Biology Chapter 15
Describe how pinocytosis is carried out
Mostly done by clathrin-coated vesicles which pinch off from plasma membrane, rapidly shed their coat, and fuse with an endosome.
Describe how the nuclear membranes and membranes of the ER, Golgi apparatus, endosomes, and lysosomes originated. What system is made up of these organelles?
By invagination of the plasma membrane. These organelles constitute the endomembrane system, which communicate with one another by vesicles.
Define coated vesicles
Vesicles that bud from membranes usually have a distinctive protein coat on their cytosolic surface. This coat is usually shed after the vesicle buds from its parent organelle, and the coat has two functions: shape membrane into a bud and capturing molecules for onward transport.
Be able to identify organelles on cartoon picture (in pre-lecture and powerpoint)
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What are the two types of COP-coated vesicles (COP short for "coat protein")?
COPII proteins go from ER to Golgi cisternae COPI proteins go from Golgi cisternae to ER
How is the amount of cell membrane kept constant?
Loss of cell membrane by endocytosis balanced by addition of cell membrane by exocytosis.
Many viruses enter cells through receptor-mediated endocytosis. Which of the following strategies could be affective in blocking entry of this class of viruses into cells and could be used to treat viral infections? Choose one or more: A.Block the function of adaptin. B.Block the receptor with an antibody. C.Block the actin filaments. D.Increase the activity of clathrin.
A,B Some viruses such as the human rhinovirus 2 gain entry into their host cells through receptor-mediated endocytosis by binding the LDL receptor on the surface of cells. Developing drugs that could block the entry of substrates into the cell by receptor-mediated endocytosis might be an effective treatment for things like colds caused by the rhinovirus or to treat other infections caused by viruses that enter the cell through receptor-mediated endocytosis. The receptor, adaptin, and clathrin are all required for viral entry so inhibition of any of these three would block receptor-mediated endocytosis. Increasing clathrin activity would not prevent viral entry. Actin filaments are not involved in receptor-mediated endocytosis, so blocking them would not affect viral entry.
Which organelle receives proteins and lipids from the endoplasmic reticulum, modifies them, and then dispatches them to other destinations in the cell? Choose one: peroxisome Golgi apparatus endosome mitochondrion nucleus
Golgi apparatus The Golgi apparatus, which is usually situated near the nucleus, receives proteins and lipids from the ER, modifies them, and then dispatches them to other destinations in the cell. Transport from the ER to the Golgi apparatus—and from the Golgi apparatus to other compartments of the endomembrane system—is carried out by the continual budding and fusion of transport vesicles. Proteins entering the Golgi can either move onward through the Golgi stack or, if they contain an ER retention signal, be returned to the ER; proteins exiting from the Golgi are sorted according to whether they are destined for lysosomes (via endosomes) or for the cell surface.
How is exit from the ER controlled?
Proteins to be retained in the ER contain ER retention signals recognized by membrane-bound receptor proteins in the ER and Golgi. Chaperone proteins in the ER bind to proteins that fold improperly until they fold corectly; these chaperones prevent aggregation of misfolded proteins. If proper folding and assembly still doesn't occur, the protein is exported to cytosol and degraded by proteasomes.
In a typical human secretory cell, which of the following membranes has the largest surface area?Choose one: nuclear inner membrane smooth ER lysosome plasma membrane rough ER
Rough ER On average, the membrane-enclosed organelles together occupy nearly half the volume of a eukaryotic cell, and in a typical mammalian cell, the area of the endoplasmic reticulum membrane is 20 to 30 times greater than that of the plasma membrane. This organelle is folded over to form an extensive maze of interconnected spaces.Cells can adjust the size of their ER to accommodate the volume of proteins entering the secretory pathway. So, in cells specialized for secretion, the ER can expand and, on its own, compose about half of the total membrane present in the cell.
Describe the nuclear envelope and the nuclear pore
The nuclear envelope consists of the inner nuclear membrane, which contains proteins that bind to chromosomes and provide anchorage for nuclear lamina, and the outer nuclear membrane, which is continuous with the ER membrane. Nuclear pores are lined by many proteins with extensive, unstructured regions that form a soft, tangled meshwork. This meshwork prevents passage of larger molecules but allows small, water-soluble molecules to pass freely and nonselectively between nucleus and cytosol.
Describe how proteins enter mitochondria and chloroplasts. What about lipids for the membranes?
The proteins usually have a signal sequence at their N-terminus. Proteins destined for either organelle are translocated simultaneously across both the inner and outer membranes at specialized sites where two memrans are close together (they are brought there by import receptor proteins on the outer membrane, for mitochondria at least, and possibly a first translocator). The proteins are unfolded as they are transported, and their signal sequences are removed after translocation is complete. Chaperone proteins help pull in proteins and fold them again once inside. Any subsequent transport to a specific site requires further sorting signals exposed after first sequence is removed. Most membrane phospholipids are imported from ER by lipid-carrying proteins.
How did mitochondria and chloroplasts originate?
They evolved from bacteria that were engulfed by primitive eukaryotic cells with which they initially lived in symbiosis. Thus, they remain separate from the extensive vesicular traffic of the endomembrane system. Note: Mitochondria and chloroplasts have their own ribosomes and DNA and can synthesize some proteins, but most mitochondrial and chloroplast proteins made in cytosol.
Where do proteins/lipids usually go from the ER?
To the golgi apparatus via vesicular transport
Describe the three routes taken by receptors once they have entered an endosome.
1. Most return to same plasma membrane domain from which they came (LDL receptor is an example) 2. Some go to lysosomes where they are degraded 3. Some go to a different domain of plasma membrane, thereby transferring bound cargo molecules across the cell from one extracellular space to another; known as transcytosis. Cargo proteins which remain bound to receptors share the same fate as those receptors, while those which disassociate are moved to lysosomes/late endosomes.
Describe the three main mechanisms by which membrane-bound organelles import proteins
1. Transport through nuclear pores. This occurs for proteins moving from cytosol into nucleus. The pores actively transport specific macromolecules but also allow free diffusion of smaller molecules. 2. Proteins moving from the cytosol into the ER, mitochondria, or chloroplasts (and peroxisomes usually) are transported across the organelle membrane by protein translocators. Transported proteins must usually unfold, unlike in the nucleus, for the translocator to guide it across hydrophobic interior. 3. Proteins moving onward from the ER-and from one compartment of endomembrane system to another- are transported by transport vesicles that deliver soluble cargo proteins along with proteins and lipids part of the vesicle membrane.
Which of the following components of receptor-mediated endocytosis of LDL is incorrectly matched with its function? Choose one: A. lysosome: releases LDL from the receptor B. LDL receptors: form bridges between the LDL particle and adaptin C. adaptin: binds to the specific receptors and recruits clathrin D. clathrin: forms the coated vesicle
A. When LDL binds to the specific LDL receptor on the plasma membrane, an adaptin binds to the receptor on the cytosol side. Adaptin recruits clathrin proteins, which form the coated vesicle. After vesicle formation, the clathrin is released and the naked transport vesicle is transported to the endosome. The endosome is the specific compartment where the LDL is released from the receptor as a result of the compartment's low pH. The empty LDL receptors are then recycled back to the plasma membrane, where the process can repeat. The LDL particles travel from the endosome to the lysosome where the LDL is broken down, finally releasing cholesterol to the cytosol for use in the cell.
Describe endosomes
Act as main sorting station in endocytic pathway. They maintain an acidic environment through an ATP-driven H+ pump; this acidic environment causes most receptors to disassociate from bound cargo. Two sets of endosomes are early endosomes and late endosomes. Early endosomes are located right next to the membrane, and molecules first go there. These early endosomes mature into late endosomes as they fuse with each other or a preexisting late endosome. Late endosomes are found closer to the nucleus and contain some lysosomal enzymes; digestion of cargo proteins and other macromolecules begins and continues as the late endosome matures into a lysosome.
How do clathrin-coated vesicles select their cargo? What are different adaptin types?
Adaptins, which secure the clathrin coat to the vesicle membrane, help capture specific cargo molecules by trapping the cargo receptors that bind them (cargo molecules have transport signals recognized by cargo receptors). Adaptin 1 goes from Golgi apparatus to lysosomes (via endosomes), while Adaptin 2 goes from plasma membrane to endosomes.
Which statements are true of receptor-mediated endocytosis?Choose one or more: A. Internalized endocytic vesicles fuse with lysosomes, which can return empty receptors to the plasma membrane. B. It allows hemoglobin to be taken up by immature red blood cells. C. It allows the internalization of extracellular substances in clathrin-coated vesicles. D. The process can be hijacked by viruses to gain entry into cells. E. It allows cholesterol-carrying low-density lipoproteins (LDLs) to be taken up by cells.
C,D,E In most animal cells, specific macromolecules can be taken up from the extracellular fluid via clathrin-coated vesicles. The macromolecules bind to complementary receptors on the cell surface and enter the cell as receptor-macromolecule complexes in clathrin-coated vesicles. This process, called receptor-mediated endocytosis, provides a selective concentrating mechanism that increases the efficiency of internalization of particular macromolecules more than 1000-fold compared with ordinary pinocytosis. Such is the case when animal cells import the cholesterol they need to make new membranes. Cholesterol is transported in the bloodstream bound to proteins in the form of particles called low-density lipoproteins, or LDL. Cholesterol-containing LDLs bound to receptors on the cell surface are ingested by receptor-mediated endocytosis and delivered to endosomes. In the acidic interior of endosomes, the LDL dissociates from its receptor. These empty receptors are returned, via transport vesicles, to the plasma membrane for reuse. The LDL is delivered to lysosomes, where it is broken down, allowing free cholesterol to enter the cytosol. Receptor-mediated endocytosis is also used to import many other essential metabolites: the vitamin B12 and iron required to make hemoglobin is taken up by immature red blood cells. At the same time, receptor-mediated endocytosis can also be exploited by viruses, such as the influenza virus, to gain entry into cells.
Through which of the following do proteins travel from one cisterna to the next in the Golgi apparatus?Choose one: A. membranes via osmosis B. bridges that link the cisternae C. transport vesicles that bud from one cisterna and fuse with the next D. transporters in the cisternal membranes E. pores in the cisternal membranes
C. Each Golgi stack has two distinct faces: an entry, or cis, face, which is adjacent to the ER, and an exit, or trans, face, which points toward the plasma membrane. The outermost cisterna at each face is connected to a network of interconnected membranous tubes and vesicles. Soluble proteins and pieces of membrane enter the cis Golgi network via transport vesicles derived from the ER. The proteins travel through the cisternae in sequence in two ways: (1) by means of transport vesicles that bud from one cisterna and fuse with the next, and (2) by a maturation process in which the Golgi cisternae themselves migrate through the Golgi stack. Proteins finally exit from the trans Golgi network in transport vesicles destined for either the cell surface or another organelle of the endomembrane system.
How do cis and trans Golgi networks function in protein sorting?
Cis network-proteins that contain an ER retention signal are returned to ER. Trans network-sorts proteins according to whether they are destined for lysosomes (via endosomes) or the cell surface. Additionally in the golgi, the oligosaccharide chains added to proteins in ER are modified (more complex chains are created, for example).
Describle clathrin and dynamin
Clathrin forms clathrin-coated vesicles that bud from the Golgi on the secretory pathway and from the plasma membrane on the endocytic pathway. These vesicles start as a clathrin-coated pit, with a GTP-binding protein called dynamin assembling as a ring around the neck of each invaginated pit. The dynamin causes the neck to constrict, thereby pinching the vesicle off.
Differentiate between the constitutive exocytosis pathway and the regulated exocytosis pathway
Constitutive-operates continuously in all eukaryotic cells. Steady stream of vesicles bud from trans Golgi network and fuse with plasma membrane, supplying it with newly made lipids and proteins. It also carries soluble proteins to be released to the outside, a process called secretion. Regulated-operates only in cells specialized for secretion. The particular product, whether it be a hormone or mucus, is stored in concentrated levels in secretory vesicles which bud off from trans Golgi network and accumulate near the plasma membrane. There, they wait for an extracellular signal which will stimulate them to fuse with plasma membrane and release their contents to cell exterior by exocytosis. Note: These proteins have surface properties that allow them to tolerate acidic pH and high Ca2+ conc. in trans Golgi network.
Phagocytosis is a process by which cells do which of the following?Choose one: engage in receptor-mediated endocytosis secrete hormones and neurotransmitters ingest extracellular fluid and macromolecules digest their own worn-out organelles consume large particles, such as microbes and cell debris
Consume large particles, such as microbes and cell debris. The most dramatic form of endocytosis, phagocytosis, was first observed more than a hundred years ago. In protozoa, phagocytosis is a form of feeding: these unicellular eukaryotes ingest large particles such as bacteria by taking them up into phagosomes. The phagosomes then fuse with lysosomes, where the food particles are digested. Few cells in multicellular organisms are able to ingest large particles efficiently. In the animal gut, for example, large particles of food have to be broken down to individual molecules by extracellular enzymes before they can be taken up by pinocytosis by the absorptive cells lining the gut. Nevertheless, phagocytosis is important in most animals for purposes other than nutrition. Phagocytic cells—including macrophages, which are widely distributed in tissues, and other white blood cells, such as neutrophils—defend us against infection by ingesting invading microorganisms.
Most mitochondrial and chloroplast proteins are made within which part of the cell? Choose one: mitochondrion or chloroplast itself Golgi apparatus peroxisome cytosol endoplasmic reticulum
Cytosol The synthesis of virtually all proteins in the cell begins on ribosomes in the cytosol. Although a few mitochondrial and chloroplast proteins are synthesized on ribosomes inside these organelles, most are made in the cytosol and subsequently imported. The proteins include a sorting signal that directs them to the correct intracellular location. Proteins moving from the cytosol into mitochondria or chloroplasts are transported across the organelle membrane by protein translocators located in the membrane. Unlike the transport through nuclear pores, the transported protein must usually unfold for the translocator to guide it across the hydrophobic interior of the membrane.
Botulism is a potentially fatal foodborne disease caused by the bacterium Clostridium botulinum. C. botulinum produces different toxins, several of which are proteases that cleave neuronal SNARE proteins. What normal process is blocked by cleavage and inhibition of SNARE proteins?Choose one: A. docking of vesicles to target membranes B. budding of vesicles from the endoplasmic reticulum C. entry of proteins with ER signal sequences into the ER lumen D. fusion of vesicles with target membranes
D. Many different proteins help vesicles dock and fuse with the correct target membrane (see below). Rabs interact with tethering proteins during the docking phase, bringing v-SNAREs and t-SNAREs into close proximity. The SNAREs then intertwine, aiding vesicle fusion with the target membrane. In neurons, the t-SNARE synaptosomal nerve-associated protein 25 (SNAP-25) is important for the fusion of neurotransmitter-containing vesicles with the plasma membrane. Cleavage of SNAP-25 by botulinum toxin prevents vesicle fusion (PeerJ. 2015; 3: e1065) and neurotransmitter release, leading to paralysis.
ATP is important for chaperone protein function. Why would protein import into mitochondria be disrupted if ATP were depleted from inside mitochondria? Choose one: A. The translocation apparatus would be unable to function without ATP hydrolysis. B. The signal sequence would not be recognized on the mitochondrial protein. C. The protein would be blocked from entering the translocation apparatus. D. The protein could slip back out of the mitochondria during transport.
D. The chaperones in mitochondria bind to the unfolded protein as it is inserted through the mitochondrial translocation apparatus. Binding of chaperones helps pull the protein into the mitochondria and prevents the unfolded protein from exiting back out. ATP is required for chaperone function, so a lack of ATP would inhibit the function of chaperones and would allow proteins to slip back out of the mitochondria.
The outer membrane of the nucleus is continuous with the membrane of which other organelle? Choose one: A. mitochondrion B. endosome C. peroxisome D. endoplasmic reticulum E. Golgi apparatus
D. The nucleus, generally the most prominent organelle in eukaryotic cells, is surrounded by a double membrane known as the nuclear envelope. The outer nuclear membrane is continuous with the membrane of the endoplasmic reticulum (ER), a system of interconnected membranous sacs and tubes that often extends throughout most of the cell. The ER is the major site of synthesis of new membranes in the cell.The nuclear membranes and the membranes of the ER, Golgi apparatus, endosomes, and lysosomes most likely originated by invagination of the plasma membrane.
How do the interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other? Choose one: A. They do not communicate with one another.B . by fusing with one another C. by excreting hormones and other small signaling molecules D. by small vesicles that bud off of one organelle and fuse with another E. by open pores that allow ions to exit and enter the organelles
D. Transport from the ER to the Golgi apparatus—and from the Golgi apparatus to other compartments of the endomembrane system—is carried out by the continual budding and fusion of transport vesicles. This vesicular transport extends outward from the ER to the plasma membrane, where it allows proteins and other molecules to be secreted by exocytosis, and it reaches inward from the plasma membrane to lysosomes, allowing extracellular molecules to be imported by endocytosis. Together, these pathways thus provide routes of communication between the individual organelles within the endomembrane system and between the interior of the cell and its surroundings.
Researchers studying yeast discovered that, for some mutants, when the temperature at which the cells are grown is elevated from 25ºC to 37ºC, their secretory pathway no longer functions and the cells grow dense with unsecreted protein.When these cells are examined microscopically, they can be divided into groups that vary in terms of where the unsecreted proteins accumulate. In some of the mutants, proteins accumulate in the ER; in others, the Golgi; in others, they accumulate in vesicles near the plasma membrane.What is the likely explanation for this difference in appearance? Choose one: Different temperature-sensitive mutations promote an increase in protein synthesis. Different temperature-sensitive mutations disrupt the integrity of cell membranes. Different temperature-sensitive mutations disrupt protein synthesis. The temperature-sensitive mutant proteins accumulate in different compartments. Different temperature-sensitive mutations affect different stages of the transport process.
Different temperature-sensitive mutations affect different stages of the transport process. Movement of proteins between different cell compartments via transport vesicles has been studied extensively using genetic techniques. Studies of mutant yeast cells that are defective for secretion at high temperatures have identified numerous genes involved in carrying proteins from the ER to the cell surface. Many of these mutant genes encode temperature-sensitive proteins. These mutant proteins may function normally at 25°C, but when the yeast cells are shifted to 37°C, the proteins are inactivated. As a result, when researchers raise the temperature, the various proteins destined for secretion instead accumulate inappropriately in the ER, Golgi apparatus, or transport vesicles—depending on the particular mutation.
How are proteins covalently modified in the ER (for exocytosis)?
Disulfide bonds are formed by the oxidation of pairs of cysteine side chains, which helps stabilize a protein's structure. Many proteins entering the ER lumen or ER membrane are glycosylated by glycosylating enzymes in the ER but not in the cytosol. A preformed oligosaccharide is transferred from the lipid dolichol to the amino group of an asparagine side chain on the protein (the sugar is said to be N-linked). This oligosaccharide is further processed in the ER and golgi before the glycoprotein reaches the cell surface. Attached oligosaccharides can protect proteins from degradation, hold them in ER until properly folded, or help guide it to appropriate organelle by serving as a transport signal. On the cell surface, these sugars can form part of outer carbohydrate layer called glycocalyx that is important in cell-cell recognition.
Proteins destined for the Golgi apparatus, endosomes, lysosomes, and even the cell surface must pass through which organelle? Choose one: ER peroxisome nucleus mitochondrion
ER Unlike the nucleus, mitochondria, and peroxisomes, the ER serves as an entry point for proteins destined for other organelles, as well as for the ER itself. Proteins destined for the Golgi apparatus, endosomes, and lysosomes, as well as proteins destined for the cell surface, all first enter this extensive system of membranes from the cytosol. Once inside the ER lumen, or embedded in the ER membrane, individual proteins will not re-enter the cytosol during their onward journey. They will instead be ferried by transport vesicles from organelle to organelle within the endomembrane system, or to the plasma membrane.
Describe how transmembrane proteins get inserted into ER membrane.
For single-pass protein: Hydrophobic stop-transfer sequence further along chain in addition to N-terminal signal sequence. Result is stop-transfer sequence remains in membrane while carboxyl end is in cytosol and amino end is in ER lumen (signal sequence cleaved off) For multi-pass proteins: An internal start-transfer sequence is combined with a stop-transfer sequence (one pair of sequences translates to two spanning segments). For one pair of sequences, the C-terminus and N-terminus are both in cytosol. Signal sequence not cleaved off Refer to powerpoint for images
What is the unfolded protein response (UPR)?
If builduip of misfolded proteins in ER is high enough, the UPR program is triggered, which causes more ER and thus more chaperones and other proteins concerned with quality control to be produced. If the expanded ER cannot keep up with volume of proteins entering secretory pathway, then the UPR directs the cell to self-destruct by undergoing apoptosis.
Define endocytosis and differentiate b/w pinocytosis and phagocytosis
In endocytosis, cells ingest materials by enclosing them around the membrane and creating an endocytic vesicle which then is delivered to endosomes from which the materials are recycled to plasma membrane or sent to lysosomes for digestion. Pinocytosis-ingestion of fluid and molecules via small pinocytic vesicles. Phagocytosis-ingestion of large particles, like microorganisms and cell debris, via large vesicles called phagosomes. All cells performing pinocytosis, but specialized phagocytic cells for phagocytosis.
Describe phagocytic cells and their function in the body
Includes macrophages and neutrophils, which are white blood cells that defend against infection by ingesting microorganisms. Binding of antibody-coated bacteria to surface receptors induce the phagocytic cell to extend projections of cell membrane called pseudopods which engulf the bacterium and fuse to form a phagosome. This phagosome then fuses with a lysosome, where the microbe is destroyed. Phagocytic cells also help scavenge cell debris; macrophages ingest over 10^11 worn-out red blood cells per day.
Describe protein sorting for the following organelles: Nucleus (interior vs. inner membrane), mitochondria, chloroplasts, Golgi, lysosomes, endosomes, peroxisomes
Interior of nucleus, mitochondria, chloroplasts - proteins delivered directly from cytosol. Golgi, lysosomes, endosomes, inner nuclear membrane-proteins and lipids delivered indirectly via the ER. Peroxisomes-use both pathways, though it mostly gets digestive enzymes from the cytosol.
Describe the golgi apparatus and how it receives vesicles from ER
It consists of a collelection of flattened sacs called cisternae, which are piled like stacks. Each Golgi stack has an entry or cis face adjacent to the ER and a trans face that points toward the plasma membrane. Soluble proteins and membrane pieces enter the cis Golgi network via transport vesicles derived from the ER. These proteins can travel through the cisternae by vesicles or by a maturation process in which the cisternae themselves migrate through the Golgi stack. Proteins finally exit from trans Golgi network in transport vesicles destined for the cell surface or another organelle in the endomembrane system.
Describe the sorting signal/signal sequence
It is a continuous stretch of amino acid which directs the protein to the organelle in which it is required. Proteins that do not have a particular signal sequence remain in the cytosol. This sequence is usually removed once the protein has been sorted (moved to its proper destination), but not always as in the case of proteins in the nucleus.
Describe how proteins enter peroxisomes
Most proteins enter in a similar manner as that for mitochondria and chloroplasts; the import receptor proteins are in the cytosol, and the protein does not have to unfold, however. Some proteins embedded in peroxisomal membrane arrive via vesicles that bud from ER. Function: Peroxisomes have enzymes that digest toxins and they make certain phospholipids, including those present in myelin sheath surrounding axons.
Define autophagy
Obsolete parts of the cell are degraded. An organelle, for example, is enclosed by a double membrane, creating an autophagosome, which then fuses with a lysosome. These organelles and proteins are sometimes tagged with ubiquitin. Autophagy occurs more often when the cell is starved or is remodeling itself extensively during development.
Describe how vesicle docking works
Rab proteins (which are GTPases), tethering proteins, and SNAREs (transmembrane proteins) help direct transport vesicles to their target membranes. Interaction between Rab proteins on the vesicle and tethering proteins on the target membrane provide the initial recognition. A v-SNARE on the vesicle then binds to a complementary t-SNARE on the target membrane, ensuring that the transport vesicle docks at an appropriate target membrane. Following vesicle docking, SNARE proteins catalyze the fusion of the vesicle and target membranes. Once appropriately triggered, the tight pairing of v-SNAREs and t-SNAREs draws the two lipid bilayers into close apposition. The force of the SNAREs winding together squeezes out any water molecules that remain trapped between the two membranes, allowing their lipids to flow together to form a continuous bilayer.
Which proteins play a central role in the fusion of a vesicle with a target membrane? Choose one: adaptin SNAREs tethering proteins Rab proteins clathrin
SNAREs Rab proteins, tethering proteins, and SNAREs help direct transport vesicles to their target membranes. Interaction between Rab proteins on the vesicle and tethering proteins on the target membrane provide the initial recognition. A v-SNARE on the vesicle then binds to a complementary t-SNARE on the target membrane, ensuring that the transport vesicle docks at an appropriate target membrane. Following vesicle docking, SNARE proteins catalyze the fusion of the vesicle and target membranes. Once appropriately triggered, the tight pairing of v-SNAREs and t-SNAREs draws the two lipid bilayers into close apposition. The force of the SNAREs winding together squeezes out any water molecules that remain trapped between the two membranes, allowing their lipids to flow together to form a continuous bilayer.
Describe lysosomes. How do the digestive enzymes and membrane proteins reach lysosomes?
Sacs of hydrolytic enzymes that are optimally active in a pH of 5 maintained by an H+ pump. Lysosomes carry out controlled intracellular digestion of extracellular materials and worn-out organelles. Metabolite transports in membrane helps products of digestion move out into cytosol. Membrane proteins unusually glycosylated on interior to protect them from digestion by lysosomal proteases. These enzymes and proteins are synthesized in ER and transported through Golgi to lysosomes. While in ER and cis Golgi face, enzymes are tagged with mannose 6-phosphate; when they arrive at trans Golgi face, the mannose 6-phosphate receptor recognizes them and allows them to be packaged into vesicles delivered to lysosomes via endosomes.
Define exocytosis
Secretion of molecules caused by the fusion of a transport vesicle with the cell membrane Ex. cell products like insulin in the pancreas
Describe the major secretory pathway and endocytic pathway
Secretory-ER through Golgi to plasma membrane Endocytic-Moves materials from plasma membrane through endosomes to lysosomes
Define receptor-mediated endocytosis. How is cholesterol taken in this manner?
Specific macromolecules bind to receptors on cell surface and enter the cell as receptor-macromolecule complexes via clathrin-coated vesicles. It is selective unlike pinocytosis and is far more efficient. Cholesterol-containing LDLs secreted by liver are taken up in receptor-LDL complexes and delivered to endosomes, where the acidic environment causes the LDL to disassociate from the receptor. The receptor is returned via vesicles to plasma membrane for reuse while the LDL is delivered to lysosomes for breakdown. Freed from LDLs, cholesterol escapes into cytosol, where it can be used to synthesize new membrane.
To determine whether a signal sequence directs proteins to a particular organelle, researchers prepare two versions of the same protein: one version contains the signal sequence, while the other lacks it. They label the protein that contains the signal sequence with a radioactive marker, and then incubate both of the proteins with the organelle of interest.After allowing enough time for any of the proteins to be transported into the organelle, a protease is added to the mixture.If the signal sequence is the correct one for the selected organelle, what would the researchers likely see? Choose one: The radioactive label would be associated with a collection of protein fragments. The radioactive label would be associated with an intact protein. The radioactive label would be destroyed. The radioactive label would be associated with one particular protein fragment. The radioactive label would be associated with the protease.
The radioactive label would be associated with an intact protein. A protein bearing a signal sequence can be introduced to a preparation of isolated organelles in a test tube. This mixture can then be tested to see whether the protein is taken up by the organelle. The protein is usually produced in vitro by cell-free translation of a purified mRNA encoding the polypeptide; in the process, radioactive amino acids can be used to label the protein so that it is easy to isolate and to follow. The labeled protein is incubated with a selected organelle and its translocation is monitored by one of several methods. In one approach, the labeled protein can be incubated with the organelle and a protease can be added to the preparation. If the protein bearing the signal sequence is transported into the organelle, it will be selectively protected from digestion by the organelle membrane; adding a detergent that disrupts the organelle membrane will eliminate that protection, and the transported protein will also be degraded.
How do larger molecules gain entry to a nuclear pore? How is nuclear import/export powered?
They must contain a nuclear localization signal, consisting of one or two short sequences containing many positively charged lysines or arginines. This signal is recognized by cytosolic proteins called nuclear import receptors which bring the proteins through the pore by disrupting the gel meshwork. They bind to the repeated sequences in pore proteins and bounce from one to the next until they enter the nucleus and deliver the cargo; the empty receptor returns to cytosol via the nuclear pore for reuse. Import of nuclear proteins powered by hydrolysis of GTP mediated by monomeric GTPase called Ran. Ran-GTP is present in high concentrations in nucleus (Ran-GEF or guanine nucleotide exchange factor in nucleus) while Ran-GDP is produced in the cytosol (Ran-GAP or GTPase activating protein in cytosol). Ran-GTP displaces protein from its receptor, allowing the receptor bound to Ran-GTP to return to cytosol where GTP is hydrolyzed; this releases the receptor for reuse. Nuclear export receptors recognize nuclear export signals and also use Ran as an energy source.
How are newly made lipids supplied to the plasma membrane? Choose one: via the constitutive pathway of exocytosis via secretory vesicles produced by the regulated exocytosis pathway via enzymes that synthesize phospholipids, which are attached to the plasma membrane via lysosomes via vesicles that bud from the ER and fuse with the plasma membrane
Via the constitutive pathway of exocytosis In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network and fuses with the plasma membrane in the process of exocytosis. This constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins, enabling the plasma membrane to expand prior to cell division and refreshing old lipids and proteins in nonproliferating cells.The regulated exocytosis pathway also adds phospholipids to the plasma membrane; however, this pathway only operates in cells specialized for secretion.
Describe how proteins enter the endoplasmic reticulum (describe water-soluble)
Water-soluble proteins are completely translocated and enter the ER lumen, while prospective transmembrane proteins are only partly translocated and become embedded in the membrane. Water-soluble proteins will either be secreted at cell surface or go to lumen of an organelle in endomembrane system, while transmembrane proteins will either reside in the ER membrane or move to another organelle's membrane in the endomembrane system. These proteins are directed to ER by ER signal sequence. When a ribosome happens to be making a protein with an ER signal sequence, the sequence directs the ribosome to the ER membrane, where the protein is translocated as it is made. The ER signal sequence is recognized by a signal-recognition particle (SRP) in the cytosol which binds to the sequence and the ribosome; this slows down protein synthesis until the SRP interacts with an SRP receptor in the ER membrane. This releases the SRP, and the polypeptide is then threaded through a channel in the protein translocator as it is being made. The signal sequence remains bound to the translocator, keeping it open while the rest of the chain is thread through. For water-soluble proteins, the signal sequence is almost always at N- tigil sequence is cut by signal peptidase and degraded in lipid bilayer.
Which membrane-enclosed organelles most likely evolved in a similar manner? Choose one: mitochondria and the ER chloroplasts and peroxisomes mitochondria and the Golgi apparatus the nucleus and the ER mitochondria and the nucleus
the nucleus and the ER Nuclear membranes and the ER likely arose through invagination of the plasma membrane. In modern bacteria and archaea, a single DNA molecule is typically attached to the plasma membrane. It is possible that, in a very ancient anaerobic archaeon, the plasma membrane, with its attached DNA, could have invaginated and, in subsequent generations, formed a two-layered envelope of membrane completely surrounding the DNA. This envelope is presumed to have eventually pinched off completely from the plasma membrane, ultimately producing a nuclear compartment. Other portions of the invaginated membrane may have formed the ER, which would explain why the space between the inner and outer nuclear membranes is continuous with the ER lumen. Mitochondria and chloroplasts are thought to have evolved from bacteria that were engulfed by primitive eukaryotic cells with which they initially lived in symbiosis. These cells already had a nucleus when they acquired these symbiotic inhabitants.