Define homeostasis and explain it in the context of blood glucose concentration.

Homeostasis is the maintenance of an constant internal environment around a strict set of parameters. Homeostasis is important to maintain optimal conditions for biochemical reactions within the body, especially those controlled by enzymes. Enzyme activity is dependent on factors such as temperate, pH, substrate concentration and enzyme concentration. Enzymes work by providing an alternate chemical pathway with a lower activation energy so that the chemical reaction speeds up. Enzymes do this by weakening bonds in the substrate to make reaction more likely and they effectively concentrate the substrates so that they are in closer proximity so reaction is more likely. The transition state of an enzyme is when it is structurally and geometrically ready and favourable for a reaction to occur. Enzymes aim to help the substrate mimic this transition state.

Exam note - Enzymes are denatured at extreme conditions that are not normal. Enzymes are not "killed" as they are not living.

Hypoglycaemia is the state of low blood glucose levels. This usualy occurs when fasting or going without meals for a long time or after exercise. It can also occur in Type 1 Diabetes. The low blood glucose level is detected by the Alpha Cells in the Islets of Langerhans in the Pancreas. The alpha cells then secrete glucagon, which migrates into the blood and acts on the liver and muscle cells. This increases glycogenolysis and gluconeogenesis. Both these processes function to increase the blood glucose concentration. This is an exocrine function of the Pancreas.

Glycogenolysis is the conversion of glycogen stores in the Liver into glucose. This is completed by enzymes.

Glycogen --> glucose-1-phosphate --> glucose-6-phosphate --> glucose

This process is also caused by release of adrenaline during the "fight or flight" response in increased activation of the sympathetic nervous system. Adrenaline is made in the adrenal medulla. 

Gluconeogensis is the process of converting fats and amino acids into to glucose.

Hyperglycaemia is the state of high blood glucose levels, usually after a meal or as a result of diabetes. The high blood glucose levels are detected by the beta cells and stimulate the release of insulin. Insulin acts on the liver and muscle cells. It decreases the use of fatty acid and other respiratory subtrates in order to favour the use of glucose. This is to decrease the blood glucose concentration. It also increases the number of glucose channels in the cell membrane of the most metabolically active cells. It stimulates glycogenesis and reduced gluconeogenesis and glycogenolysis. 

Glycogenesis is the process of converting excess glucose into glycogen for storage in the liver and muscle cells for later use during exercise or when the blood glucose concentration falls too low. 

Insulin is secreted by the beta cells. When the blood glucose concentration increased, glucose is taken up by the beta cells and this is phosphorylated and enters the glycolytic pathway. This causes gated potassium channels to close. The increase in intracellular ion concentration causes the inside to be less negative than the outside. This causes voltage-gated calcium channels to open and calcium ions move in by facilitated diffusion. This makes the inside of the beta cell more positive. This stimulates the secretion of insulin-containing vesicles by exocytosis at the Beta cell plasma membrane. This is an example of the regulatory secretory pathway.

EXAM NOTES:

1 - Enzymes are denatured at extreme conditions that are not normal. Enzymes are not "killed" as they are not living.

2- Maintenance of a constant internal environment within narrow limits is called homeostasis.

3- Hypoglycaemia - alpha cells - glucagon - gluconeogensis and glycogenolysis - fasting, diabates and post-exercise.

4 - Hyperglycaemia - beta cells - insulin - increased glycogenesis and decreased gluconeogensis and glycogenolysis - post-meal and diabates.

5 - BETA CELLS - glucose in --> potassium channels close --> calcium channels open and ions move in --> vesicles containing insulin move to plasma membrane --> secreted by exocytosis.