage
*Cell-cell recognition: ex- sperm and egg mostly use carbohydrates to determine they’re different
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Glycocalyx – Carbohydrate-Rich Cell Coating
Structure:
Made of glycoproteins, glycolipids, and proteoglycans with attached sugar units.
Covers the cell membrane (lipid bilayer).
Functions:
Cell Recognition – Helps immune system identify cells.
Protection – Shields cell from damage.
Signaling – Aids in cell communication.
Adhesion – Helps cells stick to surfaces and each other
Why are carbohydrates found only on the extracellular (non-cytosolic face) of the plasma membrane lipid bilayer?
A. Carbohydrates are added to proteins and lipids by enzymes in the extracellular space.
B. Carbohydrates on the cytosolic face of the plasma membrane are removed by enzymes.
C. Carbohydrates are added to proteins and lipids on the non-cytosolic face in the ER lumen before travelling to the plasma membrane by transport vesicles.
D. Carbohydrates are added to proteins and lipids on
the cytosolic of the plasma membrane, then “flipped” by enzymes to the non-cytosolic face.
C. Carbohydrates are added to proteins and lipids on
the non-cytosolic face in the ER lumen before travelling to the plasma membrane by transport
vesicles.
carbohydrates are found in the non-cytosolic part of the vesicle because that’s where correct enzymes are
Membrane carbohydrates are located on the
extracellular side of the plasma membrane only
Membrane proteins are fluid, but..
there are mechanisms to anchor proteins when needed
Membrane proteins cross the lipid bilayer by
folding to mask polar peptide bonds and presenting hydrophobic amino acids on the surface
the hydrophobic region of the phospholipid bilayer presents ____ to most molecules
a barrier
remember this
Membrane structure and function is a direct outgrowth of the
interaction of membrane components with each other and with the aqueous environment.
do the homework assignment
escribe the function of the following enzymes in ultimately creating asymmetry in
the lipids of the plasma membrane.
a. Scramblase
b. Flippase
scramblase: catalyzes transfer of random phospholipids from one monolayer to another in the ER
Flippase: catalyzes transfer of a specific phospholipids to cytosolic monolayer in the golgi
do membrane proteins homework
Explain why transmembrane proteins can only have an helix or a ß barrel at the positions
where the protein crosses the lipid bilayer.
This is because the peptide bonds are polar. the alpha and beta are secondary structures formed H-bonding and H-bonding allows these structures to be hydrophobic peptide bonds
Explain why carbohydrates found only on the extracellular (non-cytosolic face) of the plasma
membrane lipid bilayer?
Made in ER & Golgi – Carbohydrates are added inside these organelles, and when vesicles fuse with the membrane, the sugars end up on the outside of the cell.
Cell Recognition & Communication – Help in cell-cell recognition (immune system, signaling, etc.).
Protection – Forms a glycocalyx that shields the cell from damage, pathogens, and chemicals.
Adhesion – Helps cells stick to each other and the extracellular matrix.
Key Idea:
Carbohydrates are on the extracellular face because they’re needed for recognition, protection, and interactions with the environment!
List three major components of all cell membranes in order of most abundant to least abundant:
- lipids
- proteins
- carbohydrates
Describe at least 3 functions of cell membranes.
- Selective permeability: Only small hydrophobic molecules diffuse freely through the membrane
- Cell signaling membrane proteins and glycoproteins help cells communicate by transmitting and receiving signals either from the environment or other cells.
- Structural support: The membrane helps maintain the shape of the cell and helps anchor the cytoskeleton and other extracellular activities.
Why are phospholipids said to be amphipathic. Be specific as to the parts of the phospholipid that give it this designation.
Phospholipids are amphipathic because they a polar head which consists of a phosphate group attached to a glycerol group which has a negative charge making it hydrophilic. They have two fatty acid tails that are nonpolar and don’t interact with water making them hydrophobic.
What causes one hydrocarbon tail of each phosphphlipid to usually be “kinked”?
A double bond in the fatty acid tail between two carbons.
- The movement of water across a membrane is called
osmosis
A solution containing more solutes than the inside of a cell is called____ and will cause the cell to
hypertonic, shrink
A solution containing less solutes than the inside of a cell is called___ and will cause the cell to___
hypotonic, swell
A solution containing the same total concentration of solutes as inside of a cell is called____and will cause the cell to ___
isotonic, maintain its shape
Passive transport
goes from high to low concentration
-does not require energy
-can diffuse directly across the lipid bilayer
-can use a transporter protein
Active transport
goes from low to high concentration
-does require energy
-cannot diffuse directly across the lipid bilayer
-can use a transporter protein
When grown at higher temperatures, bacteria and yeast maintain an optimal membrane fluidity by doing which of the following?
1.producing membrane lipids with tails that are longer and contain more double bonds
2.adding cholesterol to their membranes
3.producing membrane lipids with tails that are shorter and contain more double bonds
4.producing membrane lipids with tails that are shorter and contain fewer double bonds
5.producing membrane lipids with tails that are longer and contain fewer double bonds
5.producing membrane lipids with tails that are longer and contain fewer double bonds
How fluid a lipid bilayer is at a given temperature depends on its phospholipid composition—particularly the nature of the hydrocarbon tails. The closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be. In bacterial and yeast cells, which have to adapt to varying temperatures, both the lengths and the degree of saturation of the hydrocarbon tails in the bilayer are adjusted constantly to maintain a membrane with a relatively consistent fluidity. At higher temperatures, for example, the cell makes membrane lipids with tails that are longer and that contain fewer double bonds. This allows the membrane lipids to maximize their interactions and thus to pack more tightly, which keeps the membranes from becoming too fluid. Although animal cells do not generally have to cope with large ranges of temperature, they can modulate membrane fluidity by the inclusion of the sterol cholesterol. This option is not available to bacteria and yeast, which do not produce cholesterol.
Choose one
A
B
C
D
E
B
A typical membrane phospholipid molecule has a hydrophilic head and two hydrophobic tails. For example, shown below are the structures of phosphatidylcholine (left and center) and phosphatidylserine (right).
The hydrophobic tails of both phospholipids are long hydrocarbon chains, which are uncharged. Glycerol, here outlined in green, is also uncharged. The hydrophillic head includes a chemical group (blue) characteristic of the specific phospholipid: serine in phosphatidylserine and choline in phosphatidylcholine. As shown in the figure, choline contains a positively charged nitrogen atom; serine, with its positively charged nitrogen and negatively charged carboxyl oxygen, is overall uncharged. The remaining component of the hydrophilic head, the phosphate group (yellow), includes oxygen atoms. One of these oxygens always carries a negative charge.
All of the carbohydrates in the plasma membrane face the cell exterior. Which direction do the carbohydrates on internal cell membranes face?
Choose one:
-the plasma membrane
-the glycocalyx
-the cell exterior
-the lumen of the vesicle or organelle
-the cytosol
The lumen of the vesicle or organelle
The lipids that show the most dramatically lopsided distribution in cell membranes are the glycolipids, which are located mainly in the plasma membrane and only in the noncytosolic half of the bilayer. The sugar groups of these membrane lipids face the cell exterior, where they form part of a continuous coat of carbohydrate (called the glycocalyx), which surrounds and protects animal cells. Glycolipid molecules acquire their sugar groups in the Golgi apparatus, where the enzymes that engineer this chemical modification are confined. These enzymes are oriented such that sugars are added only to lipid molecules in the noncytosolic half of the bilayer. Once a glycolipid molecule has been created in this way, it remains trapped in this monolayer, as there are no flippases that transfer glycolipids to the cytosolic side. Thus, when a glycolipid molecule is finally delivered to the plasma membrane, it displays its sugars to the exterior of the cell. On internal cell membranes, the noncytosolic half of the lipid bilayer faces the lumen of the vesicle or organelle. For an internal cell membrane, half of the bilayer that faces toward the plasma membrane would face the cytosol, as would the part of the membrane that faces the direction of the cell exterior.
What type of protein moves randomly selected phospholipids from one monolayer of a lipid bilayer to the other?
Choose one:
-phospholipase
-scramblase
-none; such movement occurs spontaneously and relatively quickly
-none; phospholipids cannot move from one monolayer to another
-flippase
Scramblase
Although newly synthesized phospholipids are deposited into the cytosolic half of the ER bilayer, the membrane manages to grow evenly. The movement of lipids from one monolayer of the membrane to the other rarely occurs spontaneously. Instead, lipids are relocated by scramblases, which remove randomly selected phospholipids from one half of the bilayer and insert them into the other. As a result of this scrambling, newly made phospholipids are redistributed equally between each monolayer of the ER membrane. Although flippases also transport membranes from one side of the bilayer to the other, these transporters remove specific phospholipids from the side of the bilayer facing the exterior space and flip them into the monolayer that faces the cytosol. The action of flippases thereby promotes membrane asymmetry.
Which of the following would produce the most fluid lipid bilayer?
Choose one:
A. phospholipids with tails of 20 carbon atoms and two double bonds
B. phospholipids with fully saturated tails of 20 carbon atoms
C. large amounts of cholesterol
D. phospholipids with fully saturated tails of 18 carbon atoms
E. phospholipids with tails of 18 carbon atoms and two double bonds
E. phospholipids with tails of 18 carbon atoms and two double bonds
The fluidity of a membrane—the ease with which its lipid molecules move within the plane of the bilayer—depends on the nature of the lipids’ hydrocarbon tails: the closer and more regular the packing of the tails, the more viscous and less fluid the membrane will be. A shorter chain length and the presence of double bonds both reduce the tendency of the phospholipid tails to interact with one another and pack tightly, thereby increasing the fluidity of the membrane.
When a vesicle fuses with the plasma membrane, which way will the monolayer that was exposed to the interior of the vesicle face?
Choose one:
-It depends on where, along the plasma membrane, the vesicle fuses.
-the cell cytoplasm
-the endomembrane system
-The direction the monolayer will face will be established randomly.
-the cell exterior
The cell exterior
Most cell membranes are asymmetric and have distinct “inside” and “outside” faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or the interior space (lumen) of an organelle. This asymmetry is preserved as membranes, in the form of vesicles, which bud from one organelle and fuse with another or with the plasma membrane.
Why must all living cells carefully regulate the fluidity of their membranes?
Choose one or more:
1. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules
- to allow cells to function at a broad range of temperatures
- to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell
- to constrain and confine the movement of proteins within the membrane bilayer
- to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides
- to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules
- to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell
- to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides
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?
Choose one:
-by activating a scramblase and inactivating a flippase in the plasma membrane
-by boosting the activity of a flippase in the plasma membrane -by inactivating a scramblase in the plasma membrane -by inverting the existing 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
All cells are separated from the extracellular environment by the plasma membrane. This cell membrane plays a key role in cell communication, presenting signals that relate information about the state of the cell, including its relative health. In healthy cells, the distribution of phospholipids in the plasma membrane is asymmetric. Some phospholipids, such as phosphatidylcholine and sphingomyelin, are confined to the noncytosolic half of the plasma membrane, while others such as phosphatidylserine and phosphatidylethanolamine are present only in the membrane’s cytosolic monolayer.
When cells are no longer needed or are damaged beyond repair, they can activate a form of programmed cell death called apoptosis. A cell undergoing apoptosis actively destroys itself from within, digesting its proteins and degrading its DNA. It also displays signals that direct circulating phagocytic cells to engulf its remains.
One of these signals involves the relocation of phosphatidylserine. An apoptotic cell displays phosphatidylserine—normally confined to the cytosolic monolayer of the plasma membrane—on its surface.
This reversal involves manipulating the activity of both flippases and scramblases in the plasma membrane. First, the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other must be activated. This scrambling 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 must be inactivated. Together, these actions cause phosphatidylserine to rapidly accumulate at the cell surface.
Boosting the activity of flippases causes phosphatidylserine to be selectively transferred to the cytosolic half of the membrane. This distribution is the sign of a healthy cell—the opposite of what happens when cells undergo programmed cell death.
If flippases were inactivated, any phosphatidylserines that had already made it to the extracellular side of the plasma membrane (through the random action of scramblases) would, indeed, remain there. But if scramblase were also inactivated, any newly synthesized phosphatidylserines would remain trapped in the cytosolic half of the bilayer. So, a limited number of phosphatidylserines would be exposed at the cell surface.
Why do phospholipids form bilayers in water?
Choose one:
-The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.
-The hydrophobic head is attracted to water, while the hydrophilic tail shuns water. -The hydrophobic tail is attracted to water, while the hydrophilic head shuns water. -The hydrophilic head is insoluble in 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.
When scientists were first studying the fluidity of membranes, they did an experiment using hybrid cells. Certain membrane proteins in a human cell and a mouse cell were labeled using antibodies coupled with differently colored fluorescent tags. The two cells were then coaxed into fusing, resulting in the formation of a single, double-sized hybrid cell. Using fluorescence microscopy, the scientists then tracked the distribution of the labeled proteins in the hybrid cell.
Which best describes the results they saw and what they ultimately concluded?
Choose one:
-Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer.
-The mouse and human proteins remained confined to the portion of the plasma membrane that derived from their original cell type. This suggests that cells can restrict the movement of their membrane proteins to establish cell-specific functional domains.
At first, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became divided such that half faced the cytosol and half faced the hybrid cell exterior. This suggests that flippases are activated by cell fusion.
-Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins recombined such that they all fluoresced with a single, intermediate color.
The mouse and human proteins began to intermix and spread across the surface of the hybrid cell, but over time, one set of proteins became dominant and the other set was lost. This suggests that cells can ingest and destroy foreign proteins.
-Initially, the mouse and human proteins intermixed, but over time, they were able to resegregate into distinct membrane domains. This suggests that cells can restrict the movement of membrane proteins.
Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer.
The orange monolayer will face the cytosol.
Most cell membranes are asymmetric, as the two halves of the bilayer often include strikingly different sets of phospholipids. This asymmetry is preserved as membranes bud from one organelle and fuse with another, or with the plasma membrane. This means that all cell membranes have distinct “inside” and “outside” faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or the interior space (lumen) of an organelle. Maintaining this asymmetric organization is essential for preserving the asymmetric distribution of phospholipids and glycolipids, which may be confined to one or another monolayer to carry out their physiological function.
he plasma membrane is involved in which activities?
Choose one or more:
RNA interference
cell signaling
import and export of nutrients and wastes
cell recognition
DNA replication and repair
cell growth and motility
cell signaling
import and export of nutrients and wastes
cell recognition
cell growth and motility
What effect do double bonds have on phospholipid hydrocarbon tails and on the fluidity of the membrane?
Choose one:
-Double bonds have little effect on membrane fluidity.
-Double bonds increase the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer less fluid. -Double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer more fluid. -Double bonds decrease the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer less fluid. -Double bonds increase the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer more fluid.
Double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer more fluid.
How does the inclusion of cholesterol affect animal cell membranes?
Choose one:
-It tends to make the lipid bilayer more fluid.
-It makes the lipid bilayer more permeable.
-It makes the lipid bilayer wider.
-It has little effect on the properties of the lipid bilayer.
-It tends to make the lipid bilayer less fluid.
-It tends to make the lipid bilayer less fluid.
In a lipid bilayer, where do lipids rapidly diffuse?
Choose one:
-within the plane of one monolayer and back and forth between the monolayers
-not at all, because they remain in place within the bilayer
-back and forth from one monolayer to the other in the bilayer
-within the plane of their own monolayer
-in and out of the bilayer
within the plane of their own monolayer
