Phloem Loading in Companion Cells

Steps:

• Companion cells in phloem actively load sugars for transport.
• Sucrose, a disaccharide, is the primary form moved into cells via symporters.
• Monosaccharides like glucose enter through facilitated diffusion or active transport.
• Polysaccharides and large polymers remain outside as they cannot cross membranes easily.

Why A is correct:

• Companion cells use proton-sucrose symporters (e.g., SUT1) to import disaccharides like sucrose and channels for monomers like glucose, per plant transport physiology.

Why the others are wrong:

• B: Polysaccharides like starch are insoluble storage molecules, not transported into cells.
• C: Polymers are macromolecules too large for direct cellular uptake.
• D: Polymers cannot enter companion cells due to size exclusion.

Final answer: A