Question 4b

Explain why there isn’t an arrow pointing in the reverse direction of arrow F. [1]

• No Reverse Movement: Glucose cannot move from muscles back to the blood.
• Reason: Muscle cells lack the enzyme glucose-6-phosphatase, which is needed to convert glycogen into free glucose that can exit the cell.
• Storage Form: Muscles store glucose as glycogen and use it internally for energy during muscle activity, rather than releasing it back into the bloodstream.
• Result: Glucose only flows into muscle cells (from blood), not out of them, hence no arrow in the opposite direction to arrow F.