Question #1e769
1 Answer
Starch is too large to cross the plasma membrane.
Explanation:
The starch can't cross the lipid bilayer of the membrane. This is mainly because of the size of the starch molecule, as it is just a lot of sugars linked together (ie it is a polysaccharide). Smaller (uncharged) molecules can pass through the membrane.
Try to imagine this molecule:
passing through this bilayer:
This molecule of starch is a lot larger than the individual parts of the membrane (which aren't that small themselves).
Even in multicellular organisms, polysaccharides (like starch and glycogen), need to be broken down to monosaccharides (simple sugars) before crossing the plasma membrane. Even these can't cross the membrane with the assistance of transport proteins.
