 Various molecules pass through the cell membrane by means of various mechanisms known as transport across cell membrane mechanisms and these mechanisms are classified into different types. So these types include passive transport mechanisms and active transport mechanisms. Now these classification is based fundamentally on whether energy that is biological energy ATP is being required for this or not. So passive transport processes do not require ATP while active transport processes require ATP and why is that? Because all passive transport processes occur from their higher concentration to the lower concentration. So if suppose sodium is being transported it will from the higher concentration of sodium to the lower concentration of sodium. On the other hand active is against the concentration gradient that is uphill transport. So in passive we have a simple diffusion, facilitated diffusion then there is non-ionic diffusion then there is osmosis and there is another one known as ultra-filtration. And in active transport we have primary active transport and secondary active transport. And reactive transport again further has two mechanisms that is simport and anti-port. So let us quickly see what are the characteristics of these types of transport mechanisms and then we will sort some MCQs. So in simple diffusion what are the requirements for simple diffusion to occur? First of all that simple diffusion remember the word simple when we are saying then it is across the lipid bilayer. It happens across the lipid bilayer. See cell membrane has both lipids and proteins but when the molecule moves through the lipid bilayer then it is known as simple diffusion. Okay and for this what will be the requirement that the substance which is moving should be lipid soluble. More the substance is lipid soluble more will be the rate of simple diffusion. So that is simple diffusion and which direction will it be? It will be from its higher concentration to lower concentration. Example what are the substances which can move like that? There is oxygen, carbon dioxide, nitrogen basically all the gases they move by simple diffusion. Then any other lipid soluble substances like there are steroids okay then alcohol they move by simple diffusion. Then what are the factors which affect the rate of simple diffusion? Well this is given by a law known as Fick's law. What is that? Fick's law state that the rate of diffusion rate is equal to that is the it depends on three main things that is the characteristics of the membrane then the pressure or concentration gradient and the characteristics of the substance which is being transported. So it also depends on the solubility of the substance in the membrane that is the lipid solubility and molecular weight of the substance. So let us write the formula rate will be directly proportional to the area of the membrane that is more the area more will be the rate of the transport inversely proportional to the thickness of the membrane. So these are the properties of the membrane then it is directly proportional to delta C that is the concentration gradient or when we are talking about gases this will be written as delta P right and then there is something known as diffusion coefficient which is basically the solubility of the substance and on denominator there will be square root of molecular weight. So more the thickness of the membrane or larger the substance more the molecular weight of the substance that rate of simple diffusion will be less. But one thing you note here that this rate is directly proportional to the concentration gradient that means if we keep all other things constant right the membrane properties are constant and the substance we talk about a single substance suppose then what will happen that the rate of diffusion will increase linearly with the concentration gradient. So here is the rate of transport rate of transport is increasing linearly as a concentration gradient is increasing and this is known as linear kinetics ok. So simple diffusion follows linear kinetics. So with this let us solve one MCQ on simple diffusion quickly the rate of simple diffusion increases with is it concentration gradient, thickness of membrane, molecular weight of the substance or water solubility of substance when we are talking rate of simple diffusion increases. It increases with concentration gradient as we have seen that there is a linear kinetics it is following. With thickness of the membrane fix a law of diffusion we saw that rate of simple diffusion is actually inversely proportional to the thickness of the membrane, molecular weight of the substance right again it is in denominator it is inversely proportional to square root of the molecular weight of the substance and water solubility of the substance. So here we said that lipid solubility rate of diffusion is directly proportional to the lipid solubility. So this is like opposite of the lipid solubility more the substance is lipid solubility that means less it is water solubility is not it. So if water solubility is more that means lipid solubility is less right and less lipid solubility means the rate of diffusion will be less fine. Now let us move on to next one that is the facilitated diffusion. What are the requirements of facilitated diffusion? First of all just note here that it is diffusion ok. Diffusion means that the transport of molecules will always be from higher concentration to lower concentration and again it does not require ATP because of this the downhill transport of substances is occurring does not require ATP. However there are certain proteins which are facilitating this transport that means the substances which are not lipid soluble ok they can move from their higher concentration to lower concentration by facilitated diffusion and this is facilitated by means of transport proteins. There are transport proteins so proteins are required and these transport proteins can be of two types that is carrier and there can be channels ok. So remember passive process no ATP required and it requires facilitation by means of the transport proteins that is carrier and channels. And what kind of substances will be transported by means of facilitated diffusion obviously as I said that the substances should not be lipid soluble. So there can be ions, there can be larger molecules larger substances ok larger substances polar substances that need to be transported by facilitated diffusion. Another example is glucose transporter glute, glute is there glute is for glucose T for transporter so that is glucose transporter that is causing facilitated diffusion. And what are the characteristics of facilitated diffusion? Well remember that this is using the transport proteins and whenever a substance is using the transport proteins there are certain characteristics which are being followed because these transport proteins are also required for active transport. So what I am going to discuss right now the characteristics will be same for facilitated diffusion as well as for active transport. So first of all that it follows saturation kinetics in simple diffusion we saw it was linear kinetics here it is saturation kinetics ok what is that? That as the concentration gradient increases ok so as it is increases the rate of transport rate of transport increases first but then it becomes plateau. So it is not that if concentration gradient keeps on increasing continuously rate of transport will also increase linearly no it increases for some time then it plateaus because what happens the transport proteins are limited in number ok they become saturated. So actually this name for this is saturation kinetics and this is the maximum rate of transport that is GM maximum rate of transport. So that is saturation kinetics then second characteristic is that it is specific the carrier or channels then specific for a particular substance other substances cannot be transported via these particular transport proteins. Third because they bind to certain substances so that is why there can be inhibition of these transport proteins so these transport proteins can be inhibited ok by competitive inhibition or non-competitive inhibition ok. So those are the characteristics of facilitated diffusion and what are the examples as I already told you one example is glute or glucose transport fine. So quickly let us see one MCQ on this true about facilitated diffusion which of these is true about facilitated diffusion it requires energy it requires carrier protein rate of transport is proportional to concentration gradient and it is non-specific. Well as I told you that C option will be wrong because it follows saturation kinetics so it is not that it is permanently proportional to concentration gradient it rises first and then it plateaus. Then it is non-specific obviously this is also wrong because wherever transport proteins are there then the transport becomes specific. It requires energy is also wrong because it is a passive transport it requires carrier protein is the correct answer fine. Let us move to the third type of transport mechanism that is osmosis. We said that the third type of transport mechanism non-ionic diffusion well this kind of transport mechanism happens for certain substances which are basically lipid soluble form and they can diffuse when in lipid soluble form but they cannot diffuse when they are in ionic form one example of it is the transport of ammonia in kidneys so when there is ammonia it can cross to various parts of the nephron right and in the interstitium also there is ammonia so concentration of ammonia is present everywhere but once it enters the collecting depths this ammonia binds with the hydrogen ions which are being secreted okay and then it forms the ammonium ion so this ammonium ion now cannot cross the membrane and hence this ammonium ion is then excreted out so this is an example of non-ionic diffusion. Fine moving on to osmosis what is osmosis? Osmosis is movement of water molecules from low concentration of solute to high concentration of solute so if I mark here a and b right here you see there is no concentration of solute in b but in a the solute is present so water will move from b to a right and what will happen that this level because of the water movement will start rising right so that is known as osmosis the movement of water and a very important requirement for osmosis is that the membrane should be permeable only to water it should not be permeable to solute see if it is permeable to solute then the solute will also cross and what will happen that the solute concentration on both sides will become equal so there will be no water movement understanding or not so for osmosis to occur we want that the membrane should be permeable only to the water or what is known as the presence of effective osmosis so the the solute which is present in this case a if it is not able to cross the membrane then it is known as effective osmosis because then the water can move and what is ineffective osmosis? Ineffective osmosis is the molecules which can cross the membrane one example being urea urea so if there is a solution in a with urea what will happen that urea will cross on the other side i'm talking about in body also that happens that urea glucose they can cross the membrane okay so these are known as ineffective osmosis and what is osmotic pressure osmotic pressure is the pressure required to prevent the osmosis so i have to put pressure on a right on the a side so that the osmosis can stop and now you can understand that if the concentration of the solutes in a is more then the water will be drawn much more towards a and then the osmotic pressure required to stop osmosis will be more okay so that is osmotic pressure the pressure required to stop osmosis and it is more when the solute concentration is higher more details about this osmosis and osmotic pressure we'll see in another video on where we will discuss the concept on moles equivalence etc fine let's move on to active transport well active transport as i said before is of two types primary active transport and secondary active transport first of all the basic characteristics of both the types of active transports are same that is it requires energy ATP is required right and the movement of molecules is from low to high concentration gradient low to high concentration gradient that is against their concentration gradient and what is this primary active versus secondary active transport see if this is a cell okay and say here is a transport mechanism a common example of primary active transport is sodium potassium ATPase which throws out three sodium ions from the cell and brings into potassium ions inside the cell and you see the name what is the name it is sodium potassium ATPase so this pump has a ATPase attached to it so this ATPase will break down ATP directly so when the energy is utilized directly by the transport protein it is known as primary active by the way this utilization of energy directly by the protein the transport protein also gives them another name so these transport proteins are known as pumps okay pumps so for primary active transport the proteins are pumps then for secondary active transport what happens that the energy is utilized indirectly what does that mean say suppose the sodium potassium ATPase is there so that there will be three sodium moving out and two potassium moving in fine and what happens because of this the sodium ions inside the cell become less if sodium ions are less that means you see if on this side on the other side if there is high concentration of sodium ions then these sodium ions can move inside okay there is a concentration gradient if the path is open there is a concentration gradient for sodium to move from outside to inside and this is being used by secondary active transport for example there is SGLT sodium glucose transporter so the concentration gradient for sodium is being maintained by sodium potassium ATPase and this causes sodium to bind to this SGLT and causes the movement of sodium from its high concentration to low concentration and of glucose against the concentration gradient that means from its low concentration to intracellularly high concentrations so the energy which is stored in the gradient of sodium transport is being used by glucose to be transported against its concentration gradient one substance is transported along concentration gradient other substance is being transported against concentration gradient so that is about secondary active transport whether energy is being utilized indirectly by sodium potassium ATPase for maintenance of gradient of sodium now in secondary active we said that there are two types simport and antiport simple actually simport is when both the molecules are being transported in the same direction so here you see both sodium and glucose are moving from outside to inside so that is simport and antiport is when they are moving in opposite direction okay that means sodium will move from outside to inside while the other substance will move from inside to outside so that is basically calcium so antiport example is sodium calcium exchanger and when the substances are moving in opposite direction right so there is an exchange of ions taking place so that is why there is another term for this proteins that is exchanger okay so simport example SGLT antiport example sodium calcium exchanger let us see certain details about sodium potassium ATPase because it's very common mcqs and very important also sodium potassium ATPase is what basically primary active transport okay it's primary active transport so it directly use utilizes energy ATPase it is associated with ATPase let's come to its structure it has two units alpha unit and beta unit alpha subunit and beta subunit and these subunits have binding sides so suppose this is extracellular and this is intracellular okay now alpha subunit should have binding site of sodium inside because sodium is moving from inside to outside right so inside on the alpha subunit we have sodium binding site we have ATP binding site right and then we also have phosphate binding site phosphate binding site okay so ATP is broken down into ATP and phosphate by ATPase and this phosphate also goes and binds to this alpha subunit intracellularly and when this happens there is change in confirmation such that the sodium is released outside man potassium is released inside so outside we have the binding site for potassium actually okay so binding site for potassium and also there is another binding site for ua bain act ua bain or similar kind of chemicals like digitalis digitalis which is used as a drug so how this digitalis acts is basically it blocks this activity of sodium potassium ATPase so once it blocks that activity what will happen that sodium will start accumulating inside the cell and if sodium start accumulating inside the cell what we have seen in our secondary active transport like so this is sodium potassium ATPase and here there is sodium calcium exchanger so if digitalis blocks this sodium will start accumulating inside the cell and if sodium starts accumulating this will not work because we want the gradient for the sodium and hence calcium will not be thrown out of the cell so calcium accumulates inside the cell that is the muscle digitalis is used for heart failure isn't it so calcium accumulates inside the cardiac muscle cell and with calcium there is increase in force of contraction so that is how digitalis acts by blocking sodium potassium ATPase then now another mcq is asked that glycosylation site so beta unit has glycol 3 glycosylation sites now you should also know about certain functions of sodium potassium ATPase first of all you see that it is throwing out three sodium ions and bringing into potassium ions so there is unequal movement of ions isn't it the positive ions which is moving in is less compared to that which are moving out so it creates small negativity inside right so creates a negativity of four millivolts inside the cell so it contributes to generation of resting membrane potential plus it also maintains the concentration of these ions right what happens that if sodium potassium ATPase stop stops working then you see the amount of sodium which is going to accumulate inside the cell which will be much more isn't it because three sodiums are being thrown out so much more sodium accumulates inside the cell so the osmolarity inside the cell increases and if osmolarity inside the cell increases then water will move start moving inside into the cell and what will happen ultimately the cell will break down so one function of sodium potassium ATPase is contribution to resting membrane potential how much it contributes minus four millivolts second is it maintains cell volume also cell volume very important and third is as I told you that maintenance of the intracellular concentration of these ions so those are very important functions of sodium potassium ATPase and also remember we have seen that if sodium potassium ATPase stops working then how secondary active transport will also suffer right so let us solve one mcq on active transport true about calcium transport is calmodulin mediated it is a simport it is active process and maintains very high intracellular calcium well as we have seen about the sodium calcium exchanger the calcium transport outside the cell it is my sodium calcium exchanger but there are other calcium transport mechanism also for example in sarcoplasmic reticulum there is calcium ATPase that is also an active process but that is a primary active process because as I am telling calcium ATPase the term itself is including the word ATPase okay so what could be the answer of this well calmodulin mediated no calmodulin is a calcium binding protein it is a simport well it can be an anti-port if we see certain calcium transporters it is anti-port not simport it is an active process yes definitely it is an active process this is the correct option it maintains very high intracellular calcium well this is absolutely wrong intracellular calcium concentration is extremely low so that is the reason that calcium can act as a signaling molecule so that was all about the concepts on transport across cell membrane thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology you thank you