Explain how negative feedback control mechanisms regulate blood glucose concentration, with reference to the effects of insulin on muscle cells and liver cells and the effect of glucagon on liver cells

SouravNovember 1, 2024

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Negative feedback control mechanisms are essential in regulating blood glucose levels, ensuring they remain within a narrow, healthy range. The hormones insulin and glucagon, both secreted by the pancreas, play complementary roles in maintaining this balance. Here’s how these hormones use negative feedback to regulate blood glucose levels, acting on muscle cells and liver cells:

1. High Blood Glucose Levels and the Role of Insulin

After eating, blood glucose levels rise as glucose from digested food enters the bloodstream. To prevent hyperglycemia (high blood sugar), the pancreas detects this increase and responds by releasing insulin from the beta cells in the islets of Langerhans.

Effects of Insulin on Muscle Cells and Liver Cells

• Muscle Cells: Insulin binds to specific receptors on the surface of muscle cells, triggering a series of intracellular reactions that allow GLUT4 glucose transporters to move to the cell membrane. This increases the uptake of glucose from the blood into the muscle cells, where it can be used immediately for energy or stored as glycogen for later use.
• Liver Cells: In liver cells, insulin also binds to its receptors, promoting the uptake of glucose and activating enzymes that convert glucose to glycogen (a process called glycogenesis). Insulin inhibits enzymes involved in glycogen breakdown, effectively storing glucose as glycogen and preventing more glucose from entering the bloodstream.

As glucose is absorbed by muscle and liver cells, blood glucose levels fall back toward normal. This drop in blood glucose reduces the stimulus for insulin release, so insulin secretion decreases, thus maintaining glucose homeostasis. This mechanism demonstrates negative feedback, as insulin release decreases when blood glucose returns to normal, preventing further glucose uptake that could lead to hypoglycemia (low blood sugar).

2. Low Blood Glucose Levels and the Role of Glucagon

Between meals or during fasting, blood glucose levels decrease as cells use glucose for energy. To avoid hypoglycemia, the pancreas detects the fall in blood glucose and releases glucagon from the alpha cells in the islets of Langerhans.

Effect of Glucagon on Liver Cells

• Liver Cells: Glucagon primarily acts on liver cells, binding to specific receptors on their surface and initiating a signaling pathway (as previously described). This pathway activates enzymes that stimulate the breakdown of glycogen into glucose (a process called glycogenolysis). Additionally, glucagon promotes gluconeogenesis, where non-carbohydrate substrates (like amino acids) are converted into glucose.

These actions increase glucose release from the liver into the bloodstream, raising blood glucose levels back toward normal. As blood glucose rises, the stimulus for glucagon release is reduced, so glucagon secretion decreases. This negative feedback ensures that blood glucose does not rise excessively.