Eat a bowl of pasta and a large amount of glucose floods into your blood over about half an hour. An hour later the level is almost back where it started. Nobody measured anything and nobody made a decision — two hormones from the pancreas did it automatically. Here is exactly how, and what goes wrong when the system breaks.
📘 What you need to know
Blood glucose is kept within narrow limits, around 5 mmol dm−3, by negative feedback.
The controlling tissue is the islets of Langerhans in the pancreas, which contain α cells and β cells.
β cells release insulin when glucose is too high. Insulin makes cells take glucose in and makes the liver store it as glycogen (glycogenesis).
α cells release glucagon when glucose is too low. Glucagon makes the liver break glycogen back down (glycogenolysis) and make glucose from amino acids (gluconeogenesis).
Type 1 diabetes: the β cells are destroyed, so little or no insulin is made.
Type 2 diabetes: insulin is still made, but target cells stop responding to it — insulin resistance.
Why glucose has to be controlled
There are two reasons, and a good answer gives both.
Too low is dangerous. Glucose is the main respiratory substrate. If the level drops too far, cells — especially brain cells — cannot make enough ATP.
Too high is also dangerous. Glucose is soluble, so it affects the osmotic concentration of the blood. Very concentrated blood draws water out of cells by osmosis, and over time high glucose damages blood vessels and organs.
So the body is not just topping glucose up. It is squeezing it between an upper and a lower limit, which is exactly the negative feedback pattern from the previous page.
The pancreas: two cell types, two hormones
The pancreas does two completely separate jobs. Most of it is exocrine tissue that makes digestive enzymes and pours them into the gut. Scattered through it are small patches of endocrine tissue called the islets of Langerhans, which release hormones straight into the blood. Only the islets are involved in glucose control.
Each islet contains two cell types you must be able to name:
β (beta) cells — detect a rise in blood glucose and secrete insulin.
α (alpha) cells — detect a fall in blood glucose and secrete glucagon.
Notice something neat here: these cells are both the receptor and the effector gland. They detect the change themselves and they secrete the hormone themselves, so the loop is short and fast. That is why glucose control does not need the brain in the way temperature control does.
Insulin: bringing glucose down
After a meal containing carbohydrate, glucose is absorbed from the small intestine and blood glucose rises. The β cells detect this and secrete insulin, which travels in the blood to target cells all over the body. The main targets are liver and muscle cells.
Insulin has several effects, and they all pull in the same direction — glucose out of the blood:
Glucose channels open in the cell surface membranes, so glucose moves out of the blood and into cells by facilitated diffusion.
Liver and muscle cells convert glucose into glycogen for storage. This is glycogenesis.
The rate of respiration increases, so more glucose is used up.
Excess glucose is converted into fatty acids and stored as fat.
Insulin, in one line
glucose too high → β cells → insulin → cells take glucose in and liver stores glycogen → glucose falls
Glucagon: bringing glucose back up
Glucose falls after a period of fasting, or during exercise when muscles are using it quickly. The α cells detect the fall and secrete glucagon, which also travels in the blood to the liver.
Glucagon activates enzymes that hydrolyse glycogen back into glucose, which is released into the blood. This is glycogenolysis.
It causes amino acids to be converted into glucose. This is gluconeogenesis — literally “making new glucose”.
The rate of respiration decreases, so less glucose is used up.
Three words that look alike.Glycogenesis = making glycogen (storing). Glycogenolysis = splitting glycogen (releasing). Gluconeogenesis = making glucose from something that was not a carbohydrate. Read the middle of the word, not the start.
Both arms start and end in the same place. If you can redraw this from memory with the correct cell type on each side, you can answer almost any glucose question.
What the graph looks like
A glucose tolerance test is a favourite data question. A person drinks a measured glucose solution and their blood glucose is measured for the next few hours. In someone without diabetes the level rises, peaks, and is back to normal within about two hours. In someone with untreated diabetes it starts higher, climbs higher, and is still high hours later.
Both curves fall after the peak, so “it goes down” is not enough. What separates them is how far it falls and how long it takes.
Diabetes: when the control loop fails
Diabetes is a condition where the homeostatic control of blood glucose has failed or got worse. Insulin no longer does its job properly, so glucose stays high in the blood. That produces a set of symptoms you should be able to explain, not just list:
Glucose in the urine. The kidneys cannot reabsorb all of the excess glucose, so some passes out.
Thirst and dehydration. The glucose in the kidney tubules makes large volumes of urine, so the body loses water.
Tiredness. Glucose stays in the blood instead of entering cells, so less respiration happens and less ATP is made.
Long-term organ damage if the level stays dangerously high after meals.
Type 1 and type 2 compared
Type 1
Type 2
Cause
An autoimmune response destroys the β cells of the islets of Langerhans, so insulin production stops
β cells still make insulin, but the receptors on target cells are fewer or no longer respond — insulin resistance
When it usually starts
Usually begins in childhood
More common than type 1, and usually develops in older adults
Insulin level in the blood
Very low or none
Normal or high at first, as the pancreas secretes more and more to compensate
Main risk factor
Not linked to diet or body mass
Obesity and a diet high in rapidly digested carbohydrate
Treatment
Regular blood glucose testing, insulin injections, and a modified diet
Medication, a low carbohydrate diet, and regular exercise to lower blood glucose
The key difference to hold on to: in type 1 the hormone is missing; in type 2 the hormone is there but the message is not received. That is why injecting insulin fixes type 1 but is not the first treatment for type 2 — adding more of a signal that cells are ignoring does not help much.
Glucagon and glycogen are the single most mixed-up pair of words in this topic. Glucagon ends in “-on”, like hormone. Glycogen is the storage polysaccharide. Write them slowly in the exam; a slip here can lose a mark even when your biology is right.
Worked examples
WORKED EXAMPLE 1
Describe how blood glucose concentration is returned to normal after a meal containing carbohydrate. [4]
Step 1: the change and the detection
Glucose is absorbed from the intestine, so blood glucose rises; the β cells of the islets of Langerhans detect this.
Step 2: the hormone
The β cells secrete insulin into the blood; it travels to target cells, mainly liver and muscle.
Step 3: what insulin does
Glucose channels open so cells take glucose up by facilitated diffusion, and the liver converts glucose to glycogen (glycogenesis). Respiration also increases.
Step 4: close the loopBlood glucose falls back towards the set pointfour clear stages: rise, detection, hormone, effect — then always finish by saying the level returns to normal
WORKED EXAMPLE 2
A runner has not eaten for six hours and then runs for 40 minutes. Explain how their blood glucose is kept from falling too low. [4]
Step 1: the change
Respiring muscle uses glucose quickly, so blood glucose starts to fall.
Step 2: detection and hormone
The α cells of the islets of Langerhans detect the fall and secrete glucagon.
Step 3: what glucagon does in the liver
It activates enzymes that hydrolyse glycogen into glucose (glycogenolysis), and it causes amino acids to be converted into glucose (gluconeogenesis). Glucose is released into the blood.
Step 4: close the loopBlood glucose rises back towards the set pointthe glycogen store is the reason a person can go hours without eating and still think clearly
WORKED EXAMPLE 3
Two people take the same glucose drink. Person A’s blood glucose returns to 5 mmol dm−3 after 2 hours. Person B’s is still 11 mmol dm−3 after 4 hours. Suggest an explanation for the difference. [3]
Step 1: read the data, do not guess
Person B starts higher, peaks higher and does not recover within four hours.
Step 2: link the pattern to the mechanism
Glucose is not being taken out of the blood, so either not enough insulin is secreted or target cells are not responding to it.
Step 3: name the condition, carefullyPerson B is likely to have diabetessay “suggests” and give both possible causes — the graph alone cannot tell you whether it is type 1 or type 2
💡 Exam tip
Always name the cell type as well as the hormone: β cells with insulin, α cells with glucagon.
Say islets of Langerhans, not just “the pancreas”, when the question asks where.
Use the proper process names — glycogenesis, glycogenolysis, gluconeogenesis. They are quick marks.
Insulin does more than one thing. If a question is worth 3 or 4 marks, give uptake into cells, glycogen storage, and increased respiration.
For data questions, quote numbers from the graph with units, including the time taken to return to normal.
For type 2, the key phrase is insulin resistance or “receptors no longer respond” — not “no insulin is made”.
⚠ Common mix-up
Glucagon and glycogen. One is the hormone, one is the storage polysaccharide.
Swapping α and β cells. Beta cells make insulin. A memory hook: B for “brings it down”.
Saying insulin “turns glucose into energy”. Energy is not a substance. Insulin causes uptake, storage and respiration.
Claiming type 2 diabetics make no insulin. They usually make plenty at first — the cells stop listening.
Forgetting that glucose is also an osmotic problem, not only a respiration problem.
Writing that the pancreas is only an endocrine gland. Most of it is exocrine tissue making digestive enzymes.
Up next: Thermoregulation — the same negative feedback shape again, but this time with the hypothalamus in charge and the skin doing most of the work.
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