The action of insulin by: attaching to receptors on the surfaces of target…

The idea

Insulin is a protein hormone secreted by the beta cells of the islets of Langerhans in the pancreas when they detect a rise in blood glucose concentration. It travels in the blood and attaches to specific receptors on the cell-surface membranes of target cells, mainly liver, muscle and fat cells, whose binding sites are complementary in shape to insulin. Binding has two effects. First, vesicles carrying glucose transporter channel proteins fuse with the cell-surface membrane, so more channel proteins are present and glucose enters the cell faster by facilitated diffusion. Second, enzymes inside the cell are activated that catalyse the conversion of glucose to glycogen, which is glycogenesis, and to fat, while glycogenolysis and gluconeogenesis are inhibited. Both effects lower the blood glucose concentration, and that fall is detected by the beta cells, which reduce insulin secretion.

What earns the marks

• Insulin is secreted by the beta cells of the islets of Langerhans when blood glucose concentration rises.
• Insulin attaches to specific receptors on the cell-surface membranes of target cells.
• The number of channel or transporter proteins in the membrane increases, so more glucose is taken up by facilitated diffusion.
• Enzymes are activated that catalyse the conversion of glucose into glycogen.
• The resulting fall in blood glucose concentration reduces insulin secretion, which is negative feedback.

Common mistakes and how to put them right

• Saying insulin converts glucose into glycogen itself, when it activates the enzymes that catalyse the conversion.

• Describing insulin as entering the target cell, when it attaches to a receptor on the outside of the membrane.

• Claiming glucose enters cells by active transport under insulin, when it is facilitated diffusion through channel proteins.

• Writing islets of Langerhans without specifying beta cells, when the question distinguishes the two cell types.

• Assuming insulin acts on every cell in the body equally, which ignores the idea of target cells with specific receptors.

Exam tips

• Use the phrase attaches to receptors on the cell-surface membrane, because that wording is the mark.
• Two separate marks sit in insulin's action, more transporter proteins and activated enzymes; give both.
• If the question concerns a diabetic patient, check whether the problem is insulin production or receptor sensitivity before writing.