Which of the following best explains how the phospholipid bilayer of a transport vesicle contributes to cellular functions?
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How do lipid bilayer components move?steve2018-02-06T13:29:31+08:30 How do lipid bilayer components move?One of the tenets of the Fluid-Mosaic membrane model is that the components of the bilayers are free to move. Before describing the differences between lipid and protein movement in the bilayer, it is important to consider the types of movement possible. Using a phospholipid as an example, the first type of movement is rotational. Here the phospholipid rotates on its axis to interact with its immediate neighbours. The second type of movement is lateral, where the phospholipid moves around in one leaflet. Finally, it is possible for phospholipids to move between both leaflets of the bilayer in transverse movement, in a “flip-flop” manner. Phospholipids in the lipid bilayer can either move rotationally, laterally in one bilayer, or undergo transverse movement between bilayers. Lateral movement is what provides the membrane with a fluid structure. By labelling single particles and following their movement via high speed video, researchers were able to discover that phospholipids did not move via Brownian motion but rather by “hop diffusion”. Phospholipids stay in one region for a short while before hopping to another location. This compartmentalization of lateral movement appears to be linked to contacts between the actin cytoskeleton and the membrane which form the regions that the phospholipids hop between. As described above, membrane asymmetry is critical for membrane functions. Transverse movement is what allows the asymmetry to be maintained. Uncatalysed movement of phospholipids between the bilayers is possible, but this is slow and cannot be relied upon to maintain the asymmetry equilibrium. Instead, lipid translocator proteins catalyse phospholipid movement between the bilayers. Flippases move phospholipids from the outer leaflet to the inner leaflet. In order to maintain the charge gradient across the membrane, flippases predominantly transport phosphatidylserine and to a lesser extent phosphatidylethanolamine. Floppases move phospholipids in the opposite direction, particularly the choline derived phospholipids phospatidylcholine and sphingomyelin. Floppases also mediate cholesterol transport from the intracellular monolayer to the extracellular monolayer. These catalyzed movements are typically dependent on ATP hydrolysis. A third class of protein are the scramblases, which exchange phospholipids between the two leaflets in a calcium activated, ATP-independent process. In the case of membrane proteins, they are able to undergo rotational and lateral movement. However, there is no transverse movement of proteins between the leaflets. Intrinsic membrane proteins are tightly embedded in the hydrophobic core, whereas extrinsic membrane proteins associate with their required leaflet. The energy requirements to move either type of membrane protein across the bilayer would be excessive. More Questions FAQ
What is Fast Endophilin-Mediated Endocytosis?Sruthi Jagannathan2018-01-04T15:48:16+08:30
FEME is a novel clathrin-independent endocytic pathway, regulated by the BAR domain protein endophilin, where tubulo-vesicular carriers form within seconds at the plasma membrane upon activation of specific G-protein coupled receptors (GPCRs) by their ligands, internalizing GPCRs and moving rapidly towards the perinucleolar area. Read more.. How does exocytosis affect cell mechanics?Sruthi Jagannathan2018-01-04T15:25:09+08:30
The recycling and delivery of membrane by exocytosis can be used by the cell to enlarge the membrane and surface area during cell shape changes.The interaction between membrane tension, unfolding and trafficking has been demonstrated by a number of studies. Read more.. What is exocytosis?Sruthi Jagannathan2018-01-04T15:18:47+08:30
Exocytosis is defined as the transport and fusion of secretory vesicles with the plasma membrane and the extracellular space. There are three exocytosis pathways that deliver vesicles to the plasma membrane. Read more.. How do clathrin-coated vesicles invaginate and mature?Sruthi Jagannathan2018-01-04T15:07:36+08:30
Clathrin-coated vesicle maturation incorporates the activities of a range of proteins. Actin, myosin and WASP all have important roles in the formation and stabilization of clathrin-coated pits (CCPs). Read more.. What causes the narrowing of clathrin-coated vesicle necks?Sruthi Jagannathan2018-01-04T13:24:09+08:30
In the final stages of clathrin-coated vesicle (CCV) formation, Phosphatidylinositol-4,5-bisphosphate (PIP2) undergoes a dephosphorylation by phosphatases such as synaptojanin 1 (Synj1). This promotes further membrane curvature at the vesicle bud, dissociation of the BAR domain proteins (BDPs) and closing of the membrane bud neck. Read more.. How do clathrin-coated pits form?Sruthi Jagannathan2018-01-04T13:10:24+08:30
Adaptor proteins such as AP-2, AP180 and CALM (Clathrin-assembly lymphoid myeloid leukaemia protein), which accumulate within the lipid bilayer, are responsible for the recruitment of the triskelion shaped Clathrin trimer. This trimer does not interact with the membrane directly but instead forms a reinforcing lattice structure that acts as a mold in which membrane vesicles may develop. Read more.. How does dynamin help in the scission of clathrin-coated vesicles?Sruthi Jagannathan2018-01-04T12:58:31+08:30
In mammalian CME the GTPase dynamin is believed to play important roles in invagination and clathrin-coated pit (CCP) maturation. Its binding partners endophilin and amphiphysin, can also induce tubulation of the vesicles and have been shown to copolymerize with dynamin. Read more.. How are clathrin-coated vesicles uncoated?Sruthi Jagannathan2018-01-04T12:51:56+08:30
Uncoating is the process by which clathrin is removed from clathrin-coated vesicles (CCVs). In mammals, this ATP dependent process is driven by the 70kDa molecular chaperone ‘Heat shock cognate protein’. Read more.. How are clathrin-coated vesicles targeted to endosomes for fusion?Sruthi Jagannathan2018-01-04T12:43:47+08:30
Different cytoskeletal networks have been implicated in the transport of clathrin-coated vesicles (CCVs). In yeast, the actin cytoskeleton traffics dissociated CCVs, whilst in mammals the microtubule network transports is involved in the sorting of CCVs to distinct populations of early endosomes. Read more.. What is clathrin-mediated endocytosis?Sruthi Jagannathan2018-01-04T12:25:09+08:30
Clathrin-mediated endocytosis (CME) is a vesicular transport event that facilitates the internalization and recycling of receptors engaged in a variety of processes, including signal transduction (G-protein and tyrosine kinase receptors), nutrient uptake and synaptic vesicle reformation. Two classical examples of CME are iron-bound transferrin recycling and the uptake of low-density lipoprotein (LDL). Read more.. How is caveolar endocytosis mechanically regulated?Sruthi Jagannathan2018-01-04T12:19:30+08:30
The cell cytoskeleton plays a role in caveolar organization and trafficking. Actin stress fibers influence the linear distribution of caveolae at the plasma membrane in many cell types. Read more.. What is caveolar endocytosis?Sruthi Jagannathan2018-01-04T12:09:13+08:30
Caveolar endocytosis is a clathrin-independent endocytic process which involves bulb-shaped, 50-60nm plasma membrane invaginations called caveolae (or ‘little caves’). Caveolae formation is driven by integral membrane proteins called caveolins as well as peripheral membrane proteins called cavins. Read more.. How do mechanical stresses regulate autophagy?Sruthi Jagannathan2018-01-04T11:36:05+08:30
When autophagy is acting as a pro-survival mechanism primarily induced by stress, it can be naturally regulated by mechanical stresses such as compression, stretching or shear stress due to fluid flow. Consistent with this, a number of studies have highlighted how cells respond to mechanical stresses by regulating autophagy levels and how this could have implications in both physiological as well as pathophysiological conditions. Read more.. What is the physiological relevance of autophagy?Sruthi Jagannathan2018-01-04T11:32:57+08:30
Autophagy is a both a stress-management system and a means of homeostatic control in cells, and is therefore regulated differently under varying cellular conditions. Read more.. What is autophagy?Sruthi Jagannathan2018-02-05T15:26:10+08:30
Autophagy, meaning self-eating, is an intracellular degradation system wherein unwanted cargo, such as old or damaged organelles, unneeded proteins, as well as pathogenic agents, are digested and the macromolecular contents from the digestion are released back into the cytosol. Read more.. What is Arf6-associated endocytosis?Sruthi Jagannathan2018-01-04T11:21:45+08:30
Arf6-associated endocytosis is a clathrin-independent, plasma membrane-endosomal recycling pathway, regulated by the Arf6 protein, which is a member of the Arf family of small GTPases. In this pathway, Arf6 cycles between the inactive GDP-bound state and active GTP-bound state. Read more.. How is membrane trafficking mediated by transport vesicles?Sruthi Jagannathan2017-12-21T12:12:03+08:30
Both endocytosis and exocytosis utilize small compartments of membrane to enclose their cargo. These transport vesicles bud off from one membrane and can dynamically fuse with other membranes, or split up into smaller vesicles by fission. Residing within the interior, or lumen, of the transport vesicles, the cargo is protected from the cytoplasm. Read more.. What is membrane trafficking?Sruthi Jagannathan2017-12-21T12:00:08+08:30
Membrane trafficking encompasses the wide variety of processes that go into the movement of cargo (typically proteins, pathogens and other macromolecules) using membrane bound transport vesicles. This transport can take place within different organelles in the same cell, or across the cell membrane to and from the extracellular environment. Read more.. How is membrane curvature generated?Sruthi Jagannathan2017-12-21T11:42:40+08:30
Several mechanisms have been proposed to be responsible for the generation of membrane curvature. One of the mechanisms is attributed to the specific shape of the lipid group that makes up the membrane and to any changes in their distribution or symmetry. Read more.. What is membrane curvature?Sruthi Jagannathan2017-12-21T11:37:14+08:30
Membrane curvature refers to the physical bending of membranes to accommodate various cell morphology changes as well as the formation of membrane-bound transport intermediates like spherical vesicles or tubules. Read more.. How do membrane reservoirs alter membrane tension during cell spreading?Sruthi Jagannathan2017-12-21T11:33:19+08:30
Membrane area is very closely linked to membrane tension, cytoskeletal attachment and membrane reservoirs. During cell spreading, the initial increase in membrane area requirements are met by depleting membrane reservoirs with the morphology of large folds such as microvilli, spikes, filopodia and ruffles. Once these reservoirs are depleted, membrane tension increases and activates exocytosis as well as myosin contraction. Read more.. How do components in a lipid bilayer move?Sruthi Jagannathan2017-12-21T11:00:14+08:30
One of the tenets of the Fluid-Mosaic membrane model is that the components of the bilayers are free to move. Using a phospholipid as an example, the first type of movement is rotational. Here the phospholipid rotates on its axis to interact with its immediate neighbours. Read more.. What is the role of membranes in mediating vesicular transport?Sruthi Jagannathan2017-12-20T12:33:08+08:30
Another major biological role of cell membranes is in mediating vesicular transport, either during the secretory pathway when proteins are transported from the endoplasmic reticulum to target locations such as lysosomes, endosomes, the plasma membrane and into the extracellular space or during the endocytic pathway, during which proteins and other macromolecules such as nutrients, fluids are internalized into the cell from the extracellular space. Read more.. What functions does the plasma membrane perform at the interface between the cell and its environment?Sruthi Jagannathan2017-12-20T12:29:53+08:30
Any communication or interactions between the intracellular and extracellular spaces occurs through the plasma membrane, which forms the boundary between these two regions. Read more.. What are the physiological functions of membranes?Sruthi Jagannathan2017-12-20T12:22:54+08:30
Eukaryotic cells and their organelles are enveloped by viscoelastic layers made of lipids and proteins. These layers are generally referred to as cell membranes and when they surround the entire cell, they are specifically known as the plasma membrane. Read more.. What are membrane reservoirs?steve2018-02-06T14:51:27+08:30
Membrane reservoirs function as membrane buffers that help redistribute membrane area when cells need to stretch or change shape and size. They are found at the cell surface as membrane superstructures varying in size from large membrane folds, to tiny membrane invaginations and caveolae… Read more… What are plasma membranes?steve2018-01-04T12:30:47+08:30
Plasma membranes are subcellular structures, approximately 10nm thick, that form a protective boundary around the cell as well as the cell’s organelles. They serve to both impede foreign material from entering the cell, and prevent the cellular contents from leaking out. Read more… Related Questions
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How does the phospholipid bilayer contribute to cellular functions?The main function of the phospholipid bilayer is to allow the cell to be selectively permeable, meaning it only lets certain things in and out. The phospholipid bilayer prevents large molecules or charged molecules like ions from diffusing directly across the membrane without the use of a channel protein.
How does phospholipid bilayer allow for membrane transport?The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion. During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane.
Which of the following best explains why a cell's plasma membrane is composed of two layers of phospholipids?Which of the following best explains why a cell's plasma membrane is composed of two layers of phospholipids rather than just a single layer? Having two oppositely oriented layers of phospholipids allows only the hydrophilic heads to interact with water inside and outside of the cell.
How are phospholipids transported in the cell?Thus, mechanisms of phospholipid transport between organelles of mammalian cells via spontaneous membrane diffusion, via cytosolic phospholipid transfer proteins, via vesicles and via membrane contact sites are discussed.
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