 Inactive transport mechanism, substances are transported from their lower concentration to their higher concentration that is against their concentration gradient. This means that inherent kinetic energy of molecules will not be sufficient for transport of the substance since it will only cause the transport of substances along their concentration gradient. So, for transporting substances against concentration gradient, we need to use biological energy that is ATP. Now, this can be done by two mechanisms, either primary active transport or by secondary active transport. In primary active transport, energy is utilized directly by breaking down ATP. So, there should be an enzyme that is ATPase which breaks down ATP and the energy which is released is utilized for the transport of the substance. On the other hand, in secondary active transport, there is indirect utilization of energy. Let us see how this is happening with examples. So, one example of primary active transport is sodium potassium ATPase. Now, this transporter is present in all cells and remember as a rule of thumb, it is always present on basolateral side of membrane. So, this rectangle represents a cell and this is the epical side of the membrane and this is the basolateral side of the membrane and here this side will be lumen and the other side is the interstitial fluid. Now, remember sodium is present in higher concentration in extracellular fluid compared to intracellular fluid. While potassium is present in higher concentration in intracellular fluid compared to extracellular fluid. Now, as said, active transport means transport of substance or more than one substance against their concentration gradient. So, sodium potassium ATPase will transport both the ions, sodium and potassium against their concentration gradient. So, sodium will be transported from inside of the cell to outside, isn't it? So, because outside it is more. So, for this, this transporter is having three binding sides for sodium in the inner aspect. Then potassium, it transports two potassium ions from outside of the cell to inside of the cell. So, on the outer aspect, there will be two binding sides for potassium. So, both the ions are being transported against their concentration gradient, very important. Also, the transporter has an ATPase and a binding side for ATP. So, when all its binding sides are full, that is three sodium ions have bound to it and two potassium ions have bound on the outer side, this ATPase cleaves ATP and there is a release of energy. Now, because of this energy, there is change in configuration or a shape of the transporter such that it releases two potassium ions inside and three sodium ions outside. So, with the utilization of ATP, ions are being transported against their concentration gradient. Fine. Another example of primary active transport is hydrogen ATPase. So, it will be transporting only single ion that is hydrogen against its concentration gradient. Now, let's come to secondary active transport. In secondary active transport, energy is utilized indirectly as I said before. How? Basically, it utilizes the energy stored in concentration gradient of one substance to transport another substance against its concentration gradient. So, one substance is moving along its concentration gradient downhill and it like carries the other substance against its concentration gradient. Let us see how this is happening. So, this is the cell and this is the transporter sodium glucose transporter. Now, this transporter transports sodium ions along its concentration gradient that is from outside to inside while it transports glucose against its concentration gradient but again from outside to inside. So, here you see the difference how sodium is moving outside to inside. In primary active transport, we saw it was moving from inside to outside. Anyways, now this transporter sodium glucose transporter that is SDLT is present in a small intestine and in nephrons in kidney. So, you see the energy stored in gradient of sodium ions that is from outside to inside is used to transport glucose against its concentration gradient. So, in summary you see that one substance is moving along the concentration gradient while second substance is moving against the concentration gradient. But you may ask where is ATP being utilized? Well, it is being utilized to generate or maintain the gradient for sodium ions. See, if this transport continues what will happen? With each transport gradient for sodium ions is decreasing since more sodium ions are moving inside and maybe after sometime it will be completely lost. So, it is important to maintain sodium gradient for this transporter to keep working. And this is done by primary active transporter. Our sodium potassium ATP is which we spoke about earlier. Remember we said that it throws out sodium ions. So, by moving sodium ions out by utilizing ATP it will maintain the gradient for sodium ions and secondary active transporter will keep working. Fine. Now remember that this secondary active transport is of two types. It is a co-transport or also known as simport and counter transport or also known as anti-port. In simport both the substances are moving in the same direction. One is moving against concentration gradient one is moving along concentration gradient that is there but they are moving in the same direction. For example, in SGLT both are moving from outside to inside. In counter transport on the other hand one substance moves from outside to inside while other substance moves from inside to outside. So, they are moving in opposite direction. An example of this kind of transport is sodium calcium exchanger which causes the movement of sodium from outside to inside while calcium moves from inside to outside against its concentration gradient. In summary, primary active transport directly utilizes ATP to transport one or more substances against their concentration gradient. Secondary active transport utilizes energy indirectly and one substance is transported along its concentration gradient while other is transported against its concentration gradient. And based on the direction of transport the secondary active transport is of two types co-transport example being SGLT and counter transport example being sodium calcium exchanger. So, that was the fundamentals about active transport. Well, 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 Open. Thank you.