In this article we are focusing on “Is Endocytosis Pinocytosis: Similarities & Differences”, with detailed facts and explanations.
Endocytosis is a cell uptake or cell internalization mechanism through which the cells take in substances either solids or liquids, from the fluid stream by engulfing and forming vesicles. The substances that are usually internalized through endocytosis are Glucose, amino acids, cholesterol and other nutrients.
Pinocytosis also called fluid endocytosis is almost similar to normal endocytosis, here fluids and other dissolved substances are internalized. Pinocytosis also called fluid endocytosis is almost similar to normal endocytosis, here fluids and other dissolved substances are internalized. It is a continuous and a non-specific process of fluid internalization by the cells with the dissolved nutrients.
Why is Pinocytosis a form of endocytosis?
Pinocytosis is a form of endocytosis, where a cell internalizes fluids along with the dissolved particles which are 0.5µm or smaller than this. This process has been seen to occur in intestines in humans and is also known as “Cell Drinking”.
Pinocytosis is initiated when the desired nutrients are present in the fluid extracellularly, the desired molecules are the trigger for this process. There are binding receptors present on the cell membranes which are not specific for each molecule unlike other endocytosis processes. The initiator can be any protein, sugar, ions, lipids or any molecule.
General steps involved in the process
- It is initiated when the ligand from the extracellular fluid binds to the receptor which is present on the cell membrane. This receptor-ligand binding is crucial for further steps in the process.
- After receiving the signals from the binding, the cell membranes start making small pockets with open ends that captures the fluid to be internalized along with the dissolved molecules or the nutrients. The membrane starts forming the invaginations.
- Then small pockets in the form of vesicles are budded off from the cell membrane. These vesicles form a protective coating around the fluid containing the desired molecules in the cytoplasm.
- Depending on the fate of the molecule which is present as a signal on it, the vesicle will either fuse with the endosome or it will be moved out for exocytosis.
- Particles which are fused with the endosomes are hydrolyzed to be used up by the cells. In other case particles are simply moved to the other of the membrane by the process called exocytosis.
Similarities between Endocytosis and Pinocytosis
- Receptor mediated Endocytosis and Pinocytosis are both necessary endocytosis process that are employed by the cells to internalize variety of solutes, ions, minerals, nutrients and proteins that cannot be easily pass through the plasma membrane.
- Pinocytosis is a type of fluid endocytosis, both processes have the same basic motive of internalization only difference lies in the mechanism.
- Both Endocytosis and Pinocytosis are active transport processes which means they requires energy for their function.
- Each process requires cell membrane for their initiation.
Endocytosis vs Pinocytosis
|It refers to the internalization of solid particles, like amino acids, lipids, ions etc.||It refers to the uptake of fluid along with dissolved particles from extracellular fluid.|
|It is also known as “Clathrin mediated endocytosis”.||It is also known as “Cell Drinking”.|
|It is highly specific and selective.||It is not a selective process.|
|The vesicles are form through the invaginations of the plasma membrane.||The vesicles are budded off form the plasma membrane.|
|It is a highly efficient process.||It is comparatively less efficient process.|
|Specific nutrients are taken up by the cells using receptors by Endocytosis process.||Solutes present in the extracellular fluid are internalized through Pinocytosis.|
|This process is mediated by membrane receptors.||No membrane receptors participate in this process.|
|Molecules are ingested within vesicles which are later fused with endosomes.||Fluids containing nutrients is packed within vesicles which are budding off from the plasma membrane|
The process has been divided into two types based on the molecule size to be internalized:
Here the molecules to be internalized are larger in size due to which, invagination and pockets are large. Vesicles are called macropinosomes with around 1-2µm in size undergo maturation in the cytoplasm and either fuse with the lysosome for degradation or move to towards the cell membrane for recycling.
Here the molecules to be internalized are smaller in size. The size of the vesicles formed here is around 0.1 µm. A very well known example of micropinocytosis is Caveolin-mediated pinocytosis.
They are further divided into three types on the basis of the receptors:
- Clathrin mediated pinocytosis
- Caveolin mediated pinocytosis
- Clathrin and Caveolin independent pinocytosis
To summarize the article we can say that the processes “Pinocytosis” and “Endocytosis” are equally important with a common basic idea of internalization within a living cell.