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REVIEW ARTICLE - BIOLOGICLA MEMBRANES -

lipids are very impornta compoentoen of biolgical membranes. not jsut palsma or cell membran but also al the organells inside the eukaryotic cells right th encuelsu the mitchondria golci.

theyre all membrane bound. so this is relvant to all those memrbaen bound structures. this article talks about. three types of lipids, phoslolipids, glycolipids,a dn sterosl. your textbook goes into more depth. talks about more types of lipds. most lipids are part of hte biolgicla membare are glycerol based. glycerol , three cabron al with three hydoxyl groups. usually two of the Oh groups are used to form ane ster linkage with two fatty acids and the third one has a polar grup attache.d thsi si a the basic structure that makes it amiphatic. thisi s critial for forming biolgicla memrbanes. another class of lipids are pshonilopoids. theya re not glycerol based. s

sterosl are different ,most famous one is cholestrol. its got a fused ring system. it has a short hydropobic tail. and it has a hydrophilic head group. choelstrol is also amiphathic. choelstrol is in animals. palnts bater yeast dont have choelstrol. they hve other types of sterols.

these lipid molcules are maiphathic, when u put in aquous solution they will spontnous form a lipsooem structure. it si a three d spherial strucutre. . . u dont need input energy to make the strucutre. it is thermodynamiclaly facvored orngaiation of lipds. this liposome structure will minimze the hdyrophobic portions of thes emocluels minimzie contact withe water and isntead they interact trhough vand der wall and hydrophobic affect. the hydrophili groups are in drect contact with water.

this kind alooks like amini cells. cellmrmeba is also a lipid bilayer. although th e difference this and real cell memrbane is that eh cell emmrbaen has a lot of sugar cmolules and protins.

thes arificial memrbanes are imponrt at for studying how cell membrane fucntions.

COVI vaccine called mRNA vaccine . so the active ingreien is mRNA mcolule that codes for COVID virla protein to bosos immune systme. mRNA is fragile moclule, it is hihgly charged and hydrophilic. so scientist wanted to proect it from RNase enzyme thats everywehre that tries to break down the mRNA. u need an intact mRNa and how to deliver the large hydrophilic moclule to the inside of hte cell. they did this by encapsulatin the mRNA and the core of a lipsome moclule like this. so it proects the mRNA and dleiver the cargo .

lipsoomes wer eused in cosmetic industry. they are hdyrophilic mocluels and hard to dleiver ot he skin cells so they are going to encapusaolte hte active ingrediten inside a liposome. we descirebthe cell memrbaen using the gluid mosaid cmoelduel. the mosaic part refers to the fact htat he cell memrane does not contain a singel tyep of moclule but multiple. lipids , and dif tyep of it and sugars, protiens etc. sugar mjcoluels are usually nort parot o fhte lipid bilayer and instea dhtey are attached ot he lipids or hte proitens on top they arent directly part of hte memrbane. not all cell membranes are identail. if u comare neuronal cells ot heart cell to muscle cells. to skin they are all different. . the tyeps of lipids u find in them the percentage is different.

membrena comspitons is diferent dpeingin if its a old or youn cells or ding. withing a singel cell if u analzuye the ocmpsiton its different whether u analzye the outer or iner layer. if u compare the palms amembrea vs the mcithocndoal memrbaen they are all diferent.

the fluid part of hte lfuid mosidac model refers to the fact that thes ecopeotns are usually not locke din psotions. they move back and forth. membrane is a 2d strucutre withign that 2d they are constnaly moving. there ar small numebr of protins or lipdis that are locked, not all are dynamic but most are. this is because when tehy are locked it is for a speicif cbiolgical funciton. to attach those protien and lipids ot the cytoskelign inside.

belaching expeiremnt, they engineer one o fhte memrbane proitns to be attached to a GFP. the GFp is on the cell surface. under the fouscnece the whole clel will flouscne. and then u take a lazer light and shin that anywhere on the location ont he cell surface and this si a belaching effect to kill the flouscent. so u cann see that there iwll be a dark spotr and then five to ten mineutes the flroucsnet recovers because the GFP protiens are going to move into the area. the non bleache dpotiens mov eot another locaiton. this shows that lipids and proteins are fluid . they also calcualted at what speed the y move. it takes about 12 seconds to move ofrm one end of hte cell to the opposite end. they are moving fast. if u comapre lipids to proitrens prteins move slwoly than lipids because they are larger..

where are lipids syntehiszed that are part of hte mermbrane, thsi is n the ER.. the smooth ER. they are going to be made then transported to ERGIC which is the endopalmsic reticulum golci intermediate complex. it goes to the strucutre first then golci to the verious memrbaens. so the ER is wehre hte memrbane lipids ar emade. not jsut phosplipds but also sterols.

memrbae proitnes sytnethis. on page 8.

the protiens are made by ribsoomes. memrbae proitnes get a speicla treamtne. just like cytososlic proeitns their syntehisze start in the cytoplams byt he ribsooems that is in the cytoplams and as they are made at the early stages of tranlsaiton, the memrbean eproiants are going to contain a singling sequnce at the N temrinsu. that singals to the cell that it is. amembrane proitne, it will bidn the singling sequnce that is 20 amin acids long with hydrophiboc amino acids, that pepide as hte prien is made is recognvized by a singal rcogniation particle. it wil bind to the sequcne and that complex will bind ot he SRp receptor preoitn on the surface of ER. and then the rest of hte tranlation takes place while the ribsooem is attached ot he ER.rough ER. then the pritne is fully made and gets embeeded into the mermabre of hte ER.

one imporntat role of membrane proiten is transport cells have to move matierla from outside ot he isnide of hte clel. and also move watse mateiral form inside to the outside. but hte memrbae is a lipid bilayer and nonpermabel to msot . and only a handful can cross direly which are small and hydrophibic. like water which is hydrophilic but its small enought to diffuse through. co2 is small and hydrophibic can directly diffuse out of hte cell . most mocluesl cant do this bc they are too large or HYDROPHILIC. this is improntat becase if it shydrophilic it needs to cross the hydrophobic area dn it cant do that , memrbena have protien that aid in the trnaport o f various mocluels.

there are tranport process there is passive vs active . passive mean the trnaport does not require inpurt of enregy. its possibel whenve u are trnaporting mateiral along its conentrioan gradient . if its going hight to low that is a spontnous process and passive trnaportn. but sometimes they need to transport materil that is again the coenntraion gradient from lot o hihg. that does require input of ernegy in one form or another. that is active trnapirt.

passive tranpirt is simpel diffusion liek water and co2 jsut diffse along the concentriaon gradient. then there ar echannel protiens that aid in the pasive tronatrion of some mocluesl like Ca, which is small chargedna dhdyrophilic and they express channel protiens. so the calcium ions cna go through to avoid the lipid bilayer . then there are carrier proitens but channel protiens just provide a hdryophilic bathwa and the carrier proitens undergo large conformation change such that they bind the molcul efrom one side and then that causes a conformation change and it will open the cavity on the other side. if u comapre the pasive carrier proitnes chanenls vs carrier suuallyt eh mvoemt of mocluels is faster if it uses channel protiens. SO CHANNELS ARE FASTER.

also some channel and carrie rproients are cosntnaly active and others are gated. so they will do this when they receive a ceritan singl thsi is different from simple diffusion because simpel diffusion is not directly controlled .

so active trnapirtser are going fro low conetrntiaon to high conetino thsi si thermodyanmciall unfaboabel theres an energy barrier. to do this usually u ahve to put in energy. soem active trnaproters use ATP and some use light. free enrgy is needed to make this happen.

specifi examples -

ligan gated ion channel . its a pentameric proitn each polypeptide chain has four alpha ehlcies. by orgnaize ing hte five chian u make a seciton in the middle the maino acids that line the central caviti is hydrophilic amino acids polar or ionzibel . so the cavity itself is hydrophilic enviometn wehr ethe amino acdis that line the periphery are hydrophibic so the compelx cna be embdeeded withing hte lipid bilayer.

to be placed int he mermabe it has to come. into contact with the hydrophiboc tails.

so thsi particule one is ligand gated. that means that hte chennel is usually closed. so if a ligan singlingmocluel bind it will open up.

ATP DRIVE ACTIVE TRNAPORTN -

calcium ion trnapotner. this is the ER memrbaen in the slide. so ur cell trnaptirn clcium ions form the cytopalms to the ER. this has to be an active tnraportn because the ER lumen is rich in. calcium ions. so it goes against the concentriaon gradient. this protien uses atp . hydrloysis of atp. the enzyem bind atp and gets hydrlozyed to adp and phosphate and the product is free enrgy and that changes teh conformation fo the proitn

ot all active trnaporters use atp as an ernegy source. rhodopisin use light energy and there are trnapotrer sthat use coneitnor fradient of another mcoluelt o drive the active trnaportn of the target mcoluels. so there are tewo types, Simporter s and anitporters

So lets say that the goal is to tranportn a form the left ot eh right and this is agianst he concnetiron gradient. some proitens coupel this active trnaport process to a passive trnaport of a seocn mcolule. so like B its going from a hihg coentrion to the low conentiron spontenously. that is going ot be couple the A trnaportn of A.

in antibport the directions are different . so a is going from left ot righ and B is going from right ot left.

another way that ur cells move materials form one cell to anotehr cell but also form one roganell to anothe roganell inside the cell is vesicles called liposoem like strucurtres. so the cargo moclule , so the donot membrane is going to bud . it eventuall cleaves off to form an indepntene mino rganell. it will contain the cargo. thenthe vescile will diffese freely to the target organell or will be actively traported by various motor proetins that walk along the cytoskelton either way it willr eacht the target and a priten called Vsnare thats part of hte vesicl ena T snare that is on teh sruface of the target orgenll will bind to one another and tightly and that release the large mount of free enrgy, and that energy used to overcome the activiation barreir for memrbaen diffusion.

so what shpening is oppostie at the top . the mino roganell will fuse witht eht memrabe of hte target organell and relase the cargo.

membrane. protiens are improtnat in singling. so cells need to clel to cell singling are achieved by electric singal like neurons or singling mcoleusl like hroemosnes. tehre is water soulabel and lipid osluble. the lipid soluble can diffsue thoruhg the lipid bilayer and then drectly act on the target proeitn insde of hte clel. so singiling is not ocmplicated. but if u have water soluabel hormoens those cannot cross the lipid bilayer. how they are recieve dby cells is through memrbae proietns called G groups or GPCR’s. ¼ of our drugs target GPCR. s

its a memrbea spanning proiten. it has a ligan binding site ont he extraculalr sige and the intraceullar side it has a g protein that are made up of three chain Aalpha beta gamma.

so the heterotrimeric proitens usuallyt eh G bdingin proeitns is atahcedo to eh