Chapter 11 Molecular Smartwork

Which characteristic describes the tails of phospholipids?

hydrophobic

Which of the following would produce the most fluid lipid bilayer?

phospholipids with tails of 18 carbon atoms and two double bonds

Which statements are true about the differences between phospholipids and detergents?

1. Detergents are shaped like cones, whereas phospholipids are more cylindrical. 2. Phospholipids have two hydrocarbon tails, whereas detergents have just one 3. Phospholipids form bilayers in water, whereas detergents tend to form micelles

How does the cortex of a typical animal cell differ from that of a mature red blood cell?

1. It allows the cell to selectively take up material from the environment. 2. It allows the cell to move. 3. It is richer in actin and in the motor protein myosin.

Why must all living cells carefully regulate the fluidity of their membranes?

1. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules 2. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides 3. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell

Which of the following is a function of proteins in the plasma membrane?

1. transport molecules across the membrane 2. transmit extracellular signals to the cell interior 3. allow specific ions to cross the plasma membrane, thereby controlling its electrical properties 4. serve as anchors to attach the cell to the extracellular matrix

How thick is the plasma membrane?

50 atoms

Shown is a schematic diagram of a membrane phospholipid. Which segment will always carry a negative charge?

B

Mutation in the hemoglobin gene can cause sickle-cell anemia. The defective protein found in sickle-cell anemia causes red blood cells to "sickle"—become a misshapen C shape. These misshapen cells abnormally stick to each other and can become trapped by leukocytes (white blood cells) that are rolling or paused on the endothelial cells lining the vessel. This causes blockages of small blood vessels, causing severe pain and strokes called vaso-occlusive crisis. A new drug that binds and blocks selectin proteins is in phase III clinical trials to test for improvement in patients' symptoms. Why might this be an effective treatment for vaso-occlusive crisis?

Blocking selectins would block the ability of selectin to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells.

The following graphs show the number of adherent leukocytes found on the blood vessel wall in control conditions and after adding a selectin inhibitor, which blocks the function of selectin. Which of the following graphs correctly shows the effect of a selectin inhibitor on adherence of leukocytes to the vessel wall?

Graph B

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?

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.

How does the inclusion of cholesterol affect animal cell membranes?

It tends to make the lipid bilayer less fluid.

In an electron transport chain, electrons are passed from one transmembrane electron carrier to another, driving proton movement across a membrane (see image below). The protons then flow through ATP synthase (not shown) to generate ATP.

See Pic

To study the structure of a particular membrane protein, the target protein is usually removed from the membrane and separated from other membrane proteins. Shown below are three different proteins associated with the cell membrane. Treatment with high salt would release which protein or proteins from the bilayer?

See Pic

Fluorescence recovery after photobleaching (FRAP) is used to monitor the movement of fluorescently labeled molecules within the plane of a cell membrane. The molecules labeled are often proteins, but lipids can be labeled too.How would the curve that represents FRAP for labeled proteins compare to the curve representing labeled lipids?

The FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence.

In 1925, scientists exploring how lipids are arranged within cell membranes performed a key experiment using red blood cells. Using benzene, they extracted the lipids from a purified sample of red blood cells. Because these cells have no nucleus and no internal membranes, any lipids they obtained were guaranteed to come from the plasma membrane alone.The extracted lipids were floated on the surface of a trough filled with water, where they formed a thin film. Using a movable barrier, the researchers then pushed the lipids together until the lipids formed a continuous sheet only one molecule thick.The researchers then made an observation that led them to conclude that the plasma membrane is a lipid bilayer.Which of the following would have allowed the scientists to come to this conclusion?

The extracted lipids covered twice the surface area of the intact red blood cells.

Why do phospholipids form bilayers in water?

The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.

When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol?

The orange monolayer will face the cytosol.

In a patch of animal cell membrane about 10 μm in area, which will be true?

There will be more lipids than proteins.

Which membrane would show a more rapid recovery of fluorescence in a FRAP study?

a membrane containing a larger proportion of unsaturated fatty acids

Which of the following would be most likely to disrupt lipid bilayer formation?

addition of a phosphate to the end of the lipid tail

Which term correctly describes the entire phospholipid molecule?

amphipathic

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?

amphipathic molecules

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 activating a scramblase and inactivating a flippase in the plasma membrane

The shape of a cell and the mechanical properties of its plasma membrane are determined by a meshwork of fibrous proteins called what?

cell cortex

The plasma membrane is involved in which activities?

cell signaling import and export of nutrients and wastes cell recognitioncell growth and motility

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

In a 2018 article (Budin, et al., Science vol. 362) researchers probed how membrane fluidity affects electron transport chain activity and ATP production in E. coli by manipulating membrane fluidity and measuring respiration. How could researchers have increased membrane fluidity?

increase the proportion of phospholipids with unsaturated fatty acids

What is a functionally specialized region of a cell membrane, typically characterized by the presence of specific proteins, called?

membrane domain

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

In eukaryotic cells, phospholipids are synthesized by enzymes bound to which of the following?

the cytosolic face of the endoplasmic reticulum

On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located?

the extracellular side

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 hydrophilic side chains of the transmembrane α helices

In the α helices of transmembrane proteins, the hydrophobic side chains face which direction?

the outside of the membrane-spanning helix

In a lipid bilayer, where do lipids rapidly diffuse?

within the plane of their own monolayer