GLUCOSE AND THE PANCREAS

First posted on 20/04/22.

Glucose concentration regulation is a subject which is a great example of one of my least favourite things about biology. Biology is absolutely rife with stupid names for things. For example, who the hell decided on mitosis and meiosis? They're completely different processes! If you make a simple spelling mistake you can end up talking about completely the wrong thing and be none the wiser. The pancreas is chock-full of these stupid words.

The pancreas is an organ shaped a little like a fat tadpole, with a head, body and tail. It is nestled behind the stomach. It acts as both an exo- and endocrine gland, secreting enzymes into the duodenum and hormones into the blood.

Most of the pancreas is exocrine glandular tissue. This is organised into acini which show up as small, dark, berry-like clusters under the microscope. It produces digestive enzymes and pancreatic juice - yes that's the scientific term - which are secreted into the pancreatic duct, a tube running down the length of the pancreas which empties into the duodenum. Pancreatic juice is an alkaline fluid, and the enzymes include amylases (starch into sugars), proteases (proteins into amino acids) and lipases (lipids into fatty acids and glycerol).

The endocrine tissue and function of the pancreas is a little more complex. Histology-wise, the endocrine tissue is organised into regions called the islets of Langerhans which appear lighter than the pancreatic acini, and are generally larger. This tissue is comprised of two different cell types - α (alpha) and β (beta) cells - based on the hormone they secrete. The α cells produce glucagon and the β cells produce insulin. I'll explain the function of these hormones in a minute. While they are hard to differentiate between on a micrograph, α cells are generally larger and more numerous.

Pretty simple right? Now onto the regulation of blood glucose concentration, or as I like to call it, a special hell for dyslexic people.

Blood glucose concentration is a vital part of homeostasis, because if it comes out of balance you will suffer from hyper- or hypoglycaemia and then die. Hyper meaning too much, and hypo meaning too little. Diabetics will be familiar with the process. The body has a range of different ways of maintaining optimal blood sugar levels.

To increase blood glucose, you can eat food heavy in carbohydrates (which can be broken down into glucose) and sugar (which is often sucrose). These are digested and the resulting glucose is absorbed into the blood. Glucose levels can also be increased by our first pair of irritatingly similar words: glycogenolysis and gluconeogenesis.

Glycogenolysis is when glycogen (a highly branched starch molecule made of glucose) is broken down into glucose. Glycogen is stored in the liver and muscles so it can be broken down in times of need. You can remember it because it starts with the word glycogen, and the word lysis means breaking. So it's breaking glycogen.

Gluconeogenesis is the production of glucose from non-carbohydrate sources such as glycerol and amino acids. It occurs in the liver. You can remember it because it starts with gluco, meaning glucose. Neo means new, and genesis means creation. So it's creating new glucose.

Moving onto the ways the body can reduce blood sugar levels brings us our third confusing word. Glycogenesis is the production of glycogen from glucose. It is not glycoNEOgenesis because nothing new is created, you're just connecting glucose together to form glycogen. It also happens mostly in the liver, and takes glucose out of the blood for storage.

Of course, respiration also takes glucose out of the bloodstream, so if you're exercising your blood sugar will fall faster.

Alright! Now let's talk about insulin and glucagon.

Insulin reduces blood glucose concentration. It does this by:

• binding to glycoprotein receptors on almost all cell surfaces, opening glucose channels and increasing uptake rate.
• increasing respiratory rate.
• increasing the rate of glycogenesis in the liver (that's the formation of glycogen from glucose).
• increasing the rate of glucose to fat conversion.
• inhibiting the release of glucagon.

The liver constantly breaks down insulin, so the β cells have to constantly produce it. Insulin secretion happens in much the same way that neuronal communication occurs. Normally, potassium ions are free to diffuse in and out of the cell through channels. The normal potential is -70mV with respect to the outside of the cell. When blood glucose concentration rises, glucose is transported into the cell. It is metabolised into ATP which binds to and closes the potassium channels. This reduces the potential difference to -30mV and depolarisation occurs. This opens voltage-gated calcium channels. The calcium ions cause the secretory vesicles containing insulin to release the insulin by exocytosis.

Glucagon increases blood glucose concentration. It does this by:

• stimulating gluconeogenesis in the liver (that's producing new glucose from other substances).
• stimulating glycogenolysis in the liver (that's breaking down glycogen into glucose).
• reducing the rate of glycogenesis (that's the creation of glycogen).

And now you see how confusing it gets. Glucagon affects the rates of gluconeogenesis, glycogenolysis and glycogenesis in order to control the amounts of glucose and glycogen in the body. At least glucagon only affects the liver and fat cells, which are the only cells with the right receptors for it.

As a final note, the control of blood glucose concentration is a textbook example of negative feedback. When blood glucose levels rise, insulin is released and it reduces them. When blood glucose levels fall, glucagon is released and it increases them. The concentration of glucose in the blood constantly fluctuates around the optimum level.

So yeah! That's a whistle-stop tour of the function of the pancreas. I'm going to go read a book with nicer words now. Like The Hungry Caterpillar.

Sources:

OCR A Level Biology A textbook