permeability
permeabiity
permeability
permeabiity
permeabiity
permeability
They must either be lipid soluble (e.g. steroids) or very small (e.g. ions).
mass
The speed doesn't necessarily determine the permeability, but the size does. Smaller molecules such as O2 can easily enter the cell while CO2 leaves the cell. There are other criteria that also determines whether a molecule can pass through the plasma membrane such as its solubility. Fat soluble molecules such as steroids can easily pass through the membrane.
Lipid solubility determines if it will diffuse across. The presence of specific protein carrier molecules determines if it will be transported across the membrane.
One factor is the channel of the molecule, without them the ions and polar molecules would not be able to pass across. Another factor is the size of the molecule, they determine how fast it will defuse.
Whether molecules are able to pass through the membrane depends on the size of the molecules. Smaller ones can, and larger ones cannot. Glucose can pass through a cell membrane because it is a monomer, which is a smaller molecule than the polymer molecules of starch.
Osmosis has taken place when there is a net movement of solvent molecules from an area of higher concentration to an area of lower concentration across a semi-permeable membrane. This can be observed by changes in the volume or concentration of the solutions on either side of the membrane.