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The method of moving molecules manufactured within a cell to the space outside of the cell is referred to as secretion. Usually, these secreted substances are called functional proteins. However, the range can include non-protein products such as steroids. This process is opposite to excretion in which the waste products are removed from the system.
Purpose of Secretion
The main purpose of secretion is to produce signals. The secreted substances act as short and long-distance signals for another cell. For instance, neurotransmitters are secreted by neurons send messages to neighboring neurons.
Similarly, several types of hormones are secreted by the pituitary gland travel via the bloodstream and act throughout the body. There are diverse functions of these long-distance signals such as they have roles in the kidney, reproductive organs, and metabolism.
The secreted substances also have important functional roles within an organ or tissue. For example, in the stomach, three different cell types occur in gastric glands and the components of gastric acid are secreted from the gland.
The lubricating mucus is secreted by the mucus cells and hydrochloric acid is secreted by parietal cells and the chief cells secrete the precursor to the protein-digesting enzyme, pepsin. All these substances help in the digestion of food inside the stomach.
How Does Secretion Occur?
Depending upon the cell type and the substance, the secretion can occur by many pathways. In some cases, the secreted material or substance inserts into the cell membrane while in others it must cross the plasma membrane. The examples of some major pathways are as follows-
i. ER-Golgi Pathway and Porosomes
In this pathway, the endoplasmic reticulum produces secretory products, which are then inserted into transport vesicles. The spherical containers made of the lipid bilayer are called vesicles.
The vesicles then travel to the Golgi apparatus, where the modification, packaging of the secretory material takes place. The products are marked for export and packed in special vesicles in the Golgi apparatus. The process is much similar to shipping and labeling a package and loading it into a delivery truck.
The chemical environment in the cytosol can cause chemical reactions and change the structure of proteins thus the compartmentalization of the secretory pathway is essential. The secretory vesicles then leave the Golgi apparatus and interact with the porosomes in the cytoskeleton while traveling.
The structure of the porosome is conical-shaped which is embedded in small pits in the plasma membrane. The pits swell after docking of vesicles and the content expel into the space outside due to resulting pressure. The material in the vesicle exit due to the increased diameter of the porosome.
A fusion ring is formed around the narrow portion of the porosome by several proteins that facilitate the release of vesicle content. The process is called exocytosis, in which a material is moved outside from inside of the cell membrane.
Some cells use a specialized form of exocytosis. The specific fusion proteins found in neurons allow rapid and synchronous neurotransmitter release that send a signal from one neuron to another.
ii. Membrane Transporters
Instead of exocytosis, some proteins in the cytosol move across the cell membrane via transporter proteins. In this pathway, the proteins are not packed in vesicles and are transported individually by specialized proteins found in the plasma membrane.
The major function of lysosomes is degradation but they also serve some other important roles such as secretion. The lysosomal secretory pathway is frequently used in certain specialized cell types such as the blood stem cells and pigment cells. Like secretory vesicles, the lysosomes fuse with the cell membrane and release its content. However, in this process, different proteins are used.
iv. Secretion in Prokaryotes
The membrane-bound organelles are not found in prokaryotes. Thus they lack ER and Golgi apparatus and secretion occurs by another pathway. For example, six different methods are used by gram-negative bacteria, labeled types I-VI.
All the methods move products across the cell membrane by using unique molecular structures. However, the substances secreted by bacteria are harmful to others, but the research is ongoing to use these substances in developing antibiotic treatments.
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