Questions: Glucagon increases blood glucose levels by stimulating - muscle cells to make glycogen - muscle cells to break down muscle glycogen - liver cells to break down liver glycogen - both liver and muscle cells to break down their glycogen stores

The answer is the third one: liver cells to break down liver glycogen.

Explanation for each option:

1. Muscle cells to make glycogen: This is incorrect. Glucagon does not stimulate muscle cells to make glycogen. Instead, it works to increase blood glucose levels, which would not be achieved by making glycogen in muscle cells.

2. Muscle cells to break down muscle glycogen: This is incorrect. While muscle cells do break down glycogen, this process is primarily stimulated by adrenaline (epinephrine) rather than glucagon. Glucagon mainly targets the liver.

3. Liver cells to break down liver glycogen: This is correct. Glucagon primarily acts on liver cells to break down glycogen into glucose, which is then released into the bloodstream to increase blood glucose levels.

4. Both liver and muscle cells to break down their glycogen stores: This is incorrect. Glucagon primarily targets liver cells, not muscle cells, for glycogen breakdown.

Summary: Glucagon increases blood glucose levels by stimulating liver cells to break down liver glycogen.