Cells perform various physiological functions while actively maintaining life. Some such functions include actively bringing substances into the cell. These substances could be:
- Cell debris
The result of both endocytosis and phagocytosis mechanisms is a vesicle surrounding the engulfed material. Matter is brought into the cell in different ways, depending on what that matter is and what the ultimate destination of the matter is.
What is Endocytosis?
Endocytosis is an intracellular process that involves taking matter into the cell through the formation of vesicles around the material. The matter is essentially surrounded by a section of the plasma membrane of the cell, which then proceeds to pinch off inside the cell forming what is termed an endocytic vesicle.
There are four categories of endocytosis:
- Receptor-mediated endocytosis
During the process of pinocytosis, the fluids surrounding the particle, generally 0.5-5 nm, or macromolecule are also taken in, resulting in a small vesicle called a pinosome. Pinocytosis is primarily used for expelling extracellular fluids and in contrast to phagocytosis, it actually generates small quantities of adenosine triphosphate from the waste products of compounds such as lipids. Pinocytosis is non-specific in the substances it brings across the cell membrane and includes the fluids and solutes.
Some pinocytic physiological pathways are rather mediated by receptors. There are clathrin-coated pits in the plasma membrane which are involved in the vesicle formation and the pinosome subsequently comprises a clathrin-coated membrane. Specific receptors recognise macromolecules, typically 100 nm in diameter, and thus bind and form a protein coated vesicle with the bound macromolecule and the receptor on the inside.
For example, in the human body iron is transported in the blood bonded to a protein called transferrin which mediates its movement across cell membranes of cells that have these transferrin receptors on their surfaces.
Caveolae are a specialized type of lipid raft which comprise 50 – 100 nm invaginations in the cell membrane which have important functionality in signal transduction, mechano-protection, mechano-sensation, endocytosis, oncogenesis and the uptake of pathogenic bacteria and some viruses. They comprise proteins and lipids and are presents in many cell types, such as endothelial cells, adipocytes and embryonic notochord cells. They typically absorb particles that are smaller than 50 nm in diameter.
What is Phagocytosis?
Phagocytosis is the mechanism whereby cells take up material, usually large solid particles typically with a size of 0.75 nm in diameter, by a process involving the ingestion of the particle by the cell resulting in a phagosome. During this process, particles present in the extracellular fluid such as cell debris, mineral particles, apoptotic cells, dust and bacteria are ingested by the cell.
There are various immunological cells that are considered phagocytes and these include macrophages, neutrophils and monocytes. There are also phagocytic cells found in the Langerhans cells of the skin, Kupffer cells within the liver, the pigmented epithelial cells of the eye, as well as the microglia of the brain. Phagocytosis is usually a defence mechanism which destroys any pathogenic matter or particles in the body.
This process is receptor-mediated and involves various receptors:
- Immunoglobulin G (Ig G)
- Complement CR1 and CR3
The process is triggered by the receptors binding to the protein on the surface of the molecule being taken up, this prompts the cell to form pseudopodia which then surround the material to be up taken and the membrane is pinched off into a phagosome. Phagosomes have the capability to fuse with lysosomes forming phagolysosomes which can then digest the material due to the presence of enzymes in the lysosome. The digested waste is then excreted from the cell by means of exocytosis.
Phagocytosis is known “cell eating” and provides a means for simple celled organisms, such as protozoans such as amoeba to absorb nutrients.
The Difference between Endocytosis and Phagocytosis
- By Definition
Endocytosis refers to a cell taking up material by formation of a vesicle by the plasma membrane, whereas phagocytosis refers to the taking in of large solid particles into the cell through the formation of phagosomes.
The words themselves give some indication to their processes and functionality with phagocytosis coming from the Greek work “phagein” which means “to devour”, “kytos” means cell and “-osis” means process which essentially means the process of engulfing solid particles. Endocytosis derives from the word “endo” which means “within”, “cyt” refers to the “cell” and again “-osis” means process.
Endocytosis is composed of three categories, namely phagocytosis, pinocytosis and receptor-mediated endocytosis, whereas phagocytosis is actually a category of endocytosis.
- Matter taken up
Endocytosis involves the taking up of both macromolecules and particles into the cell, whereas phagocytosis comprises the uptake of solid particles only. Phagocytosis engulfs entire particles which are then subjected to enzymatic action, for example cathespins, and then absorbed, whereas pinocytosis engulfs already broken down particles. Pinocytosis is non-specific; although receptor mediated endocytosis is molecule specific.
Comparison between Endocytosis and Phagocytosis
- Endocytosis refers to the taking in of matter into a cell through the formation of a vesicle by the plasma membrane, whereas phagocytosis refers to the taking in of a large solid particle into the cell through the formation of phagosomes.
- Phagocytosis is actually a sub-category of endocytosis and has a typically defensive role in cell physiology, whereas endocytosis has more metabolic functions such as hormone movement and ingestion of nutrients.
- Endocytosis involves the engulfing of macromolecules and particles, including the extracellular fluid around them, whereas phagocytosis engulfs the particle only.