 Now, we talk about the movements across the cell membrane. This is very important actually one of the most important features of the cell membrane that it is semi permeable. It allows certain molecules to pass, it does not allows other molecules to pass and also this actually very technically separate the internal and external environments. There are certain molecules which are lipid like, as we know from the structure that the cell membrane overall so if a molecule is like a lipid for example there is a fat type hormone it can directly pass through the cell membrane and come inside or it can pass out but the other things the other molecules like proteins like carbohydrates they cannot pass through like ions water they cannot pass through directly through the cell membrane because it is a fluid it is a lipid and lipids they actually repel those molecules. So what happen that for movement of this type of molecules cell membrane have channels the ways from which these molecules can pass and these channels these ways are made up of proteins. These proteins are very very specific in their structure. One type of channel usually allows only one type of molecule or ion to pass for example there are channels called water channels or aqua for ins aqua or water for ins force. So these are the protein channels channels made up of proteins present inside the integrated inside the cell membrane which allows only water molecules to pass. There are certain other channels for example sodium ion is a very important ion present inside the cells and in their extracellular environment sodium ion have particular channels through which it can pass we call them sodium channels just like that there are protein channels there are calcium channels they are potassium channels and so on and these channels are not many of these channels are not always open there are few channels which are always open we call them leak channels from this from these channels ions or molecules can pass against a concentration gradient for example if they are more outside they will move towards inside if they are more inside they will move towards outside but there are other channels which allow only specific type of molecule to pass and they open and close as required by the cell certain signals come and tell these channels to open or other signal comes and tell these channels to close so these are open and close upon the requirement of the cell for example during nerve impulse when a message from brain is transmitted for example to the muscles our message from muscle is going to the brain there are sodium channels which need to be open and these are opened with the help of a stimulus that is some signal when a stimulus come signal come these channels open and allows the sodium to move in and the cell transmit a message here you can see a diagram in the diagram that you can see on your screen right now it can show you various types of channels view on your left are open open protein channels view on the other side shows when they are close so channels open and close and how they open and close they open and close by a conformational change I hope you understand the conformational change but I simply describe it conformational change is this that arrangement of molecules of a macro molecule or arrangement of atoms of a molecule in space is changed this is called a conformational change for example if they were towards right they move towards left with the result channel open or close