Liver and its role

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Maciej Pogorzelski                 Biology coursework

Liver coursework


I will investigate how bile salts aid the digestion of fats by enzyme lipase, by varying the concentration of the bile salts added to the mixture containing food source and the lipase enzyme.

Theoretical knowledge:

The human body is adapted to many functions, including detoxification. Another important function is the metabolism, which includes the breakdown fats (triglycerides). The organ responsible for this is the liver which is found on the bottom right side of the ribcage. Fig. 1.1 is the basic structure of the liver. As we can see it is boomerang shaped and consists of many blood vessels.

'The liver is the largest internal organ in the human body, and is an organ present in vertebrates and some other animals. It plays a major role in metabolism and has a number of functions in the body, including glycogen storage, decomposition of red blood cells, plasma protein synthesis, and detoxification. The liver is also the largest gland in the human body. It lies below the diaphragm in the thoracic region of the abdomen. It produces bile, an alkaline compound which aids in digestion, via the emulsification of lipids. It also performs and regulates a wide variety of high-volume biochemical reactions requiring very specialized tissues.'

ref. 1.1

Fig. 1.1

The interior of liver is made out of many canalicules and lobules. There is a blood vessel called hepatic portal vein. This blood vessel is very unique vessel, as it is capable of transporting three times more blood per minute than the hepatic artery (main source of oxygenated blood for the liver). Like every vein in the body, the hepatic portal vein carries deoxygenated blood. Normally veins carry blood away from organ to heart (pumps blood), while this vessel carries the blood into the liver. It brings blood rich in amino acids and glucose from the small intestine.

The Liver has got more than one function in the body. One of the most important is the production and secretion of bile, which can aid digestion of fats and then absorption may occur. When substances such as phospholipids, cholesterol, water, and bilirubin are absorbed from the intestine they are being transported to liver (via hepatic portal vein) where they are being converted into bile. Bile is transported to the gall bladder (storage) via many lobules. When the bile is needed, it will diffuse into the small intestine to digest lipids. This is done by the action of cholecysokinin on gall bladder, by making it release stored bile into the small intestine. There are other organs that aid the liver in the breakdown of lipids, especially pancreas.

‘The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both exocrine (secreting pancreatic juice containing digestive enzymes) and endocrine (producing several important hormones, including insulin, glucagon, and somatostatin)’, ref. 1.1.

Each of the substances secreted and produced has got distinctive role in the digestion of substances within the human body, such as

  • Pancreatic juice neutralises the acid from the stomach, so the enzymes have a better environment, so they can work more effectively. As we know the stomach acid (gastric juice) has very low concentration of pH 2 or even pH 1. The stomach is adapted to cope with such strong acid (folds which produces a lot of mucus). However the intestines have not got these adaptations, which is why it is important that the pH of the substances that enter small intestine have got pH closer to the neutral (which is 7)

Insulin is the hormone, which is involved in the storage of the glucose in form of glycogen, when the glucose level in the blood is high. The insulin is made and stored in the beta cells in the islet of Langerhans. The problem that may occur in the body is when the insulin is either not produced by the body (or in small amounts), or the insulin resistance. This disease is called diabetes mellitus. Group one diabetes are dependent on the injections of insulin, as their body don’t produce it at all, which is life threatening, as the glucose will not be converted to the glycogen for storage, so after all of the glycogen is used as the energy source the cells will not have any more “fuel” to carry on their functions.

Glucagon is involved in the carbohydrate metabolism, in the way of converting the glucagon into the glucose, when glucose level in blood is low. Glucagon is made released by the alpha cells in islet of Langerhans. Glucagon is also involved in the process of the gluconeogenesis, which is making the glucose out of non-carbohydrate substance (such as fats, lactate or amino acids). This process occurs when the glucose level in blood is low and all of the Glucagon storages are not available. The liver will convert the amino acids into the glucose, so the body has the source of the energy. This process includes deamination of amino acid (nitrogen part of amino acid is converted into urea, as nitrogen can form ammonia which is toxic in high concentrations), while rest of the molecule is converted into the pyruvate which is then converted into glucose. Liver is also able to use a protein, in way of converting it into an amino acid, if there is shortage in the particular amino acid.

Insulin is also involved in the process called glycogenolysis. This is the process in which glucose is converted into glycogen, as glucose is small and soluble it is not useful for storage (liver is the organ in which glycogen is stored). When glucose level in blood falls, the alpha cells in islets of Langerhans release hormone called glucagon which binds to glycoprotein receptors of hepatocytes (liver cells) and initiate processes which eventually break glycogen into glucose. This causes release the glucose into the bloodstream and the glucose level in blood rises.

Fats (lipids) are molecules made out of compounds called triglycerides. The name triglyceride suggests that these molecules are made out of one molecule of glycerol and 3 molecules of fatty acids.

Above is the simple structure of lipid molecule. The hydrocarbon tails repeal water molecules (they are hydrophobic), which makes lipids non-soluble in water. All of the triglycerides are formed by a condensation reaction and broken by hydrolysis (opposite reaction to fig. 1.5).

An enzyme is the biological molecule, globular proteins (proteins that has spherical shape of globe), known for catalysing reaction, by offering the alternative route for the reaction and lowering the activation energy (minimum energy needed for reaction to occur, during collision between substrates) for the reaction, without being used up. The basic structure of an enzyme is shown in the picture below.

figure 1.2

As you can see the enzyme consist of region called active site. This is the part of the enzyme that is complimentary for particular substrate. This means that only particular substrate can bind to this active site, as the active site of an enzyme has specific shape. There are situations in which more than 2 substrates that bind to the enzyme to form one product. This is called “lock and key” theory, as only particular key will open lock.

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 Emil Fisher in 1894, stated that ‘that this was because both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. This is often referred to as "the lock and key" model.

ref. 1.2

 When substrate binds to the active site of an enzyme, we commonly refer to it as substrate-enzyme complex. Referring to figure 1.3 activation energy changes when using enzyme and without using it. You can clearly see that activation energy without energy is much higher, than activation energy when enzyme is used. Higher activation energy means that there must ...

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