Contents
The study of cellular Biology Topics is essential to understanding the workings of all living organisms.
Endocytosis and Exocytosis: Differences and Similarities
Endocytosis – Definition and Examples
Endocytosis is the ingestion of material by the cells through the plasma membrane. It is a collective term that describes three similar processes: phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis. These processes are pathways to specifically internalize solid particles, small molecules, and ion, and macromolecules, respectively. All of them require energy, so they may be regarded as different forms of active transport.
Three types of Endocytosis
In phagocytosis, the plasma membrane binds to a large particle (adsorption) and extends to engulf it. In potocytosis small areas of the plasma membrane, bearing specific receptors for a small molecule or ion, invaginate to form caveolae. Receptor-mediated endocytosis is a mechanism for the selective uptake of large molecules (the ligands) in clathrin-coated pits. The binding of the ligand to the receptor on the surface/plasma membrane stimulates the invagination of pits.
Phagocytosis: A Fundamental Process in Immunity
It literally means “cell eating”. It is a common method of feeding among the protozoa (Amoeba) and lower agent Phagocyte metazoa (e.g., sponges). It is also the way in which white blood cells (leucocytes) engulf cellular debris and uninvited microbes (viruses and bacteria) in the blood. Like WBCs, there are other phagocytes, such as the macrophages found in connective tissue and liver sinusoids.
In phagocytosis, an area of the plasma membrane, coated initially with actin-myosin, forms a pocket that engulfs the solid material (e.g., bacteria, debris). The membrane-enclosed vesicle, phagosome, then detaches from the cell surface (= plasma membrane) into the cytoplasm where its contents are digested by lysosomal enzymes.
Differences between Endocytosis and Phagocytosis
| Endocytosis | Phagocytosis |
| 1. It is the intake of extra-cellular fluid, droplets, and macromolecules. | 1. It is the intake of extracellular particles. |
| 2. Plasma membrane invaginates to take up the materials. | 2. Plasma membrane grows, around the particle as pseudopodia. |
| 3. It is a nutritive process. | 3. It is a nutritive and defensive process. |
| 4. Endocytotic vesicles are only 0.1 µm wide. | 4. Phagocytotic vesicles are 1 to 2 µm or more wide. |
Exocytosis – An Overview
Just as materials can be brought into a cell by the invagination and formation of a vesicle, the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium, ibis process is called cell vomiting or exocytosis.
Exocytosis occurs in various cells to
- remove undigested residues of substances brought in by endocytosis
- secrete substances such as hormones, enzymes
- transport a substance completely across a cellular barrier.
For example, a substance (e.g., IgA or immunoglobulin/antibody) may be picked up on one side of the wall of a blood vessel by phagocytosis, moved across the cell, and released by exocytosis.
In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from a Golgi apparatus is first moved by the cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicle discharges its contents outside the cell.