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The phospholipid bilayer is semipermeable, meaning in cases other than bulk transport-endocytosis&exocytosis-, it does not compromise its own integrity to allow molecules to enter. The molecules that can most easily diffuse through a cell membrane are small, nonpolar molecules such as N2, O2, and CO2. Ions and polar molecules will have a difficult time crossing the membrane even if they are small because of the middle, hydrophobic layer. They can still pass through, just not as easily. Large molecules, however, cannot pass through the membrane due to their size, regardless or their polarization, and so they rely on those proteins embedded in the bilayer to transport them across (or, in some cases, endocytosis, which is when the cell membrane forms a kind of pocket - looks like a little mouth- and just engulfs the molecules). In this way the proteins are the only gap in what you could imagine as a sort of very fine and very picky filter. Larger molecules cannot get through the mesh of the filter and so they need to be recognized and passed through by those helpful little proteins.

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16y ago
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8y ago

Because they are large. Hard to diffuse and toxic potential

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8y ago

Cell membranes arepartially permeable. They do not allow laarge molecules to pass

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11y ago

because it is to big

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Q: Why can't big molecules pass through cell wall?
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Molecules can pass through the cell membrane of the human cell?

The molecules that can pass through the cell membrane of the human cell include water. Other molecules include fat soluble vitamins.


Why cant big molecules pass through wall guts?

The cell walls of the intestinal cells do not allow the passage of large molecules through them; in addition, the tight junctions between the cells blocks the transfer of large molecules through the interstitial space.


Do chemicals pass in and out of a cell through openings of the nucleus?

No they pass through the cell membrane. The cell membrane is selectively permeable to ions and organic molecules


How do you large molecules and ions pass through the cell membrane?

Recently had a homework on the cell membrane, i do know that larger molecules that cant fit through the polar heads into the cell (like gases can...and small molecules?!) can get through only if they qualify the shape fitting of the channel protein- the one that reaches all the way from the outside to the inside of the membrane. hope this helps


How can polar molecules pass through cell membranes?

Through facilitated diffusion


What part of the cell membrane allows small molecules to pass through?

the cell membrane


What allows water molecules to pass through?

water can pass through cell membranes by osmosis- similar to diffusion


What is meant by cell membrane is semipermeable?

It allows only certain molecules to pass through.


What kinds of molecules pass through a cell membrane most easily?

small and hyrdophobic molecules


What kind of substance cannot pass through the cell membrane?

lipids, and ribosomeslipids


Which molecues cross the membrane of a cell easily and which do not?

Non-polar molecules (such as fatty acids, steroid hormones and O2) pass freely through the cell membrane. Small uncharged molecules (such as H2O) also pass freely, but are slower. Large, polar molecules and ions (such as Na+ and K+) do not pass freely. Macromolecules (such as proteins and polysaccharides) do not pass through the cell membrane. Molecules and ions that cannot pass freely through the cell membrane rely on other means, such as protein transporters, to move in to the cell.


The process by which glucose can pass through a cell membrane by combining with special carrier molecules is called?

Facilitated diffusion is the process by which glucose can pass through a cell membrane by combining with special carrier molecules.