How Organelles Work Together to Make and Secrete the Protein Insulin

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Human Biology Unit 1 Assignment
The human body is made up of 100 trillion cells. All cells have the same basic structure (see figure 1) however some cells are specialised to suit a specific purpose. An example of this is the pancreatic beta cells found in the islets of Langerhans. These cells are specialised to synthesise the protein insulin that is involved in the metabolism of glucose in the cells. (Layden, 2010) Figure 1 - Picture to show basic structure of a cell.

(Farrabee, M J. 2007)
But how is insulin made? The production of insulin starts in the nucleus which is the cellular organelle where the DNA (deoxyribonucleic acid) is found. They form a series of multiple linear molecules which are then folded in the nucleolus in to chromosomes as a gene. Genes are sections of DNA which contain instructions on how to make proteins and in this case insulin. The DNA molecule however is too large to get through the nuclear pores of the membrane so by the process of transcription a section of the instruction is copied in to RNA (ribonucleic acid). The RNA then takes the section of the instruction and leaves the nucleus and joins with a ribosome where it can be used to synthesise the protein. (Hickman and Thain 2004) Ribosomes are either free in the cytoplasm or are attached to the ER. Each ribosome has one conformational groove to fit the growing polypeptide chain and another for the messenger RNA. It has a gap between both of its sub units to permit the entry of transfer RNA. The tRNA is then bonded to an amino acid from the amino acid pool. The amino acid pool is composed by amino acids that the body has broken down and put in to the bloodstream via facilitated diffusion.

Figure 2- Passive Transport, Diffusion & Osmosis

(Coli, E. 2007)
Diffusion is the process that is used in oxygen entering a cell, and carbon dioxide leaving. These molecules will move from where they are at a high concentration to where they are at a lower concentration they diffuse down a concentration gradient. The blood system in humans continually brings more oxygen to the cell and takes carbon dioxide away. This maintains a high concentration gradient. Since the movement is always down the concentration gradient, it requires no energy. The small molecules pass from one side of the membrane to the other by moving between the lipid molecules. Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of high water potential a higher concentration of water molecules to an area of low water potential a lower concentration of water molecules. (S-cool, 2013)

Facilitated diffusion is a type of passive transport that allows substances to cross membranes down a concentration gradient with the assistance of special transport proteins. Through the use of ion channel proteins and carrier proteins that are embedded in the cell membrane these substances can be transported in to the cell. Ion channel proteins allow specific ions to pass through the protein channel. The ion channels are regulated by the cell and are either open or closed to control the passage of substances in to the cell. Carrier proteins bind to specific molecules, change shape and then deposit the molecules across the membrane. Once this complete the proteins return to their original position. (Bailey, 2013) ‘The ribosome then translates the mRNA in to a single chain precursor called preproinsulin; thereafter the removal of its signal peptide during insertion in to the endoplasmic reticulum generates proinsulin.’ (Barret, Brooks, Boitano, Barman, 2009)

Figure 3 – Pre/Pro Insulin Diagram.
(Noske, A. 2010)
Figure 4 – Rough & Smooth ER’s, Golgi and Vesicles.
Proinsulin consists of three parts: an amino- terminal B chain, a carboxy-terminal A chain and a connecting peptide in the middle known as a C peptide. In the endoplasmic reticulum, the proinsulin is exposed to several specific endopeptidases which excise the C peptide; this forms...
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