Histology (plasma membrane)
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When examined by transmission electron microscopy (TEM), the plasma membrane displays a .............(unit membrane) structure
What is true about semipermeable plasma membrane?
IT is freely permeable to small, lipid-soluble, nonpolar molecules but is impermeable to charged ions.
What are the component of the lipid bilayers plasma membrane?
The lipid bilayer is composed of phospholipids, glycolipids, and cholesterol, of which, in most cells, phospholipids constitute the highest percentage.
What are the characteristics of phospholipid in lipid bilayer?
- Amphipathic molecules, consisting of one polar (hydrophilic) head and two nonpolar (hydrophobic) fatty acyl tails, one of which is usually unsaturated.
- The two leaflets are not identical; instead the distribution of the various types of phospholipids is asymmetrical.
- The polar head of each molecule faces the membrane surface, whereas the tails project into the interior of the membrane, facing each other.
- The tails of the two leaflets are mostly 16–18 carbon chain fatty acids, and they form weak noncovalent bonds that attach the two leaflets to each other.
Glycolipids of plasma membrane are restricted to ..............
the extracellular aspect of the outer leaflet
Which part of plasma membrane form part of the glycocalyx?
Polar carbohydrate residues of glycolipids extend from the outer leaflet into the extracellular space and form part of the glycocalyx.
What are the role of cholesterol in lipid bilayer?
- Cholesterol, constituting 2% of plasmalemma lipids, is present in both leaflets, and helps maintain the structural integrity of the membrane.
- Cholesterol and phospholipids can form microdomains, known as lipid rafts, that can affect the movement of integral proteins of the plasmalemma.
What are the determinants of high plasma membrane fluidity?
- Lower cholesterol component
- Higher temperature
- decreased saturation of the fatty acyl tails
Proteins constitute about .....% of the membrane composition
What are the two types of plasma membrane proteins?
- Integral proteins are dissolved in the lipid bilayer
- Peripheral proteins do not extend into the lipid bilayer
What are the features of integral proteins?
- Dissolved in the lipid bilayer.
- Transmembrane proteins span the entire thickness of the plasma membrane and may function as membrane receptors, enzymes, cell adhesion molecules, cell recognition proteins, molecules that function in message transduction, and transport proteins.
- Most transmembrane proteins are glycoproteins, amphipathic and contain hydrophilic and hydrophobic amino acids
- Are folded so that they pass back and forth across the plasmalemma
- Integral proteins may also be anchored to the inner (or occasionally outer) leaflet via fatty acyl or prenyl groups
In freeze-fracture preparations, integral proteins remain preferentially attached to the ..............................................
- P-face, the outer (protoplasmic face) surface of the inner leaflet, rather than the E-face (extracellular face)
Most transmembrane proteins are ..............
Where are the peripheral proteins located?
cytoplasmic aspect of the inner leaflet
How do some peripheral proteins may attach to outer surface of the cell?
The outer leaflets of some cells possess covalently linked glycolipids to which peripheral proteins are anchored; these peripheral proteins thus project into the extracellular space.
Peripheral proteins bind to which portion of plasmalemma?
Phospholipid polar groups or integral proteins of the membrane via noncovalent interactions
What are the functions of peripheral proteins?
- electron carriers (e.g., cytochrome c)
- part of the cytoskeleton
- part of an intracellular second messenger system
What are the three important examples of peripheral proteins?
- Annexins: a group of anionic, calcium-dependent, lipid-binding proteins which act to modify the relationships of other peripheral proteins with the lipid bilayer and also to function in membrane trafficking and the formation of ion channels;
- Synapsin I, which binds synaptic vesicles to the cytoskeleton;
- Spectrin, which stabilizes cell membranes of erythrocytes
Which peripheral protein binds synaptic vesicles to cytoskeleton?
The highest lipid/protein ratio is seen in ...............
Normal lipid/protein ratio in membrane is......
What are the morphological features of glycocalyx?
- Located on the outer surface of the outer leaflet of the plasmalemma
- Consists of polar oligosaccharide side chains linked covalently to most proteins and some lipids (glycolipids) of the plasmalemma. It also contains proteoglycans (GAG bound to integral proteins).
What are the functions of glycocalyx?
- The glycocalyx aids in attachment of some cells (e.g., fibroblasts but not epithelial cells) to extracellular matrix components.
- It binds antigens and enzymes to the cell surface.
- It facilitates cell-cell recognition and interaction.
- It protects cells from injury by preventing contact with inappropriate substances.
- It assists T cells and antigen-presenting cells in aligning with each other in the proper fashion and aids in preventing inappropriate enzymatic cleavage of receptors and ligands.
- In blood vessels, it lines the endothelial surface to decrease frictional forces as the blood rushes by and it also diminishes loss of fluid from the vessel.
Glycocalyx aid in attachment of ...... to ECM
Which component prevent friction of EC with blood cells?
What are the types of passive transport?
- simple and facilitated diffusion.
- Neither of these processes requires energy because molecules move across the plasma membrane down a concentration or electrochemical gradient
What are the features of passive diffusion?
- transports small nonpolar molecules (e.g., O2 and N2) and small, uncharged, polar molecules (e.g., H2O, CO2, and glycerol).
- Little specificity
- The diffusion rate is proportional to the concentration gradient of the diffusing molecule.
What are the features of facilitated diffusion?
- Via ion channels and/or carrier proteins
- Faster than simple diffusion
- Responsible for providing a pathway for ions and large polar molecules to traverse membranes that would otherwise be impermeable to them
What are some examples of Facilitate diffusion?
- Ion channel proteins are multipass transmembrane proteins that form small aqueous pores across membranes through which specific small water-soluble molecules and ions pass down an electrochemical gradient (passive transport).
- Aquaporins are channels designed for the rapid transport of water across the cell membrane without permitting an accompanying flow of protons to pass through the channels. They accomplish this by forcing the water molecules to flip-flop halfway down the channel, so that water molecules enter aquaporins with their oxygen leading into the channel and leave with their oxygen trailing the hydrogen atoms.
- Carrier proteins are multipass transmembrane proteins that undergo reversible conformational changes to transport specific molecules across the membrane; these proteins function in both passive transport and active transport
What is the cause of cystinuria?
abnormal carrier proteins that are unable to remove cystine from the urine
What are the features of active transport?
Active transport is an energy-requiring process that transports a molecule against an electrochemical gradient via carrier proteins
What are some examples of active transport?
- Na+–K+ pump
- Glucose transport
- ATP-binding Cassette transporter
What are the functions of Na K pump?
- Three Na+ ions are pumped out of the cell and two K+ ions are pumped into the cell by hydrolysis of a single ATP molecule
- Primary function is to maintain constant cell volume by decreasing the intracellular ion concentration (and thus the osmotic pressure) and increasing the extracellular ion concentration, thus decreasing the flow of water into the cell.
- Also plays a minor role in the maintenance of a potential difference across the plasma membrane
What are ABC transporters?
- ATP-binding cassette transporters (ABC-transporters) are transmembrane proteins that have two domains, the intracellularly facing nucleotide-binding domain (ATP binding domain) and the membrane-spanning domain (transmembrane domain).
- In eukaryotes, ABC-transporters function in exporting materials, such as toxins and drugs, from the cytoplasm into the extracellular space, using ATP as an energy source.
- ABC-transporters of the placenta, presumably protect the developing fetus from xenobiotics, macromolecules such as antibiotics, not manufactured by cells of the mother.
Multidrug-resistant proteins (MDR proteins) are ................... that are present in certain cancer cells that are able to transport the cytotoxic drugs administered to treat the malignancy
Facilitated diffusion of ions can occur via........................
ion channel proteins or ionophores
What are the types of ion channels?
- Mechanically gated
- Ligand gated
- Voltage gated
What is an example of ungated ion channels?
K+ leak channels are the most common ion channels. These channels are ungated and leak K+, the ions most responsible for establishing a potential difference across the plasmalemma.
............... open only transiently in response to various stimuli
Gated ion channels
What are the examples of gated ion channels?
- Voltage-gated channels open when the potential difference across the membrane changes (e.g., voltage-gated Na+ channels, which function in the generation of action potentials).
- Mechanically gated channels open in response to a mechanical stimulus (e.g., the tactile response of the hair cells in the inner ear).
- Ligand-gated channels open in response to the binding of a signaling molecule or ion. These channels include neurotransmitter-gated channels, nucleotide-gated channels, and G protein–gated K+ channels of cardiac muscle cells.
......................... are probably the location where anesthetic agents act to block the spread of action potentials
Ligand-gated ions channels
What are ionophores?
- Ionophores are lipid-miscible molecules that form a complex with ions and insert into the lipid bilayer to transport those ions across the membrane.
- There are two ways in which they perform this function:They enfold the ion and pass through the lipid bilayer.They insert into the cell membrane to form an ion channel whose lumen is hydrophilic
What is the structure of membrane receptors?
- integral membrane glycoproteins
- have three domains, an extracellular domain that protrudes into the extracellular space and has binding sites for the signaling molecule, a transmembrane domain that passes through the lipid bilayer, and an intracellular domain that is located on the cytoplasmic aspect of the lipid bilayer and contacts either peripheral proteins or cellular organelles, thereby transducing the extracellular contact into an intracellular event.
How does snake venom cause paralysis?
inactivate acetylcholine receptors
What are types of receptors?
- Intracellular--> NO, steroid
- Receptors Located on Membrane-Spanning Enzymes--> Insulin
- Receptors Located on Membrane-Spanning Molecules That Bind Separate Intracellular Tyrosine Kinase Molecules--> JAK STAT (cytokines)
- Receptors Located on Membrane Ion Channels--> Nicotinic Ach receptor, GABA receptor
- Receptors Linked to Effectors via G Proteins--> adrenergic
What are the ligands for receptor tk?
What are the features of G proteins?
- 1)G protein–linked receptors are transmembrane proteins associated with an ion channel or with an enzyme that is bound to the cytoplasmic surface of the cell membrane.2) These receptors interact with heterotrimeric G protein (guanosine triphosphate [GTP]-binding regulatory protein) after binding of a signaling molecule.
- 3) The heterotrimeric G protein is composed of three subunits: α and β and γ complex. The binding of the signaling molecule causes eitherthe dissociation of the α subunit from the β and γ complex where the α subunit interacts with its target orthe three subunits do not dissociate, but either the α subunit and/or the β and γ complex become activated and can interact with their targets.
- 4) This interaction results in the activation of intracellular second messengers, the most common of which are cyclic adenosine monophosphate (cAMP), Ca2+, and the inositol phospholipid–signaling pathway
What are examples of heterotrimeric and monomeric G proteins?
- Heterotrimeric G proteins which are folded in such a fashion that they make seven passes as they penetrate the cell membrane. These are stimulatory G protein (Gs), inhibitory G protein (Gi), phospholipase C activator G protein (Gq), olfactory-specific G protein (Golf), transducin (Gt), Go which acts to open K+ channels and closes Ca2+ channels, and G12/13 which controls the formation of the actin component of the cytoskeleton and facilitates migration of the cell.
- Monomeric G proteins (low-molecular-weight G proteins) are small single-chain proteins that also function in signal transduction. Various subtypes resemble Ras, Rho, Rab, and ARF proteins.
What are the functions of Gs, Gi, and Gq?
- Gs: Activates adenylate cyclase, leading to formation of cAMP, Activation of protein kinases, Binding of epinephrine to β-adrenergic receptors increases cAMP levels in cytosol
- Gi: Inhibits adenylate cyclase, preventing formation of cAMP, Protein kinases remain inactive, Binding of epinephrine to α2-adrenergic receptors decreases cAMP levels in cytosol
- Gq: Activates phospholipase C, leading to formation of inositol triphosphate and diacylglycerol, Influx of Ca2+ into cytosol and activation of protein kinase C, Binding of antigen to membrane-bound IgE causes the release of histamine by mast cells
What is the function of Go?
- Opens K+ channels and closes Ca2+channels
- Inhibits adenylate cyclase Influx of K+ and limits Ca2+movement
- Inducing contraction of smooth muscle
What is the function of Golf?
- Activates adenylate cyclase in olfactory neurons
- Opens cAMP-gated Na+channels
- Binding of odorant to G protein–linked receptors initiates generation of nerve impulse
What is the function of Gt?
- Activates cGMP phosphodiesterase in rod cell membranes, leading to hydrolysis of cGMP
- Hyperpolarization of rod cell membrane
- Photon activation of rhodopsin causes rod cells to fire
What is the function of G12/13?
- Activates Rho family of guanosine triphosphatases
- Regulates cytoskeleton assembly by controlling actin formation
- Facilitating cellular migration
Which is considered an intergrin of RBC?
The plasmalemma and cytoskeleton associate through ..................
How does integrin work in associating plasmalemma and cytoskeleton?
The extracellular domain of integrins binds to extracellular matrix components, and the intracellular domain binds to cytoskeletal components. Integrins stabilize the plasmalemma and determine and maintain cell shape.
The cytoskeleton of a red blood cell consists mainly of ...................
spectrin, actin, band 4.1 protein, and ankyrin.
What are the functions of each cytoskeletal component of RBC?
- Spectrin is a long, flexible protein , composed of an α-chain and a β-chain, that forms tetramers and provides a scaffold for structural reinforcement.
- Actin attaches to binding sites on the spectrin tetramers and holds them together, thus aiding in the formation of a hexagonal spectrin latticework.
- Band 4.1 protein binds to and stabilizes spectrin–actin complexes.
- Ankyrin is linked to both band 3 proteins and spectrin tetramers, thus attaching the spectrin–actin complex to transmembrane proteins
...........is linked to both band 3 proteins and spectrin tetramers, thus attaching the spectrin–actin complex to transmembrane proteins
What are the component of cytoskeleton of non RBC cells?
- Actin (and perhaps fodrin), which serves as a nonerythroid spectrin.
- α-Actinin, which cross-links actin filaments to form a meshwork.
- Vinculin, which binds to α-actinin and to another protein, called talin, which, in turn, attaches to the integrin in the plasma membrane
..........binds vinculin to integrins
Hereditary spherocytosis results from a .......................................
defective spectrin that has a decreased ability to bind to band 4.1 protein
Diffuse axonal injury is associated with irreparable cleavage of .......................
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