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Exam 3: Membrane Transport through Protein Trafficking Practice Questions

Terms in this set (67)

An aqueous pore in a lipid membrane, with walls made of protein, through which selected ions or molecules can pass
Channel
The movement of a small molecule or ion across a membrane due to a difference in concentration or electrical charge.
Passive transport
Movement of a molecule across a membrane that is driven by ATP hydrolysis or other form of metabolic energy
Active Transport
Driving force for ion movement that is due to differences in ion concentration and electrical charge on either side of the membrane
Electrochemical gradient
Membrane carrier protein that transports two different ions or small molecules across a membrane in opposite directions, either simultaneously or in sequence
Antiporter
Quantitative expression that relates the equilibrium ratio of concentrations of an ion on either side of a permeable membrane to the voltage difference across the membrane
Nernst Equation
Voltage difference across a membrane due to the slight excess of positive ions on one side and of negative ions on the other (typically -60 mV, inside negative, for an animal cell)
Membrane Potential
General term for a membrane protein that selectively allows cations such as Na+ to cross a membrane in response to changes in membrane potential
Voltage gated ion channel
. That part of an ion channel structure that determines which ions it can transport
Selectivity filter
Long, thin nerve cell process capable of rapidly conducting nerve impulses over long distances so as to deliver signals to other cells
Axon
Small signaling molecule such as acetylcholine, glutamate, GABA, or glycine, secreted by a nerve cell at a chemical synapse to signal to the post-synaptic cell
Neurotransmitter
Technique in which the tip of a small glass electrode is sealed onto an area of cell membrane, thereby making it possible to record the flow of current through individual ion channels
Patch-Clamp Recording
The plasma membrane is highly impermeable to all charged molecules
False: permeable to specific ions and charged solutes under certain circumstances
Transport by transporters can be either active or passive, whereas transport by channels is always passive
True: Transporters: conformational changes, passively down an electrochemical gradient or active transport via ATP hydrolysis

channels: aqueous pores that are open or shut, always downhill; interact more weakly with solutes: always passive
A symporter would function as an antiporter if its orientation in the membrane were reversed; that is, if the portion of the protein normally exposed to the cytosol faced the outside of the cell instead.
False: a symporter binds two different solutes on the same side of the membrane; turning an antiporter around will not change it into a symporter
The co-transport of Na+ and a solute into a cell, which harnesses the energy in the Na+ gradient, is an example of primary active transport
False; this is an example of secondary active transport (when a solute uses ATP generated by Na+ gradient)
Transporters saturate at high concentrations of the transported molecule when all their binding sites are occupied; channels, on the other hand, do not bind the ions they transport and thus the flux of ions through a channel does not saturate
False; transporters and channels saturate: narrow selectivity filter, limits rate of passage
The membrane potential arises from movements of charge that leave ion concentrations practically unaffected, causing only a very slight discrepancy in the number of positive and negative ions on the two sides of the membrane
True; only takes a minute amount of ions to dramatically set up the membrane potential
The aggregate current crossing the membrane of an entire cell indicates the degree to which individual channels are open
False; Channels open in all or nothing fashion; indicates the total number of channels that are open at any one time
Order the molecules on the following list according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your order.
a) Ca2+
b) CO2
c) Ethanol
d) Glucose
e) RNA
f) H2O
CO2 > Ethanol > H2O > Glucose > Ca2+ > RNA

CO2 is small and nonpolar (highly effective)

Ethanol is small and slightly polar

H2O is a small and uncharged polar molecule (somewhat diffusible)

Glucose (large uncharged polar molecule; very inefficient)

Ca2+ (small and charged, charged ion requires a transmembrane protein)

RNA (very large and charged, requires transport assistance)

size (small > large)
polarity (nonpolar > polar > charged)
Why are the maximum rates of transport by transporters and channels thought to be so different?
Transporters: slow because must bind the solute and undergo a series of conformational changes

Channels: much faster because they are ion specific pores that neither bind the ion nor undergo any conformational changes
How is it possible for some molecules to be at equilibrium across a biological membrane and yet not be at the same concentration on both sides?
Depends on the chemical gradient (concentration) and on the electrical gradient (membrane potential). Difference in concentration is balanced by the membrane potential (negative inside), which opposes the movement of cations to the outside of the cell (K+ example)
Excitatory neurotransmitters open Na+ channels, while inhibitory neurotransmitters open either Cl- or K+ channels. Rationalize this observation in terms of the effects of these ions on the firing of an action potential.
Excitatory neurotransmitters: opening of Na+ channels allows the cell to depolarize (more positive), which makes it more likely that an action potential will occur. Move Na+ in

Inhibitory neurotransmitters: opening of Cl- or K+ channels will repolarize the cell (more negative), which make it more negative, decreasing the chance of an action potential occuring. Cl- in or K+ out.
The sub-compartment formed between the inner and outer mitochondrial membranes
Intermembrane space
Process in bacteria and mitochondria in which ATP formation is driven by the transfer of electrons from food molecules to molecular oxygen, with the intermediate generation of a proton gradient across a membrane
Oxidative Phosphorylation
The result of a combined pH gradient and membrane potential
Electrochemical gradient
Colored, heme-containing protein that transfers electrons during cellular respiration
Cytochrome
Electron-transporting group consisting of either two or four iron atoms bound to an equal number ofsulfur atoms
Iron-Sulfur Center
Enzyme in the inner membrane of a mitochondrion that catalyzes the formation of ATP from ADP and inorganic phosphate
ATP Synthase
Light-driven reactions in photosynthesis in which electrons move along the electron-transport chain in the thylakoid membrane
Photosynthetic electron transfer
Part of a photosystem that captures light energy and channels it into the photochemical reaction center
Antenna Complex
Process by which green plants incorporate carbon atoms from atmospheric carbon dioxide into sugars
Carbon fixation
The part of a photosystem that converts light energy into chemical energy
Photochemical reaction center
Organelle in green algae and plants that contains chlorophyll and carries out photosynthesis
Chloroplast
Light-absorbing green pigment that plays a central role in photosynthesis
Chlorophyll
The large space that surrounds the inner chloroplast membrane
Stroma
Pattern of mitochondrial inheritance in higher animals that arises because the egg cells always contribute much more cytoplasm to the zygote than does the sperm
Maternal Inheritance
The most important contribution of the citric acid cycle to energy metabolism is the extraction of high-energy electrons during the oxidation of acetyl CoA to CO2
True: sole products are CO2 and reduced forms of electron carriers NADH and FADH2. Electrons are passed to the ETC to generate ATP via oxidative phosphorylation
Each respiratory enzyme complex in the electron-transport chain has a greater affinity for electrons than its predecessors, so that electrons pass sequentially from one complex to another until they are finally transferred to oxygen, which has the greatest electron affinity of all
True: the natural (thermodynamic) tendency of electrons is to move from low affinity to high affinity carriers
An average person contains 50 kg of ATP, which is required to meet their daily energy needs
False: an average person carries a much smaller amount of ATP, reuse by recycling it between ATP and ADP; turns over roughly 50 kg of ATP per day in order to support their daily energy needs
In a general way, one might view the chloroplast as a greatly enlarged mitochondrion in which the cristae have been pinched off to form a series of interconnected submitochondrial particles in the matrix space
True: thylakoids and cristae have the same arrangement of membrane proteins: ATP synthase for example
When a molecule of chlorophyll in an antenna complex absorbs a photon, the excited electron is rapidly transferred from one chlorophyll molecule to another until it reaches the photochemical reaction center
False: transfers energy, not electron from one chlorophyll molecule to another by resonance energy transfer
The mitochondrial genetic code differs slightly from the nuclear code, but it is identical in mitochondria from all species that have been examined
False; mitochondrial genetic code differs slightly from the nuclear code, and also varies slightly from species to species
. Both mitochondria and chloroplasts use electron transport to pump protons, creating an electrochemical proton gradient, which drives ATP synthesis. Are protons pumped across the same (analogous) membranes in the two organelles? Is ATP synthesized in the analogous compartments? Explain your answers.
Protons are pumped across the crista membrane into the crista space in mitochondria. By contrast, they are pumped across the thylakoid membrane into the thylakoid space in chloroplasts. The thylakoid and cristae membranes and spaces are not precisely analogous, but cristae would be converted to something resembling thylakoids if they were pinched off from the inner membrane; similarly, if thylakoids fused to the inner membrane, they would resemble cristae. ATP is synthesized in the corresponding compartments in the two organelles: in the matrix in mitochondria and in the stroma in chloroplasts.
A suspension of the green alga Chlamydomonas is actively carrying out photosynthesis in the presence of light and CO2. If you turned off the light, how would you expect the amounts of ribulose 1,5-bisphosphate and 3-phosphoglycerate to change over the next minute? How about if you left the light on but removed the CO2?
In the absence of light, but in the presence of CO2, the amount of ribulose 1,5- bisphosphate will decrease and 3-phosphoglycerate will increase. The presence of CO2 allows ribulose 1,5-bisphosphate to be converted to 3-phosphoglycerate but, in the absence of light (and therefore with decreased amounts of NADPH and ATP), 3- phosphoglycerate will accumulate because subsequent reactions require NADPH and ATP. In the absence of CO2, ribulose 1,5-bisphosphate will accumulate (and 3- phosphoglycerate will decrease) because its conversion to 3-phosphoglycerate is dependent on CO2.
. In chloroplasts, protons are pumped out of the stroma across the thylakoid membrane, whereas in mitochondria, they are pumped out of the matrix across the crista membrane. Explain how this arrangement allows chloroplasts to generate a larger proton gradient across the thylakoid membrane than mitochondria can generate across the inner membrane.
Protons pumped across the crista membrane into the crista space can exit to the intermembrane space, which equilibrates with the cytosol, a huge H+ sink. Both the mitochondrial matrix (pH 8) and the cytosol (pH 7.4) house many metabolic reactions that require a pH around neutrality. The largest H + concentration difference between the mitochondrial matrix and cytosol that is compatible with function is therefore relatively small (less than 1 pH unit). Much of the energy stored in the mitochondrial electrochemical gradient is instead due to the membrane potential (about 140 mV of the 200 mV potential difference is due to the membrane potential). By contrast, chloroplasts have a smaller, dedicated compartment—the thylakoid space—into which H+ ions are pumped. Much higher concentration differences can be achieved (more than 3 pH units), and nearly all of the energy stored in the thylakoid electrochemical gradient is due to the pH difference between the stroma and the thylakoid space.
Examine the variegated leaf shown in Figure 14-15. Yellow patches surrounded by green are common, but there are no green patches surrounded by yellow. Propose an explanation for this phenomenon.
Variegation occurs because the plants have a mixture of normal and defective chloroplasts.
Contents of a cell that are contained within its plasma membrane but, in the case of eukaryotic cells, outside the nucleus
Cytoplasm
Contents of the main compartment of the cell, excluding the nucleus and membrane-bounded compartments such as endoplasmic reticulum and mitochondria
Cytosol
Membrane-enclosed compartment in a eukaryotic cell that has a distinct structure, macromolecular composition, and function
Organelle
Protein sorting signal that consists of a short continuous sequence of amino acids
. Signal sequence
Sorting signal contained in the structure of macromolecules and complexes that are transported from the nucleus to the cytosol through nuclear pore complexes
Nuclear export signal
Large multiprotein structure forming a channel through the nuclear envelope that allows selected molecules to move between nucleus and cytoplasm
Nuclear pore complex (NPC)
Monomeric GTPase present in both cytosol and nucleus that is required for the active transport
of macromolecules into and out of the nucleus through nuclear pore complexes
Ran
Fibrous meshwork of proteins on the inner surface of the inner nuclear membrane
Nuclear lamina
Protein that binds nuclear localization signals and facilitates the transport of proteins with these signals from the cytosol into the nucleus through nuclear pore complexes
Nuclear import receptor
Sorting signal found in proteins destined for the nucleus and which enable their selective transport into the nucleus from the cytosol through the nuclear pore complexes
Nuclear localization signal
The portion of the nuclear envelope that is continuous with the endoplasmic reticulum and is studded with ribosomes on its cytosolic surface.
Outer nuclear membrane
2. Multisubunit protein assembly that transports proteins across the mitochondrial outer membrane
TOM complex
Region of the ER not associated with ribosomes
Smooth ER
Ribonucleoprotein complex that binds an ER signal sequence on a partially synthesized polypeptide chain and directs the polypeptide and its attached ribosome to the ER
Signal-recognition particle (SRP)
. Describes import of a protein into the ER before the polypeptide chain is completely synthesized
Co-translational
Short amino acid sequence on a protein that keeps it in the ER
ER retention signal
Any protein with one or more oligosaccharide chains covalently linked to amino acid side chains.
Glycoprotein
Ribosome in the cytosol that is unattached to any membrane.
Free ribosome
. General term for a membrane-enclosed container that moves material between membraneenclosed compartments within the cell.
Transport vesicle
Any of a large family of monomeric GTPases present in the plasma membrane and organelle membranes that confer specificity on vesicle docking.
. Rab protein
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