This selectively permeable barrier is essential to the cells daily function as it allows substances in such as amino acids, oxygen and glucose. It also ensures all waste products such as carbon dioxide are released. There are proteins embedded within the bilayer which performs various tasks, such as; carrier and channel proteins, these are used to move materials across the surface of the cell membrane in active transport, receptors for hormones and a way of communication between cells.
Two ways substances can be transported are passively which requires no energy and actively which requires energy in the form of ATP. The transport method used is dictated by the size of the molecule. The most common method, and used in the Beta cell, is diffusion; a passive method in which small or lipid soluble molecules, such as oxygen, carbon dioxide and water, can pass through the cell membrane to get in and out of the cell. The particles always move from an area of high concentration to low concentration and will continue until equilibrium is achieved.
Another method of passive transport is facilitated diffusion, in which the embedded proteins in the bilayer act as channels. Each channel is specific to the passage of a certain substance. Larger molecules such as glucose and amino acids that are lipid insoluble can pass through this way, as they are too big to fit through the phospholipid bilayer. The molecule attaches itself to a channel protein in the membrane, the protein changes shape releasing the molecule on the opposite side of the membrane. Just like diffusion, it also follows the concentration gradient from an area of high to low concentration until equilibrium is achieved.
Osmosis is the passive transportation of water which is a vital substance for the cell to function; the water diffuses through the cell membrane to follow the concentration gradient from a high water potential to a low water potential.
Active transport works in a similar way of facilitated diffusion as it requires a channel protein that changes shape to release the molecule on the other side of the membrane. However, it needs energy in the form of ATP because it moves the molecule against the concentration gradient from an area of low to high concentration.
Once these vital substances have been transported into the Beta cell; it can begin to make insulin. The Beta cell is a eukaryotic cell; the nucleus is the most obvious organelle and the starting point of protein synthesis (fig. 2.) It is surrounded by a nuclear envelope and contains many pores which allow substances to move between the nucleus and cytoplasm. Inside the nucleus is Deoxyribonucleic Acid (DNA) which contains the ‘instructions’ for making proteins, in order to extract and process these, an intermediate molecule is created called Messenger Ribonucleic Acid (mRNA), which also contains the genetic information to make the protein. The mRNA serves as a link to ribosomes. Also inside the nucleus is the nucleolus, this is a dense and spherical structure that plays an indirect role in protein synthesis by producing ribosomes.
Ribosomes are a small organelle not bound by a membrane, and are composed of protein and RNA (Ribonucleic Acid), they are the only site where proteins are made and play a huge role in protein synthesis. Ribosomes can sometimes float free in the cytoplasm or are attached to the membrane of the Rough Endoplasmic Reticulum (RER). Insulin is initially synthesised as preproinsulin in the ribosomes of the RER.
Endoplasmic reticulum is a complex network forming inter-connecting sacs or sheets called cisternae that work as a packaging system. There are two forms; smooth and rough. RER is covered in ribosomes and its main purpose is to manufacture proteins; in the Beta cells the main function of RER is to produce insulin. After the ribosomes have constructed the protein it is moved to the interior channels of the RER where enzymes further modify the protein to make it pro-insulin.
Once the protein is complete, it leaves the RER via transport vesicles that fuse to the Golgi Apparatus (GA), a stack of cistrnae flattened sacs formed by membranes. This is the main transport system within a cell; its main function is to modify proteins. As the protein travels between the layers of the cisternae some sugar groups are added or taken away, the proteins are then labelled for delivery. Pro-insulin is exposed to specific enzymes which generate the mature form of insulin; it is then packed in membranous sacs called secretary vesicles which leave the GA and head to the cell membrane for secretion.
Mitochondria are found in the cytoplasm of the cell, it is usually oval-shaped and has a double membrane; the inner one is folded to form cristae. This is the site of respiration and where energy is produced in the form of ATP required protein synthesis.
In the Beta cell, once insulin is ready to be secreted, it is done by exocytosis; an active transport method which requires ATP. It migrates packaged substances to the inner surface of the membrane, which fuse and release their contents outside the cell. Insulin is released when a high level of glucose has been detected in the blood. It causes cells to store glucose which reduces the blood sugar levels.