Phagocytosis and pinocytosis are biological processes wherein a cell senses and engulfs nearby material. These processes enable cells to intake substances that can’t easily pass through the cell membrane. However, phagocytosis and pinocytosis are forms of endocytosis that differ in several ways including how they intake material and the type of material they engulf. In this guide, we explore the basics and key differences between phagocytosis vs pinocytosis in more detail.
What is phagocytosis?
Phagocytosis is a specialized process by which cells engulf relatively large, solid material. These particles, generally larger than 0.5 μm in diameter, may include apoptotic cells or foreign substances. Unicellular organisms such as amoebas use phagocytosis to acquire nutrition while cell types of multicellular organisms use this universal process for preventative functions such as tissue homeostasis. Among these cells, certain types are capable of greater efficiency in the phagocytic process, such as monocytes or macriophages. Other, like epithelial cells, are capable of phagocytosis, but at a less efficient rate.Phagocytosis function
The cellular processes of phagocytosis consist of four distinct phases that include 1) the detection of target material, 2) activation of phagocytosis, 3) the formation and 4) maturation of the phagosome.
Image 1: Phagocytosis: The detection of target material
The detection of target material begins when a cell senses target material on its surface (Image 1). Most cell types have a dedicated set of cell surface receptors that recognize various substances. For example, immune cells can sense invasive microbes with receptors that recognize pathogen-associated markers. After the target material has been detected, the cell surface receptor initiates a series of signals that prepare the cell for phagocytosis.
Image 2: Activation of phagocytosis
Activation of phagocytosis causes the cell membrane area in contact with the target material to rearrange into a cavity called the phagocytic cup (Image 2). This process begins when the material-stimulated receptors send molecular signals that prepare the cell for target material intake.
Image 3: Phagosome formation
The formation of the phagosome is the third phase of phagocytosis that involves the complete internalization and closure of the phagocytic cup (Image 3). Once the phagocytic cup fully covers and internalizes into a cellular compartment, the newly formed phagosome can begin its maturation process.
Image 4: Phagosome maturation. Here, the phagosome has merged with a lysosome, forming a phagolysosome.
Maturation of the phagosome is the final phase of phagocytosis (Image 4). Phagosome maturation consists of a series of material handoffs between the original phagosome and cellular compartments specialized in digestion. The early stages of phagosome maturation include a combination of fusion and fission events that prepare the cellular membrane of the maturing phagosome for material digestion. During the final stages of phagocytosis, the internal environment of the mature phagosome becomes more acidic and reactive, which aids the digestion of target material such as microorganisms.
What is pinocytosis?
While phagocytosis involves the ingestion of solid material, pinocytosis is the ingestion of surrounding fluid(s). This type of endocytosis allows a cell to engulf dissolved substances that bind to the cell membrane prior to internalization. Unlike phagocytosis, pinocytosis is a “drinking” mechanism wherein a cell actively engulfs external fluids over time. Even though pinocytosis differs from other forms of receptor-mediated endocytosis, these terms overlap with one another due to their similarities.
Pinocytosis function
Image 5: Pinocytosis is initiated when the cell senses a soluble substrate.
In humans, pinocytosis primarily occurs when cells absorb nutritional or waste droplets suspended in external fluid. The nutritional molecules that can activate pinocytosis include fats, sugars, proteins, ions or other small molecules. This process begins when a soluble substrate binds to the surface of a cell (image B1)
Image 6: A: External fluid is gathered into a membrane-bound pocket. B: The pocket begins to pinch off.
After a substrate opportunistically binds to the cell membrane, the cell continues pinocytosis by gathering external fluid into a membrane-bound pocket called a pinosome (image 6A).Image 7: The pinosome is internalized.
This pinosome continues to pinch off from the cell membrane (Image 6B) within the cell until it is fully internalized (Image 7).
Image 8: The pinosome can now merge with other vacuole, such as the early endosome, to enable breakdown of its contents.
Once the cell fully internalizes this cellular pocket, the newly formed compartment can swap its material with specialized vesicles that can break down substrate into smaller molecules (Image 8).
The size of these vesicles during the intake of fluid material can establish whether a cell performs macropinocytosis vs micropinocytosis. Macropinocytosis is a form of non-specific endocytosis that is common for all cell types. This process includes the intake of fundamental nutrients such as amino acids, carbohydrates and fats.
Micropinocytosis involves the ingestion of fluid material by small vesicles that are typically no larger than 0.1 micron. This process is typically initiated by cell surface lipid domain once they bind to their specific target molecules.1
Phagocytosis vs pinocytosis: chart
| Phagocytosis | Pinocytosis | |
| Definition | Cellular intake of solid material | Cellular intake of fluid |
| Role | Immune response, nutrient uptake | Immune surveillance, nutrient uptake |
| Cell Types | Most commonly immune cells | Almost all cell types |
| Example | Macrophages | Skin cells |
Examples of phagocytes and phagocytic cells
Phagocytes are a specialized group of cells that perform phagocytosis efficiently. This group includes immune cells such as macrophages, neutrophils and monocytes. Each type of phagocyte has a set of cell surface receptors that can initiate phagocytosis whenever they sense a target particle or pathogen.
Opsonization in phagocytosis
Opsonization is an immune process where phagocytes bind to foreign molecules that are linked to host-derived proteins such as antibodies. This process involves the binding of free-floating antibodies to foreign materials so that these potential threats can be identified by the immune system. The tail of each antibody has a segment called the Fc region. This region specifically binds to Fc cell surface proteins found on the surface of phagocytic cells. Opsonization can also occur via the complement system, where a fragment of the system, C3b, acts as a binding site for phagocytes.
Opsonization is completed when a phagocyte identifies the foreign molecules of interest and initiates phagocytosis.
Pinocytosis is the method by which a cell absorbs small particles outside the cell and brings them inside. The word pinocytosis comes from the Greek for “cell drinking”. During this process, the cell surrounds particles and then “pinches off” part of its membrane to enclose the particles within vesicles, which are small spheres of the membrane. This process is usually used for taking in extracellular fluid (ECF).
Pinocytosis is a type of endocytosis. Endocytosis is when a cell takes in particles by engulfing them with its membrane. The other types of endocytosis are phagocytosis (“cell eating”), which is nearly the same as pinocytosis but involves the cell takes in a greater amount of particles, and receptor-mediated endocytosis, which is when particles are taken into a cell by binding to receptors.
Small particles of substances in the ECF are absorbed into the cell via pinocytosis. It is a process that requires active transport, which means that it requires energy on the part of the cell (as opposed to a process like simple diffusion). It is non-specific; although it is triggered by the presence of certain substances outside the cell such as amino acids or certain ions, it results in the absorption of many different substances by the cell at the same time, such as water and various solutes like sugars and proteins.
This figure depicts pinocytosis:
- An inducer substance, such as a protein, binds to a receptor on the cell membrane. Note that this is different from receptor-mediated endocytosis because multiple types of molecules will be taken in by pinocytosis instead of just one specific type.
- The cell membrane forms a small open-ended pocket, or invagination, around the part of the ECF that is going to be absorbed into the cell.
- The cell membrane starts to reconnect at the open end of the invagination, or “pinch off”.
- When the cell membrane has fully pinched off the invagination, a vesicle is formed. This is a small sphere of the cell membrane that surrounds the materials that the cell has just taken in. It prevents the molecules from disrupting the rest of the cell as they are transported.
- The molecules inside the vesicle are eventually released to be used by other parts of the cell.
Pinocytosis is widely used among the cells of the body, but there are specific situations where pinocytosis plays a major role. Microvilli in the gut use this process to absorb nutrients from food. Cells in the kidney can use pinocytosis to separate nutrients and fluids from the urine that will be expelled from the body. In addition, human egg cells also use it to absorb nutrients prior to being fertilized.
Pinocytosis is broken down into macropinocytosis or micropinocytosis based on the size of the vesicle that is formed. Vesicles formed from macropinocytosis are about 1-2 µm in length (0.0001-0.0002 cm), while those formed from micropinocytosis are even smaller, about 0.1 µm, and are formed from the tiniest indentations in the cell membrane.
Pinocytosis and phagocytosis are similar processes, but they have some key differences. Phagocytosis is used to absorb materials much bigger than the particles that can be absorbed by pinocytosis, such as bacteria. Phagocytosis involves the ingestion of solid materials, while pinocytosis involves the ingestion of liquids and solute particles. During pinocytosis, the contents of the vesicles are emptied directly into the cell, but this doesn’t happen in phagocytosis because the contents of the vesicles are too big. Lysosomes must combine with the vesicles to break down the contents. This process does not occur in pinocytosis.
- Endocytosis – When a cell takes in particles through invagination of the cell membrane.
- Phagocytosis – Also known as “cell eating”; when a cell absorbs large particles such as whole bacteria.
- Invagination – The process by which a structure (in this case, the cell membrane) is folded back or turned inside out to form a pocket or cavity.
- Vesicle – A small sphere of membrane inside the cell that can uptake and transport materials such as nutrients or waste products.
1. Which is NOT a step of pinocytosis?
A. An inducer molecule binds to a receptor on the cell membrane.
B. An invagination forms, surrounding part of the ECF.
C. The particles are brought into the cell via a vesicle.
D. Lysosomes digest the contents of the vesicle.
D is correct. A, B, and C are all steps. D is not involved in pinocytosis, but it is involved in phagocytosis.
2. What is NOT a type of endocytosis?
A. Receptor-mediated
B. Pinocytosis
C. Cytosine
D. Phagocytosis
C is correct. The three types of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis. Cytosine is one of the nitrogenous bases found in DNA.
3. Which of these would not be absorbed by pinocytosis?
A. Proteins
B. Bacteria
C. Sugars
D. Water
B is correct. Pinocytosis takes in small molecules, such as proteins, sugars and water found in extracellular fluid. An entire bacterium is too big to be absorbed by pinocytosis and would instead be absorbed by phagocytosis.