Glucose (circles) needs to enter the cell for energy. Based on the diagram, how is glucose going to cross the membrane?

Question

BlackTom AI · Accepted Answer

Glucose is a polar, relatively large molecule, so it cannot cross the hydrophobic interior of the phospholipid bilayer by simple diffusion.

1) Relevant knowledge points and concepts
- Cell membranes are phospholipid bilayers with hydrophobic fatty-acid tails in the interior; small nonpolar molecules diffuse directly, polar or charged molecules do not.
- Facilitated diffusion uses specific membrane proteins (channels or carrier/transport proteins) to move molecules down their concentration gradient without ATP.
- Active transport moves substances against their gradient and requires energy (ATP). Osmosis specifically refers to water movement across a membrane.

2) Step-by-step reasoning
- Determine glucose properties: polar, relatively large (monosaccharide) → low permeability through hydrophobic core.
- Determine direction/energy requirement from diagram context: glucose needs to enter for energy (implies movement down a typical extracellular → intracellular gradient in many cases), and no ATP source or pump is shown.
- Match transport mechanism: polar + down gradient + membrane crossing by protein → facilitated diffusion via a carrier/transporter (e.g., GLUT proteins).

3) Calculations
- No numeric calculation is needed for this conceptual question.

4) Why the correct answer is correct
- Facilitated diffusion with a carrier protein fits all constraints: glucose cannot pass directly through phospholipid tails, it does not require energy if moving down its concentration gradient, and it uses specific transporters that bind glucose and change conformation to move it across the membrane.

5) Why other options are incorrect
- (a) Active transport: would require ATP and move against a gradient; not indicated here. 
- (b) Diffusion through phospholipid tails: incorrect because glucose is polar and too large for simple diffusion across the hydrophobic core. 
- (c) Osmosis: describes water movement only, not glucose transport.

6) Conclusion
- Therefore, glucose crosses the membrane by facilitated diffusion using a carrier protein or transporter, matching answer d.

Glucose (circles) needs to enter the cell for energy. Based on the diagram, how is glucose going to cross the membrane?单项选择题

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