biol 314 chapter 11

The movement of an individual protein in a cell membrane can be followed using a technique called single-particle tracking (SPT). what type of movement would actin proteins that are part of the cell cortex be expected to show in an SPT study?

Actin is tethered firmly to attachment proteins anchored in the cell membrane; as such, it is essentially immobile.

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 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.

Naïve B cells found in our immune system each contain a specific cell-surface receptor capable of binding to a specific foreign antigen such as a protein from a pathogen like a bacterium or virus. Binding to an antigen triggers a process whereby the naïve B cell can differentiate into a plasma cell that secretes large quantities of an antibody that specifically binds to an antigen from the pathogen. Because plasma cells are long-lived cells, the immune system is then primed to rapidly respond to that antigen in the future if the person is again exposed to the pathogen. During differentiation, the cell rapidly expands the amount of endoplasmic reticulum and Golgi membranes needed for secreting the antibodies. Which organelle(s) will become particularly active in synthesizing new membrane lipids?

Endoplasmic reticulum New membrane lipids are synthesized by enzymes on the cytosolic surface of the endoplasmic reticulum.

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.

Increase the proportion of phospholipids with unsaturated fatty acids Unlike saturated fatty acids, unsaturated fatty acids contain kinks (see image below). These kinks prevent tight packing of adjacent phospholipids, thus increasing membrane fluidity.

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.

The lamin B receptor is found in the inner membrane of the nuclear envelope. It connects the nuclear envelope to the heterochromatin (chromosome) and nuclear lamina proteins, which provide structure to the nucleus. In normal cells, the lamin B receptor protein is stably locked in place by these interactions and shows very little movement. Infection of cells by the Herpes Simplex Virus Type I (HSV-1) can disrupt the lamin B receptor interactions when the virus capsids exit from the nucleus by budding through the inner nuclear membrane of the nucleus. This causes some of the lamin B receptor to move between the inner nuclear membrane and the ER membrane. FRAP was completed for lamin B or a control protein under different conditions. Match the three cellular treatments with the correct FRAP graph.

The lamin B protein in uninfected cells is anchored and does not move much, as seen in graph A. Lamin B in HSV-1-infected cells is moved to the ER and recycled back to the inner nuclear envelope in small amounts and will show some recovery, as in graph C. A control protein that normally cycles between the inner nuclear envelope and other organelles will show a more rapid recovery, as in graph B.

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

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 For all living cells, maintaining optimal membrane fluidity permits the diffusion of newly synthesized membrane lipids and proteins, ensures that membrane molecules are distributed evenly when a cell divides, and, under appropriate conditions, allows membranes to fuse with one another and mix their molecules.

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?

Treatment with high salt or changing the pH of the solution can disrupt protein-protein interactions. High salt would therefore release this peripheral membrane protein from the bilayer.The other proteins are integral membrane proteins; their attachment to the membrane would not be altered by salt alone.

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

addition of a phosphate to the end of the lipid tail

Organisms that live in cold climates adapt to low temperatures by doing which of the following?

increasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid Antarctic fishes have an unusually high percentage of unsaturated phospholipids in their membranes. The double bonds in these phospholipids help these organisms keep their membranes fluid at very low temperatures.

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 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.

Multipass transmembrane proteins can form pores across the lipid bilayer. The structure of one such channel is shown in the diagram.

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.

The FRAP technique occurs in a series of steps. Select every statement that correctly describes a step in the FRAP procedure.

the molecule of interest is fluorescently labeled the relative mobility of the fluorescently labeled molecule is measured

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

the phosphate group, which is always negatively charged.