For many functions, the cell needs to take in substances, from outside to inside. Very small particles, can penetrate the lipid bilayer itself, or pass through the transport proteins on the membrane. However, very large particles, cannot cross the membrane. So they are taken inside the cell by a special process, called endocytosis. Basically in endocytosis, the cell membrane wraps around the particle, and then breaks off a vesicle containing that particle, inside the cell. Based on how it is formed, you can understand, that the fluid inside this vesicle, is actually extracellular fluid from the outside, and the wall of the vesicle, is a detached portion of the cell membrane. These were some basics. Now let's go into more detail. So, there are two main forms of endocytosis: pinocytosis, and phagocytosis! First, let's talk about pinocytosis. Pinocytosis, means drinking by the cell. It occurs to take in macromolecules. The cell expresses receptors for such molecules on the membrane. Such receptors, are generally concentrated in a small pit on the membrane. It is called a coated pit. Beneath this pit, on the inner side of the cell, there is a latticework of fibrillar proteins called clathrin, and other proteins, like contractile filaments: actin and myosin. All these proteins, drive the movement of the cell membrane during endocytosis. What happens is: when the particle attaches to the receptor on the outer side of the cell, these proteins pull the membrane inside the cell, thus deepening the pit. Then they close the mouth of this pit, forming a vesicle inside the cell. Then these proteins dissociate from the vesicle, and go back to the membrane for next round of endocytosis. The entire process requires energy from ATP. The vesicle formed by pinocytosis, is called a pinocytotic vesicle. Such vesicles are very small. Pinocytosis occurs in almost all the cells in the body. So this was about pinocytosis. Now let's talk about phagocytosis. It means eating by the cell. It occurs for the intake of large particles, such as bacteria, or whole cells, etc. And only some cells are capable of doing phagocytosis, like tissue macrophages, and some white blood cells. The process of phagocytosis, is similar to pinocytosis. First, some ligand on the particle to be engulfed, binds with the receptor on the cell. This initiates the process of phagocytosis. In this, the membrane around the points of attachment, evaginates outwards, to surround the entire particle. At the tip, the membrane merge, creating a vesicle inside the cell. It's called a phagocytic vesicle. So this is how phagocytosis happens. Now, the cell has organelles called lysosomes, containing various hydrolase enzymes. The vesicle formed by pinocytosis or phagocytosis, merges with this lysosome. This empties the enzymes in lysosomes, in the vesicle. Now it's called digestive vesicle. And as per the name, it starts digesting the proteins, carbohydrates, lipids, and other things in the vesicle. Small molecules like amino acids, glucose, phosphate, etc formed during this digestion, cross the membrane and enter the cytoplasm. Indigestible things are left inside the vesicle. They are excreted out of the cell, by exocytosis. So this was all about ingestion by the cell. Now let's have a quick summary. Entry of large particles inside the cell, happens by endocytosis. For this, the cell membrane wraps around the particle to be taken in, and then breaks off a vesicle containing that particle, inside the cell. Pinocytosis is drinking by the cell. Large macromolecules are taken in, by this process. It occurs in most cells. Phagocytosis is eating by the cell. It takes very large things, like bacteria or whole cells. Only some cells are capable of phagocytosis. The vesicle formed by endocytosis, merges with a lysosome, to form a digestive vesicle. In this, the ingested particles are broken down. Small molecules like amino acids and glucose thus formed, enter the cytoplasm. And indigestible material is exerted out of the cell. That's it for this video. Click here, to read the notes on this video at our website. You can also explore our entire animation video library over there. To support our free content creation, first leave a nice comment, and then share the video with all your friends and colleagues. It helps us a lot. Thanks for watching, see you in the next video!