Exam 4
How and when do proteins enter the lumen of the endoplasmic reticulum (ER)?
Binding of a signal recognition particle (SRP, a complex of proteins and RNA) signals the ribosomes to pause after assembling the first part of the polypeptide SRP binds to a receptor on the ER End of the polypeptide is fed to a translocator The translocator holds onto the signal sequence as the rest of the polypeptide chain slides past into the ER lumen as a long loop, and begins to fold, often with the help of chaperone proteins who encourage it fold correctly Once the finished carboxyl end is through, the signal sequence is clipped off, and the folded protein is free within the ER, where it is likely to be modified further
Why must all living cells carefully regulate the fluidity of their membranes?
1. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell 2. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides 3. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules
How thick is the plasma membrane?
50 atoms
Approximately what percentage of the volume of a typical eukaryotic cell is comprised of cytosol?
50%
Describe the structure of a membrane-spanning beta-barrel transmembrane protein.
A 16-stranded beta sheet folded around itself is filled with water to form a channel.
How does an action potential spread along the cell membrane?
A change in membrane potential triggers the opening of nearby voltage-gated sodium channels in a one-way direction.
Why would a cell express the aquaporin protein if water can cross the membrane in the absence of aquaporin?
Aquaporin facilitates the faster movement of water molecules across the membrane.
How "fussy" is the ER quality control process for releasing proteins?
As proteins proceed through the ER process, they are continually checked for quality. Chaperone proteins keep them away from "bad influences" of other molecules with which they could react, and especially away from other unfolded proteins, with which they could stick and clump so that hopefully they can refold correctly. If they still do not, then after a while they will be broken down and recycled.
A sodium-potassium antiport maintains the extracellular concentration of sodium at levels that are about 20-30 times higher than inside the cells. What directly supplies the energy for maintaining this gradient?
ATP hydrolysis drives the function of the pump.
What are three sources of energy used by membrane pumps? Give examples of each.
ATP hydrolysis → Na+/K+ pumps Gradient-driven pumps → Na+-Glucose Symport Light-powered pumps → bacteriorhodopsin and retinal opsins
Chloroplasts
ATP synthesis and carbon fixation by photosynthesis
Mitochondria
ATP synthesis by cell resp
Where and when are the polysaccharide trees built and modified?
In the ER Many proteins, especially those which will be embedded in the cell membrane or part of the extracellular matrix, get turned into glycoproteins by the attachment of branching polysaccharide trees, which can serve as a protective barrier, or as cell surface recognition signals, or as attachment points for other things. Most glycoproteins begin by the transfer of a prefabricated 14-sugar tree from the lipid dolichol to an asparagine in the protein sequence. The basic tree then gets extensively remodeled and edited and added to elsewhere in the ER and Golgi.
Describe the relationship between ion gradients and concentration gradients.
Ion gradients are electro-chemical gradients because the separation of charges creates a voltage across the membrane, which can work in either the same or opposite direction as the concentration gradient, depending on the overall distribution of all the different ions involved.
For voltage-gated channels, a change in the membrane potential has what effect on the channel?
It alters the probability that the channel will be found in its open conformation.
As a polypeptide is being translocated across the membrane of the endoplasmic reticulum, a stop-transfer sequence can halt the process. What eventually becomes of this stop-transfer sequence?
It forms an α-helical membrane-spanning segment of the protein.
To pass through the pore of an ion channel, what must be true of an ion?
It must interact with polar groups in the narrowest part of the channel.
How does the inclusion of cholesterol affect animal cell membranes?
It tends to make the lipid bilayer less fluid.
In most animal cells, which ion can move through "leak" channels?
K+
Which ion is generally maintained at a high concentration inside the cell and a low concentration outside the cell?
K+
Which of the following is NOT used as a source of energy by a transmembrane pump to actively transport a solute? a. ATP b. Na+ c. H+ d. K+
K+
What functions does the Na+/K+ serve?
Maintains osmotic equilibrium (compensates for constant leakage) and the resting membrane potential in cells.
What happens to the receptors after receptor-mediated endocytosis?
Many are recycled back to the membrane and reused Some broken down in lysosomes, along with the cargo they carried Some shift to a different phase of the same job, transporting the cargo across the membrane to other cells or through the bloodstream
Describe glycosylation of proteins in the ER.
Many proteins, especially those which will be embedded in the cell membrane or part of the extracellular matrix, get turned into glycoproteins by the attachment of branching polysaccharide trees, which can serve as a protective barrier, or as cell surface recognition signals, or as attachment points for other things
Membrane potential
Membrane potential is the charge relative to the inside of the cell as a result of the distribution of ions on either side of the cell membrane. Resting membrane potential is negative.
Compare endosomes and lysosomes
Most endosomes eventually fuse with lysosomes, and thereby mature into larger lysosomes themselves Lysosomal fluid is acidic → if broken open and enzymes released, no harm done, because they do not work in the near-neutral pH of the cytoplasm Lysosome membrane also contains transporters to move the resulting small molecules out to the cytoplasm for reuse
Explain the role of GTP-hydrolysis in the nuclear pore complex.
Nuclear transport is driven by GTP hydrolysis in the opposite direction of incoming protein transport. Once the loaded transport is inside the nucleus, a small Ran protein with bound GTP displaces the cargo and binds to the transporter itself. Once it is back in the cytoplasm, the GTP is hydrolyzed and Ran is released.
Double membrane-bound organelles
Nucleus, mitochondria and chloroplast
Explain how water diffuses across membranes by osmosis.
Osmosis: water diffuses across the cell membrane down its own concentration gradient (more to less). The concentration gradient of water is the reverse of the concentration gradient for the solute.
Describe how O2 and CO2 cross the plasma membrane. What characteristics of each substance determine its mode of transport?
Oxygen and CO2 are small, hydrophobic, and nonpolar substances that can pass straight through the membrane by simple diffusion.
Outline how the asymmetric distribution of phospholipids, characteristic of different membrane types, is established and maintained.
Phospholipids and glycolipids are distributed asymmetrically in animal cell membranes. Flippase transporters flip particular types of phospholipids preferentially to one side or another. Membranes are made by enzymes attached to the cytosolic surface of the ER, get inserted into the outer side, and scramblase transporters scramble them, randomly moving about half of them across to the inside of the membrane
Describe the ways the phospholipids can and cannot move on their own within membranes.
Phospholipids can move rotationally, laterally in one bilayer, or undergo transverse movement between bilayers. Lateral movement provides the membrane with a fluid structure.
Explain the function of K+ leak channels.
Play a major role in maintaining the resting membrane potential in animal cells. The strong net negative charge inside the cell, which strongly attracts the positively charged potassium, has a tendency to keep it there. This allows the resting membrane potential to remain fairly constant.
What causes substances to diffuse?
Random kinetics from higher concentration to lower concentrations
What is/are the difference(s) between cytoplasmic and membrane-bound ribosomes?
Ribosomes in the cytoplasm and on the ER are identical and move back and forth between one another. They may produce different proteins. ER ribosomes synthesize proteins that are being translocated into the ER, while free-floating ribosomes synthesize all other proteins encoded by the nuclear genome.
Which proteins play a central role in the fusion of a vesicle with a target membrane?
SNAREs
Golgi Apparatus Structure
Series of broad, flat, membranous sacs called cisternae
What do signal sequences do, and where are they located?
Signal sequences direct proteins to the correct destination outside of the cell membrane by tagging the protein for transport. They are located on the N-terminus of some proteins.
In muscle cells, which organelle sequesters Ca2+ from the cytosol?
Smooth ER
What contributes the largest share of the negative charge inside the cell?
Sodium potassium pump. The negative charge is created by the potassium movement out of the cell, making the inside more negative.
How is the Na+/K+ pump maintained?
The Na+-dependent phosphorylation and K+-dependent dephosphorylation arrange the conformational shifts into a one-way cycle, so that both ions are pumped against their concentration gradients, and those gradients are maintained until needed for other processes later.
How and why is a membrane self-healing?
The amphipathic nature of phospholipids forms the bilayer. This bilayer spontaneously closes in on itself to form sealed compartments. This is what makes the membrane self-healing, and resilient to small bumps, pokes, and tears.
Explain the cycle of the Na+-K+ pump to generate and maintain a gradient?
The carrier is open to the cytoplasmic side first, which allows for 3 sites for Na+ to fit After the third sodium binds, phosphorylation is triggered by ATP Phosphorylation changes the confirmation and the new shape is open to the outside of the cell and releases the sodium Potassium ions bind to the new binding sites When the second potassium binds, the carrier releases the phosphate The confirmation is reverted back to its original shape, which releases the potassium into the cytoplasm and reopens them again for sodium
Endomembrane system
The collection of membranes inside and around a eukaryotic cell, related either through direct physical contact or by the transfer of membranous vesicles. Includes endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.
Explain what is meant by a concentration gradient when dealing with a membrane.
The concentration gradient is the flow of a molecule from an area of higher concentration to one of lower concentration.
How does the mixed polarity/nonpolarity of phospholipids and membrane proteins determine the structure of the cell membrane?
The internal bonds are nonpolar, so they do not interact with extremely polar molecules of water. Therefore, the nonpolar tails are faced inward, while the polar heads are on the outer edge, which determines the overall structure of the phospholipid bilayer of the cell membrane.
What is the main way glycocalyx cells recognize each other.
The main way cells recognize each other — and one of the ways they communicate with each other — is through lectin receptor proteins which bind to these carbohydrates on other cells.
In a famous experiment, mouse cells and human cells were fused into hybrid cells and the membrane proteins of human origin and mouse origin were specifically tagged and examined. After cell fusion and incubation, what was observed by investigators?
The mouse and human proteins mixed evenly throughout the membrane of the hybrid cell.
How does the structure of the Nuclear Pore Complex select for particular proteins while allowing free diffusion of water and small solutes?
The nuclear membrane allows free passage of ions and small molecules because it is perforated with numerous nuclear pore complexes—3000-4000 in a typical mammalian cell—each of which has one or more open aqueous channels through which small water-soluble molecules can passively diffuse.
How does the structure of the cell membrane make it selectively permeable?
The phospholipids are arranged in a tightly packed bilayer with the hydrophobic tails facing inward. These tails are packed tightly which prevents any large molecules from diffusing through.
Contrast how the plasma membrane is mechanically stabilized in animal cells versus plant cells.
The plasma membrane is reinforced by the underlying cell cortex. By anchoring many of the membrane proteins, the cell cortex just beneath greatly strengthens the membrane to give animal cells their shape. Plant cells have a cell wall to stabilize the plasma membrane mechanically.
Where are transmembrane proteins made and inserted into membranes?
Transmembrane proteins are synthesized and assembled at the endoplasmic reticulum. This is where all of the membrane-spanning segments of a protein are inserted and oriented correctly into the membrane so the protein can fold properly.
Explain how the polypeptide chain of a transmembrane protein, with its hydrophilic backbone, is able to span the hydrophobic interior of the lipid bilayer.
Transmembrane proteins expose the hydrophilic regions on either side of the membrane, and cross the lipid bilayer as an alpha helix, which folds in the hydrophilic side chains so they are hidden and only the hydrophobic region is exposed to the hydrophobic part of the bilayer.
Exocytosis
Transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
Describe the sequence of events involved in an action potential's travel along the membrane.
Triggered by a depolarization of a neuron's plasma membrane Influx of sodium depolarizes the membrane so it becomes less negative inside Membrane will reach its threshold potential of -40mV, which triggers opening of more sodium channels to begin the action potential Raises MP to +50mV, with a slight delta Reversed polarity triggers voltage-gated potassium channels K+ floods out of the cell and repolarizes the cell Once triggered, action potentials are all or nothing events
How do ion channels select for specific ions?
They are shaped like an hourglass that ensures only ions of a certain size and charge are able to pass through
How do precursor proteins enter mitochondria and chloroplasts?
They are unfolded and threaded through translocater tunnel proteins. Once inside, their signal sequences are clipped off, and with the help of chaperones, the proteins refold into their final functional conformations.
What is the role of K+-gated ion channels in an action potential?
They help reverse the action potential by repolarizing the cell.
Single membrane-bound organelles
Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum
Discuss how transport vesicles keep their contents separate from the cytosol, and why this might be important.
Vesicles budding from the ER usually go next to the golgi, which shuffles its content through another series of vesicles within itself, before finally sending them - by other vesicles - to other parts of the cell, or the cell membrane, or exported from the cell entirely
How is an electrical signal converted to a chemical signal at a nerve terminal?
Voltage-gated Ca2+ channels are activated and the influx of Ca2+ triggers the release of neurotransmitters.
Describe how water crosses the plasma membrane. What characteristics of each substance determine its mode of transport?
Water passes through the membrane with the use of aquaporins.
How are voltage-gated ion channels opened by voltage sensors?
When membrane potential changes sufficiently, the electrical force causes voltage sensor domains to change conformation.
A single-pass transmembrane protein destined for one of the organelles in the endomembrane system would be marked by what type of signal sequence?
a cleaved N-terminal ER signal sequence and an internal stop-transfer sequence
Nodes of Ranvier
a gap in the myelin sheath of a nerve, between adjacent Schwann cells, that allows for the diffusion of ions
Which membrane would show a more rapid recovery of fluorescence in a FRAP study?
a membrane containing a larger proportion of unsaturated fatty acids
Scramblase
an enzyme that randomly mixes phospholipids from one face of the ER membrane to the other
Inhibitory neurotransmitters such as glycine and GABA make a postsynaptic cell harder to depolarize by allowing what?
an influx of Cl-
In a fluorescence recovery after photobleaching (FRAP) experiment, a fluorescently tagged membrane protein, A, shows very little recovery of fluorescence ten minutes after photobleaching, while membrane protein B shows a rapid increase in fluorescence after bleaching, recovering nearly 80% of its original fluorescent signal by ten minutes. Based on this information, which of the following statements can be made? a. Protein A has faster mobility in the membrane. b .Protein B is diffusing in a more fluid membrane. c. Proteins A and B interact with each other in a complex. d. Protein A is easily denatured.
b .Protein B is diffusing in a more fluid membrane.
Which of the following is a common transmembrane protein structure that can traverse the membrane to form a pore or channel by alternating hydrophobic and hydrophilic amino acids? a. single-pass alpha helix b. multipass alpha helix c. beta barrel d. beta sheet
c. beta barrel
If the backbone of a polypeptide is hydrophilic, how can a transmembrane alpha helix span the hydrophobic portion of the lipid bilayer?
because amino acid side chains in a transmembrane helix are hydrophobic and interact with the hydrophobic interior of the bilayer
Receptors
bind signaling molecules and pass the message to the inside
Which of the following is a difference between transporters and channels? a. Transporters move solutes against their concentration gradient, whereas channels can move solutes with or against their concentration gradient. b. Channels do not discriminate between ions, whereas transporters bind their solute with extreme specificity. c. Transporters can facilitate both active or passive transport of solutes; channels facilitate only passive transport. d. Channels are single-pass transmembrane proteins, whereas transporters are multipass transmembrane proteins.
c. Transporters can facilitate both active or passive transport of solutes; channels facilitate only passive transport.
What determines the direction that glucose is transported across the membrane, through a glucose transporter?
concentration gradient
Phagocytosis is a process by which cells do what?
consume large particles, such as microbes and cell debris
Nucleus
contains main genome; DNA and RNA synthesis
Cytosol
contains many metabolic pathways; protein synthesis; the cytoskeleton
Cell body (soma)
contains nucleus and cytoplasm; organizes and keeps cell functional
Lipoproteins
covalently anchored to membrane lipids
Most mitochondrial and chloroplast proteins are made within which part of the cell?
cytosol
Which of the following would be able to cross a protein-free lipid bilayer most rapidly? a. a chloride ion (charged, small) b. glucose (uncharged polar, large) c. ethanol (uncharged polar, small) d. a steroid hormone (nonpolar, large)
d. a steroid hormone (nonpolar, large)
What inhibits inorganic ions, such as Na+ and Cl-, from passing through a lipid bilayer?
the hydrophobic interior of the lipid bilayer
In the α helices of transmembrane proteins, the hydrophobic side chains face which direction?
the outside of the membrane-spanning helix
Synaptic transmission
the process at synapses by which a chemical signal (a transmitter) is released from one neuron and diffuses to other neurons or target cells where it generates a signal which excites, inhibits or modulates cellular activity.
Infolding of the plasma membrane
the process of the plasma membrane of a prokaryotic cell infolds to form internal membranes in creating membrane-bound compartments. Overtime, these compartments can become specialized and develop into distinct organelles. For example, the endoplasmic reticulum and Golgi apparatus likely evolved from the infolding of the plasma membrane.
What is the molecular target of the antidepressant Prozac?
the symport that drives reuptake of serotonin
What happens to proteins with no signal sequence that are made in the cytosol?
they remain in the cytosol
Why do cells regulate their membrane fluidity?
to allow membrane proteins to diffuse to where they are needed for their function
Axon
transmites information away from the nucleus to other cells/organs
What do proteins travel through from one cisterna to the next in the golgi apparatus?
transport vesicles that bud from one cisterna and fuse with the next
Plasma membrane proteins that move ions in and out of cells using active transport are called
transporters
Which type of membrane transport protein can perform either passive or active transport?
transporters
Receptor-mediated endocytosis
uses clusters of receptor proteins for some particular kind of molecule Molecules binds to the ligand → once enough copies of a ligand bind to a cluster of these receptors, they're all taken into a vesicle and engulfed
Active transport
uses energy (ATP) to move a substance from an area of low concentration to high concentration (against its concentration gradient). Directly uses a source of chemical energy to move molecules across the membrane against its gradient Sodium-Potassium Pump
How do the interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other?
vesicle fusion -- small vesicles that bud off of one organelle and fuse with another
In which process do Rab proteins function?
vesicle tethering
How are newly made lipids supplied to the plasma membrane?
via the constitutive pathway of exocytosis
Hypotonic
water enters the cell (more solute inside the cell)
Hypertonic
water leaves the cell (more solute on the outside)
Cotransport (secondary active transport)
when you indirectly, actively transport an ionic solute in one direction, then as those ions diffuse back in the other direction, they drive the transport of a different kind of solute (Na+/Glucose cotransport) Uses an electrochemical gradient - generated by active transport - as an energy source to move molecules against their gradient, and does not require a chemical source of energy like ATP Na+-Glucose Symporter
In a lipid bilayer, where do lipids rapidly diffuse?
within the plane of their own monolayer
In an average cell, about how much of the total cell volume is taken up by membrane-bound organelles?
On average, the membrane-enclosed organelles together occupy nearly half the volume of a eukaryotic cell, and the total amount of membrane associated with them is enormous. In a typical mammalian cell, for example, the area of the endoplasmic reticulum membrane is 20-30 times greater than that of the plasma membrane. In terms of its area and mass, the plasma membrane is only a minor membrane in most eukaryotic cells.
Ions in solution are found in a hydration shell of water. This shell must be removed for an ion to pass through the channel. How does the K+ channel accomplish removal of the water from the shell around the ion?
Carbonyl groups lining the wall of the pore can interact with the unsolvated K+ ion, balancing the energy needed to remove the hydration shell.
Explain some of the different ways that cells deal with the problem of osmoregulation.
Conforming and regulating. Osmoconformers use active or passive processes to match their internal osmolarity to that of the environment. Osmoregulators have a greater concentration in their body fluids than the surrounding fluids to prevent water and salt loss. Excretion of salts/water through sweat, urine, and feces.
Glucose enters the cell by which process? a. osmosis b. facilitated diffusion c. active transport d. simple diffusion
facilitated diffusion
Which characteristic describes the tails of phospholipids?
hydrophobic
Phospholipids assemble into in a membrane using
hydrophobic forces
What types of products are suited for constitutive secretion?
in busy phagocytes and secretory cells
Proteins encoded by nuclear genes and destined for the mitochondrial matrix are
in possession of a single sequence for targeting to the mitochondria
What is the conformation of the voltage-gated Na+ channel that keeps the action potential from traveling backward along the axonal membrane?
inactivated
Myelin Sheath
increases the speed of a signal
Organisms that live in cold climates adapt to low temperatures by doing what?
increasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid
Nuclear pores restrict larger molecules from traversing the membrane due to their
interwovem meshwork of protein fibrils
Lysosomes
intracellular digestion
How do excitatory and inhibitory neurotransmitters differ in their mechanisms?
Excitatory neurotransmitters open ligand-gated cation channels to move the membrane potential towards threshold. Inhibitory neurotransmitters open ligand-gated Cl- channels to hyperpolarize the membrane moving it away from threshold.
Peripheral proteins
lie completely outside the bilayer but are non-covalently linked to other single-pass embedded proteins
An extracellular molecule binds to a channel and triggers it to move more often to the open conformation than the closed conformation, as shown in the figure. This is referred to as a ___________ channel.
ligand-gated
________ are the most abundant molecules in the animal cell membrane, whereas ________ make up 50% of the membrane by mass.
lipids; proteins
Autophagy
lysosomes break down damaged organelles even cells which don't engulf things use lysosomes to break down and recycle worn out organelles and other cellular debris and molecular junk that needs cleaning up.
Adaptin
mediates contact between the receptor and another component
What is a functionally specialized region of a cell membrane, typically characterized by the presence of specific proteins, called?
membrane domain
What is the voltage difference across a membrane of a cell called?
membrane potential
Golgi Apparatus
modification, sorting, and packaging of proteins and lipids for either secretion or delivery to another organelle
Cargo
molecules packaged into vesicles for transport
Transporters
move materials across the membrane
Isotonic
no net movement of water into or out of the cell
Which proteins bind to nuclear localization signals on newly synthesized proteins?
nuclear import receptors
Which of these macromolecules is NOT commonly found in the plasma membrane? a. carbs b. proteins c. nucleic acids d. fatty acids
nucleic acids
Fully folded proteins can be transported into which organelle?
nucleus
Protoeukaryote
occurred when an anaerobic Archaean cell developed infoldings of the outside membrane, which later became the ER and nuclear envelope
Ligand-gated channels
open in response to binding a signal molecule, typically a neurotransmitter, on the outside, or a different signal molecule intensity
Inhibitory neurotransmitter mechanism
open ligand-gated Cl- channels to hyperpolarize the membrane moving it away from threshold.
Excitatory neurotransmitter mechanism
open ligand-gated cation channels to move the membrane potential towards threshold.
Constitutive secretion
operates in all cells, and is on all the time at a fairly steady level. New membrane components are continually added to replace those lost to endocytosis and to wear and tear, and likewise with components of the extracellular matrix
Regulated secretion
operates in cells specialized for secretion of things like hormones, neurotransmitters, or digestive enzymes Highly concentrated amounts of these molecules are stored, on standby, in vesicles just inside the plasma membrane. External signals can then trigger rapid release of large amounts of products for larger-scale or faster acting effects.
Peroxisomes
oxidative breakdown of toxic molecules
When a neuron has been stimulated by a signal, the change in membrane potential first spreads locally to adjoining regions of the plasma membrane by what means?
passive spread
Proteins that are associated with the membrane by noncovalent interactions with other membrane proteins are called ___________ proteins.
peripheral membrane
Which of the following compartments receives proteins directly from cytosol? a. golgi apparatus b. lysosomes c. endosomes d. peroxisomes
peroxisomes
Which of the endocytic pathways involves the ingestion of large particles or microorganisms and is performed mainly by specialized cells?
phagocytosis
All other factors (concentration, solute size, etc.) being equal, which type of solute does a cell tend to pull inside?
positively charged solutes
Schwann cells
produce myelin in PNS
Transmembrane proteins
stretch all the way across, with hydrophilic regions exposed on either side → cross the lipid bilayer as an alpha helix
Endosymbiont Theory
suggests that an early anaerobic ancestor of eukaryotes engulfed an oxygen-using prokaryotic cell → led to the formation of mitochondria/chloroplasts
What is the name of the specialized junction between a neuron and a target cell?
synapse
Endoplasmic Reticulum (ER)
synthesis of most lipids; synthesis of proteins for distribution of many organelles and to the plasma membrane
Endocytosis
taking things into the cell in bulk → membrane forms a pocket that deepens and pinches off, forming a vesicle around the imported material.
Proteins destined for the Golgi apparatus, endosomes, lysosomes, and even the cell surface must pass through which organelle?
the ER
How does the original direction of transmembrane protein within the membrane relate to their direction in the membrane at their destination?
Exocytic pathways go in the opposite direction, starting with the synthesis of proteins on the ER membrane and leading through the Golgi apparatus to the cell surface.
What is a main difference between exocytic and endocytic pathways?
Exocytic pathways often start with synthesis of proteins, whereas endocytic pathways involve breaking down macromolecules like proteins.
Contrast the actions of flippases and scramblases.
Flippase transporters flip particular types of phospholipids preferentially to one side or the other. Scramblases will scramble the phospholipids by randomly moving about half of the phospholipids across the inside of the membrane.
Electrochemical gradient
the combined forced of the concentration gradient of solutes and the electrical gradient of charge separation. In other words, positive charges are attracted to negative ones and vice versa.
When glucose moves across a phospholipid bilayer by passive transport, which factor determines the direction of its transport?
the concentrations of glucose on either side of the membrane
If a phospholipid is located in the outer layer of the bilayer in a vesicle, where will it end up when the vesicle fuses with the plasma membrane?
the cytosolic face of the bilayer
In eukaryotic cells, phospholipids are synthesized by enzymes bound to what?
the cytosolic face of the endoplasmic reticulum
What factors influence a membrane's degree of fluidity, and how?
Fluidity depends on the membrane's composition and temperature. Higher temperature increases the fluidity of the membrane. Colder temperatures can freeze the membrane. Temperatures that are too high can cause the membrane to rupture and melt. Unsaturated phospholipids have kinked tails which keeps the layer more fluid at lower temperatures. Longer tails make the membrane stiffer and less fluid.
Describe how clathrin and similar coat-proteins help to form endocytic vesicles.
Form basketlike cages that help shape membranes into vesicles and transport selected cargo molecules.
Describe how glucose crosses the plasma membrane. What characteristics of each substance determine its mode of transport?
Glucose travels through the plasma membrane with Na+ as a cotransport system (gradient-driven pump)
Review the structure and functions of the glycocalyx.
Glycocalyx is the carbohydrate layer when eukaryotic cells are coated with sugar. They have short oligosaccharide chains attached to their outer sides as part of the extracellular matrix. They help protect and strengthen the cell, and serve as a lubricant for the surface of the cell. They also help it attack and interact with other cells. (Protection; lubrication; cell-cell recognition)
On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located?
the extracellular side
Explain the process by which nerve signals travel across a synapse.
Impulse travels down the axon of a presynaptic neuron. Reaches the synaptic end bulb and opens voltage sensitive Ca 2+ channels. Calcium enters the cell and through a series of reactions, cause the synaptic vesicles to fuse to cell membrane. Vesicles release neurotransmitters into the synaptic cleft.
How are misfolded proteins and incompletely assembled proteins retained in the ER?
chaperone proteins bind them and prevent their entry into vesicles
Which organelle is the site of steroid hormone synthesis in endocrine cells?
smooth endoplasmic reticulum
Endosomes
sorting of endocytosed material
How does a substance's concentration gradient influence its diffusion?
A solute that is more concentrated will move to an area where it is less concentrated. Water will move from areas of lower solute to higher solute.
Which of the following is a function of proteins in the plasma membrane? Select all that apply. -allow specific ions to cross the plasma membrane, thereby controlling its electrical properties -generate the energy required for lipids to diffuse within the membrane -serve as anchors to attach the cell to the extracellular matrix -transmit extracellular signals to the cell interior -transport molecules across the membrane
-allow specific ions to cross the plasma membrane, thereby controlling its electrical properties -serve as anchors to attach the cell to the extracellular matrix -transmit extracellular signals to the cell interior -transport molecules across the membrane
The plasma membrane is involved in which activities? Select all that apply. a. import and export of nutrients and wastes b. cell growth and motility c. cell signaling d. RNA interference e. cell recognition f. DNA replication and repair
-cell signaling -cell growth and motility -cell recognition -import and export of nutrients and wastes
Which of the following organelles are surrounded by a double membrane? Select all that apply. a. nucleus b. chloroplast c. peroxisome d. mitochondrion e. golgi apparatus f. lysosome
-nucleus -chloroplast -mitochondrion
Contrast the structure and potential function of single-pass and multipass transmembrane proteins.
A multipass protein crosses the bilayer multiple times, whereas a single-pass protein only crosses one time. Multipass proteins can help with communication and are often targeted by drugs. Multipass proteins contain two or more parallel stretches of nonpolar amino acids traveling back and forth, typically in the form of multiple alpha helices, plus more hydrophilic terminal and linking segments exposed to the aqueous environments
Which of the following would produce the most fluid lipid bilayer? A. phospholipids with tails of 18 carbon atoms and two double bonds B. phospholipids with fully saturated tails of 18 carbon atoms C. phospholipids with tails of 20 carbon atoms and two double bonds D. large amounts of cholesterol E. phospholipids with fully saturated tails of 20 carbon atoms
A. phospholipids with tails of 18 carbon atoms and two double bonds
In a typical human secretory cell, which of the following membranes has the largest surface area? A. rough ER B. nuclear inner membrane C. lysosome D. plasma membrane E. smooth ER
A. rough ER
Which of the following would be most likely to disrupt lipid bilayer formation? A. addition of a hydroxyl group to the head group of the lipid B. addition of cholesterol to the membrane C. addition of a phosphate to the end of the lipid tail D. addition of a methyl group to the end of the lipid tail
C. addition of a phosphate to the end of the lipid tail
What is typically true of ion channels? A. They operate by active transport. B. They are gated. C. They are open all the time. D. They hydrolyze ATP. E. They are nonselective.
B. They are gated.
Outline the ways that cells restrict the lateral movement of their membrane proteins.
By tight junctions which restrict membrane proteins. Cell junctions link cells permanently. Lateral movement of membrane proteins can also be restricted by anchoring to the cytosolic cortex inside, to the extracellular matrix outside, or to a neighboring cell.
Which of the following accurately describes a step in GTP-driven nuclear transport? A. GTP-bound cargo interacts specifically with the protein fibrils of the pore. B. Ran-GDP escorts the nuclear receptor back to the cytosol. C. Binding of Ran-GTP to the receptor releases the cargo protein. D. GTP hydrolysis powers a membrane-bound transporter protein.
C. Binding of Ran-GTP to the receptor releases the cargo protein.
Which of the following ions has a low cytosolic concentration so that a flood of this ion into the cell can be used as a signal for cell processes like fertilization? a. Ca2+ b. H+ c. K+ d. phosphate
Ca2+
How do clathrin-coated vesicles select their cargo molecules?
Cargo receptors bind specifically to cargo proteins and to clathrin.
What role does cholesterol play in animal cell membranes, and how?
Cholesterol fills the gaps between tails so the lipid bilayer is less leaky to smaller molecules. It also acts as antifreeze. The bulkiness of cholesterol reduces the tight packing of the phospholipids, to keep membranes more fluid when it is cold.
What are the two ways which proteins move through the golgi body?
Cis face: receives vesicles from the ER → vesicles move batches of the contents between cisternae toward the trans face → then buds off vesicles containing the final products
In general, which of the following will diffuse across a lipid bilayer most rapidly? A. a large hydrophobic molecule B. a small hydrophilic molecule C. a large hydrophilic molecule D. a small hydrophobic molecule E. water
D. a small hydrophobic molecule
Which of the following mechanisms prevents osmotic swelling in plant cells? A. the activity of Na+ pumps B. the collection of water in contractile vacuoles C. the expulsion of water from contractile vacuoles D. turgor pressure E. tough cell walls
E. tough cell walls
What would the final destination be for a protein bearing both an ER sorting signal and a nuclear localization signal?
ER
Which organelle is the major site of new membrane synthesis in a cell?
ER
Unfolded Protein Response
ER can get too cluttered with unfolded/misfolded proteins → triggers the unfolded protein response → stimulates the expression a whole suite of genes to enhance protein folding capacity, producing more chaperones, enlarge the ER, and reduce the rate of other new protein synthesis for a while until the rest of the assembly line can catch up If this is not enough, UPR can trigger apoptosis.
Distinguish between integral membrane proteins and peripheral membrane proteins.
Integral proteins are embedded deeply in the membrane due to large hydrophobic regions. Peripheral proteins lie completely outside of the bilayer but are non-covalently linked to other single-pass embedded proteins.
endosome
Membrane-enclosed compartment of a eukaryotic cell through which material ingested by endocytosis passes on its way to lysosomes.
Describe how membranes retain their orientation during transfer between cell compartments.
Membranes maintain their orientation during the transfer between cell components. For example, cell recognition carbs and hormone receptors face outward; domains that anchor to the cytoskeleton face inward (on the inside). They maintain the established orientation as they move through other vesicles and organelles, or out to the cell surface.
What does it mean that membranes are fluid mosaics?
Molecules are always in motion. There is random thermal shaking against one another, flexing of tails, spinning on axes, and sideways diffusion constantly happening.
Lipid bilayers are highly impermeable to which molecule(s)?
Na+ and Cl-
On which side of the plasma membrane are Na+, K+, and Cl- more concentrated?
Na+ and Cl- are more concentrated OUTSIDE of the cell K+ is more concentrated INSIDE the cell
Describe how Na+/K+ cross the plasma membrane. What characteristics of each substance determine its mode of transport?
Na+/K+: The sodium-potassium pump allows these ions to pass through the plasma membrane.
Explain the role of nuclear import receptors .
Nuclear import receptors are soluble cytosolic proteins that can bind to the nuclear localization signal on the protein to be transported, and to the nucleoporins (tentacle-like fibers)
What is one of the main differences in the behavior of the proteins in a vesicle destined for constitutive secretion, and the proteins in the vesicle destined for regulated secretion?
Proteins in the regulated secretion vesicle tend to aggregate and become highly concentrated in the ionic conditions in the vesicle.
Which term correctly describes the entire phospholipid molecule? a. amphipathic b. apathetic c. hydropathic d. hydrophobic e. hydrophilic
amphipathic
Golgi Apparatus Functions
Sorts, tags, and ships things in the cell. Edits and modifies proteins and other cellular products.
How and when are transmembrane proteins made and inserted into membranes?
Start and stop signals determine the arrangement of a transmembrane protein in the lipid bilayer. Transmembrane proteins also have a hydrophobic stop-transfer sequence somewhere in the middle of the chain. Sequence halts further transfer, and remains embedded within the membrane, usually as an alpha helix, while construction of the protein continues, and the remaining portion remains in the cytosol.
T/F: An action potential is an all or nothing event.
TRUE
True or False: Given enough time, virtually any molecule will diffuse across a lipid bilayer.
TRUE
In what two locations is Ca2+ more abundant?
The endoplasmic and sarcoplasmic reticulum. Calcium pumps keep cytosolic concentrations low. Stockpile of calcium gets used to trigger muscle contractions in muscle cells and as secondary messengers in the phospholipase C signaling pathway
Why is the diversity of shapes in the sugar trees so important?
The glyco- parts of glycoproteins and lipids can be assembled into a wide variety of branching patterns. That sugar forest varies among different species and individuals, and even between different cell types in the same individual.
Why do phospholipids form bilayers in water?
The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.
Which type of movement is the least common for lipids in a bilayer? a. flip-flop b. lateral diffusion c. flexion d. rotation
a. flip-flop
A less permeable membrane is likely to have a. more cholesterol b. less cholesterol c. many unsaturated fatty acids d. shorter fatty acids
a. more cholesterol
Which of the following is a cell type in humans that uses voltage-gated ion channels? a. muscle cells b. skin cells c. liver cells d. gut epithelial cells
a. muscle cells
The movement of an ion against its concentration gradient is called what?
active transport
What strategy do prokaryotic cells use to isolate and organize their chemical reactions?
aggregating proteins into multicomponent complexes that form biochemical subcompartments with distinct functions
Inside the cell, the quantity of positively charged ions is __________ the quantity of negatively charged ions. -greater than -less than -equal to
almost equal to
Detergent molecules are ___________ in nature and bind with membrane proteins and membrane lipids to disrupt their interactions and release the proteins from the membrane.
amphipathic
A cell membrane made up primarily of lipids with which characteristics would be the least fluid (i.e., most stiff)? a. short, unsaturated fatty acid tails b. long, saturated fatty acid tails c. long, unsaturated fatty acid tails d. short, saturated fatty acid tails
b. long, saturated fatty acid tails
A phospholipid is inserted into the cytosolic side of the ER membrane. Which of the following could randomly reposition this phospholipid to the other (lumen) side of the ER membrane? a. flippase b. scramblase c. glycolipids d. golgi apparatus
b. scramblase
Receptor
captures the correct cargo molecules
Transport Vesicles
carry soluble proteins and membrane between compartments
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 This meshwork of protein filaments is attached to the underside of the plasma membrane.
Pinocytosis
cell drinking --> takes up fluids and macromolecules
Phagocytosis
cell eating --> swallowing it whole
Which of the following is NOT a way that cells restrict the lateral movement of membrane proteins? a. anchoring to internal cell components like actin or other proteins b. tethering to external cell components like the extracellular matrix or adjacent cells c. establishing diffusion barriers in the membrane d. reducing the temperature of the membrane
d. reducing the temperature of the membrane
What is an action potential?
electrical impulse that travels down the axon triggering the release of neurotransmitters A rapid depolarization (upstroke) followed by repolarization of the membrane potential. A sudden, fast, transitory, and propagating change of the resting membrane potential.
Which factors determine the force driving the passive transport of charged solutes across the membrane?
electrochemical gradient
Integral proteins
embedded deeply in the membrane due to large hydrophobic regions
The movement of materials from the plasma membrane, through endosomes, and then to lysosomes describes which type of pathway?
endocytic pathway
The outer membrane of the nucleus is continuous with the membrane of which other organelle?
endoplasmic reticulum
Where are new phospholipids made?
endoplasmic reticulum
Which organelle is important for controlling the concentration of calcium ions in the cytosol?
endoplasmic reticulum
Which organelle sorts ingested molecules and recycles some of them back to the plasma membrane?
endosome
Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?
endosomes
SNARES
filaments on both the vesicle and the target membranes wrap around each other and pull the vesicle close enough that it can fuse with the destination membrane.
Tethers
filaments which latch onto the vesicle and keep it from drifting away, like tethering a balloon or a boat
Which of the following is a covalent modification that occurs mainly in the ER? A. formation of disulfide bonds B. addition of phosphate groups C. maturation of oligosaccharide chains D. methylation of side chains
formation of disulfide bonds
Axon terminal
forms junctions with other cells
Axon hilock
generates impulse in the neuron
What is responsible for moving glucose from the gut lumen into intestinal epithelial cells?
glucose-sodium symport
Which of the following organelles is surrounded by a single membrane? a. nucleus b. mitochondria c. chloroplasts d. golgi apparatus
golgi apparatus
Which organelle receives proteins and lipids from the endoplasmic reticulum, modifies them, and then dispatches them to other destinations in the cell?
golgi apparatus
What types of products are suited for regulated secretion?
horomes, neurotransmitters, or digestive enzymes
Lysosomes contain ____________ enzymes that can break down diverse macromolecules, cell parts, and microorganisms.
hydrolytic
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
Dendrites
receive messages from other neurons
How do virusis and LDLs enter the cell?
receptor-mediated endocytosis
Which of the following pathways helps selectively concentrate substances to be ingested by their binding to proteins on the cell surface?
receptor-mediated endocytosis
Voltage-gated ion channels
respond to changes in membrane potential, specifically depolarization
Mechanically-gated channels
respond to touch or pressure or physical force of some sort
Which of the following organelles is NOT a part of the endomembrane system? a. ER b. golgi apparatus c. lysosomes d. ribosomes
ribosomes
Flippase
selectively move phospholipids from one face to another.
Ion channels contain a selectivity filter that
selects for ions based on size and charge due to the width of the channel and charge of amino acids lining the channel.
Clathrin
shapes a forming vesicle
Cells, compared with the extracellular fluid are
slightly negatively charged.
