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
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