 Today, we are going to talk about the cell theory and the cellular components. First of all, I describe that what cell theory states. Cell theory states that all living organisms are made up of one or more different types of cells. Second part, it states that cells are the basic unit of life. The third point is that all the cells arise from the pre-existing cells. That is, all the newer cells are formed by divisions in the older cells. Now we look at it, how this theory developed. Cells were first described by Robert Hook was the curator of instruments of the Royal Society of London. In 1656, he made a simple type of microscope by using different types of lenses to magnify different things. He observed in the thin slices of cork some sort of cells. Now we know that he observed certain sort of dead cells. Later on, another person who was actually a textile merchant, he also developed magnifying lenses and by the setup, he also made a sort of microscope. Out of with the help of that microscope, he observed different types of single cellular or we can say unicellular organisms in the Pound's water. He was Leuwenhock. So Leuwenhock observed actually the unicellular organisms and Robert Hook observed the plant cells. After that, in about 1809, about 200 years later, there was a person Leuwenhock. He proposed that a body is living if it consists of cells. There is another person came later, Robert Brown. He identified nucleus inside the cell. He says that cell have central part that is more dense in comparison to the other parts of cell. In 1838, a botanist named Sheldon, he actually stated the first part of the cell theory, actually half part that all the plant cells, all the plants, the multicellular plants, they are aggregates of cells. Later on, another cytologist and more precisely, a zoologist, an animal scientist, Siobhan stated that all animals also consist of cells. These two theories combined, various other people actually looked at these and they combined to make the modern form of first part of cell theory. That is, all organisms, the living organisms consist of one or more cells. In 1855, another scientist virtue described that cells only arises from pre-existing cells. If there are some cells which are present, they divide and they produce the new cells. Faster later on, produce the proof for this theory, that is, cells arise from pre-existing cells. In this way, the cell theory took its current form, that is, all the living organisms consist of one or more cells. Cells are the basic unit of life and all cells arise from the pre-existing cells. This is about important features of cell theory. Now we are going to do about cellular components. Cell consists of lot many components. Cell is covered, most of all of the cells are covered by a membrane called plasma membrane or the cell membrane. This membrane protects the internal parts of the cell. Plant cells, fungal cells and the prokaryotic cells, they also have another layer outside the cell membrane which is called cell wall. So plant cells, the fungal cells and the prokaryotic cells do have a cell wall that is external to their cell membrane. Cell wall is even a more protective layer. It's more hard in comparison to the cell membrane and in part sometimes in few cells it's like that. Inside the membrane, there is a plasma, a fluid that is called cytoplasm, cyto of cell, plasm, fluid. Cytoplasm is the fluid that is present between the cell membrane and the nucleus. Cytoplasm have lot many components, all the organelles are there, all the functions like metabolism performed there and so on. Cell also have its skeleton, a framework which gives cells its shape, help it in its movement and set its different organelles on their place. Then comes the organelles, cells have organelles, mitochondria, nucleus itself, endoplasmic reticulum, Golgi bodies, vacuoles and lot many more. Here you can see a diagram of an animal cell which shows that it have cell membrane outside then inside almost in the center you can see a nucleus, then you can see surrounded nucleus, the endoplasmic reticula, a network. After that you can observe the Golgi complex, mitochondria, centrioles, cytoplasm, lot many other structures. Now we go ahead and look at these structures one by one. The cell membrane or the plasma membrane. It covers all the components of cell. This actually gives cell, individual cell a state of integrity or we can say that it encloses the contents of the cell in a specific environment. What are the functions of the cell membrane and the plasma membrane? It acts as a barrier, it gives protection to the cell and it allows transport of materials. The cell membrane act as a barrier. Cell membrane is semi permeable that is it allows few molecules to pass in or go out. It does not allow some other molecules to move in or move out. So it means that it acts as a barrier between the internal part of the cell and the external environment. It gives protection to the internal parts of the cell. Internal parts of the cell are delicate, they may damage in response to any pressure or tension exerted on the cell. Cell membrane protects it, separate or isolate the inner part from external environment. It provides transport of materials. Cells need many materials that should come in or should go out on intervals. A cell membrane at the plasma membrane is semi permeable. It manages the transport of materials which are required like food particles which are required by the cell. It allows food particles to come in, it allows the waste products to go out in a very technical way. For example, cells need water. Cell membrane by specific channels in it allows those water molecules to move in or move out. Now look at a diagram. This diagram shows you different parts of plasma membrane or a cell membrane. This diagram shows that the cell membrane, plasma membrane as it is generally said is a fluid mosaic. It is a fluid because it consists of a double layer of lipid molecules, the fat molecules. These fat molecules as you observe have their heads on one side and have their tails in the inner side. On upper side, the top, you can see the extracellular part. On the bottom, you can see the intracellular part that is inside of the cell. Cell membrane consists of a double layer of lipids which give it a fluidity. It's like a fluid but it also have various proteins integrated. As you can see in the diagram again, there are different types of proteins which are integrated at different places. For example, if we start from the left, we can see two large proteins which are close together. Both are crossing the whole of the fluid bilayer that is the lipid bilayer. Then you can see a small protein which is present down there towards intracellular side labelled as peripheral protein. Peripheral is present on the periphery. It is present towards the inside. Below that, you can see rope-like structures which are attached to these peripheral proteins. These rope-like structures are the cytoskeletal elements. These elements attach to the peripheral protein and on the other side, they attach to different organelles. For example, mitochondria, nucleus, maybe some else, endoplasmic reticulum and they give support to those organelles because you know, cytoplasm is a fluid, it's a plasma. Organelles have to be placed at, have to be kept at a particular location, at least for some time and they are placed on these locations with the help of these cytoskeletal elements and then the cytoskeletal elements, the fibres, the ropes as you can see, they are attached to the peripheral proteins. So, cell membrane gives support to the organelles with the help of these cytoskeletal elements. You can see various different types of proteins above, to which certain carbohydrate groups are attached. We call them glycoproteins, glycopacabohydrates, proteins, proteins which are attached with carbohydrate groups. Most of these proteins act as receptors. That is, when any molecule have to come in and give some message to that particular cell, they have to come and attach to a glycoprotein many times. So, glycoproteins, they receive the signals. They are receptors of the cell. Just like that, you can see that integrated inside the cell membrane in its gel or part towards the tail part, there are cholesterol molecules. You know, this lipid bilayer is like a fluid. It needs some support. It should be supported by some more strong structure. These cholesterol molecules, they give support and strength to the cell membrane. Now, in different types of cell membranes, the concentration of these cholesterol molecules, that is, quantity of these cholesterol molecules may be more to less. If membrane needs more strength, then there are more cholesterol molecule. If it needs less strength, there are less cholesterol molecule, depending upon in which environment particular cell is living. As you can see certain glycolipids, there are certain carbohydrate groups which are attached to the surface of the lipid bilayer because they are attached to the lipid. They are called glycolipid, glycol for carbohydrate, lipids for the fat molecules. This is quite a structure of the cell membrane. So, cell membrane is a fluid mosaic. It allows transport of materials inside and outside and it acts as a protective barrier.