Pinocytosis – Definition and Functions


Pinocytosis

Pinocytosis or “cell drinking” is a crucial cellular process that involves ingesting liquid and dissolved solutes. The fluid typically includes fat droplets, nutrients, and sometimes antigens.

Key Points

  • In pinocytosis, cells “drink” fluids and dissolved molecules by closing pockets in the cell membrane.
  • Pinocytosis, like phagocytosis, is a way that cells take in materials from outside the cell. But, the vesicles formed by pinocytosis are much smaller than those in phagocytosis.
  • The process occurs in all eukaryotic cells and serves multiple functions, including nutrient absorption, communication, and roles in the immune response.

Pinocytosis Definition

Pinocytosis is one form of endocytosis, where a cell expends energy (ATP) and engulfs materials with the cell membrane. The term “pinocytosis” comes from the Greek words: “pino,” meaning “to drink,” and “cyto,” meaning “cell.” Thus, pinocytosis is essentially the process where cells “drink” extracellular fluids and their contents, form small vesicles, and absorb their contents. These vesicles contain a variety of substances, including ions, fats, nutrients, and other dissolved molecules.

Functions

Pinocytosis serves multiple functions:

  • Nutrient Uptake: Pinocytosis helps cells acquire essential nutrients from their surroundings. In humans, pinocytosis helps the small intestine absorb fats and fat-soluble vitamins and the kidneys filter waste from the blood.
  • Cellular Communication: By taking in extracellular fluid, cells receive signaling molecules that influence their behavior and responses.
  • Immune Response: Pinocytosis contributes to immune surveillance by sampling the environment for potential pathogens or antigens. It also plays a role in transferring IgG antibodies to the fetus.

Pinocytosis is not restricted to a specific type of organism. It occurs in both unicellular and multicellular organisms, including bacteria, protists, plants, and animals. For example, single-celled protists use pinocytosis to obtain nutrients from their environment, while animal cells utilize it for various physiological processes.

Pinocytosis vs. Phagocytosis: Similarities and Differences

Pinocytosis and phagocytosis are both forms of endocytosis, so they are variations of active transport that require ATP. However, they differ in their functions and mechanisms:

  • Similarity: Both processes expend energy to ingest materials from the extracellular environment. Both form vesicles within the cell.
  • Difference in Ingested Material: Pinocytosis involves the uptake of liquids and dissolved substances, while phagocytosis involves the ingestion of larger particles, such as bacteria, dead cells, or cellular debris.
  • Vesicle Formation: The cell membrane invaginates or folds inward are surrounds fluid during pinocytosis. In some cases of phagocytosis, the cytoplasm and plasma membrane extend outward and surround the particle to engulf it.
  • Difference in Vesicle Size: The vesicles formed during pinocytosis are smaller (50-200 nm) than those formed during phagocytosis (500-10,000 nm).
  • Cell Types: Pinocytosis occurs in most cells, while phagocytosis mainly happens in specialized cells like macrophages and neutrophils.

The other form of endocytosis is receptor-mediated endocytosis or RME, which involves surface receptor proteins for ferrying large particles across the cell membrane.

How Pinocytosis Works

Pinocytosis occurs through several steps:

  1. Initiation: The cell membrane forms small pouches or folds as it encounters extracellular fluid. Often, this involves electrostatic attraction between ions in the fluid and the cell membrane.
  2. Vesicle Formation: These invaginations pinch off from the membrane, forming vesicles containing fluid and solutes.
  3. Vesicle Transport: The vesicles move through the cytoplasm, where they release their contents or fuse with other organelles like lysosomes for further processing or degradation.
  4. Release of Contents: The vesicles release their contents within the cell, allowing for cellular utilization or signaling. Ultimately, the vesicle returns to the cell membrane and reintegrates with it.

Types of Pinocytosis

There are two primary types of pinocytosis:

  1. Fluid-Phase Pinocytosis: This type involves the nonspecific uptake of extracellular fluid, allowing cells to ingest a wide range of substances. It’s a continuous process and generally occurs at a slower rate.
  2. Receptor-Mediated Pinocytosis: This type is more specific and involves receptors on the cell membrane that bind to particular molecules in the extracellular fluid. These receptor-bound molecules are then internalized through vesicle formation, providing a more selective mechanism of uptake. This is a clathrin-independent mechanism that only occurs in response to surface receptor activation. Note some viruses and bacteria induce this type of pinocytosis to get inside cells and then evade the degradation process so they can replicate.

References

  • Alberts, Bruce (2015). Molecular Biology of the Cell (6th ed.). New York, NY: Garland Science. ISBN 978-0-8153-4432-2.
  • Dragwidge, J.M.; Van Damme, D. (2020). “Visualising endocytosis in plants: past, present, and future”. J Microsc. 280(2):104-110. doi:10.1111/jmi.12926
  • Falcone, S.; Cocucci, E.; et al. (2006). “Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events”. Journal of Cell Science. 119 (Pt 22): 4758–4769. doi:10.1242/jcs.03238
  • Pollard, Thomas D.; Earnshaw, William C.; Lippincott-Schwartz, Jennifer; Johnson, Graham T., eds. (2017). “Chapter 22 – Endocytosis and the Endosomal Membrane System”. Cell Biology (3rd ed.). Elsevier. ISBN 978-0-323-34126-4. doi:10.1016/B978-0-323-34126-4.00022-0