(1) Membranes are sheetlike structures that are two molecules thick. (2) Membranes are composed of lipids and proteins, both of which may be decorated by carbohydrates. (3) Membrane lipids are amphipathic molecules, composed of hydrophilic and hydrophobic components, that spontaneously form closed bimolecular sheets in aqueous solution. (4) Proteins, unique to each membrane, mediate the transfer of molecules and information across the membrane. (5) Membranes are noncovalent assemblies. (6) The leaflets of the membrane bilayers are different; that is, membranes are asymmetric. (7) Membranes are fluid, rather than rigid structures. (8) Membranes are electrically polarized, with the inside of the cell negative with respect to the outside.
First, the molecule must be lipophilic, and second, the concentration of the molecule must be greater on one side of the membrane than on the other.
(a) 3 ; (b) 5; (c) 6 ; (d) 1; (e) 7 ; (f) 10; (g) 2; (h) 4; (i) 8 ; (j) 9
c, a, e, b, d
In simple diffusion, the molecule in question can diffuse down its concentration gradient through the membrane. In facilitated diffusion, the molecule is not lipophilic and cannot directly diffuse through the membrane. A channel or carrier is required to facilitate movement down the gradient.
In passive transport (facilitated diffusion), a substance moves down its concentration gradient through a channel or transporter. In active transport, a concentration gradient is generated at the expense of another source of energy, such as the hydrolysis of ATP.
Recall that most proteins are not static structures and require conformational changes to perform their biochemical tasks. If the membrane were too rigid, the required structural conformation could not be obtained. If the membrane were too fluid, the interactions with the environment (hydrophobic core of the membrane) that the protein needs to maintain its structure would be disrupted.
The heart contraction (the heartbeat) is initiated by the release of calcium from calcium stores. The contraction is terminated by the removal of calcium from the cytoplasm. This removal is accomplished, in part, by a sodium–
Ouabain, like digitalis, inhibits the Na+−K+ ATPase. The Na+−K+ ATPase is crucial to maintaining the sodium gradient that renders neurons and muscle cells electrically excitable. Inhibition of the enzyme shuts down a host of biochemical process required for life, such as cardiac and respiratory function.
Inhibition of the symporter would eventually lead to the inhibition of the ATPase. Because the sodium gradient would not be dissipated by the symporter, the sodium concentration outside the cell would become so great that the hydrolysis of ATP by the ATPase would not provide sufficient energy to pump against such a large gradient.
Selectivity and the rapid transport of ions
Ligand-
False. Although the cotransporter does not directly depend on ATP, the formation of the Na+ gradient that powers glucose uptake depends on ATP hydrolysis.
The two forms are (1) ATP hydrolysis and (2) the movement of one molecule down its concentration gradient coupled with the movement of another molecule up its concentration gradient.
Databases could be searched for proteins with stretches of 20 hydrophobic amino acids.
The hydrophobic effect. If there is a hole, the hydrophobic tails of the phospholipids will come together, freeing any associated water.
Peripheral proteins are attached to the phospholipid head groups of membrane lipids or the exposed portions of integral membrane proteins. Integral membrane proteins are embedded in the membrane.
For both sides of a membrane to become identical, the hydrophilic parts of the lipids, proteins, and carbohydrates would have to pass through the hydrophobic interior of the membrane. Such movement is energetically unfavorable.
C13
Establish a glucose gradient across vesicle membranes that contain a properly oriented Na+–glucose linked transporter. Initially, Na+ concentration should be the same on both sides of the membrane. As the glucose flows “in reverse” through the transporter, down its concentration gradient, a Na+ concentration gradient becomes established as the glucose gradient is dissipated.
An ion channel must transport ions in either direction at the same rate. The net flow of ions is determined only by the composition of the solutions on either side of the membrane.
All of the amide hydrogen atoms and carbonyl oxygen atoms are stabilized in the hydrophobic environment by intrachain hydrogen bonds. If the R groups are hydrophobic, they will interact with the hydrophobic interior of the membrane, further stabilizing the helix.
The catalytic prowess of acetylcholinesterase ensures that the duration of the nerve stimulus will be short.
Ibuprofen is a competitive inhibitor of the synthase.
The graph shows that, as temperature increases, the phospholipid bilayer becomes more fluid. Tm is the temperature of the transition from the predominantly less fluid state to the predominantly more fluid state. Cholesterol broadens the transition from the less fluid to the more fluid state. In essence, cholesterol makes membrane fluidity less sensitive to temperature changes.
This effect is important because the presence of cholesterol tends to stabilize membrane fluidity by preventing sharp transitions. Because protein function depends on the proper fluidity of the membrane, cholesterol maintains the proper environment for membrane-
Glucose displays a transport curve that suggests the participation of a carrier, because the initial rate is high but then levels off at higher concentrations, consistent with saturation of the carrier, which is reminiscent of Michaelis–
During the day, the lipids are likely to be long saturated hydrocarbon chains. With the onset of night, the chains may be shorter or contain cis double bonds or both.
Cells may be exposed to many environmental chemicals, called xenobiotics. Many of these chemicals are likely to be toxic. Cells that can remove such chemicals will survive longer.
Essentially a reverse membrane. The hydrophilic groups would come together on the interior of the structure, away from the solvent, whereas the hydrocarbon chains would interact with the solvent.
(a) Only ASIC1a is inhibited by the toxin. (b) Yes; when the toxin was removed, the activity of the acid-
This mutation is one of a class of mutations that result in slow-
The recordings would show the channel opening only infrequently. The mutation reduces the affinity of acetylcholine for the receptor.
The blockage of ion channels inhibits action potentials, leading to a loss of nervous function. These toxin molecules are useful for isolating and specifically inhibiting particular ion channels.
For either ion to pass through the channel, it must shed the water of solvation, which is an endergonic process. For potassium, the energy to compensate for the loss of water is provided by interaction between the ion and the selectivity filter. Because sodium is smaller than potassium, the energetic compensation between the ion and the selectivity filter is too small to compensate the sodium ions for the loss of the water of solvation. Thus, sodium cannot pass through the channel.