The cell is the basic component of living organisms. Cells have a plasma membrane, composed of a lipid bilayer, which acts like a barrier to the cell, controlling the movement of substances in and out of the cell. The cell has many organelles which are involved in the production and secretion of many substances; cells have to exchange nutrients continuously in order for the body to function properly. ≠
≠ Reference made from Cells and Cell Biology hand-out.
Insulin is a peptide hormone that is produced by the beta cells in the pancreas. Pancreatic beta cells are in groups situated in the islets of Langerhans. Insulin is a small protein, but produced as a larger protein and is replicated onto messenger RNA (mRNA) onto the inactive protein Preproinsulin.
Preproinsulin contains amino terminal signalling this is so insulin can pass through the endoplasmic reticulum (ER) for processing. While the Preproinsulin is being processed within the ER the signal sequence is broken down from peptides or proteins into amino acids to form proinsulin.
Three vital sulphide bonds are formed, specific peptidases sever the proinsulin. This process is known as biosynthesis. Once the biosynthesis process is developed the insulin is then active. †
† Reference made from Beta Cell Biology Consortium author Jean-Philippe Cartailler http://www.betacell.org/content/articleview/article_id/1/ 17th January 2013
The nucleus which contains chromosomal DNA and is surrounded by two membranes, also known as the nuclear envelope. The cytoplasm consists of a number of membrane bound structures collectively named organelles. They carry out very specific functions. In addition to organelles, ribosomes are floating in the cytoplasm or are attached to membranous sacs, called rough endoplasmic reticulum or rER. The rER is the site of the synthesis of enzymes destined for the degradation of particular nutrients, metabolites or spent organelles, protein destined for export from the cell (secretion), or proteins to be included into the plasma membrane. Previous to the proteins reaching their final destination, they are structurally changed in the Golgi complex. From there certain enzymes are bundled into lysosomes, which are capable of breaking down biological molecules. Mitochondrion, an organelle surrounded by two membranes is the cell's powerhouse, i.e. it is continuously producing the high energy molecule ATP. In addition an animal cell contains a network of filaments and microtubules known as cytoskeleton, which provides mechanical support and assists in the guided transport of small vesicles and organelles.*
*Reference made from http://www.prism.gatech.edu/~gh19/b1510/rergolgi.htm
The Endoplasmic Reticulum
The area near the nucleus contains two types of interconnected membranous structures, the smooth endoplasmic the reticulum (SER) and the rough endoplasmic reticulum (RER). The RER is very important for protein processing. The space enclosed by the RER membrane is called lumen. The rER is called rough, because its membrane appears rough under the microscope. The membrane is actually covered with ribosomes, protein is then secreted into the cytoplasm and will remain there or it may diffuse into an organelle. However, the later destinations require that the peptides have a destination signals.
The RER contains ribosomes; however they are not attached permanently to the RER membrane. Let's assume a protein synthesis begins on a free ribosome and a particular sequence of very hydrophobic amino acids emerges as shown below
This hydrophobic sequence called signal peptide binds to a so called signal recognition particle, which prevents further synthesis. The entire complex migrates to the RER attaches itself to a receptor protein called a docking protein. The signal particle dissociates off and protein synthesis continues, with the protein discharged into the lumen of the RER.