Introduction
Pinocytosis is the ingestion of extracellular fluids, i.e. the fluid surrounding the cell, together with its contents of small dissolved molecules (solutes). This begins with the cell forming narrow channels through its membrane that pinch off into vesicles, and fuse with endosomes resulting in the hydrolysis or breakdown of the contents. Pinocytosis can be thought of as ‘cell drinking’ as the word comes from the Greek “pino“, meaning ‘to drink’ and “cyto“, meaning ‘cell’. Pinocytosis was discovered by Warren Lewis in 1931 and is also known as fluid-phase endocytosis.
Pinocytosis is an example of endocytosis, a cellular process in which substances are brought inside a cell. Other types of endocytosis include phagocytosis and receptor-mediated endocytosis. All three are about taking in substances into the cell. However, what is the difference between phagocytosis and pinocytosis? Phagocytosis is about “engulfing” a relatively larger substance. Conversely, pinocytosis refers to “cell drinking”.
As for the difference between pinocytosis and receptor-mediated endocytosis, the latter is more specific; substances have to bind to the receptors on the cell surface to initiate endocytosis. Nevertheless, some references classify the latter under the wider, broader, pinocytosis. The uptake of fluid from outside the cell that involves pinosomes (fluid-filled vesicles), irrelevant of the size, is what defines pinocytosis.
Types of Pinocytosis
Pinocytosis can be divided by the size of the molecules to be taken up.
Micropinocytosis refers to the uptake of small molecules with a vesicle size of around 0.1µm. Caveolin-mediated pinocytosis is a common example of micropinocytosis that will be described in more detail below.
Macropinocytosis results in the formation of larger vesicles of around 0.5-5 µm. Macropinocytosis is a non-selective process. It results in the formation of large macropinosomes. The protein actin is largely involved in the formation of protrusions or ruffles in the cell membrane which results in the formation of these large vesicles. Macropinocytosis is used by immune cells such as macrophages to sample bulk extracellular fluid for soluble antigens that can evoke an immune response if necessary.
Pinocytosis can be further divided into 4 sub-types based on the mechanism of action. These are as follows:
Macropinocytosis
Clathrin-mediated endocytosis (also known as receptor-mediated endocytosis)
Caveolae-mediated endocytosis
Clathrin-independent/caveolae independent endocytosis.
Steps of Pinocytosis
What happens during pinocytosis? The membrane surrounding the cell can be described as semi-permeable. This means that it allows some molecules in or out via diffusion. The cell membrane also contains various lipids, fats, and protein channels/carriers.
Only small particles can be taken up during pinocytosis as they are usually dissolved in the extracellular fluid. The resulting vesicle contains this extracellular fluid complete with its solutes.
The vesicle can be described as a membrane-bound organelle; it is made up of the extracellular membrane of the cell enclosing the fluid in a spherical arrangement. Pinocytosis can be initiated by electrostatic interaction between a positively charged substance, such as the charged portion of a peptide or protein, and the negatively charged surface of the cell membrane. This can initiate binding to the cell membrane, altering the shape of the membrane to create a pouch around the fluid containing the charged peptide or protein.
Eventually, the membrane curls around on itself, and the pouch is ‘pinched off ‘ allowing the resulting vesicle to drift into the cytoplasm of the cell.
The pinocytotic vesicles function as carriers of the extracellular fluid into the cell. Let’s take a look at the steps involved in pinocytosis as shown in the diagram below.
Figure 2: the intake of small membrane vesicles from the extracellular fluid is called pinocytosis.
The steps of pinocytosis are shown in the diagram and cited below.
Step 1. A molecule in the extracellular fluid binds to the cell membrane which begins the pinocytosis process.
Step 2. This triggers the cell membrane to create a fold around the fluid containing the molecules to be ingested.
Step 3. The cell membrane invaginates (folds back on itself) to create a pouch.
Step 4. This pouch is then pinched off at the cell membrane and can migrate into the cytosol of the cell.
Function of Pinocytosis
The main function of pinocytosis is to absorb extracellular fluids. It plays an important role in the uptake of nutrients along with the removal of waste products and signal transduction.
Examples of Pinocytosis
What are examples of pinocytosis? In eukaryotic cells, pinocytosis is used widely, from the transport of dissolved fats (e.g. low-density lipoprotein) and vitamins to the removal of waste materials via the kidney cells. It is used by cells of the immune system to check the extracellular fluid for antigens (toxins or foreign substances). It can also be seen in the microvilli of the digestive system. Interestingly, flu viruses can use certain methods of pinocytosis to gain entry to cells as can some bacterial toxins.
