Why can glucose-6-phosphate no longer leave the cell once it has been formed?
AIt binds tightly to the nuclear envelope
BIt is instantly split by an isomerase
CIt is too large to fit any channel
DIts negative phosphate blocks membrane crossing
Answer & Solution
Correct answer: D. Its negative phosphate blocks membrane crossing
1. Phosphorylating glucose adds a negatively charged phosphate group to it.
2. The interior of the plasma membrane is hydrophobic and repels charged groups.
3. The negatively charged phosphate therefore will not allow the sugar to cross that interior.
4. The same change also stops the sugar interacting with the GLUT transporter proteins.
5. Size is not the barrier here, since glucose itself crosses by facilitated diffusion perfectly well.
6. An isomerase does act on glucose-6-phosphate, but it rearranges the molecule rather than splitting it.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.2 Glycolysis_
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