The term pinocytosis combines two Greek words, where ‘pino’ means ‘to drink’, and ‘kytos’ denotes ‘cell’. So, the word stands for ‘cell drinking’. It involves the transport of extracellular fluids along with its contents of small dissolved molecules (solutes). That is why pinocytosis is considered a form of endocytosis.
Is Pinocytosis a Form of Active or Passive Transport
What Happens During Pinocytosis
Types
Functions
Examples
Pinocytosis vs. Phagocytosis
Though it is not specific, certain ions or amino acids can trigger the process. Various molecules, such as ions, sugars, and amino acids, activate the process when they contact the plasma membrane. This phenomenon induces the development of small pockets (fold) on the membrane around ECF (and minute dissolved solutes), and the consequent internalization of these materials occurs inside the cell.
Steps of the Process
The basic steps of pinocytosis are described below: Step 1 – Initiation: In the initial stage, an inducer molecule, such as sugar, protein, or ion, comes in contact with the plasma membrane. As a result, an ionic interaction occurs between the positively charged inducer and the negatively charged cell membrane. This phenomenon triggers the binding of the molecule to the membrane. Step 2 – Folding of the Membrane: Following binding, the cell membrane gets stimulated to fold inwards and form a small open-ended pocket, or invagination, around the fluid containing the molecule. Step 3 – Invagination and Engulfment of the Fluid: The cell membrane continues to fold inwards with the fluid and dissolved solutes in small pockets. Later, they start to reconnect at the open end of the invagination to enclose the fluid and solutes. Step 4 – Detachment of the Pocket: This is the final stage when both ends of the invagination meet. It gives rise to a vesicular structure called a pinosome that contains extracellular fluid and dissolved solutes. Afterward, this pouch gets detached or pinches off from the cell membrane. The molecules present inside the vesicle are eventually released to be used by other parts of the cell, thus completing the process.
Based on the Size of the Molecule
Depending on the size of the molecules to be transferred, pinocytosis has two broad divisions:
Micropinocytosis
Going by the name, it refers to the uptake and internalization of small molecules. In this case, the vesicle or micropinosome formed measures around 0.1um in diameter. As a result, tiny indentations develop on the cell membrane. An example of this phenomenon is Caveolin-mediated pinocytosis (has been described later), formed in the epithelial cells of blood vessels.
Macropinocytosis
Cells use this process to draw larger materials from the extracellular environment inside the cell. So, in this case, the vesicle or the macropinosomes are large with a diameter of around 0.5-5 µm. Macropinocytosis is non-specific and thus involves the uptake of a variety of materials ranging from antigens to nutrients. This type of pinocytosis helps immune cells to detect soluble antigens, among other materials in ECF. As soon as the inducers trigger the process, the cell’s cytoskeleton rearranges the actin filaments in the cell membrane to produce protrusions or ruffles to form macropinosomes. The vesicle matures in the cytoplasm. Later, they either fuse with the lysosomes or migrate towards the cell membrane for recycling. This process is commonly seen in white blood cells (WBCs) like macrophages and dendritic cells.
Based on the Receptor and Their Mode of Action
Depending on the receptor involved in the process and the mechanism of vesicle formation, pinocytosis further has four divisions. Among the four types, one of them is macropinocytosis, which is described in the previous section. The rest three divisions are as follows:
Clathrin-Mediated Pinocytosis
Here, the cells take up a wide range of molecules, including proteins, hormones, and various metabolites. Like macropinocytosis, the cell membrane undergoes a conformational change following the attachment of molecules to the surface receptors. This receptor-mediated pinocytosis is the best form of endocytosis to uptake selective macromolecules. Initially, these molecules adhere to specific cell surface receptors known as clathrin-coated pits. Later the pits bud through the membrane to develop tiny clathrin-coated vesicles, which contain the receptors and their bound macromolecules. Finally, the vesicles fuse with early endosomes. They are then driven for transportation to the lysosomes or recycled to the plasma membrane.
Caveolae-Mediated Pinocytosis
It helps in the transport of small materials (solutes and molecules) inside the cell. It is an example of micropinocytosis. The adipocytes (fat cells) and endothelial cells of blood vessels exhibit this phenomenon. Caveolae are 50–80 nm cup-shaped pits usually found immobile in the plasma membrane. They are often found associated with two classes of proteins, caveolins, and cavins. When a caveolae-liked receptor is activated, it can bud off from the cell membrane to form vesicles. Various solutes or molecules present in the extracellular fluid interact with the caveolae complex and caveolin proteins on the membrane. As a result, the internalization of the complex and proteins take place.
Clathrin and Caveolae-Independent Pinocytosis
This type of pinocytosis does not depend on receptors or other coat proteins or material stimuli to develop vesicles. Due to this, it allows the transportation of a wide variety of materials, including various pathogens, receptors, and toxins. Actin and associated proteins play an essential role in the formation of vesicles. Though both of them are a variation of endocytosis, they have contrasting work mechanisms.
What is the Difference between Pinocytosis and Phagocytosis
Similarities
Despite numerous differences, they share some similarities. They are given below:
Types of endocytosis. So, they help to draw extracellular materials inside the cell.Involve the folding of the plasma membrane to form a vesicle or pouch-like structure.Require energy (ATP) for cellular transport.
