| Describe the belt of tight junctions? | Zipper like structure intersecting at a band at cell circumference between two cells, formed around each cell, plugs intercellular space and acts as a barrier to flow of material between two cells. |
| How do gap junctions occur? | aqueous channels, made up of six connexins in each cell of the two communicating cells, named connexon, extend from one cytosol to another, tightly linked by interactions with extracellular domains in intercellular space, space between two adjacent plasma membranes in 3 nm (diameter of DNA), each connexin is integral polytopic protein having four alpha-helix domains, which is hyrophobic (excepy long one), since all hydrophilic sides are directed towards channel centre, a hydrophillic channel (1.5-2 nm diameter) is formed allowing the flow of polar molecules (not selectively) However it is controlled into one direction (reverse rotation for both sides of the connexon. |
| What are cell membranes? | all membranes that are found in a cell, including plasma membrane, nuclear membrane, ER membrane. |
| What are the main roles of cell membranes? | delimit specific compartments and separate them from their surrounding. |
| How are cell membrane observed? | through EM and not LM. |
| What is the difference between prokaryotic and eukaryotic cell membranes? | they are less developed in prokaryotes, in eukaryotes they are characterized by complex cytoplasm compartmentalization. |
| What are the main roles of plasma membrane? | responsible for cell intergrity since it separates cytoplasm from surroundings, if it is damaged cell will die. Site of many biochemical reactions and physiological processes such as signal transduction, phosphrylation, cell respiration and communication and cell-cell interaction |
| Do cell membrane completely isolate the organelles from surroundings? | No since they are not watertight. |
| What are the main properties of cell membranes? | All selective and controlled permeability, since they regulate the passage of solutes between two compartments that are separated by a cell membrane, certain particles can cross the membrane at certain times, which may be active or passive, this selective permeability is responsiblefor electrical polarity, due to the lipid properties most hydrophobic molecules can easily pass through the membranes whereas hydrophilic ones require channels or transporters, proteins integrated through the membrane, however they cannot transport large macromolecules, they are rather transported in vesicles through endocytosis, phagocytosis and exocytosis. |
| What the is main composition of cell membrane? | main components forming the basic structure are phospholipids (sphingosin and glycerol derived) there is presence of glycolipids, precisely in the outer leaflet, also there may be steroids (cholesterol in most eukaryotes) In addition to these lipids, membrane comprises proteins (functionality) and carbohydrates. Theses components are different among each cells quantitatively and qualitatively. |
| How was the membrane structure known and by whom? | The were known by chemical and biochemical analysis, it were ambiguous for a long time until 1925 where Gorter and Grindle who were studying RBC structure, They suggested that polar heads of lipids are directed inwardly to the cytosol and outwardly towards the ECM whereas the non polar tails are directed towards each other, (bilayer shape maintained by hydrophobic interactions), however no clear explanation about proteins present, until 1935 (Danielli and Davson) where proteins are absorbed by two polar sides or integrated through the membrane forming a channel, which was confirmed by studies (freeze-fracturing technique) and (staining by osmium tetroxide remarking trilaminar appearance of membrane- two outer dark layers called osmophillic and one inner bright layer.) |
| Is the thickness of all cell membranes the same? | It is variable according to fatty acids and protein composition, average is from 55 A to 85 A |
| Talk about the fluid-mosaic model. | Singer and Nicolson, according to it, cell membrane is of a biomolecular lipid layer that is composed of phospholipids with small amounts of neutral fats and steroids and whose surface is interrupted by proteins that occur in a mosaic (irregular) manner. |
| How are proteins classified according to mosaic manner? | -Peripheral or extrinsic proteins, weakly attached to outer polar surface of lipid bilayer.
Peripheral or extrinsic proteins that are tightly attached to inner and outer surfaces of lipid bilayer via lipid molecules, covalently bonded to a phospholipid.
-Integral or Intrinsic proteins that penetrate the membrane partially through one layer (monotopic) or two layers (polytopic) by means of one or more helix segment, most abundant, must have at least one stretch rich in hydrophobic aa to stablize in hydrophobic region of lipid bilayer, whom usually adopt a helix shape, |
| What are membrane proteins connected to? | Cytoskeleton or other components of cytosol or extracellular components, may be constitutive (permenant) or signaled (by ligands) |
| What are the main families of integral proteins of cell membrane? | -Immunoglobulins: and integrins involved in immune activity and cell adhesion (Ca dependant) (mediate interaction between lymphocytes and macrophages, Antiibodies and T receptors are a kind of it CAM cell adhesion molecules)
-Cadherins: glycoproteins, mediate Ca cell adhesion and communication with surrounding, found at surface of many animal cells.
-Selectins : glycoproteins able to recognize and bind to specific sugar motif proturding from cell. Some CAM are selectins, mediate temporary interaction between leukocytes and endothelial cells, at site of inflammation allowing infiltertation. |
| How are cabohydrates present in fluid-mosaic model? | covalently bonded to membrane proteins and lipids exposed to the outer surface of the plasma membrane, usually exposed away from cytosol. |
| What is the main role of glycoproteins and proteins in cell membrane fluid-mosaic model? | cell physiology , metabolism (transporters or channels, some connected to cytoskeleton or ECM ligands) and cell identity, (BG is determined by different glycolipids in RBC membrane (A,B and O antigens) |
| Talk about protein asymmetry in cell membrane | they are not uniformly distrubuted between two lipid bilayers, some are at outer leaflet some at inner one, and some are in both but in different quantities all, which is logical given by specific functions of membrane. |
| Talk about lipid and carbohydrate asymmetry in cell membrane? | lipids are not uniformly and carbs are only in outer leaflet, |
| Give examples concerning asymmetry in membrane components. | Enterocyte (outer half has less proteins than inner half)
RBC (outer leaflet has phosphatidyl choline while inner has phosphatidyl ethanolamine and phosphatidyl serine (-ve charge polar head) and inner charge is more - than outer) |
| What are the criteria that also are asymmetrical in cell membrane in terms of molecules? | Fatty acids are not equal and polar heads. |
| How is phospholipid and glycolipid asymmetry achieved? | specific enzyme called flipase, that displaces them from a lipid lamina to another. |
| How is the fluid structure of cell membranes? | They appear in EM as static structure, however they are dynamic, where continuous flux motion is observed by addition or removal of components, that are either in perment motion because of non covalent bonding, so it is in continuous movement which is fluid structure. |
| Why does fluid structure occur? | Since lipids and proteins are maintained together not by covalent bonds, and because of length of fatty acids, unsaturation degree, presence of cholesterol and temperature. Unsaturation increases fluidity, because they donot behave as crystallatice. Cholesterol acts as a bidirectional regulator, which decreases fluidity at high temperature and increases it at low one, via intercalating with phospholipids. Longer CH tail of fatty acids decreases fluidity.
All cells must keep suitable fluidity by adjusting each factor, (cells at below 37 C promote activity of unsaturating enzymes to compensate loss of fluidity by temp decrease to maintain fluidity. Plants has unsaturated FA when temp drops at night and winter) |
| How was cell membrane protein mobility facilitated? | By Frye and Edidin, using sendal virus to facilitate formation of heterokaryon in cell culture, which is a cell resulting from fusion of two cells from different species, labeling cell surface antigens with fluorochromes that help distinguish them in EM, Where it was noticed that first human and mouse antigens were at opposite poles, but then both became uniformly distrubuted, so proteins are not fixed at precise positions but able to move. |
| Talk about lipid movement? | Photolabeling is done on phospholipids, which demonstrates their movement, (phospholipids can move lateraly) |
| Can a lipid or protein move from one leaflet to another? | No, lateral movement (within same layer) since hydrophilic part of them cannot easily pass through hydrophobic region. Flip-flop movement is almost impossible without flipases. |
| how is cell membrane protein movement controlled? | controlled by cytoskeleton components (connection between protein and cytoskeleton may be a cause for protein immobility) and also ECM basal lamina and membrane proteins themselves control their movement, it is restricted to one pole of polarized cells (presence of tight junctions limits the mobility) |
| Give examples of polarized cells that cell membrane differs compositionally, | -Epithelial cells have apical lateral and basal pole, who differ and might share some structural characteristics. Apical pole (transporters, absorption) Basal pole (adhesion transportation to CT) lateral pole (adhesion)
-Hepatocytes: three different poles, each with contact to two neighboring cells and bile canaliculus (site of bile secretion) one pole faces capillary sinusoids, various nutreints are exchanged with blood stream, thus it has receptors transporters enzymes...Second pole is adjacent to neighboring cells, exchange of substances and desmosomes junctions. Third pole in contact with bile canaliculus into which bile and salt pigments are inserted. |
| What are the main roles played by junctional regions of plasma membrane? | Cell-cell adhesion, cell-matrix interaction, and intercellular transport and communication. |
| What are tight junctions? | AKA zonula occludens , cell-cell adhesion characterized by plugged intercellular spaces, occur in the form of a belt, at limit between apical and lateral pole, plasma membrane of neighboring cells fuse with one another at many points due to tight interaction between intrinsic proteins (occludin) In other words, they are formed by the attachment between different occludin belonging to to neighboring cells so they function as a zipper. Actin filaments mat attach to them. |
| Where are tight junctions found? | All epithelia (intestine enterocytes, seminiferous tubules (among steroli cells), endothelia of brain blood vessels, they also contribute in selective nutreint absorption by intestine epithelium by preventing their passage between intercellular spaces. |
| How are tight junctions controlled? | Complex control, vey strong but dissosciation might occur by certain immune cells that have to pass through epithelia during immune response. |
| Talk about the intermediate junctions. | AKA belt desmosomes/ zomula adherens. Main function is adhesion of cells in many types of tissues, especially epthelia forming a circular belt under the tight junctions, however doesn't close intercellular space. The plasma membranes are linked by cadherins (intrinsic proteins react using extracellular domains) which is calcium dependant. |
| How are cadherins linked? | each cadherin in its cytoplasmic side is connected to anchor proteins linked to actin filaments, zone where actin filaments are in plasma membrane is thick and dark, actin is anchored to cadherins by a protein called catenin. |
| Talk about spot desmosomes. | AKA macula adherens, may occur anywhere between neighboring cells, punctual junctions forming a button like attachment point, strongest junction occurs in tissues frequently subjected to mechanical pressure (uterus epidermis uterus) |
| How does spot desmosome interaction occure? | cadherin molecules whose interaction is calcium dependent similar to other cadherins in other desmosomes, Plaque formed of proteins attaches actin filaments and cadherin in the cytoplasmic side made of proteins, cadherins form 3D junctions with each other making the spot desmosome which confers strength to the tissues and resistance to pressure. |
| Talk about hemidesmosomes. | Consist of half-desmosomes occur at basal pole of epithelia lies on basal lamina. Mediates attachment of epithelia to underlying CT. |
| How are hemidesmosomes interconnected? | Dense plaque of plectin, intrinsic proteins that are integrins, bundles of intermediate filaments (keratane) attached to the plaque, and interacts with integrins and other proteins which interacts with the laminins of basal lamina. |
| Talk about synaptic junctions. | Contact points between neurons and post-synaptic cells. There are more than 50 types of cadherins in the axon end plasma membrane of a neuron. |
| How are synaptic junctions occuring? | No direct contact between cells, AP transferred by chemical neurotransmitters (e.g acetylcholine) whereby the change in RP of pre-synaptic causes change in post one, neurotransmitters bind to receptors (integrins) and change permeability of the post-synaptic cell causing difference in AP. |
| Talk about gap junctions. | Pipline structures occurring between neighboring cells in clusters, made of integral proteins, named connexins, play a role in transportation and molecule exchange. Occur in nearly all tissues except skeletal and nervous ones (AP Ca dependent). Present in cardiac muscle and mediate rapid propagation which ensures synchronization of movement of all cells making them as if one cell with multiple nuclei. In smooth muscles (esophagus), ions propagating causes contractions that enable swallowing (peristaltic movement) |
| Talk about plant cell wall. | Envelopes plasma membrane, made of cellulose, hemi-cellulose (heteropolysaccharide), pectin (heteropolysaccharide) and small amount of glycoproteins. Differ according to cell types and maturation degree, cellulose is made by outer enzymes while others are made by golgi body, there may be a secondary cell wall made of lignin and the other components |
| How are plant cells interconnected? | Not by the desmosomes, rather by cell wall, specifically pectin filaments, howevre there are cytoplasmic bridges between adjacent cells allowing communication and nutrient exchange, named plasmodesmata (water, gas, salts and carbs) . |
| How are plasmodesmata found in the EM? | Cell wall is interrupted, plasma membrane of first cell is continuous with the other, diameter about 30-60 nm, in the center of the channel there is desmotubules, derived from SER, which narrows the molecule gaps, (annulus), only certain molecules can pass, but larger molecules can be transported after dilation of its pore. |
| Talk about microvilli. | Finger like permanent and stable structure, 1-2 microm length and 0.1 microm diameter, surface of epithelia, involved in absorption (enterocytes) or secretion, appear as a brush in apical pole, increase cell surface area making absorption more effective. internally supported by 40 actin filaments, connected to cytoskeleton and plasma membrane covering them, actin filaments are connected by other proteins (villin and fimbrin) and play a supportive role and mobility role, causing material flow. Usually covered by viscous glycoproteins and carbs called glycocalyx, facilitating their movement. |
| What is the main characteristic of plasma membrane concerning molecule tranasport? | Plasma membrane is selective permeable, mediating many physiological processes- propagation of nervous impulses and glucose intake.
Influenced by many factors, electrical charge of transported molecule, weight/size, distribution coefficient (relates solubility in water and lipid solubility) |
| How do small entities transport through the plasma membrane? | Non-polar ones pass easily, since they can cross the hydrophobic region of the lipid bilayer. Very small polar ones also pass easily (H2O, CO2..) However larger polar and charged entities need pumps to transport (poorly permeable). Which could be passive or active. |
| Talk about passive flow of molecules through the plasma membrane. | AKA diffusion, occurs due to concentration gradient (from side high salt concentration to low) through channels, transporters or lipids. Solute molecules move due to gradient energy and passive membrane- not making chemical or metabolic activity- thus not spending energy. Never accumelate salts instead establish equilibrium so that the media becomes isotonic. Movement continues at equal rates in both sides. |
| Talk about active transport. | Requires pumps that are polytopic integral proteins, consume ATP to transport solute against its gradient. from low to high concentration, example (proton pump of lysosome from cytosol into its lumen, making it acidic/ Na+ and K+ pumps in plasma membrane) |
| How does water diffuse in cell membrane? | From hypotonic to hypertonic membrane, causing turgor pressure in plant cells (dehydration) when surrounding medium is hypotonic. Also causes swelling of RBC when incubated in hypotonic medium and shrinking in hypertonic one. |
| How are macromolecules transported? | They don't pass like ions, monosaccharides and aa. Their transport is mediated by vesicles known as bulk transport. Enables take in and out of macromolecules. Inward transportation known as endocytosis (enter of proteins and some viruses...) and phagocytosis (large molecules 1 microm and more) restricted to animal cells since plant cell wall and turgor pressure prevent this process. Last type is outer transport called exocytosis. |
| How do macromolecule transportation occur? | Large movement of PM (psuedopod extension) which requires intervention of cytoskeleton, endocytosis removes small portions of PM, exocytosis adds portions. Endomembrane system is the complex system of the vesicles added or detached. |
