Homeostasis and the control of blood glucose

Last time

Before we start: last time, in The Calvin cycle, limiting factors and rate investigations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Keeping the inside steady

Outside, the weather swings from freezing to scorching. Inside you, blood glucose stays close to five millimoles per litre, and body temperature close to thirty-seven degrees. Holding conditions steady is homeostasis, and blood glucose is the favourite example, because it has everything the exam wants: receptors, hormones, feedback and signalling inside cells.

Principles of homeostasis

Homeostasis is the maintenance of a stable internal environment, within narrow limits, around a set point. It keeps enzymes and cells working efficiently. A receptor detects a change away from the set point. A coordinator, such as the brain or pancreas, processes the information and signals an effector. The effector produces a response that reverses the change, bringing the level back to the set point. This is negative feedback. Negative feedback needs a corrective response in both directions, so it can raise or lower the value. This gives control around the set point.

Blood glucose control: high glucose

After a meal, blood glucose rises. The beta cells in the islets of Langerhans in the pancreas detect the rise, and secrete insulin into the blood. Insulin binds to receptors on liver, muscle and fat cells. It causes more glucose transporter proteins to be added to the cell surface membrane, so more glucose enters the cells. In the liver and muscles, insulin activates enzymes that convert glucose into glycogen, which is glycogenesis. The blood glucose falls back to the set point.

Blood glucose control: low glucose

When glucose falls, as between meals or in exercise, the alpha cells in the pancreas secrete glucagon. Adrenaline from the adrenal glands acts in a similar way in stress or exercise. Both bind to receptors on liver cells, and cause the conversion of glycogen into glucose, which is glycogenolysis. The liver also makes glucose from non carbohydrates, gluconeogenesis. Glucose is released into the blood, and the concentration rises towards the set point.

A past-paper example

October November 2024, Paper 42, question 10 part a. Explain the principles of homeostasis. Four marks. There is a set point. A receptor detects a deviation from it. A coordinator sends signals to an effector. The effector brings about a response that reverses the change. This is negative feedback, and it keeps the internal environment within narrow limits.