 Now, we are going to talk about the cytoskeleton, the skeleton, the cell, cytoskeleton, the framework that supports cell. Cytoskeleton is the system that gives support to the cell and help the cell in its own movements. It also help different organelles inside the cell to move from one place to another place when this is required. Cytoskeleton consists of three types of fibers. One is called microtubules, the other is called microfilaments, the third is called intermediate filaments. We talk about them one by one. The microtubules, these are the largest in diameter and microtubules are hollow from inside. Their generalized diameter is about 20 nanometers. But remember, then when we are talking about the size of different cytoskeletal elements, we are only referring or talking about their diameter and not their length, because lengths of these fibers are different from each other, but their diameter is, their size actually refer to only their diameter. So microtubules in comparison to other are the largest, they have about 20 nanometer in diameter, but they are hollow from the inside. They consist of a protein that is called tubulin. Tubulin molecules are of two types, alpha tubulin and beta tubulin. They join together to make the tubulin diameter. Remember means that it is made up of two tubulin molecules. These diamers later on join together to make long hollow tubes called microtubules. The other one is called microfilament, these are the smallest among cytoskeletal elements. Microfilaments are about 7 nanometers in their diameter and they consist of a protein called actin. Actin molecules are arranged in the form of beads and there are two bead based chains wound around each other to make a helix. We call it an actin subunit. The diameter of this subunit is 7 nanometers. Again their lengths may vary. Then comes the third, these are called intermediate filaments. Intermediate filaments consist of a particular family of protein called the keratins. Keratins are the same proteins which make our nails, for example, the harder parts. It means these are tough proteins. The keratin family of proteins, different types of keratins join to make intermediate filaments. It specifies their function. They give support though there are now we talk about the functions of these cytoskeletal elements or we can call them cytoskeletal fibres, cytoskeletal elements they have one major function. That is giving shape to the cell, giving support to the cell, support to its different organelles, keep them in place and movement. We have a look on the diagrams which shows structures, generalized structures of these cytoskeletal elements. Here in the diagram, it shows you the structure of a microtubule. Microtubule as I described previously consists of tubulin diameter and then these diamers are arranged in a circular fashion to make a long hollow tube. Length of these tubes depends upon how and which part of cell require them for how long they are required. For example, if their cell is using these microtubules for transport of an organelle, say a mitochondria from one place to another, then the length of microtubule will depend upon the distance from the point one from where mitochondria have to be taken up and till point two where mitochondria have to be placed. In the second diagram, you can see microfilament. Microfilament is an actin subunit as you can see, it is a helical chain. In the third diagram, you can see the structure of an intermediate filament on the right. Their diameter is about 10 nanometers and they consist of fibrous proteins which initially they are monomers that is smallest molecules joined together to make a diameter. Teteramers join together to make tetramers, tetra means four and these tetramers then join to make sheets and these sheets are wrapped around each other as you can see in the diagram to make a thick and tough fiber which is called intermediate filament. Now look at the diagrams which are coloured. These are the pictures. If we want to know that in a cell where at which places the cytoskeleton is present, we can stain them because you know that the microtubule consist of tubulin which is a protein. Microfilament consist of another protein called actin and the intermediate filaments also consist of various proteins. So if we stain our cells, we give them a colour by using anti-tubulin antibody, anti-actin antibody or anti-carotene antibody then we can look at them that where these elements are present. The pictures above which are not diagrams but pictures taken through fluorescent microscopy shows that cytoskeletal elements are present almost everywhere in the cell. The first diagram shows that in blue colour you can see the nucleus in green colour surrounding them are microtubules. In the second picture you can see that we stained it this cell with anti-actin antibody and it shows that in almost all part of the cells these actin filaments are spread it. So it means that cytoskeletal elements are present almost everywhere in the cell and support all parts of the cell. In the third diagrams you can again see inside nucleus in orange colour and in green colour different types of intermediate filaments. This is another picture taken more closely in which you can see various cells which have a nuclear in blue colour and in green colour they have microtubules and in red colour they have actin that is these are stained for microtubules, actins, intermediate filaments all of these. You can see that in a cell the cytoskeletal elements are spreaded everywhere it means that they can support almost every structure of the cell the organelles, the nucleus, the cell membrane and the others and also they can help in movements of organelles of the cell itself when required. Here is a closer diagram in which you can see that there is a mitochondria inside in a green colour, upper is the cell membrane and above and below the mitochondria there are different types of filaments which are attached on one side to the mitochondria and they are attached on other side to different other filaments or to the endoplasmic reticula or to the cell membrane actually mitochondria are kept in their place with the help of these different types of cytoskeletal elements some actin filaments the micro filaments some microtubules and some of the micro filaments. Here is another view of the same thing here you can again see cell membrane, endoplasmic reticulum from another view you can say about roughly surface view you can see the same thing the endoplasmic reticulum the microtubules the micro filaments and one micro mitochondria entangled in them just like mitochondria other organelles are also kept in place they are also kept in place with the help of these cytoskeletal elements and whenever they need to move for example if energy is required because you know mitochondria produces energy molecules if energy is required in another part of cell then mitochondria will be transported from this part to the other part of cell and these mitochondria will be moved with the help of the microtubules.